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Utilization of shrubs and trees for browse, fuelwood and timber in the Tamaulipan thornscrub, northeastern Mexico
Forest Ecology and Management, 36 (1990) 61-79
61
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Utilization of shrubs and trees for browse, fuelwood and timber in the Tamaulipan thornscrub, northeastern Mexico
N i c k R e i d 1, J o r g e M a r r o q u i n a n d P e t e r B e y e r - M i i n z e l 2
Facultad de Ciencias Forestales, Universidad Aut6noma de Nuevo Lebn, Apdo. Postal 41, Linares N.L. 67700 (Mexico) (Accepted 29 August 1989)
ABSTRACT Reid, N., Marroquin, J. and Beyer-Miinzel, P., 1990. Utilization of shrubs and trees for browse, fuelwood and timber in the Tamaulipan thornscrub, northeastern Mexico. For. Ecol. Manage., 36: 6179.
The Tamaulipan thornscrub is the subtropical, semi-arid vegetation that covers the Gulf coastal plain of northeastern Mexico. Leguminous trees and shrubs constitute one-third of the diverse woody flora, which is exploited by the rural population for extensive grazing of livestock, fuelwood and timber for fencing and construction. Cutting intensity and extent of browsing damage to 53 woody species was assessed in 40 sites stratified according to habitat in the region of Linares, Nuevo Le6n. Principal uses of the timber were identified in interviews with campesinos. Unarmed shrubs and trees such as Bernardia myricaefolia, Arnyris texana, Citharexylum berlandieri and Helietta parvifolia were most heavily browsed: H. parvifolia, Condalia hookeri, Pithecellobium pallens, Cordia boissieri and Prosopis laevigata were the most frequently cut trees. Helietta parvifolia, a non-legume, was the most important multipurpose tree in the region, and three legumes - Pithecellobiurn ebano (tree), Acacia berlandieri and Eysenhardtia polystachya (shrubs) - provided both browse and timber or fuelwood. The most heavily exploited and highly valued timber species had dark, hard wood and were slower-growing than the high-yielding legumes commonly associated with multipurpose trees and shrubs in the semi-arid tropics. Sustainable, managed silvopastoral and agroforestry systems based on the indigenous forage, timber and fuelwood trees and shrubs need to be developed in the Tamaulipan thornscrub if traditional land-use practices are to be improved and the excessive exploitation and overgrazing of dwindling plant resources are to be reversed. More information is required regarding the response of valuable woody species to browsing and cutting, their ease of propagation, and their capacity to increase on landscapes where less desirable elements have been selectively removed. ~Present address: Department of Ecosystem Management, University of New England, Armidale N.S.W. 2351, Australia. 2present address: Amanstenstrasse 73, 8000 Miinchen-2, F.R.G.
0378-1127/90/$03.50
© 1990 Elsevier Science Publishers B.V.
62
N. REIDETAL.
INTRODUCTION
The Tamaulipan thornscrub is a subtropical, semi-arid shrubland covering 200 000 km 2 of the coastal plain of the Gulf of Mexico in southern Texas and northeastern Mexico (Goldman and Moore, 1946; Muller, 1947; Johnston, 1963; Udvardy, 1975). The vegetation type extends north to the Edwards Plateau (Diamond et al., 1987), south to Llera de Canales and the southern slopes of Sierra Azul, Tamaulipas (Gonz~ilez Medrano, 1985), and west to the base of the Sierra Madre Oriental and central Coahuila (Fig. 1 ). Spiny shrubs and trees dominate the natural vegetation, but grasses, forbs and succulents are also prominent. \\
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Fig. 1. (A) Location of the sampling sites near Linares, Nuevo LEon; (B) distribution of the Tamaulipan thornscrub (shaded) in northeastern Mexico and southern Texas.
UTILIZATION OF TAMAULIPAN THORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
63
In the three-and-a-half centuries since Spanish colonization and agricultural and livestock expansion in the region (Del Hoyo, 1979), the rural population of northeastern Mexico has developed a self-sufficient, subsistence economy in which the thornscrub is exploited for diverse resources (Medellin Leal and G6mez Gonzalez, 1979). Grazing of cattle and goats is the most extensive land use (Anonymous, 1988). The thornscrub also provides fuelwood, timber for construction, fences and implements, plant foodstuffs, herbs and medicines, and game (Alanis, 1981; Heiseke and Foroughbakhch, 1985; Wolf and Perales, 1985; Foroughbakhch and Martinez, 1986; G. Alanis, personal communication, 1987). Campesinos clear areas of natural vegetation to grow rain-fed maize for grain and forage for domestic use or local sale (Berenschot, 1986). Alanis ( 1981 ) and Foroughbakhch and Martinez (1986) published lists of native plants which provide forage or construction materials in the state of Nuevo Le6n, northeastern Mexico, and T611ez (1986) studied the dietary preferences of goats in a thornscrub site in the southeast of Nuevo Le6n. However, quantitative data describing the importance of thornscrub species for browse, fuelwood and construction materials at a regional level are lacking (MedeUin Leal and G6mez Gonzalez, 1979). We therefore conducted an inventory of the degree of utilization of woody plants in the principal habitats near Linares, Nuevo Le6n. This paper identifies the most valuable browse, fuelwood and timber species, and emphasizes the multipurpose trees and shrubs with potential for agroforestry and reforestation in the region. METHODS Study area
The study was conducted within 20 km of Linares (24°47'N 99°32'W; 350 m above sea level) in the piedmont of the Sierra Madre Oriental (Fig. 1 ). The orographic effect of the Sierra and the gradual increase in elevation from east to west produce a strong climatic gradient across the region. Conditions are relatively cool and humid at the base of the Sierra, southwest of Linares (mean annual precipitation and temperature, > 1000 mm and 19 ° C, respectively; Anonymous, 1981 ), grading to warm and dry to the northeast of Linares ( < 700 mm and 23°C, respectively). The regional climate is capricious. Precipitation is variable, and prolonged droughts and wet spells occur in cycles of 10-25 years, affecting agriculture (Norwine, 1978). Severe frosts at intervals of up to 15 years plague the regional citrus industry (Synnott, 1986), and hurricanes at intervals of 20-25 years as well as severe hailstorms contribute to agricultural losses. The principal landforms are plains and gently undulating slopes, inter-
64
N. REID ET AL.
TABLE 1 Morphological characteristics, abundance and browsing-damage indices for 53 woody species in the Tamaulipan thornscrub near Linares, Nuevo IAon Species
Family
Habit b
Max. height (m)
Acacia berlandieri Benth. Acacia constricta Benth. Acaciafarnesiana (L.) Willd. Acacia rigidula Benth. Acacia schaffneri (Wats.) Herin. Acacia wrightii Benth. A myris madrensis Wars. Amyris texana (Buckl.) P. Wils. Berberis chochoco Schlecht. Bernardia myricaefolia ( Scheele ) Watts. Bumelia celastrina H.B.K. Caesalpinia atropunctata (Gray) Fisher Caesealpinia mexicana Gray Calliandra conferta Gray Calliandra eriophylla Benth. Castela texana (Torr. & Gray) Rose Cehis pallida Torr. Cercidium macrum I.M. Johnst. Chamaecrista greggii (Gray) Pollard ex A. Heller var. greggi Citharexylum berlandieri Robins. Condalia hookeri M.C. Johnst. Cordia boissieri DC. Croton torreyanus Muell. Arg. Diospyros palmeri Scheele Diospyros texana Scheele Ehretia anacua (Ter~in & Berl.) I.M. Johnst. Eysenhardtia polystachya (Ort.) Sarg. Forestiera angustifolia Tort. Forestiera racemosa S. Wats. Fraxinus greggii A. Gray Gochnatia hypoleuca DC. Gymnosperma glutinosum (Spreng.) Less Helietta parvifolia (Gray) Benth. Heliotropium calcicola Fernald Karwinksia humboldtiana (R. & S.) Zucc. Krameria ramosissirna (Gray) Wats. Leucophyllumfrutescens (Teran & Bed. ) I.M. Johnst. Malpighia glabra L. Neopringlea integrifolia (Hemsl. ) Wats. Parkinsonia aculeata L. Phaulotharnnus spinescens A. Gray Pistacia texana Swingle
Leguminosae Leguminosae Leguminosae Leguminosae Leguminosae Leguminosae Rutaceae Rutaceae Berberidaceae Euphorbiaceae Sapotaceae Leguminosae Leguminosae Leguminosae Leguminosae Simaroubaceae Ulmaceae Leguminosae Leguminosae Verbenaceae Rhamnaceae Boraginaceae Euphorbiaceae Ebenaceae Ebenaceae Boraginaceae Leguminosae Oleaceae Oleaceae Oleaceae Compositae Compositae Rutaceae Boraginaceae Rhamnaceae Krameriaceae Scrophulariaceae Malpighiaceae Sapindaceae Leguminosae Achatocarpaceae Anacardiaceae
S S T S T T T S T S T U T U U S S T S T ST ST S T T T S S T T T U T U U U S U S T S ST
7 4 10 8 6 9 5 4 6 5 10 1 5 1 1 2 8 4 2 7 6 3 11 9 14 6 5 9 8 1 9 1 2 1 4 1 6 6 2 9
UTILIZATION OF TAMAULIPAN THORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
Sample Spines frequency ~ or (%) thorns
40 8 20 100 10 30 15 63 33 90 83 30 10 15 10 28 70 8 30 30 88 100 73 58 90 13 85 88 8 30 23 40 65 13 100 10 70 75 38 10 8 13
Regional browse index
Habitat Dry caliche
Dry deep
Humid deep
Humid skeletal
Cover (%)
Browse index
Cover (%)
Browse index
Cover (%)
Cover (%)
4.2 0.3 1.3 4.7 0 -
9.2 . 7.4 7.5 <0.1 2.1 <0.1 0.6 <0.1 1.8 2.0 0.1
1.0 . 0 1.0 0 0.9 0 4.8 0 4.3 0.5 2.0
<0.1
+ + + + + + + -
2.3 0.3 0.4 0.9 0 0.7 3.0 4.7 2.0 4.7 0.3 0.9
-
0
-
-
-
1.0
1.0
1.0 4.0 2.0 2.0 3.9
2.3 0.4 17.7 0.3 0.1 -
-
3.3
<0.1
+ + + +
0.2 1.6 0.7 2.7 4.2 0.4 0.3 0.4 1.1 1.4 1.5 3.5 1.2 0 1.6 1.7 0 3.8 2.6 0.1 1,3 1,5 3,2 2.5 1.3 1.0 0
<0.1 <0.1 4.4 . <0.1 11.4 0.8 . 0.7 1.7 0.6 . 0.8 . 0.4 9.9 0.4 6.6 0.3 0.5 0.3 .
-
-+ + -
65
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.
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.
. 0.3 3.7 <0.1
1.9 <0.1 <0.1 7.8
t.9 2.0 1.3 2.3
0 0 . 0 3.1 0 1.0
1.5 0.2 0 0 2.5 4.4 1.8 4.3 0.6 0 0
.
.
4.1
1.7
12.5 <0.1 1.4 4.0 9.5 2.3 <0.1
1.0 5.0 4.9 2.6 4.9 0 0
0.3
0
. 0.1 <0.1 0.2 1.5 0.1 3.2 13.2 0.3 9.2 11.1 5.9 2.5 4.4 9.0 0.2 8.7 0.2 1.2
0 0.8 0
.
