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Comparative population analysis of desert ironwood (Olneya tesota) in the Sonoran Desert
Journal of Arid Environments 74 (2010) 173–178
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Comparative population analysis of desert ironwood (Olneya tesota) in the Sonoran Desert ˜ iga-Tovar*, H. Suza´n-Azpiri B. Zun ´ uregui, C.P. 76230 Quere´taro, Qro., Mexico ´ noma de Quere´taro, Av. De las Ciencias s/n. Juriquilla, Delegacio ´n Santa Rosa Ja Facultad de Ciencias Naturales, Universidad Auto
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 October 2008 Received in revised form 31 July 2009 Accepted 4 August 2009 Available online 11 September 2009
Olneya tesota (ironwood) is a keystone tree species that modifies the structure and function of Sonoran desert ecosystems. Presently, extraction of trees and branches in the region constitutes a severe risk for ironwood populations. We analyzed the size structure, two damage indicators (damaged basal area or ‘‘DBA’’ and percents of damaged individuals or ‘‘PDI’’) for eleven ironwood populations in Sonora and Baja California, Me´xico. In four populations, we compared damage indicators obtained in 1992 and 2004. We found that in unexploited populations, small size categories (<500 cm2 in basal area) and juveniles were better represented. In contrast in exploited populations large size individuals were severely damaged and lacked recruitment. Populations on the coast of Sonora have greater damage (up to 50% of the individuals), while those in the Baja California Peninsula exhibited less damage (smaller than 20%), with high juvenile recruitment (up to 40%). Historical comparisons of DBA and PDI exhibited non significant differences between 1992 and 2004 for three populations, and only in Bahia Kino population (Sonora) did the PDI decreased significantly. One population surveyed in 1992 (Sonoyta) was completely removed. Even that the damage seems to be constant, the damage indicators remain high, affecting the ecosystems dynamics. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Damage indicators Desert legume trees Size structure Wood harvesting
1. Introduction Olneya tesota (Fabaceae) is an endemic arboreal legume tree of the Sonoran Desert. Commonly, it’s called ironwood due to the hardness of the wood. Numerous studies have demonstrated their cultural and ecological importance as a key species that modifies the structure and function of desert ecosystems (Suza´n et al., 1996; West et al., 2000a, West et al., 2000b). Ironwood is used frequently by various Native American groups including Pimas, Mayos, Yaquis and Comca´acs or Seris (Nabhan and Plotkin, 1994). Felger and Moser (1985) reported that ironwood is an important element in Comca´ac ceremonies. Monti et al. (2000) mentioned that the seeds are ground into flour, and some parts of the plant are used in traditional medicine. The wood is also used for tool manufacturing, farm implements, and in building houses (Monti et al., 2000; St. Antoine, 1994). The most important use of this tree is the manufacturing of wood figures. This activity began in the 1960´s within Comca´ac communities (St. Antoine, 1994). Because the figures made by the tribe were handmade, the production numbers were initially small.