-
Browse index
.
.
. 0.8 0.4 0
<0.1 2.5 . 0.1 1.0 <0.1 1.1 .
.
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.
0.4 2.2 -
2.3 9.7 0.I
-
0 5.1 0 0.3 1.3 3.6 3.3 .
0
.
2.0 4.9 3.3
. 0.4 13.8 0.3 <0.1 0.1 14.9 0.2 6.0 2.4 0.3
.
0 0.9 0.9 .
5.2 .
. .
Browse index
0.3 4.0 7.1 0 6.2 0 0.6 0.4 16.4 1.7 12.6 0.2 0.1 1.3 7.7 2.2 15.3 0.2 0.5 0 0.4 1.7 <0.1 5.0 1.3 0 0.3 2.9 <0.1 5.0 0.6 0 . . . 0.6 1.8 1.0 2.0 <0.1 1.7 0.3 2.5 . . . 0.3 0
1.0 0.4 7.4 -
3.0 0 2.0 1.0 4.2 0.2 0
0.3 0.7 1.7 4.3 0.8 1.4 1.7 0 3.5 2.7 0
0.7 4.2 2.4 -
. <0.1
0
66
N. REID ET AL.
TABLE 1 (continued)
Species
Family
Habitb
Max.
height
(m)
Pithecellobium ebano (Berl. ) Muller Pithecellobium pallens (Benth.) Standl. Porlieria angustifolia (Engelm. ) Gray Prosopis laevigata (Willd.) M.C. Johnst. Randia spp. Rhus pachyrrachis Hemsley Schaefferia cuneifolia Gray Viguiera stenoloba Blake Xylosmaflexuosa (H.B.K.) O. Ktze. Zanthoxylumfagara (L.) Sarg. Ziziphus obtusifolia (Torr. & Gray) Gray
Leguminosae Leguminosae Zygophyllaceae Leguminosae Rubiaceae Anacardiaceae Celastraceae Compositae Flacourtiaceae Rutaceae Rhamnaceae
T TS S T S T U S S S S
14 8 3 10 5 3 1 2 9 3
aVoucher specimens are deposited in the Herbarium of the Facultad de Ciencias Forestales, UANL, Linares, N.L., Mexico. DU, undershrub; S, multi-stemmed shrub; TS, tall shrub; ST, small tree; T, tree. cPercentage occurrence of the species in 40 sites.
spersed with ridges and hills up to 50 m above the surrounding terrain. Soils of the plains and lower slopes are chiefly dark grey, silty-clay vertisols. On gentle hills and upper slopes, outcrops of Upper Cretaceous mudstone or shale (lutita) occur, often overlain by silty-clay loams. Scattered Pliocene and Quaternary palaeoriver terraces of conglomerate, often consolidated in a limestone matrix (caliche), exist throughout the region. Caliche terraces usually cap ridges and rises or occur as raised benches, being more resistant to weathering than the lutita bedrock (M. Ruiz, personal communication, 1987). All soil types are low in nitrogen (consistently < 1%, Kjeldahl; N. Reid and P. Beyer-Miinzel, unpublished data, 1987) and are probably deficient in phosphorus as well (Foroughbakhch et al., 1987 ). The native vegetation is a diverse, spiny, often dense scrub dominated by woody plants (Heiseke, 1986). Some 80 species of shrubs and trees, ranging in height from 1 to 15 m, are commonly encountered. Wildfire does not occur in the thornscrub near Linares and prescribed burning is rare. Thus the present vegetation has probably developed in the absence of fire (Jurado and Reid, 1989).
Sampling Thirty-nine sites were sampled in four major habitats defined by climate and substrate (Reid et al., 1987). Sites were located in humid and xeric
UTILIZATION OF TAMAULIPANTHORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
Sample Spines frequency ¢ or (%) thorns
33 93 50 45 78 8 48 10 28 85 25
Regional browse index
67
Habitat Dry caliche
Dry deep
H u m i d deep
H u m i d skeletal
Cover (%)
Browse index
Cover (%)
Browse index
Cover (%)
Browse index
Cover (%)
Browse index
0.7 0.1 <0.1 <0.1 . 0.1
2.2 1.5 0 5.0 . 3.6
2.6 0.8 3.8 0.8 2.8 . 4.8
1.8 20.5 <0,1 0.5 24.1
1.3 1.0 2.0 0 1.1
20.7 <0.1 3.9 <0.1 <0.1 . 0.4 5.9 -
0.7 3.3 2.0 1.3 4.0
+ + + + -
1.9 1.1 2.9 0.4 2.0 1.3 3.6
-
0
-
+ + +
0.4 1.8 0
. 0,1 -
3.1 7.9 0.7 14.2 2.3 . 0.2
.
-
.
1.2
. 1.5 -
0
. 14.8 0.8
1.9 0
. 0,3 1.8 . . . 0.2 0.8 22.5 1.2 0.1 0
0 2.1 -
subregions (Fig. 1 ), in areas with either deep soils (vertisols, >0.5 m deep) or skeletal soils (leptosols and calcisols, ~<0.5 m deep, over caliche hardpan or mudstone). A 40th site with intermediate soil characteristics was sampled in the humid subregion. At each site, a 0.5-ha area homogeneous in physical and vegetational attributes was delimited. A list of the woody plants was compiled at each site (see Table 1 for scientific names and authorities). Cover of all woody species was measured by point interception (Mueller-Dombois and Ellenberg, 1974) in dense vegetation, or by estimating canopy area in quadrats in more open vegetation. Dung of large herbivores was counted to identify the principal herbivores and assess current grazing pressure in each site (Reid et al., 1987 ). The dung pats of cattle, horses and donkeys were counted individually. For goats, the number of occurrences of dung > 5 m from other dung was counted per site. Degree of browsing was rated from 0 (not browsed ) to 6 (heavily browsed ) for each woody species (Table 2). Where conspecifics showed varying degrees of browsing damage within a site, the most severe damage class was recorded. An index of browsing damage, B, was calculated for the ith species, using the expression: Bi = ( ~ I,.)lm, n
where:/in is grade of browsing damage of the ith species in the nth site; and m~, the number of grazed sites in which the ith species occurred; B thus em-
68
N. REIDET AL.
TABLE 2 Criteria for classifying levels of browsing on shrubs and trees Index browsing damage
Appearance of plant
O: Not browsed
Plants normal in height and form; no evidence of utilization of current or previous years growth
1: Scarcely browsed
Browsed stem over a small part of the canopy; otherwise plant normal in height and form
2: Light
Normal in height and form but stems or shoots browsed over most of the canopy; without disfigurement of the woody portions
3: Light to moderate
Complete canopy but some stems gnarled and disfigured owing to repeated browsing
4: Moderate
Normal in height but with irregular, incomplete canopy; the wood portions of stems disfigured and twisted
5: Moderately heavy
Stunted, but with aerial stems, much disfigured and twisted by repeated biting, above a low, browsed crown
6: Heavy
Pruned to a low crown near ground level (unless part of canopy protected by other plants
phasizes the species which were heavily browsed, regardless of their frequency of occurrence. Browsing indices were calculated for each woody species across all sites, as well as for each habitat. Intensity of cutting was measured by counting the number of cut stems > 3 cm diameter at the point of removal in each site. Cut stems were recorded as trunks if cut ~ 1 m above the ground, and as branches if > 1 m from the ground. For each species, a regional cutting index was calculated as the sum of cut stems across all sites, and habitat-specific indices were computed as the sum of cut stems of the species in each habitat. An index of cutting preference was calculated for each species as the (regional cutting i n d e x ) / ( m e a n cover across all sites). Information about uses of wood of different species for timber, fuelwood and other purposes was obtained by interviewing a former contract woodcutter, the owner/manager of a 40-ha ranch, a carpenter/cart-maker and the manager of a 200-ha ranch. The informants were selected for their knowledge of thornscrub shrubs and trees and the various uses made of each species.
UTILIZATION OF TAMAULIPAN THORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
69
Informants were asked to rate the value of 22 shrubs and trees for fuelwood and fence-posts, and to specify other uses. RESULTS
Floristics and vegetation A total of 53 woody species were encountered in three or more of the 40 sites inventoried in thornscrub near Linares (Table 1 ). Legumes constituted 32% of the woody flora and contributed a third of the vegetative cover. Other prominent families were the Rutaceae, Boraginaceae, Oleaceae, Rhamnaceae and Ebenaceae. Armed species bearing spines or thorns comprised 38% of the flora but contributed 52% of the vegetative cover. Among the Leguminosae, the 11 armed species (65% of 17 species) accounted for 87% of the total leguminous cover. The most widely distributed species were multi-stemmed shrubs and small trees. Acacia rigidula, Cordia boissieri and Karwinskia humboldtiana occurred in all 40 sites (Table 1 ). Pithecellobiumpallens, Diospyros texana, Bernardia myricaefolia, Forestiera angustifolia and Condalia hookeri were also widespread, with sample frequencies > 88%. Species with the highest mean regional cover values were mainly shrubs: P. pallens (13.6%), A. rigidula (12.8%), Zanthoxylurnfagara ( 11.9%), Cordia boissieri (9.9%), Randia spp. (8.9%), Diospyros palmeri (7.4%) and F. angustifolia (7.3%). Dry caliche sites supported a predominantly unarmed, depauperate shrubland or scrub, to 4-5 m, dominated by A. rigidula, Cordia boissieri and Helietta parvifolia in the upperstorey and K. hurnboldtiana and Chamaecrista greggii in the understorey. Krameria ramosissima, Calliandra conferta and Jatropha dioica were conspicuous undershrubs in open shrubland, and H. parvifolia and A. berlandieri dominated taller, denser scrub. Dry deep-soil sites supported moderately diverse, dense, spiny scrub to 5 m, although some sites were grassland/scrub mosaics. The dominant upperstorey trees and shrubs were A. berlandieri, Cordia boissierL A. rigidula, and in the mid-shrub layer, F. angustifolia. Better water relations close to arroyos promoted the development of low, closed forests to 8 m, dominated by arborescent Prosopis laevigata and A. farnesiana and the tall shrub Pithecellobium pallens, with abundant Celtis pallida and Z. fagara in the mid-shrub layer. Humid deep-soil sites supported very dense, diverse spiny scrubs, to 5-6 m, dominated by thin, multi-stemmed shrubs and trees. A. rigidula, D. texana, Eysenhardtia polystachya and P. pallens were dominant, with abundant Z. fagara, Randia spp., F. angustifolia and Amyris texana in the mid-shrub layer. Arroyos were fringed with dense, closed, gallery forests, 7-14 m, dominated by D. palmeri and P. pallens, with Randia spp. and Z. fagara abundant in the mid-shrub layer.
70
N. REID ET AL.
Humid sites with skeletal soils overlying caliche were characterized by moderately diverse and floristically distinct scrub or low forest to 6 m, dominated by arborescent Cordia boissierL D. texana, H. parvifolia and Gochnatia hypoleuca as well as the tall shrub, P. pallens in the upperstorey, and Neopringlea integrifolia and Acacia rigidula in the mid-shrub layer. Skeletal soils overlying mudstone in the humid zone supported scrub to 5 m, in which the tree, Fraxinus greggii was a character species. Pithecellobium pallens, Acacia rigidula and Cordia boissieri were dominant in the upperstorey, and Bernardia myricaefolia and Z. fagara dominated the mid-shrub layer.
Browsing damage Dung counts indicated that cattle and occasionally horses and mules were the most abundant large herbivores in all four habitats (Table 3). Cattle and equid dung counts varied considerably among sites in most habitats, but were generally highest in dry caliche sites and lowest in humid deep-soil sites. Goat dung was only encountered in dry caliche sites. Evidence of browsing damage and large herbivore dung was found in all but two humid deep-soil sites. Species exhibiting low levels of browsing damage at the regional level ( B < 1.25; Table 1 ) were often armed, whereas more heavily browsed species tended to be unarmed (Table 4). A chi-square test (2"2= 4.37, 1 OF, P < 0.05) supported the hypothesis that level of browsing TABLE 3 Dung counts and browsing damage indices among habitats (mean ± SE) Dry caliche Dung counts per site Cattle, horse and mule 35.4 ± 14.9 Goat 3.3 ± 1.36 Browsing damage index per species 2.2±0.32
Dry deep
Humid deep
Humid skeletal
25.6±7.0 0
13.2±5.6 0
27.0±3.4 0
1.3 + 0.24
1.3 + 0.22
1.8 + 0.27
TABLE 4 Relationship between browsing damage and spinescence (see Table 1) in the woody flora of the Tamaulipan thornscrub Regional index of browsing damage
Armed species Unarmed species
>t 1.25
< 1.25
6 21
14 12
UTILIZATION OF TAMAULIPAN THORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
71
damage was related to spinescence. The species with the highest levels of browsing damage were unarmed shrubs and trees. A large array of armed and unarmed species showed intermediate levels of browsing damage. Another group of common or widespread spiny shrubs and trees suffered low levels of browsing damage. Seven species registered no browsing damage at all, most being uncommon trees that grew beyond the browsing range of livestock, or spiny or otherwise unpalatable shrubs. However, three common unarmed species showed little or no browsing damage, the toxic shrub Karwinskia humboldtiana, the leathery-leaved small tree Cordia boissieri, and the viscid
Gymnosperma glutinosum. Mean browsing damage per species was highest in dry caliche sites and lowest in humid deep-soil sites (Table 3 ). When browsing damage to particular species was compared among habitats, three trends emerged (Table 1 ). Highly palatable species such as the unarmed B. myricaefolia and H. parvifolia were moderately or heavily browsed in all habitats in which they occurred. Other species, such as Acacia berlandieri and Leucophyllumfrutescens, were heavily browsed in dry caliche sites, but were less damaged in more mesic habitats. Finally, a large group of species registered little or no browsing damage across all habitats.
Cutting of trees and shrubs Evidence of cutting was found in all but one site. Species with hard, dark timber such as Condalia hookeri, Prosopis laevigata, Gochnatia hypoleuca and Piochnatia ebano were heavily cut, had high values of the index of cutting preference, were the preferred species for fuel, fencing or construction, and were less abundant in the region than tall shrubs and trees with soft, pale wood, such as Pithecellobium pallens and Diospyros spp. (Tables 1, 5 ), which were said to be most valuable in carpentry (e.g. for handles of farm implements, light furniture, handicrafts and children's toys). Due to the fruticose growth-form of most thornscrub species, the trunks of shrubs and trees were typically cut (Table 5). However, among larger tree species (e.g. Prosopis laevigata, Fraxinus greggii and Acacia farnesiana ), branches were about as frequently cut as trunks. The intensity of cutting was highest in humid, skeletal-soil sites (mean of 94 cut stems/ha) and lowest in the dry subregion (62 cut stems/ha and 69 cut stems/ha in caliche and deep-soil sites, respectively). The heavily exploited species varied widely among sites, owing to differences in the composition of vegetation between habitats (Table 5 ). The most heavily cut species on a regional basis, H. parvifolia, was only abundant in caliche sites (Table 1 ). Prosopis laevigata and Pithecellobium ebano were more common in dry, deep-soil sites and were the most heavily exploited species there. Condalia
0.7 1.5 0.8 1.5 0.5 0.9 15.0 6.9 3.1 2.9 0.2 1.2 0.1 2.4 6.4 29.1 0.8 5.9 11.8 0.1 6.7 1.4
Acacia berlandieri Acaciafarnesiana Acacia rigidula Acacia wrightii Bumelia celastrina Celtis pallida Condalia hookeri Cordia boissieri Diospyrospalmeri Diospyros texana Ehretia anacua Eysenhardtiapolystachya Forestiera angustifolia Fraxinus greggii Gochnatia hypoleuca Heliettaparvifolia Neopringlea integrifolia Pithecellobium ebano Pithecellobiumpallens Porlieria angustifolia Prosopis laevigata Zanthoxylumfagara 56 55 80 90 100 50 94 88 70 68 100 94 I00 45 86 77 100 84 93 100 60 44
Trunks (%)
1.0 3.6 0.5 0.5 94.4 -
Dry caliche 6.9 0.5 7.4 0.9 2.8 6.0 5.6 0.5 26.4 6.5 0.5 34.7 1.4
Dry deep 2.3 1.3 1.6 1.0 1.0 1.6 36.5 7.8 8.0 5.2 0.8 1.6 0.3 1.0 0.5 5.2 17.4 3.1 3.9
Humid deep 8.5 8.3 1.1 3.4 1.9 4.4 17.4 40.0 2.3 12.7 -
Humid skeletal
Cutting intensities by habitat a (%)
excellent good good excellent poor excellent excellent good poor poor poor regular regular poor poor excellent regular excellent regular not used excellent poor
0.1
Fuelwood
not used
Fenceposts
regular poor excellent poor n o t used excellent regular not used not used not used poor not used not used good excellent regular excellent poor not used excellent not used
Uses o f w o o d
0.9 0.1 3.1 0.3 0.3 4.3 0.7 0.4 0.4 0.3 0.0 1.7 2.8 6.6 0.4 4.6 0.9 0.4 2.0 0.1
Index o f cutting preference
1, 2, 4 3, 4, 5, 6 1, 4, 6 4, 6 1, 4 2, 5, 6 5, 6 3, 5, 6 4, 6 2,5,7 2, 5 1 1, 2, 3, 4 2, 5 1, 4 2, 5, 6, 7 3 I, 4 5, 6, 7
7
Otherb
a Sample sizes for n u m b e r o f cut stems and b r a n c h e s were: dry caliche, 195; dry deep, 216; h u m i d deep, 386; h u m i d skeletal, 472; regional, 1297. bl, vertical posts in rural housing and constructions; 2, roofing in rural dwellings; 3, w o v e n branch walls in rural habitations and corrals; 4, cart construction; 5, childrens' toys, handicrafts, wooden spoons; 6, handles o f axes, machetes and animal-drawn ploughs; 7, furniture, ladders.
Regional cutting intensity ~ (%)
Species
Intensity o f cutting, p r o p o r t i o n o f cut trunks to branches, and uses o f t i m b e r species in the T a m a u l i p a n t h o r n s c r u b near Linares, N u e v o Le6n, northeastern Mexico
TABLE 5 t,~
UTILIZATION OF TAMAULIPAN THORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
73
hookeri was most abundant and was the most heavily cut species in humid deep-soil sites. DISCUSSION
Browse The browsing index, B, quantifies damage to woody plants on a site. The degree of browsing damage is dependent on herbivore dietary preferences and the relative and absolute abundance of the forage species; therefore B should not be Confused with the importance of the forage species in the diet. A heavily browsed species may be highly preferred by stock, but the species may not form a principal component of the diet if it is uncommon. Alternatively, if browsing is spread uniformly over many individual shrubs of an abundant species, the damage inflicted by browsing may be relatively light and the species will have a low browsing index. A small group of unarmed shrubs and trees (e.g.B. myricaefolia, Amyris
texana, Citharexylum berlandieri, H. parvifolia, Schaefferia cuneifolia, Calliandra eriophylla, E. polystachya and Porlieria angustifolia) were heavily browsed in most or all habitats. Some of these species are known to be preferred browse for livestock in the Linares region or elsewhere (Rovalo et al., 1983; Warren et al., 1984; Foroughbakhch and Martinez, 1985; Heiseke, 1986; G. Villegas Durfin, personal communication, 1986; Gonz~ez Ochoa and Ochoa Garcifi, 1987 ). Most heavily browsed species were widespread in the region but had low cover values and, with the exception of H. parvifolia, were not dominants in thornscrub vegetation. Spines and thorns are clearly an evolutionary adaptation against mammalian browsers (Janzen, 1987), restricting the loss of foliage by reducing the bite size and harvest rates of large herbivores (Cooper and Owen-Smith, 1986 ). Armed species exhibited significantly less browsing damage than unarmed species (Table 4 ). Spiny or unpalatable unarmed species had the highest mean cover values in the regional vegetation and armed species contributed disproportionately to total plant cover. In the 17th and 18th Centuries, huge flocks of sheep, totalling more than a million head, were depastured in the Linares region for 6 months each year (Del Hoyo, 1979). Reid et al. ( 1987 ) reported evidence of grazing-induced changes in the vegetation in favour of spiny succulents and unpalatable shrubs at the expense of unarmed, heavily browsed species. Thus, the present dominance of spiny and unpalatable shrubs and trees near Linares may reflect past overgrazing by livestock. Browsing damage to particular species was variable among habitats. Acacia berlandieri and Leucophyllum frutescens were heavily browsed in dry-caliche sites but not elsewhere, and most species showed less browsing damage in humid, deep-soil sites than in other habitats. Several factors were responsible.
74
N. R E I D ET AL.
Grazing pressure, as judged by dung counts, was heaviest in dry-caliche sites and least in humid deep-soil sites. The vegetation on skeletal soils was more open, shorter in stature and less spiny than the dense scrub of deep soils, and probably provided more accessible pasture for livestock. Thus, A. berlandieri and E. polystachya were within browsing range of livestock in skeletal-soil sites, but out of range on deep soils. Dry-caliche sites supported fewer woody species, and so plants of intermediate palatability may have been heavily utilized in the absence of preferred browse species. The heavily tomentose L. frutescens may fall in this category. Alternatively, since goats favor L. frutescens (Warren et al., 1984), its heavy utilization in dry-caliche sites may have been due to goat browsing in that habitat. Management was responsible, in certain instances, for a reduction in grazing on deep-soil sites. Vertisols are the preferred agricultural soils in the region, and some remnant thomscrub on deep soils was ungrazed or grazed only seasonally because of the potential for damage to adjacent, unfenced crops.