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Because of good market value, other craftsmen began to imitate the Comca´ac figures using industrialized methods that allowed the manufacture in larger volumes, with shorter production time and lower costs. This asymmetry in the production techniques lead to a reduction in the numbers of Comca´ac craftsmen (only fifteen in 1996; Durand, 1996), because the hand crafted products could not compete with the industrialized production of wood figures. In 2003, 250 workshops that elaborate ironwood figures were estimated to be active in Hermosillo city, Sonora. Durand (1996) points out that this industry requires approximately 5000 tons of wood annually, and that most of this wood was extracted illegally. The production of figures is one of the main reasons for the decrease of the ironwood populations in Sonora (Suza´n et al., 1999). In an evaluation of the damage caused by tree cutting Suza´n, Nabhan and Patten (1994) detected that in some populations 80% of the intermediate size plants were damaged. Nevertheless, there are other serious threats for this species, including extraction for firewood and charcoal, and the conversion of extensive desert areas into grasslands for livestock. These threats are compounded by the distorted population structure with limited presence of juveniles throughout all the ironwood distribution range, particularly in the sites undergoing wood extraction (Solı´s, 1997; Suza´n et al., 1997). Because ironwood is a keystone species, the decrease of natural populations set in risk the conservation of the Sonoran Desert (Nabhan and Plotkin, 1994). This species is considered a key
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(damaged area percentage and damaged individuals percentage) of ironwood populations in Baja California and Sonora, Mexico; b) describe and compare the population structure of ironwood in exploited and non-exploited sites, and; c) compare the damage indicators estimated in 1992 (Suza´n, 1994) with the more recent data obtained in this study for four populations (2004). 2. Methods 2.1. Study sites
Fig. 1. Sites of study and regional division of Sonora’s desert based on Shreve (1951) and modified by Turner and Brown (1982). Sites identification: (1) Comita´n (110.8416 W, 24.2511 N); (2) Bahı´a de los A´ngeles (113.6700 W, 28.9475 N); (3) San Felipe (114.9611 W, 30.906 N); (4) El Pinacate Biosphere Reserve (113.3597 W, 31.8964 N); (5) Sonoyta (112.8194 W, 31.8725 N); (6) Puerto Lobos (112.2474 W, 30.6040 N); (7) Puerto Libertad (112.3100 W, 29.6500 N); (8) Rancho Lobos (112.0398 W, 29.562 N); (9) Bahı´a Kino (111.4300 W, 28.900 N); (10) Pozo Hondo (110.9027 W, 29.9463 N); (11) Cuesta Blanca (110.8611 W, 29.6722 N).
modifier of the structure and function of this ecosystem (Mills et al., 1993), generating microhabitats that benefit many other species of the flora and fauna (Bu´rquez and Quintana, 1994; Nabhan and Suza´n, 1994; Suza´n et al., 1996; 2000). Ironwood and other legumes such as velvet mesquite (Prosopis velutina) under their canopies create ‘‘resources or fertile islands’’ where the biodiversity and the productivity are higher when compared with open sites (Bu´rquez and Quintana, 1994; Carrillo-Garcı´a et al., 1999;Tewksbury and Petrovich, 1994). This situation has multifactor origins, such as: the non-random dispersion of seeds, protection from sunlight, higher humidity and higher soil fertility, a reduction in the risk of freezing, and a general buffering of extreme environmental conditions (McAuliffe, 1984; Suza´n et al., 1994; Tewksbury et al., 1999). Nabhan and Plotkin (1994) indicated that more than 160 plant species were associated with ironwood, in particular the columnar cacti, Peniocereus striatus, Carnegia gigantea, Stenocereus thurberi and Lophocereus schotti. The distribution and densities of these cacti are correlated with the presence of ironwood nurse plants (Suza´n et al., 1994; Suza´n et al., 1996). West et al. (2000a), West et al. (2000b) reported that 424 species of fauna use ironwood for refuge, perching or nesting. In addition ironwood and mesquite also promote the growth of AM-fungi that enrich the soils beneath their canopies (Bashan et al., 2000). Intensive ironwood extraction and the shortage of information on the status of populations motivated to us to propose the following objectives: a) Estimate present damage indicators