Timber The most heavily exploited trees (e.g.H. parvifolia, Condalia hookeri, Prosopis laevigata and Pithecellobium ebano) were cut in disproportionate relation to their abundance, as judged by high values of the index of cutting preference, and were the preferred species for fuelwood, charcoal, fence-posts and the large upright posts in rural dwellings. Less-frequently-cut species were least preferred for these same uses, and were more abundant (Tables 1 and 5). According to informants, the darker wood of the most heavily exploited species burned to coals and the timber was hard and durable. Field and laboratory studies confirm that the wood of H. parvifolia, Condalia hookeri, Pithecellobium ebano, Prosopis laevigata and Acacia wrightii has a higher specific gravity and is more resistant to decomposition in soil than that of other thornscrub species (Wolf and Perales, 1985 ). In addition, Prosopis laevigata has a low equilibrium moisture content, conferring high dimensional stability even in climatic extremes, making it a preferred timber in construction for doors, window-frames and cart-wheels (Wolf, 1986 ). The small tree Cordia boissieri was highly regarded as fuelwood but not for fence-posts, despite a high natural resistance to soil decomposers (Wolf and Perales, 1985 ). The disfavour shown towards the species as a fencing material appears to reflect the difficulty in finding long, straight stems and fixing metal staples or nails in the posts (Wolf and Perales, 1985 ). Gochnatia hypoleuca was moderately exploited and was a preferred timber for fence-posts, but was avoided as firewood because its smoke has a foul odour which taints food. Although timber species with pale wood and lower specific gravities (Wolf and Perales, 1985 ) were reputed to burn quickly without producing coals and to rot quickly in damp soil, Pithecellobium pallens was heavily exploited (Ta-
UTILIZATION OF TAMAULIPAN THORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
75
ble 5 ) due to its abundance and provision of long, straight posts. A small number of species which produce excellent timber or firewood were not heavily cut, owing either to their relative scarcity in the sites sampled (e.g. Acacia wrightii) or to the fact that they rarely produce a stem of exploitable diameter ( Celtis pallida). Timber extraction was greater in humid than dry sites owing to the greater development of arborescent vegetation in the former. Timber extraction in humid skeletal-soil sites exceeded that in humid deep-soil sites, because the dominant shrubs and trees in thornscrub on deep soils were generally lowquality timber species. Moreover, on deep soils away from arroyos, the thin and multi-stemmed habit of the shrubs and trees did not provide as many stems per unit area suitable for fence-posts as the arborescent dominants on skeletal soils.
Silvopastoral and agroforestry potential of thornscrub species The most abundant woody plants in thornscrub in the Linares region were the less-preferred browse, fuelwood and timber species. There is a considerable need, therefore, for experimenting with silvopastoral and silvicultural practices to increase livestock and wood production. Future research should focus on the highly preferred forage and timber species identified in this study. The most valuable species is the multi-stemmed tree H. parvifolia (bar° reta), which produces durable fenceposts and is heavily browsed by both cattle and goats. About 40% of the fence-posts in the region are of this species (N. Reid, H. Stienen and H. Hempel, unpublished data, 1988 ). Rovalo et al. ( 1983 ) reported a high fungicide and insecticide potential in leaf extracts of barreta but found no evidence of toxicity to chickens nor reports of toxicity to livestock. The species occurs in association with caliche, normally forming extensive stands on caliche rises and hills (Table 1; Rovalo et al., 1983; Reid et al., 1987 ). Foroughbakhch et al. ( 1987 ) were unable to germinate the seeds of H. parvifolia and reported slow growth rates and high mortality among seedlings transplanted from the field. However, growth of the species in plantation improved markedly after the second year (R. Foroughbakhch, personal communication, 1988 ). The problem of propagating H. parvifolia must be resolved before its timber and forage productivity can be further evaluated. Three leguminous shrubs and trees browsed by livestock also produced high quality fuelwood or timber: the spiny tree Pithecellobium ebano, the thorny shrub Acacia berlandieri, and the unarmed shrub E. polystachya. Considering their potential to fix nitrogen and the relatively low level of N in soils of the region, these species could play an important role in agroforestry and silvopastoral systems. The two shrub species can be managed to maintain a bushy crown within reach of livestock (R. Foroughbakhch (1987) and G. Alanis ( 1987 ), personal communications), although their crown height is normally
76
N. REID ETAL.
> 2 m. Pithecellobium ebano produces an annual crop of large seeds which are roasted and eaten or used as coffee substitute. In plantation, juvenile Pithecellobium ebano and Acacia berlandieri exhibited 100% survival despite frosts and had growth rates of 0.9 and 0.6 m/year in height, respectively, over 2 years (Foroughbakhch et al., 1987 ). The seeds of all three species germinate readily (R. Foroughbakhch ( 1988 ) and J. Castafieda ( 1988 ), personal communications). After heavy browsing or cutting, the suckers of both spiny species are more heavily armed than undamaged shoots, a common characteristic of spinescent shrubs and trees (Cooper and Owen-Smith, 1986 ). Several other species either produce excellent timber and fuelwood or are heavily browsed. The legumes in this category are the heavily browsed, unarmed undershrubs Callandria eriophylla and Chamaechrista greggii, and the timber-producing trees Prosopis laevigata and Acacia wrightii. These species should be screened for nitrogen fixation. Under plantation conditions, Prosopis laevigata did not grow well in a 3-year study near Linares because of repeated defoliation by locusts and jackrabbits (Foroughbakhch et al., 1987 ). However, Acacia wrightii performed well, exhibiting 100% survival and a height increment of 0.9 m/year. The valuable browse, timber and fuelwood species identified in this study provide an interesting contrast to the multipurpose trees and shrubs attracting attention in the semi-arid regions of central America and elsewhere. High population pressure and extensive land clearance in much of central America have produced critical shortages of fuelwood and timber (Dulin, 1984). Accordingly, most emphasis to date has been given to fast-growing, high-producing multipurpose species such as Leucaena, Gliricidia, Prosopis and certain Acacia spp. (Ch~aro Bas~ifiez and Pardo Tejeda, 1976; Anonymous, 1980, 1983, 1986; Hughes and Styles, 1984; Withington et al., 1987). However, high specific gravity, timber durability and frost hardiness are sacrificed in fast-growing legumes (Hughes and Styles, 1984; Synnott, 1986; Foroughbakhch et al., 1987 ). Fast-growing multipurpose species in northeastern Mexico (e.g.L. leucocephala, Parkinsonia aculeata, Acacia farnesiana ) are not heavily exploited by the rural population, since timber species with a high specific gravity and greater durability are still available for exploitation and are preferred for firewood, construction and fence-posts. Improvement of traditional land-management practices in the Tamaulipan thornscrub should involve the development of sustainable silvicultural, silvopastoral and agroforestry systems based on the extant natural vegetation. The high-quality browse and timber species need to be evaluated with respect to productivity, resistance to varying levels of exploitation, and ability to increase in abundance after the selective elimination from natural vegetation of species of lesser value. Less-common species need to be evaluated for ease of propagation and establishment, ability to persist in thornscrub after introduction, and suitability for incorporating in artificial grazing systems that employ both woody and herbaceous components. The degraded and ever de-
UTILIZATIONOF TAMAULIPANTHORNSCRUBFOR BROWSE,FUELWOODANDTIMBER
77
creasing remnants of thornscrub will be unable to meet the traditional needs of the rural population if population growth and Government-sponsored landclearance programmes in northeastern Mexico are not drastically curtailed. ACKNOWLEDGEMENTS
We are indebted to Steve Archer, Manuel Fem~indez Brondo, Colin Hughes, Jorge Landa, Rub6n Pefiazola, Holger Stienen and Brian Styles for their encouragement, information and criticisms during the course of this study. The work was funded by a grant to Glafiro Alanis F. from the Mexican Secretaria de Educaci6n Pfblica.
REFERENCES
Alanis, G., 1981. Aprovechamiento de la flora nativa en el estado de Nuevo Le6n. In: C.E. Gonz~ilez Vicente, I. Casas Diaz and R. Padilla Ibarra (Editors), Memoria de la Primera Reuni6n sobre Ecolog/a, Manejo y Domesticaci6n de Plantas Utiles del Desierto. Instituto Nacional de Investigaciones Forestales, SARH, Monterrey, Publ. Espec. No. 31, pp. 220227. Anonymous, 1980. Firewood Crops, Shrubs and Tree Species for Energy Production. National Academy of Sciences, Washington, DC, Vol. 1,236 pp. Anonymous, 1981. Carta de Climas. 1 : 1 000 000. Secretar/a de Programaci6n y Presupuesto, Monterrey, Mexico. Anonymous, 1983. Leucaena research in the Asian-Pacific region. International Development Research Centre, Ottawa. Anonymous, 1986. Silvicultura de Especies Promisorias para Producci6n de kefia en America Central. Centro Agron6mico Tropical de Investigaci6n y Ensefianaza, Turrialba, Inf. T6c. No. 86, 220 pp. Anonymous, 1988. Primera Reuni6n Cientifica Forestal y Agropecuaria. Centro de Investigaciones Forestales y Agropecuarias en Nuevo Le6n, Secretaria de Agricultura y Recursos Hidraulicos, M6xico, 89 pp. Berenschot, L., 1986. The distribution of cattle grazing pressure in matorral vegetation near Linares, Nuevo Le6n. M.Sc. Thesis, Agricultural University, Wageningen. Ch~izaro Baszlfiez, M. and Pardo Tejeda, E., 1976. E1 Huizache. Instituto Nacional de Recursos Bi6ticos, Xalapa, Informa No. 8, 3 pp. Cooper, S.M. and Owen-Smith, N., 1986. Effects of plant spinescence on large mammalian herbivores. Oecologia, 68: 446-455. Del H oyo, E., 1979. Historia del Nuevo Reino de Le6n ( 1577-1723 ). Editorial Lib ros de M Oxico, M6xico, (2nd edition). Diamond, D.D., Riskind, D.H. and Orzell, S.L., 1987. A framework for plant community classification and conservation in Texas. Tex. J. Sci., 39: 203-221. Dulin, P., 1984. Situaci6n lefiera en los paises centroamericanos. Centro Agron6mico Tropical de Investigaci6n y Ensefianaza, Turrialba, Inf. T6c. No. 51, 52 pp. Foroughbakhch, P., R and Martinez M.A., 1986. Estudio sobre diferentes tipos de agostaderos y la producci6n caprina en la regi6n de Linares y Hualahuises, Nuevo Le6n, MOxico. Schr. Forst. Fak. Univ. GOtt. Nieders~ichs. Forst. Versuchsanst., 84:159-183.
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N. REIDETAL.
Foroughbakhch, R., Pefialoza, R. and Stienen, H., 1987. Increasing productivity in the matorral of northeastern Mexico: domestication of ten native multipurpose tree species. In: E.F. A1don, C.E. Gonz~ilez Vicente and W.H. Moir (Technical Coordinators), Strategies for Classification and Management of Native Vegetation for Food Production in Arid Zones. USDA For. Serv. Gen. Tech. Rep. RM-150, pp. 90-98. Goldman, E.A. and Moore, R.T., 1946. The biotic provinces of Mexico. J. Mammal., 26" 347360. Gonzfilez Medrano, F., 1985. E1 limite sur de la Provincia Bi6tica Tamaulipeca. In: Resumenes de los Trabajos, II Simposio Internacional sobre la Provincia Bi6tica Tamaulipeca, 6-8 March 1985, Universidad Aut6noma de Tamaulipas, Victoria, Mrxico, p. 7. Gonz~ilez Ochoa, R.I. and Ochoa Garcia, J.G., 1987. Estimaci6n de las deficiencias nutricionales en los pastizales del norte de Durango. In: L.C. Fierro (Editor), Resultados de Investigaci6n sobre Ecologia y Manejo de Pastizales en el Norte de Durango, 1978-1987. Centro de Investigaciones Forestales y Agropecuarias del Estado de Durango, Durango, Spec. Publ. No. 2., pp. 71-86. Heiseke, D., 1986. Regeneraci6n por rebrotes en dos tipos de matorral del noreste de Mrxico. Schr. Forst. Fak. Univ. Grtt. Nieders~ichs. Forstl. Versuchsanst., 84" 184-199. Heiseke, D. a n d Foroughbakhch, R., 1985. E1 matorral como recurso forestal. Facultad de Silvicultura y Manejo de Recursos Renovables, Universidad Aut6noma de Nuevo Le6n, Linares, Rep. Cient. No. 1, 31 pp. Hughes, C.E. and Styles, B.T., 1984. Exploration and seed collection of multiple-purpose dry zone trees in central America. Int. Tree Crops J., 3: 1-31. Janzen, D.H., 1987. Chihuahuan desert nopaleras: defaunated big mammal vegetation. Annu. Rev. Ecol. Syst., 17: 595-636. Johnston, M.C., 1963. Past and present grasslands of southern Texas and northeastern Mexico. Ecology, 44: 456-466. Jurado, Y.E. and Reid, N., 1989. Asociaci6n entre especies vegetales, factores ed~ificos, topogr~ificos y perturbaci6n en el matorral Tamaulipense cerca de Linares, Nuevo Le6n. Facultad de Ciencias Forestales, Universidad Aut6noma de Nuevo Le6n, Linares, Rep. Cient, 10:29 pp. Medellin Leal, F. and G6mez Gonzfilez, A., 1979. Management of natural vegetation in the semi-arid ecosystems of Mexico. In: B.H. Walker (Editor), Management of Semi-Arid Ecosystems. Elsevier, Amsterdam, pp. 351-376. Mueller-Dombois, D. and Ellenberg, H., 1974. Aims and Methods of Vegetation Ecology. Wiley, New York, 547 pp. Muller, C.H., 1947. Vegetation and climate of Coahuila, Mexico. Madrofio, 9" 1-32. Norwine, J., 1978. Twentieth-century climates and climatic fluctuations in Texas and northeastern Mexico. J. Arid. Environ., l: 313-325. Reid, N., Stafford Smith, M., Beyer-Miinzel, P. and Marroquin, J., 1987. A research strategy for ecological survey: floristics and land use in the Tamaulipan thornscrub, north-eastern Mexico. In: E.F. Aldon, C.E. Gonz~llez Vicente and W.H. Moir (Technical Coordinators), Strategies for Classification and Management of Native Vegetation for Food Production in Arid Zones. USDA For. Serv. Gen. Tech. Rep. RM-150, pp. 32-38. Rovalo, M., Graue, B., Gonzfilez, M.E., Gonzfilez, L., Rojas, D.B., Covarrubias, M.L. and Magallanes, E., 1983. La barreta o barreto, Helietta parvifolia, recurso vegetal desaprovechado del semidesierto del noreste de M6xico. Instituto Nacional de Investigaciones sobre Recursos Bioticos, Xalapa, Cuad. Divulg. No. 11, 19 pp. Synnott, T., 1986. Los efectos de las heladas de 1983 sobre las plantas de la regi6n de Linares, N.L. Facultad de Silvicultura y Manejo de Recursos Renovables, Universidad Aut6noma de Nuevo Le6n, Rep. Cient. No. 5, pp. l - 13.
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T611ez, R., 1986. Nachhaltige Nutzung der nordostmexikanischen Strauchsteppe (Matorral) durch Ziegen. Diplom. Thesis, Justus-Liebig Universit~it, Giessen, West Germany, 110 pp. Udvardy, M.D.F., 1975. A classification of the biogeographical provinces of the world. IUCN, Morges, Switzerland, Occas. Pap. No. 18, 48 pp. Warren, L.E., Ueckert, D.N., Shelton, M. and Chamrad, A.D., 1984. Spanish goat diets on mixedbrush rangeland in the south Texas plains. J. Range Manage., 37: 340-342. Withington, D., Glover, N. and Brewbaker, J.L., 1987. Gliricidia sepium (Jacq.) Walp.: management and improvement. Nitrogen-Fixing Tree Assoc., Waimanalo, Hawaii, Spec. Publ. 87-01. Wolf, F., 1986. Htimedad de equilibrio de la madera para el noreste de M~xico. Facultad de Silvicultura y Manejo de Recursos Renovables, Universidad Aut6noma de Nuevo Le6n, Linares, Rep. Cient. No. 5, pp. 14-25. Wolf, F. and Perales, F., 1985. Durabilidad natural de la madera de algunas especies del matorral del noreste de M6xico. Facultad de Silvicultura y Manejo de Recursos Renovables, Universidad Aut6noma de Nuevo Le6n, Linares, Rep. Cient. No. 3, 15 pp.
61
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Utilization of shrubs and trees for browse, fuelwood and timber in the Tamaulipan thornscrub, northeastern Mexico
N i c k R e i d 1, J o r g e M a r r o q u i n a n d P e t e r B e y e r - M i i n z e l 2
Facultad de Ciencias Forestales, Universidad Aut6noma de Nuevo Lebn, Apdo. Postal 41, Linares N.L. 67700 (Mexico) (Accepted 29 August 1989)
ABSTRACT Reid, N., Marroquin, J. and Beyer-Miinzel, P., 1990. Utilization of shrubs and trees for browse, fuelwood and timber in the Tamaulipan thornscrub, northeastern Mexico. For. Ecol. Manage., 36: 6179.
The Tamaulipan thornscrub is the subtropical, semi-arid vegetation that covers the Gulf coastal plain of northeastern Mexico. Leguminous trees and shrubs constitute one-third of the diverse woody flora, which is exploited by the rural population for extensive grazing of livestock, fuelwood and timber for fencing and construction. Cutting intensity and extent of browsing damage to 53 woody species was assessed in 40 sites stratified according to habitat in the region of Linares, Nuevo Le6n. Principal uses of the timber were identified in interviews with campesinos. Unarmed shrubs and trees such as Bernardia myricaefolia, Arnyris texana, Citharexylum berlandieri and Helietta parvifolia were most heavily browsed: H. parvifolia, Condalia hookeri, Pithecellobium pallens, Cordia boissieri and Prosopis laevigata were the most frequently cut trees. Helietta parvifolia, a non-legume, was the most important multipurpose tree in the region, and three legumes - Pithecellobiurn ebano (tree), Acacia berlandieri and Eysenhardtia polystachya (shrubs) - provided both browse and timber or fuelwood. The most heavily exploited and highly valued timber species had dark, hard wood and were slower-growing than the high-yielding legumes commonly associated with multipurpose trees and shrubs in the semi-arid tropics. Sustainable, managed silvopastoral and agroforestry systems based on the indigenous forage, timber and fuelwood trees and shrubs need to be developed in the Tamaulipan thornscrub if traditional land-use practices are to be improved and the excessive exploitation and overgrazing of dwindling plant resources are to be reversed. More information is required regarding the response of valuable woody species to browsing and cutting, their ease of propagation, and their capacity to increase on landscapes where less desirable elements have been selectively removed. ~Present address: Department of Ecosystem Management, University of New England, Armidale N.S.W. 2351, Australia. 2present address: Amanstenstrasse 73, 8000 Miinchen-2, F.R.G.
0378-1127/90/$03.50
© 1990 Elsevier Science Publishers B.V.
62
N. REIDETAL.
INTRODUCTION
The Tamaulipan thornscrub is a subtropical, semi-arid shrubland covering 200 000 km 2 of the coastal plain of the Gulf of Mexico in southern Texas and northeastern Mexico (Goldman and Moore, 1946; Muller, 1947; Johnston, 1963; Udvardy, 1975). The vegetation type extends north to the Edwards Plateau (Diamond et al., 1987), south to Llera de Canales and the southern slopes of Sierra Azul, Tamaulipas (Gonz~ilez Medrano, 1985), and west to the base of the Sierra Madre Oriental and central Coahuila (Fig. 1 ). Spiny shrubs and trees dominate the natural vegetation, but grasses, forbs and succulents are also prominent. \\
A
\ \
~
I \\*
~1
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,,
\\ ~ ~ M ~
t~
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"~', ".": '~
u
s
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"
MEXICO
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i
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Fig. 1. (A) Location of the sampling sites near Linares, Nuevo LEon; (B) distribution of the Tamaulipan thornscrub (shaded) in northeastern Mexico and southern Texas.
UTILIZATION OF TAMAULIPAN THORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
63
In the three-and-a-half centuries since Spanish colonization and agricultural and livestock expansion in the region (Del Hoyo, 1979), the rural population of northeastern Mexico has developed a self-sufficient, subsistence economy in which the thornscrub is exploited for diverse resources (Medellin Leal and G6mez Gonzalez, 1979). Grazing of cattle and goats is the most extensive land use (Anonymous, 1988). The thornscrub also provides fuelwood, timber for construction, fences and implements, plant foodstuffs, herbs and medicines, and game (Alanis, 1981; Heiseke and Foroughbakhch, 1985; Wolf and Perales, 1985; Foroughbakhch and Martinez, 1986; G. Alanis, personal communication, 1987). Campesinos clear areas of natural vegetation to grow rain-fed maize for grain and forage for domestic use or local sale (Berenschot, 1986). Alanis ( 1981 ) and Foroughbakhch and Martinez (1986) published lists of native plants which provide forage or construction materials in the state of Nuevo Le6n, northeastern Mexico, and T611ez (1986) studied the dietary preferences of goats in a thornscrub site in the southeast of Nuevo Le6n. However, quantitative data describing the importance of thornscrub species for browse, fuelwood and construction materials at a regional level are lacking (MedeUin Leal and G6mez Gonzalez, 1979). We therefore conducted an inventory of the degree of utilization of woody plants in the principal habitats near Linares, Nuevo Le6n. This paper identifies the most valuable browse, fuelwood and timber species, and emphasizes the multipurpose trees and shrubs with potential for agroforestry and reforestation in the region. METHODS Study area
The study was conducted within 20 km of Linares (24°47'N 99°32'W; 350 m above sea level) in the piedmont of the Sierra Madre Oriental (Fig. 1 ). The orographic effect of the Sierra and the gradual increase in elevation from east to west produce a strong climatic gradient across the region. Conditions are relatively cool and humid at the base of the Sierra, southwest of Linares (mean annual precipitation and temperature, > 1000 mm and 19 ° C, respectively; Anonymous, 1981 ), grading to warm and dry to the northeast of Linares ( < 700 mm and 23°C, respectively). The regional climate is capricious. Precipitation is variable, and prolonged droughts and wet spells occur in cycles of 10-25 years, affecting agriculture (Norwine, 1978). Severe frosts at intervals of up to 15 years plague the regional citrus industry (Synnott, 1986), and hurricanes at intervals of 20-25 years as well as severe hailstorms contribute to agricultural losses. The principal landforms are plains and gently undulating slopes, inter-