Fieldwork was carried out from February 2004 to May 2005 at eleven sites in the Sonoran Desert. In the Peninsula of Baja California three sites were selected: Comita´n (1), Bahia de Los Angeles (2), and San Felipe (3). In Sonora eight sites were selected: El Pinacate Biosphere Reserve (4), Sonoyta (5), Puerto Lobos (6), Puerto Libertad (7), Rancho Lobos (8), Bahia Kino (9), Pozo hondo (10) and Cuesta Blanca (11) (Fig. 1). The sites were located in the following vegetation subdivisions of Shreve (1951) for the Sonoran Desert (Fig. 1): Sites 1, 6, 7, 8 and 9 were placed in the Central Gulf Coast subdivision. This region receives an average precipitation between 100 and 250 mm annually, generally occurring at the end of the winter and in the middle of the summer, although occasionally there are several successive years without recorded rainfall. Sites 2, 3, 4 and 5 occur in the Lower Colorado Valley subdivision. This is the largest and most arid subdivision of the Sonoran Desert, characterized by its high temperatures and low precipitation (<250 mm). Sites 10 and 11 corresponded to the Plains of Sonora subdivision, which is the smallest and less diverse subdivision with an annual average precipitation that fluctuates between 250 and 375 mm. 2.2. Population structure and cutting damage In each study site, 15 circular parcels of 250 m2 were outlined according to the methodology proposed by McAuliffe (1990). In each parcel, we registered the number of ironwood trees, their height, canopy cover (calculated from two radii assuming ellipsoidal canopies), and base diameter. To evaluate the damage, the numbers of cut or dead branches were noted, as well as the number and diameter of branches with evidence of cutting. We compared the cover, height and basal area between sites using a multivariate analysis of variance (Manly, 1994), with a completely randomized with fixed effects model. Trees densities for all the sites were compared with a completely randomized analysis of variance. We measured the damaged basal area (DBA) in each site (obtained from the diameter of each stem and branches beneath 1.3 m with evidence of cutting with a Hagloff caliper), and we compared them by an analysis of covariance with total basal area as a covariate. We also compared the percentages of damaged and dead individuals (PDI) in each site by an analysis of variance of the arcsine transformed percentages with a randomized fixed effects design (Daniel, 2002; Zar, 1984). In order to compare densities among exploited and non-exploited sites we classified them according to the percent of damaged trees (<20% as unexploited, and >20% as exploited) and compared them with a completely randomized analysis of variance. Size structures were determined, in reference to diameter categories, using the basal area according to Suza´n (1994). Ten categories were considered: individuals smaller to 50 cm2, 50–200 cm2, 201–500 cm2, 501–800 cm2, 801–1100 cm2, 1101– 1400 cm2, 1401–1700 cm2, 1701–2000 cm2, 2001–2300 cm2 and finally those larger than 2300 cm2. Juvenile plants were those smaller or equal to one meter in height and five cm in diameter. The
˜iga-Tovar, H. Suza´n-Azpiri / Journal of Arid Environments 74 (2010) 173–178 B. Zun Table 1 Multivariate analysis of variance for height, canopy cover, basal area and density of ironwood adult individuals for all study sites. Data per site in mean, standard error. Sites numbers according to Fig. 1. Population (Sites) 1 2 3 4 5 6 7 8 9 10 11
Height (m)
Covering (m2)
Basal area (cm2)
Density per plot (250 m2)
3.5, 0.2 2.2, 0.1 2.9, 0.1 4.5, 0.2 3.6, 0.1 3.27, 0.2 4.4, 0.2 3.7, 0.1 3.8, 0.1 2.3, 0.1 3.0, 0.1
13.8, 4.3 14.5, 2.3 13.5, 3.2 47.0, 3.8 23.1,3.3 21.2, 4.8 16.8, 4.0 17.5, 2.1 23.0, 3.2 8.1, 2.0 13.1, 3.0
349.3, 303.3, 401.9, 1346.8, 527.0, 880.9, 770.4, 617.5, 908.1, 179.3, 317.1,
3.6, 1.8, 4.5, 2.4, 2.3, 2.2, 2.1, 2.9, 2.0, 5.7, 4.5,
142.0 77.9 16.6 125.8 108.0 156.9 130.6 69.0 105.6 85.6 99.2
0.4 0.1 0.9 0.3 0.2 0.4 0.3 0.2 0.1 0.3 0.2
Test
Value
F
Significance (P)
Wilks Lambda Pillai´s Trace Hotelling–Lawley Roy´s Max Root
0.612480 0.431642 0.561962 0.394099
9.1101 8.6384 9.5659 20.2567
<0.0001 <0.0001 <0.0001 <0.0001
relative frequency of healthy, damaged or dead plants was determined for each size category. All statistical analysis was performed with JMP version 5.0.1. (SAS, 2002). 2.3. Historical comparison of cut damage We analyzed four populations (4, 5, 7 and 9) previously monitored in 1992 by Suza´n (1994), comparing the percentage of dead or damaged trees and the percentage damaged basal area between the data collected in 1992 and those of 2004 with an analysis of variance of a single factor (Daniel, 2002). 3. Results 3.1. Population structure and damage Trees in the populations differed significantly in mean height, cover and basal area according to the multivariate analysis of variance (Table 1). Therefore, differences in the size of individual trees among the populations were the result of two distinct patterns of growth. In the first pattern, the individuals had a predominantly bushy pattern. These plants have many branches from the base, with thin trunks generally smaller than three meters tall. The populations
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displaying these characteristics were 1, 2, 3, 10 and 11. The second pattern had trees with distinct arboreal growth, with one or two well defined trunks, and generally reaching height over four meters. This pattern was observed in populations 4, 5, 6, 7, 8, and 9. The analysis of variance illustrates a significant difference in the density among populations P(F ¼ 10j10,160 d.f.) < 0.001. Highest densities were observed in sites 3, 10 and 11 with an average density of 4.56, 5.7 and 4.5 plants per plot (250 m2) respectively (Table1). Sites 2 and 9 had low densities, with an average of 1.85 and 2 plants per plot. The remaining sites (1, 4, 5, 6, 7 and 8) had an average density between 2 and 3.6 plants per plot. The comparison of densities among exploited and non-exploited sites showed significant differences P(F ¼ 27.69j1,169 d.f.) < 0.001. The average mean in exploited and non-exploited sites were 2.4 and 3.9 plants per plot respectively, converting to 96 and 156 plants per hectare, respectively. There was a wide range of values for the damage to the basal area of ironwood trees among sites. Sites 10 and 11 did not present any evidence of cutting (Fig. 2a) and differed significantly from the remaining sites. With the rest of the sites, significant differences were found (Fig. 2a). Sites 2 and 3 had a significantly smaller DBA than the rest of the sites with P(F ¼ 5.62j8,1134 d.f.) < 0.001. The populations with smaller percentages of DBA were sites 2 and 3 with a mean of 6.42% and 3.3%, respectively. Sites with the highest percentage of DBA were located in the Central Coast of California Gulf (sites 8 and 9) with a DBA of 26% and 31.54% each. Populations with an intermediate range of damage were found in sites 6, 7, and 4 with DBA of 18%, 21.2%, and 25%, respectively (Fig. 2a). The percentage of damaged plants (PDI) showed significant differences for nine exploited populations (P (F ¼ 3.28j8,133 d.f.) ¼ 0.0019). The sites with lower PDI were 2 and 3 with 15% and 14% respectively. The sites with higher PDI were 4, 8, and 9 with 49%, 47.5%, and 44.6%, respectively (Fig. 2b). Two groups with contrasting characteristics were observed (Table 2). The first group consisting of sites 1, 2, 3, 10 and 11 which were not subjected to commercial extraction. The dominant growth forms were bush with a basal area less than 500 cm2 (Figs. 3a, 3d). The sites with the highest percentages of juveniles plants were site 11 (15.5%), site 10 (29%) and site 2 (44.5%). The populations located in the Central Coast of the California Gulf extending to the Mexico–USA border (sites 4, 5, 6, 7, 8 and 9) made up the second group. These sites have endured high wood extraction. The size distributions differ greatly from the first group (Table 2). The intermediate categories, between 500 and 1400 cm2 of basal area, were best represented (Figs. 3b and 3c). It is noteworthy that no juvenile plants were observed within the sites. Within these populations, the largest plants with basal areas
Fig. 2. 95% Confidence intervals of the arcsine transformed percents of basal damaged area (a), and damaged and dead trees (b) per plot (250 m2). Site numbers according to Fig. 1. Sites with the same letter do not have significant differences.