64
N. REID ET AL.
TABLE 1 Morphological characteristics, abundance and browsing-damage indices for 53 woody species in the Tamaulipan thornscrub near Linares, Nuevo IAon Species
Family
Habit b
Max. height (m)
Acacia berlandieri Benth. Acacia constricta Benth. Acaciafarnesiana (L.) Willd. Acacia rigidula Benth. Acacia schaffneri (Wats.) Herin. Acacia wrightii Benth. A myris madrensis Wars. Amyris texana (Buckl.) P. Wils. Berberis chochoco Schlecht. Bernardia myricaefolia ( Scheele ) Watts. Bumelia celastrina H.B.K. Caesalpinia atropunctata (Gray) Fisher Caesealpinia mexicana Gray Calliandra conferta Gray Calliandra eriophylla Benth. Castela texana (Torr. & Gray) Rose Cehis pallida Torr. Cercidium macrum I.M. Johnst. Chamaecrista greggii (Gray) Pollard ex A. Heller var. greggi Citharexylum berlandieri Robins. Condalia hookeri M.C. Johnst. Cordia boissieri DC. Croton torreyanus Muell. Arg. Diospyros palmeri Scheele Diospyros texana Scheele Ehretia anacua (Ter~in & Berl.) I.M. Johnst. Eysenhardtia polystachya (Ort.) Sarg. Forestiera angustifolia Tort. Forestiera racemosa S. Wats. Fraxinus greggii A. Gray Gochnatia hypoleuca DC. Gymnosperma glutinosum (Spreng.) Less Helietta parvifolia (Gray) Benth. Heliotropium calcicola Fernald Karwinksia humboldtiana (R. & S.) Zucc. Krameria ramosissirna (Gray) Wats. Leucophyllumfrutescens (Teran & Bed. ) I.M. Johnst. Malpighia glabra L. Neopringlea integrifolia (Hemsl. ) Wats. Parkinsonia aculeata L. Phaulotharnnus spinescens A. Gray Pistacia texana Swingle
Leguminosae Leguminosae Leguminosae Leguminosae Leguminosae Leguminosae Rutaceae Rutaceae Berberidaceae Euphorbiaceae Sapotaceae Leguminosae Leguminosae Leguminosae Leguminosae Simaroubaceae Ulmaceae Leguminosae Leguminosae Verbenaceae Rhamnaceae Boraginaceae Euphorbiaceae Ebenaceae Ebenaceae Boraginaceae Leguminosae Oleaceae Oleaceae Oleaceae Compositae Compositae Rutaceae Boraginaceae Rhamnaceae Krameriaceae Scrophulariaceae Malpighiaceae Sapindaceae Leguminosae Achatocarpaceae Anacardiaceae
S S T S T T T S T S T U T U U S S T S T ST ST S T T T S S T T T U T U U U S U S T S ST
7 4 10 8 6 9 5 4 6 5 10 1 5 1 1 2 8 4 2 7 6 3 11 9 14 6 5 9 8 1 9 1 2 1 4 1 6 6 2 9
UTILIZATION OF TAMAULIPAN THORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
Sample Spines frequency ~ or (%) thorns
40 8 20 100 10 30 15 63 33 90 83 30 10 15 10 28 70 8 30 30 88 100 73 58 90 13 85 88 8 30 23 40 65 13 100 10 70 75 38 10 8 13
Regional browse index
Habitat Dry caliche
Dry deep
Humid deep
Humid skeletal
Cover (%)
Browse index
Cover (%)
Browse index
Cover (%)
Cover (%)
4.2 0.3 1.3 4.7 0 -
9.2 . 7.4 7.5 <0.1 2.1 <0.1 0.6 <0.1 1.8 2.0 0.1
1.0 . 0 1.0 0 0.9 0 4.8 0 4.3 0.5 2.0
<0.1
+ + + + + + + -
2.3 0.3 0.4 0.9 0 0.7 3.0 4.7 2.0 4.7 0.3 0.9
-
0
-
-
-
1.0
1.0
1.0 4.0 2.0 2.0 3.9
2.3 0.4 17.7 0.3 0.1 -
-
3.3
<0.1
+ + + +
0.2 1.6 0.7 2.7 4.2 0.4 0.3 0.4 1.1 1.4 1.5 3.5 1.2 0 1.6 1.7 0 3.8 2.6 0.1 1,3 1,5 3,2 2.5 1.3 1.0 0
<0.1 <0.1 4.4 . <0.1 11.4 0.8 . 0.7 1.7 0.6 . 0.8 . 0.4 9.9 0.4 6.6 0.3 0.5 0.3 .
-
-+ + -
65
.
. . 2.0 9.5 . .
.
.
.
.
.
.
4.0 .
.
. 0.3 3.7 <0.1
1.9 <0.1 <0.1 7.8
t.9 2.0 1.3 2.3
0 0 . 0 3.1 0 1.0
1.5 0.2 0 0 2.5 4.4 1.8 4.3 0.6 0 0
.
.
4.1
1.7
12.5 <0.1 1.4 4.0 9.5 2.3 <0.1
1.0 5.0 4.9 2.6 4.9 0 0
0.3
0
. 0.1 <0.1 0.2 1.5 0.1 3.2 13.2 0.3 9.2 11.1 5.9 2.5 4.4 9.0 0.2 8.7 0.2 1.2
0 0.8 0
.
-
Browse index
.
.
. 0.8 0.4 0
<0.1 2.5 . 0.1 1.0 <0.1 1.1 .
.
.
0 .
<0.1
.
0.4 2.2 -
2.3 9.7 0.I
-
0 5.1 0 0.3 1.3 3.6 3.3 .
0
.
2.0 4.9 3.3
. 0.4 13.8 0.3 <0.1 0.1 14.9 0.2 6.0 2.4 0.3
.
0 0.9 0.9 .
5.2 .
. .
Browse index
0.3 4.0 7.1 0 6.2 0 0.6 0.4 16.4 1.7 12.6 0.2 0.1 1.3 7.7 2.2 15.3 0.2 0.5 0 0.4 1.7 <0.1 5.0 1.3 0 0.3 2.9 <0.1 5.0 0.6 0 . . . 0.6 1.8 1.0 2.0 <0.1 1.7 0.3 2.5 . . . 0.3 0
1.0 0.4 7.4 -
3.0 0 2.0 1.0 4.2 0.2 0
0.3 0.7 1.7 4.3 0.8 1.4 1.7 0 3.5 2.7 0
0.7 4.2 2.4 -
. <0.1
0
66
N. REID ET AL.
TABLE 1 (continued)
Species
Family
Habitb
Max.
height
(m)
Pithecellobium ebano (Berl. ) Muller Pithecellobium pallens (Benth.) Standl. Porlieria angustifolia (Engelm. ) Gray Prosopis laevigata (Willd.) M.C. Johnst. Randia spp. Rhus pachyrrachis Hemsley Schaefferia cuneifolia Gray Viguiera stenoloba Blake Xylosmaflexuosa (H.B.K.) O. Ktze. Zanthoxylumfagara (L.) Sarg. Ziziphus obtusifolia (Torr. & Gray) Gray
Leguminosae Leguminosae Zygophyllaceae Leguminosae Rubiaceae Anacardiaceae Celastraceae Compositae Flacourtiaceae Rutaceae Rhamnaceae
T TS S T S T U S S S S
14 8 3 10 5 3 1 2 9 3
aVoucher specimens are deposited in the Herbarium of the Facultad de Ciencias Forestales, UANL, Linares, N.L., Mexico. DU, undershrub; S, multi-stemmed shrub; TS, tall shrub; ST, small tree; T, tree. cPercentage occurrence of the species in 40 sites.
spersed with ridges and hills up to 50 m above the surrounding terrain. Soils of the plains and lower slopes are chiefly dark grey, silty-clay vertisols. On gentle hills and upper slopes, outcrops of Upper Cretaceous mudstone or shale (lutita) occur, often overlain by silty-clay loams. Scattered Pliocene and Quaternary palaeoriver terraces of conglomerate, often consolidated in a limestone matrix (caliche), exist throughout the region. Caliche terraces usually cap ridges and rises or occur as raised benches, being more resistant to weathering than the lutita bedrock (M. Ruiz, personal communication, 1987). All soil types are low in nitrogen (consistently < 1%, Kjeldahl; N. Reid and P. Beyer-Miinzel, unpublished data, 1987) and are probably deficient in phosphorus as well (Foroughbakhch et al., 1987 ). The native vegetation is a diverse, spiny, often dense scrub dominated by woody plants (Heiseke, 1986). Some 80 species of shrubs and trees, ranging in height from 1 to 15 m, are commonly encountered. Wildfire does not occur in the thornscrub near Linares and prescribed burning is rare. Thus the present vegetation has probably developed in the absence of fire (Jurado and Reid, 1989).
Sampling Thirty-nine sites were sampled in four major habitats defined by climate and substrate (Reid et al., 1987). Sites were located in humid and xeric
UTILIZATION OF TAMAULIPANTHORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
Sample Spines frequency ¢ or (%) thorns
33 93 50 45 78 8 48 10 28 85 25
Regional browse index
67
Habitat Dry caliche
Dry deep
H u m i d deep
H u m i d skeletal
Cover (%)
Browse index
Cover (%)
Browse index
Cover (%)
Browse index
Cover (%)
Browse index
0.7 0.1 <0.1 <0.1 . 0.1
2.2 1.5 0 5.0 . 3.6
2.6 0.8 3.8 0.8 2.8 . 4.8
1.8 20.5 <0,1 0.5 24.1
1.3 1.0 2.0 0 1.1
20.7 <0.1 3.9 <0.1 <0.1 . 0.4 5.9 -
0.7 3.3 2.0 1.3 4.0
+ + + + -
1.9 1.1 2.9 0.4 2.0 1.3 3.6
-
0
-
+ + +
0.4 1.8 0
. 0,1 -
3.1 7.9 0.7 14.2 2.3 . 0.2
.
-
.
1.2
. 1.5 -
0
. 14.8 0.8
1.9 0
. 0,3 1.8 . . . 0.2 0.8 22.5 1.2 0.1 0
0 2.1 -
subregions (Fig. 1 ), in areas with either deep soils (vertisols, >0.5 m deep) or skeletal soils (leptosols and calcisols, ~<0.5 m deep, over caliche hardpan or mudstone). A 40th site with intermediate soil characteristics was sampled in the humid subregion. At each site, a 0.5-ha area homogeneous in physical and vegetational attributes was delimited. A list of the woody plants was compiled at each site (see Table 1 for scientific names and authorities). Cover of all woody species was measured by point interception (Mueller-Dombois and Ellenberg, 1974) in dense vegetation, or by estimating canopy area in quadrats in more open vegetation. Dung of large herbivores was counted to identify the principal herbivores and assess current grazing pressure in each site (Reid et al., 1987 ). The dung pats of cattle, horses and donkeys were counted individually. For goats, the number of occurrences of dung > 5 m from other dung was counted per site. Degree of browsing was rated from 0 (not browsed ) to 6 (heavily browsed ) for each woody species (Table 2). Where conspecifics showed varying degrees of browsing damage within a site, the most severe damage class was recorded. An index of browsing damage, B, was calculated for the ith species, using the expression: Bi = ( ~ I,.)lm, n
where:/in is grade of browsing damage of the ith species in the nth site; and m~, the number of grazed sites in which the ith species occurred; B thus em-
68
N. REIDET AL.
TABLE 2 Criteria for classifying levels of browsing on shrubs and trees Index browsing damage
Appearance of plant
O: Not browsed
Plants normal in height and form; no evidence of utilization of current or previous years growth
1: Scarcely browsed
Browsed stem over a small part of the canopy; otherwise plant normal in height and form
2: Light
Normal in height and form but stems or shoots browsed over most of the canopy; without disfigurement of the woody portions
3: Light to moderate
Complete canopy but some stems gnarled and disfigured owing to repeated browsing
4: Moderate
Normal in height but with irregular, incomplete canopy; the wood portions of stems disfigured and twisted
5: Moderately heavy
Stunted, but with aerial stems, much disfigured and twisted by repeated biting, above a low, browsed crown
6: Heavy
Pruned to a low crown near ground level (unless part of canopy protected by other plants
phasizes the species which were heavily browsed, regardless of their frequency of occurrence. Browsing indices were calculated for each woody species across all sites, as well as for each habitat. Intensity of cutting was measured by counting the number of cut stems > 3 cm diameter at the point of removal in each site. Cut stems were recorded as trunks if cut ~ 1 m above the ground, and as branches if > 1 m from the ground. For each species, a regional cutting index was calculated as the sum of cut stems across all sites, and habitat-specific indices were computed as the sum of cut stems of the species in each habitat. An index of cutting preference was calculated for each species as the (regional cutting i n d e x ) / ( m e a n cover across all sites). Information about uses of wood of different species for timber, fuelwood and other purposes was obtained by interviewing a former contract woodcutter, the owner/manager of a 40-ha ranch, a carpenter/cart-maker and the manager of a 200-ha ranch. The informants were selected for their knowledge of thornscrub shrubs and trees and the various uses made of each species.