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Table 2 Comparisons of arboreal and bush growing types. Study sites from Study sites
1, 2, 3, 10 and 11
4, 5, 6, 7, 8 and 9
Geographical location
Baja California Peninsula Hermosillo Plains Predominantly bush-shape Minor diametrical types < 500 cm2 basal area Dead or damaged plants: 0–15% Damaged basal area: 0–18% No-commercial exploitation, occasionally firewood domestic consume Percentages between 16 and 40 juveniles
Principally located at Central Gulf Coast and Lower Colorado Valley
Growth patterns Size types Damage indices Ironwood use history Recruitment evidence
greater than 2000 cm2 and heights between 4 and 7 m were found. Site 4 stands out where the majority of plants with basal areas of more than 1700 cm2 represent 35% of the population (Fig. 3b). 3.2. Historical comparison of damage indices Damaged basal area percent (DBA) and percentage of damaged individuals (PDI) for site 4 (El Pinacate Biosphere Reserve) did not show significant differences between 1992 and 2004 (P(F ¼ 3.13j 1,19 df) > 0.09 for DBA; P(F ¼ 4.97j 1,19 df) > 0.09 for PDI). However, the DBA mean for Pinacate decreased from 1992 to 2004 (33.5, 24.98, respectively). Similar trend occurs for PDI (73.33 and 49.22 respectively). Site 7 (Puerto Libertad) had non significant differences for DBA (P(F ¼ 1.79j 1,19 df) > 0.19), and PDI (P(F ¼ 0.531j 1.19 df) > 0.74). Both indices remain with low values, in 1992, 8.0 for DBA and 23.91 for PDI; in 2004, 21.22 for DBA and 35.80 for PDI.
Predominant tree-shape Intermedium types, between 500 and 1400 cm2 de basal area Dead or damaged plants: 30–49% Damaged basal area: 11–32% Wood extraction for commercial purposes: coil, firewood, handicraft manufacturing Almost null
In site 9 (Bahia Kino), non significant differences in the percentage of DBA were detected (P(F ¼ 0.09j 1,36 df) > 0.76). This region was the origin of ironwood exploitation for handcrafts, and in addition is the closest site to the city of Hermosillo, therefore, exhibited bigger damage indicators. Nonetheless, there was a significant difference in the PDI that had diminished from 76.2% in 1992 to 44.6% in 2004 (P(F ¼ 4.97j 1,36 df) < 0.0009). Finally, the site 5 studied by Suza´n (1994) was completely removed and the area was totally devoid of plant covering. 4. Discussion 4.1. Population structure and cutting damage estimation Ironwood commercial exploitation for artisan crafts manufacturing began in Bahı´a Kino (site 9), around 1960 (St. Antoine, 1994). During 1990 it was evident that the wood extraction
Fig. 3. Relative frequencies of ironwood basal areas for sites 3 (a); 4 (b); 9 (c) and 10 (d). Healthy trees are those without any damage.
˜iga-Tovar, H. Suza´n-Azpiri / Journal of Arid Environments 74 (2010) 173–178 B. Zun
had extended as far as Sonoyta (site 5) located at the Mexico–US border (Suza´n, 1994). In this study, we observed that a large portion ˜ asco of the coastal region of Sonora, from Bahı´a Kino to Puerto Pen including the El Pinacate Biosphere Reserve (site 4), exhibited ironwood populations with evidence of wood extraction damage. Two conditions appear to exist concerning the population structure and wood extraction: 1) damage due to harvesting for firewood, and 2) damage as the result of wood extraction. The first condition occurs in the Baja California Peninsula and in the Plains of Sonora region where populations are not harvested for commercial use of the wood (Fig. 1). In this condition, ironwood grows predominantly as a bush, and does not reach sufficient sizes to provide adequate wood for the use by commercial craftsmen. Damage to trees in Baja California (sites 1, 2, and 3) were relatively low, associated to firewood collected for domestic purposes for rural inhabitants. The second condition was detected at sites 4, 5, 6, 7, 8 and 9, and exhibited high percentages of damage due to wood extraction for artisan crafts manufacturing and charcoal production, and significantly lower densities of recruits. The different conditions related to wood extraction coincide with remarkable differences in plant population structures. In areas where trees are used for wood sculpture whole trees and large branches are extracted and individuals in the minor size categories are absent or poorly represented. At these sites, intermediate sizes are the most frequently observed. In contrast, in areas where the trees are subjected to non commercial exploitation (firewood) the individuals are not extracted but have small basal areas (<500 cm2) and bush like architecture and greater overall densities. Therefore, the first group of sites has trees with bigger basal area, greater height and covering, whereas the second group has smaller basal area and height, and higher recruitment. Suza´n et al. (1997) reported two growth patterns, the first with individuals growing in uplands with bush architecture and the second with trees growing in washes or ephemeral streams with arboreal growth morphology and more size classes, and the differences in canopy architecture and population structure were related to water availability. The greatest frequencies of dead plants were in intermediate size categories (basal areas less to 1100 cm2). Suza´n et al. (1997) reports similar findings indicating that bigger size trees are more resilient and survive cutting damage. Moreover, these authors describe that allometric proportions change among healthy and damaged plants, modifying from arboreal to bush-type architecture. This situation was also observed in this study and as a consequence, results in a decreased area shadowed by ironwood canopies, which undoubtedly must have an effect, not yet studied, on the microenvironment available for the species associated to this keystone species. Further studies are needed on the root systems of ironwood and its relationship with soil water availability and fertility under damaged and healthy trees. Considering that ironwood is a highly interactive keystone species (Nabhan and Plotkin, 1994; Suza´n et al., 1996), the reduction of canopy shading area may have other important negative effects on the ecosystem. The 160 plant and 424 species of fauna associated with ironwood (West et al., 2000a; West et al., 2000b) might be affected in several ways. For example, phenological disruption patterns, changes in the species demography or geographical distribution. This is particularly important in the case of various cactaceae species that depend on ironwood as a nurse plant. A case in point is P. striatus, a rare species strongly associated with ironwood (Suza´n et al., 1996). The diminishing ironwood population and its lessened role as a shade provider could produce important changes in the biodiversity that might lead to degradation or even to simplification of Sonoran Desert ecosystem.
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4.2. Historical comparison The comparison between 1992 and 2004 indicate different tendencies for the four populations studied. El Pinacate Biosphere Reserve (site 4), did not show any changes in the damage indices, and a generalized lack of recruitment in the region. These results indicate that the designation as a protected natural area has been only partially effective for the non increase in damage indicators. For Puerto Libertad (site 7), no increments in the damage indices were registered, nevertheless, in this region, inhabitants mentioned that there had been an increase of illegal wood extraction, especially in areas far from highways. In population 9 (Bahı´a Kino) a decrease in the percentage of damaged trees of appropriate size for extraction was detected. However, for the period of this study we observed areas subjected to mesquite and ironwood charcoal extraction (Bahia Kino and Sonoyta) where stumps were removed. Whole tree extraction may lead to an underestimation of damaged individuals in presented data. Suza´n et al. (1997) observed that damage to populations in Bahı´a Kino was less than the other three sites studied in 1992. Therefore, data from both surveys (1992, 2004) suggest that illegal wood extraction has moved from its original exploitation region (Bahia Kino) to other areas of ironwood distribution. The situation in Sonoyta (site 5), demonstrate the continued harvesting pressure on ironwood populations. This population sampled in 1994 had higher damage in the first survey, and in the 2004 the population was completely removed, despite the location as a buffer zone between two protected areas: El Pinacate Biosphere Reserve in Me´xico and Organ Pipe Cactus National Monument in United States. These facts clearly illustrate the reasons why action must be promoted to preserve and restore this biological corridor that connects both protected areas. 