UTILIZATION OF TAMAULIPAN THORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
69
Informants were asked to rate the value of 22 shrubs and trees for fuelwood and fence-posts, and to specify other uses. RESULTS
Floristics and vegetation A total of 53 woody species were encountered in three or more of the 40 sites inventoried in thornscrub near Linares (Table 1 ). Legumes constituted 32% of the woody flora and contributed a third of the vegetative cover. Other prominent families were the Rutaceae, Boraginaceae, Oleaceae, Rhamnaceae and Ebenaceae. Armed species bearing spines or thorns comprised 38% of the flora but contributed 52% of the vegetative cover. Among the Leguminosae, the 11 armed species (65% of 17 species) accounted for 87% of the total leguminous cover. The most widely distributed species were multi-stemmed shrubs and small trees. Acacia rigidula, Cordia boissieri and Karwinskia humboldtiana occurred in all 40 sites (Table 1 ). Pithecellobiumpallens, Diospyros texana, Bernardia myricaefolia, Forestiera angustifolia and Condalia hookeri were also widespread, with sample frequencies > 88%. Species with the highest mean regional cover values were mainly shrubs: P. pallens (13.6%), A. rigidula (12.8%), Zanthoxylurnfagara ( 11.9%), Cordia boissieri (9.9%), Randia spp. (8.9%), Diospyros palmeri (7.4%) and F. angustifolia (7.3%). Dry caliche sites supported a predominantly unarmed, depauperate shrubland or scrub, to 4-5 m, dominated by A. rigidula, Cordia boissieri and Helietta parvifolia in the upperstorey and K. hurnboldtiana and Chamaecrista greggii in the understorey. Krameria ramosissima, Calliandra conferta and Jatropha dioica were conspicuous undershrubs in open shrubland, and H. parvifolia and A. berlandieri dominated taller, denser scrub. Dry deep-soil sites supported moderately diverse, dense, spiny scrub to 5 m, although some sites were grassland/scrub mosaics. The dominant upperstorey trees and shrubs were A. berlandieri, Cordia boissierL A. rigidula, and in the mid-shrub layer, F. angustifolia. Better water relations close to arroyos promoted the development of low, closed forests to 8 m, dominated by arborescent Prosopis laevigata and A. farnesiana and the tall shrub Pithecellobium pallens, with abundant Celtis pallida and Z. fagara in the mid-shrub layer. Humid deep-soil sites supported very dense, diverse spiny scrubs, to 5-6 m, dominated by thin, multi-stemmed shrubs and trees. A. rigidula, D. texana, Eysenhardtia polystachya and P. pallens were dominant, with abundant Z. fagara, Randia spp., F. angustifolia and Amyris texana in the mid-shrub layer. Arroyos were fringed with dense, closed, gallery forests, 7-14 m, dominated by D. palmeri and P. pallens, with Randia spp. and Z. fagara abundant in the mid-shrub layer.
70
N. REID ET AL.
Humid sites with skeletal soils overlying caliche were characterized by moderately diverse and floristically distinct scrub or low forest to 6 m, dominated by arborescent Cordia boissierL D. texana, H. parvifolia and Gochnatia hypoleuca as well as the tall shrub, P. pallens in the upperstorey, and Neopringlea integrifolia and Acacia rigidula in the mid-shrub layer. Skeletal soils overlying mudstone in the humid zone supported scrub to 5 m, in which the tree, Fraxinus greggii was a character species. Pithecellobium pallens, Acacia rigidula and Cordia boissieri were dominant in the upperstorey, and Bernardia myricaefolia and Z. fagara dominated the mid-shrub layer.
Browsing damage Dung counts indicated that cattle and occasionally horses and mules were the most abundant large herbivores in all four habitats (Table 3). Cattle and equid dung counts varied considerably among sites in most habitats, but were generally highest in dry caliche sites and lowest in humid deep-soil sites. Goat dung was only encountered in dry caliche sites. Evidence of browsing damage and large herbivore dung was found in all but two humid deep-soil sites. Species exhibiting low levels of browsing damage at the regional level ( B < 1.25; Table 1 ) were often armed, whereas more heavily browsed species tended to be unarmed (Table 4). A chi-square test (2"2= 4.37, 1 OF, P < 0.05) supported the hypothesis that level of browsing TABLE 3 Dung counts and browsing damage indices among habitats (mean ± SE) Dry caliche Dung counts per site Cattle, horse and mule 35.4 ± 14.9 Goat 3.3 ± 1.36 Browsing damage index per species 2.2±0.32
Dry deep
Humid deep
Humid skeletal
25.6±7.0 0
13.2±5.6 0
27.0±3.4 0
1.3 + 0.24
1.3 + 0.22
1.8 + 0.27
TABLE 4 Relationship between browsing damage and spinescence (see Table 1) in the woody flora of the Tamaulipan thornscrub Regional index of browsing damage
Armed species Unarmed species
>t 1.25
< 1.25
6 21
14 12
UTILIZATION OF TAMAULIPAN THORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
71
damage was related to spinescence. The species with the highest levels of browsing damage were unarmed shrubs and trees. A large array of armed and unarmed species showed intermediate levels of browsing damage. Another group of common or widespread spiny shrubs and trees suffered low levels of browsing damage. Seven species registered no browsing damage at all, most being uncommon trees that grew beyond the browsing range of livestock, or spiny or otherwise unpalatable shrubs. However, three common unarmed species showed little or no browsing damage, the toxic shrub Karwinskia humboldtiana, the leathery-leaved small tree Cordia boissieri, and the viscid
Gymnosperma glutinosum. Mean browsing damage per species was highest in dry caliche sites and lowest in humid deep-soil sites (Table 3 ). When browsing damage to particular species was compared among habitats, three trends emerged (Table 1 ). Highly palatable species such as the unarmed B. myricaefolia and H. parvifolia were moderately or heavily browsed in all habitats in which they occurred. Other species, such as Acacia berlandieri and Leucophyllumfrutescens, were heavily browsed in dry caliche sites, but were less damaged in more mesic habitats. Finally, a large group of species registered little or no browsing damage across all habitats.
Cutting of trees and shrubs Evidence of cutting was found in all but one site. Species with hard, dark timber such as Condalia hookeri, Prosopis laevigata, Gochnatia hypoleuca and Piochnatia ebano were heavily cut, had high values of the index of cutting preference, were the preferred species for fuel, fencing or construction, and were less abundant in the region than tall shrubs and trees with soft, pale wood, such as Pithecellobium pallens and Diospyros spp. (Tables 1, 5 ), which were said to be most valuable in carpentry (e.g. for handles of farm implements, light furniture, handicrafts and children's toys). Due to the fruticose growth-form of most thornscrub species, the trunks of shrubs and trees were typically cut (Table 5). However, among larger tree species (e.g. Prosopis laevigata, Fraxinus greggii and Acacia farnesiana ), branches were about as frequently cut as trunks. The intensity of cutting was highest in humid, skeletal-soil sites (mean of 94 cut stems/ha) and lowest in the dry subregion (62 cut stems/ha and 69 cut stems/ha in caliche and deep-soil sites, respectively). The heavily exploited species varied widely among sites, owing to differences in the composition of vegetation between habitats (Table 5 ). The most heavily cut species on a regional basis, H. parvifolia, was only abundant in caliche sites (Table 1 ). Prosopis laevigata and Pithecellobium ebano were more common in dry, deep-soil sites and were the most heavily exploited species there. Condalia
0.7 1.5 0.8 1.5 0.5 0.9 15.0 6.9 3.1 2.9 0.2 1.2 0.1 2.4 6.4 29.1 0.8 5.9 11.8 0.1 6.7 1.4
Acacia berlandieri Acaciafarnesiana Acacia rigidula Acacia wrightii Bumelia celastrina Celtis pallida Condalia hookeri Cordia boissieri Diospyrospalmeri Diospyros texana Ehretia anacua Eysenhardtiapolystachya Forestiera angustifolia Fraxinus greggii Gochnatia hypoleuca Heliettaparvifolia Neopringlea integrifolia Pithecellobium ebano Pithecellobiumpallens Porlieria angustifolia Prosopis laevigata Zanthoxylumfagara 56 55 80 90 100 50 94 88 70 68 100 94 I00 45 86 77 100 84 93 100 60 44
Trunks (%)
1.0 3.6 0.5 0.5 94.4 -
Dry caliche 6.9 0.5 7.4 0.9 2.8 6.0 5.6 0.5 26.4 6.5 0.5 34.7 1.4
Dry deep 2.3 1.3 1.6 1.0 1.0 1.6 36.5 7.8 8.0 5.2 0.8 1.6 0.3 1.0 0.5 5.2 17.4 3.1 3.9
Humid deep 8.5 8.3 1.1 3.4 1.9 4.4 17.4 40.0 2.3 12.7 -
Humid skeletal
Cutting intensities by habitat a (%)
excellent good good excellent poor excellent excellent good poor poor poor regular regular poor poor excellent regular excellent regular not used excellent poor
0.1
Fuelwood
not used
Fenceposts
regular poor excellent poor n o t used excellent regular not used not used not used poor not used not used good excellent regular excellent poor not used excellent not used
Uses o f w o o d
0.9 0.1 3.1 0.3 0.3 4.3 0.7 0.4 0.4 0.3 0.0 1.7 2.8 6.6 0.4 4.6 0.9 0.4 2.0 0.1
Index o f cutting preference
1, 2, 4 3, 4, 5, 6 1, 4, 6 4, 6 1, 4 2, 5, 6 5, 6 3, 5, 6 4, 6 2,5,7 2, 5 1 1, 2, 3, 4 2, 5 1, 4 2, 5, 6, 7 3 I, 4 5, 6, 7
7
Otherb
a Sample sizes for n u m b e r o f cut stems and b r a n c h e s were: dry caliche, 195; dry deep, 216; h u m i d deep, 386; h u m i d skeletal, 472; regional, 1297. bl, vertical posts in rural housing and constructions; 2, roofing in rural dwellings; 3, w o v e n branch walls in rural habitations and corrals; 4, cart construction; 5, childrens' toys, handicrafts, wooden spoons; 6, handles o f axes, machetes and animal-drawn ploughs; 7, furniture, ladders.