4.3. Considerations for the conservation of ironwood populations Illegal ironwood extraction for artisan crafts and coal production are not the only activities that threaten this species. Unfortunately, the extensive conversion of lands for urban development and agricultural expansion are also significant problems (Suza´n, 1994). The most serious problem of habitat transformation has occurred as a direct result of plant removal to establish pastures for livestock. In the central and south portion of Sonora’s desert this has been the case as major habitat conversion for the introduction of buffel grass (Cenchrus ciliaris) on more than 400,000 hectares, that has threatened many native plants in Sonora (Bu´rquez and Quintana, 1994). Livestock-farming activities also affect ironwood populations because the removal of plant coverage and overgrazing inhibits species recruitment. Tewksbury and Petrovich (1994) pointed out that Tiburo´n Island, a protected area where cattle was excluded, had higher proportion of juveniles compared to the areas in Bahia Kino subjected to grazing. The fact that Mexican authorities have granted ironwood legal special protection (NOM-059-ECOL-2001) does not seem to be enough to secure the long term survival of its populations. Other leguminous species in the Sonora desert are also being affected by human activities. A case in point highlights mesquite (P. velutina and P. glandulosa), severely damaged for charcoal extraction exhibiting important population decreases in different regions in Mexico, affecting specially to those inhabiting the Sonoran Desert (Leo´n de la Luz et al., 2005; Suza´n et al., 1999). Therefore, the implementation of viable management plans for keystone species such as ironwood or mesquite is a crucial activity for governmental managers. The lack of accurate management programs for desert species is a common denominator in many ecosystems. As an example Lemeninh et al. (2007) pointed out the serious risks that
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Boswelli papyfera populations under commercial extraction faces in the arid zones of Ethiopia as a result of the absence of a management plan. To enact effective conservation of ironwood populations, four important strategies must be considered: 1) It is necessary to create environmental consciousness among tourists and locals who are the main buyers of ironwood figures and charcoal; 2) It is fundamental that governmental agencies impose effective restrictions that decrease the damage of natural populations; 3) It is essential an intensive strategy of environmental education, to decrease the illegal clearing of these lands for establishment of pastures, and improve range management in the region including restoration techniques and practices; and 4) A long term evaluation of the population structure and dynamics in protected and unprotected areas is needed. Ironwood is a long-lived species that contributes to ecological processes that may not be obvious on a human time scale, but evident only in the time span in which this species lives. Acknowledgments This work was financed by the CONACYT grant CO1-2002-273. Dr. Robert Jones from UAQ helped in editing the text. References Bashan, Y., Davis, E.A., Carrillo-Garcia, A., Linderman, R.G., 2000. Assessment of VA mycorrhizal inoculum potential in relation to the establishment of cactus seedlings under mesquite nurse-trees in the Sonoran desert. Applied Soil Ecology 14, 165–176. Bu´rquez, A., Quintana, M.A., 1994. Islands of diversity: ironwood ecology and the richness of perennials in a Sonoran Desert Biological Preserve. In: Nabhan, G., Carr, J.L. (Eds.), Ironwood: An Ecological and Cultural Keystone on the Sonoran Desert. Conservation International, Washington D.C., pp. 9–27. Carrillo-Garcı´a, A.J., Leo´n de la Luz, L., Bashan, Y., Bethlenfalvay, G.J., 1999. Nurse plants, mycorrhizae and plant establishment in a disturbed area of the Sonoran Desert. Restoration Ecology 7 (4), 321–335. Daniel, W., 2002. Bioestadı´stica, fourth ed. Limusa Wiley, Me´xico. Durand, L.,1996. El palo fierro, especie clave del Desierto de Sonora. Ciencias 43, 24–26. Felger, R.S., Moser, M.B., 1985. People of the Desert and Sea: Ethnobotany of the Seri Indians. University of Arizona Press. Lemeninh, M., Feleke, S., Tadesse, W., 2007. Constraints to smallholders production of frankincense in Metema district, north-western Ethiopia. Journal of Arid Environments 71, 393–403. Leo´n de la Luz, L., Domı´nguez Cadena, R., Dı´az Castro, S.C., 2005. Evaluacio´n del peso ˜ o a partir de variables dimensionales en dos especies de mezquite Prodel len sopis articulata S. Watson y P. palmeri S. Watson, en Baja California Sur, Me´xico. Acta Botanica Mexicana 72, 17–32.