Regional cutting intensity ~ (%)
Species
Intensity o f cutting, p r o p o r t i o n o f cut trunks to branches, and uses o f t i m b e r species in the T a m a u l i p a n t h o r n s c r u b near Linares, N u e v o Le6n, northeastern Mexico
TABLE 5 t,~
UTILIZATION OF TAMAULIPAN THORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
73
hookeri was most abundant and was the most heavily cut species in humid deep-soil sites. DISCUSSION
Browse The browsing index, B, quantifies damage to woody plants on a site. The degree of browsing damage is dependent on herbivore dietary preferences and the relative and absolute abundance of the forage species; therefore B should not be Confused with the importance of the forage species in the diet. A heavily browsed species may be highly preferred by stock, but the species may not form a principal component of the diet if it is uncommon. Alternatively, if browsing is spread uniformly over many individual shrubs of an abundant species, the damage inflicted by browsing may be relatively light and the species will have a low browsing index. A small group of unarmed shrubs and trees (e.g.B. myricaefolia, Amyris
texana, Citharexylum berlandieri, H. parvifolia, Schaefferia cuneifolia, Calliandra eriophylla, E. polystachya and Porlieria angustifolia) were heavily browsed in most or all habitats. Some of these species are known to be preferred browse for livestock in the Linares region or elsewhere (Rovalo et al., 1983; Warren et al., 1984; Foroughbakhch and Martinez, 1985; Heiseke, 1986; G. Villegas Durfin, personal communication, 1986; Gonz~ez Ochoa and Ochoa Garcifi, 1987 ). Most heavily browsed species were widespread in the region but had low cover values and, with the exception of H. parvifolia, were not dominants in thornscrub vegetation. Spines and thorns are clearly an evolutionary adaptation against mammalian browsers (Janzen, 1987), restricting the loss of foliage by reducing the bite size and harvest rates of large herbivores (Cooper and Owen-Smith, 1986 ). Armed species exhibited significantly less browsing damage than unarmed species (Table 4 ). Spiny or unpalatable unarmed species had the highest mean cover values in the regional vegetation and armed species contributed disproportionately to total plant cover. In the 17th and 18th Centuries, huge flocks of sheep, totalling more than a million head, were depastured in the Linares region for 6 months each year (Del Hoyo, 1979). Reid et al. ( 1987 ) reported evidence of grazing-induced changes in the vegetation in favour of spiny succulents and unpalatable shrubs at the expense of unarmed, heavily browsed species. Thus, the present dominance of spiny and unpalatable shrubs and trees near Linares may reflect past overgrazing by livestock. Browsing damage to particular species was variable among habitats. Acacia berlandieri and Leucophyllum frutescens were heavily browsed in dry-caliche sites but not elsewhere, and most species showed less browsing damage in humid, deep-soil sites than in other habitats. Several factors were responsible.
74
N. R E I D ET AL.
Grazing pressure, as judged by dung counts, was heaviest in dry-caliche sites and least in humid deep-soil sites. The vegetation on skeletal soils was more open, shorter in stature and less spiny than the dense scrub of deep soils, and probably provided more accessible pasture for livestock. Thus, A. berlandieri and E. polystachya were within browsing range of livestock in skeletal-soil sites, but out of range on deep soils. Dry-caliche sites supported fewer woody species, and so plants of intermediate palatability may have been heavily utilized in the absence of preferred browse species. The heavily tomentose L. frutescens may fall in this category. Alternatively, since goats favor L. frutescens (Warren et al., 1984), its heavy utilization in dry-caliche sites may have been due to goat browsing in that habitat. Management was responsible, in certain instances, for a reduction in grazing on deep-soil sites. Vertisols are the preferred agricultural soils in the region, and some remnant thomscrub on deep soils was ungrazed or grazed only seasonally because of the potential for damage to adjacent, unfenced crops.
Timber The most heavily exploited trees (e.g.H. parvifolia, Condalia hookeri, Prosopis laevigata and Pithecellobium ebano) were cut in disproportionate relation to their abundance, as judged by high values of the index of cutting preference, and were the preferred species for fuelwood, charcoal, fence-posts and the large upright posts in rural dwellings. Less-frequently-cut species were least preferred for these same uses, and were more abundant (Tables 1 and 5). According to informants, the darker wood of the most heavily exploited species burned to coals and the timber was hard and durable. Field and laboratory studies confirm that the wood of H. parvifolia, Condalia hookeri, Pithecellobium ebano, Prosopis laevigata and Acacia wrightii has a higher specific gravity and is more resistant to decomposition in soil than that of other thornscrub species (Wolf and Perales, 1985 ). In addition, Prosopis laevigata has a low equilibrium moisture content, conferring high dimensional stability even in climatic extremes, making it a preferred timber in construction for doors, window-frames and cart-wheels (Wolf, 1986 ). The small tree Cordia boissieri was highly regarded as fuelwood but not for fence-posts, despite a high natural resistance to soil decomposers (Wolf and Perales, 1985 ). The disfavour shown towards the species as a fencing material appears to reflect the difficulty in finding long, straight stems and fixing metal staples or nails in the posts (Wolf and Perales, 1985 ). Gochnatia hypoleuca was moderately exploited and was a preferred timber for fence-posts, but was avoided as firewood because its smoke has a foul odour which taints food. Although timber species with pale wood and lower specific gravities (Wolf and Perales, 1985 ) were reputed to burn quickly without producing coals and to rot quickly in damp soil, Pithecellobium pallens was heavily exploited (Ta-
UTILIZATION OF TAMAULIPAN THORNSCRUB FOR BROWSE, FUELWOOD AND TIMBER
75
ble 5 ) due to its abundance and provision of long, straight posts. A small number of species which produce excellent timber or firewood were not heavily cut, owing either to their relative scarcity in the sites sampled (e.g. Acacia wrightii) or to the fact that they rarely produce a stem of exploitable diameter ( Celtis pallida). Timber extraction was greater in humid than dry sites owing to the greater development of arborescent vegetation in the former. Timber extraction in humid skeletal-soil sites exceeded that in humid deep-soil sites, because the dominant shrubs and trees in thornscrub on deep soils were generally lowquality timber species. Moreover, on deep soils away from arroyos, the thin and multi-stemmed habit of the shrubs and trees did not provide as many stems per unit area suitable for fence-posts as the arborescent dominants on skeletal soils.
Silvopastoral and agroforestry potential of thornscrub species The most abundant woody plants in thornscrub in the Linares region were the less-preferred browse, fuelwood and timber species. There is a considerable need, therefore, for experimenting with silvopastoral and silvicultural practices to increase livestock and wood production. Future research should focus on the highly preferred forage and timber species identified in this study. The most valuable species is the multi-stemmed tree H. parvifolia (bar° reta), which produces durable fenceposts and is heavily browsed by both cattle and goats. About 40% of the fence-posts in the region are of this species (N. Reid, H. Stienen and H. Hempel, unpublished data, 1988 ). Rovalo et al. ( 1983 ) reported a high fungicide and insecticide potential in leaf extracts of barreta but found no evidence of toxicity to chickens nor reports of toxicity to livestock. The species occurs in association with caliche, normally forming extensive stands on caliche rises and hills (Table 1; Rovalo et al., 1983; Reid et al., 1987 ). Foroughbakhch et al. ( 1987 ) were unable to germinate the seeds of H. parvifolia and reported slow growth rates and high mortality among seedlings transplanted from the field. However, growth of the species in plantation improved markedly after the second year (R. Foroughbakhch, personal communication, 1988 ). The problem of propagating H. parvifolia must be resolved before its timber and forage productivity can be further evaluated. Three leguminous shrubs and trees browsed by livestock also produced high quality fuelwood or timber: the spiny tree Pithecellobium ebano, the thorny shrub Acacia berlandieri, and the unarmed shrub E. polystachya. Considering their potential to fix nitrogen and the relatively low level of N in soils of the region, these species could play an important role in agroforestry and silvopastoral systems. The two shrub species can be managed to maintain a bushy crown within reach of livestock (R. Foroughbakhch (1987) and G. Alanis ( 1987 ), personal communications), although their crown height is normally
76
N. REID ETAL.
> 2 m. Pithecellobium ebano produces an annual crop of large seeds which are roasted and eaten or used as coffee substitute. In plantation, juvenile Pithecellobium ebano and Acacia berlandieri exhibited 100% survival despite frosts and had growth rates of 0.9 and 0.6 m/year in height, respectively, over 2 years (Foroughbakhch et al., 1987 ). The seeds of all three species germinate readily (R. Foroughbakhch ( 1988 ) and J. Castafieda ( 1988 ), personal communications). After heavy browsing or cutting, the suckers of both spiny species are more heavily armed than undamaged shoots, a common characteristic of spinescent shrubs and trees (Cooper and Owen-Smith, 1986 ). Several other species either produce excellent timber and fuelwood or are heavily browsed. The legumes in this category are the heavily browsed, unarmed undershrubs Callandria eriophylla and Chamaechrista greggii, and the timber-producing trees Prosopis laevigata and Acacia wrightii. These species should be screened for nitrogen fixation. Under plantation conditions, Prosopis laevigata did not grow well in a 3-year study near Linares because of repeated defoliation by locusts and jackrabbits (Foroughbakhch et al., 1987 ). However, Acacia wrightii performed well, exhibiting 100% survival and a height increment of 0.9 m/year. The valuable browse, timber and fuelwood species identified in this study provide an interesting contrast to the multipurpose trees and shrubs attracting attention in the semi-arid regions of central America and elsewhere. High population pressure and extensive land clearance in much of central America have produced critical shortages of fuelwood and timber (Dulin, 1984). Accordingly, most emphasis to date has been given to fast-growing, high-producing multipurpose species such as Leucaena, Gliricidia, Prosopis and certain Acacia spp. (Ch~aro Bas~ifiez and Pardo Tejeda, 1976; Anonymous, 1980, 1983, 1986; Hughes and Styles, 1984; Withington et al., 1987). However, high specific gravity, timber durability and frost hardiness are sacrificed in fast-growing legumes (Hughes and Styles, 1984; Synnott, 1986; Foroughbakhch et al., 1987 ). Fast-growing multipurpose species in northeastern Mexico (e.g.L. leucocephala, Parkinsonia aculeata, Acacia farnesiana ) are not heavily exploited by the rural population, since timber species with a high specific gravity and greater durability are still available for exploitation and are preferred for firewood, construction and fence-posts. Improvement of traditional land-management practices in the Tamaulipan thornscrub should involve the development of sustainable silvicultural, silvopastoral and agroforestry systems based on the extant natural vegetation. The high-quality browse and timber species need to be evaluated with respect to productivity, resistance to varying levels of exploitation, and ability to increase in abundance after the selective elimination from natural vegetation of species of lesser value. Less-common species need to be evaluated for ease of propagation and establishment, ability to persist in thornscrub after introduction, and suitability for incorporating in artificial grazing systems that employ both woody and herbaceous components. The degraded and ever de-
UTILIZATIONOF TAMAULIPANTHORNSCRUBFOR BROWSE,FUELWOODANDTIMBER
77
creasing remnants of thornscrub will be unable to meet the traditional needs of the rural population if population growth and Government-sponsored landclearance programmes in northeastern Mexico are not drastically curtailed. ACKNOWLEDGEMENTS
We are indebted to Steve Archer, Manuel Fem~indez Brondo, Colin Hughes, Jorge Landa, Rub6n Pefiazola, Holger Stienen and Brian Styles for their encouragement, information and criticisms during the course of this study. The work was funded by a grant to Glafiro Alanis F. from the Mexican Secretaria de Educaci6n Pfblica.
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