Manly, B., 1994. Multivariate Statistical Methods, second ed. Chapman & Hall, United Kingdom. McAuliffe, J.R., 1984. Prey refugia and distributions of two Sonoran Desert cacti. Oecologia 65, 82–85. McAuliffe, J.R., 1990. A rapid survey method for the estimation of density and cover in desert plants communities. Journal of Vegetation Science 1, 653–656. Mills, S.L., Soule´, M.E., Doak, D.F., 1993. The keystone-species concept in ecology and conservation. Bioscience 43, 219–223. Monti, L., Nabhan, G.P., Solı´s-Garza, G., 2000. Cultural and traditional uses of ironwood in the Sonoran Desert. In: Nabhan, G., Behan, M. (Eds.), Desert Ironwood Primer. Arizona-Sonora Desert Museum, pp. 1–10. Nabhan, G.P., Plotkin, M.J., 1994. Introduction. In: Nabhan, G., Behan, M. (Eds.), Desert Ironwood Primer. Arizona-Sonora Desert Museum, pp. 4–5. Nabhan, G.P., Suza´n, H., 1994. Boundary effects on endangered cacti and their nurse plants in and near a Sonoran Desert Biosphere Reserve. In: Nabhan, G.P., Carr, J.L. (Eds.), Ironwood: An Ecological and Cultural Keystone of the Sonoran Desert. Conservation International, Washington, D.C., pp. 55–68. NOM-059-ECOL-2001, 2002. Proteccio´n ambiental Especies nativas de Me´xico de flora y fauna silvestre. Diario Oficial de la Federacio´n. SAS Institute, 2002. JMP Users Guide Cary, New York. Shreve, F., 1951. Vegetation of the Sonoran Desert. Carnegie Institute Washington Publication, Washington DC. Solı´s, G. 1997. Evaluacio´n poblacional actual del mezquite y palo fierro en ambientes a´ridos sujetos a un aprovechamiento continuo. Reporte final. DICTUS, Departamento de Investigaciones Cientı´ficas y Tecnolo´gicas Universidad de Sonora. St. Antoine, S., 1994. Ironwood and art: lessons in cultural ecology. In: Nabhan, G.P., Carr, J.L. (Eds.), Ironwood: An Ecological and Cultural Keystone of the Sonoran Desert. Conservation International, Washington, DC, pp. 69–85. Suza´n, H., Nabhan, G.P., Patten, D.T., 1994. Nurse plants and floral biology of a rare night-blooming cereus, Peniocereus striatus (Brandegee) F. Buxbaum. Conservation Biology 8, 461–470. Suza´n, H., Nabhan, G.P., Patten, D.T., 1996. The importance of Olneya tesota as a nurse plant in the Sonoran Desert. Journal of Vegetation Science 7, 635–644. Suza´n, H., Patten, D.T., Nabhan, G.P., 1997. Exploitation and conservation of ironwood (Olneya tesota) in the Sonoran Desert. Ecological Applications 7, 948–957. Suza´n, H., Malda, G., Patten, D.T., Nabhan, G.P., 1999. Effects of exploitation and park boundaries on legume trees in the Sonoran Desert. Conservation Biology 13, 1497–1501. Suza´n, H., West, P., Nabhan, G.P., 2000. Ironwood as a buffer from extreme heat and damaging radiation. In: Nabhan, G.P., Behan, M. (Eds.), Desert Ironwood Primer. Arizona-Sonora Desert Museum, p. 19. Tewksbury, J., Petrovich, C.A., 1994. The influences of ironwood as a habitatmodifier species: a case study on the Sonoran Desert coast of the Sea of Cortez. In: Nabhan, G.P., Carr, J.L. (Eds.), Ironwood: An Ecological and Cultural Keystone of the Sonoran Desert. Conservation International, Washington, DC, pp. 29–54. Tewksbury, J.J., Nabhan, G.P., Norman, D., Suza´n, H., Tuxill, J., Donovan, J., 1999. In situ conservation of wild chiles and their biotic associates. Conservation Biology 13 (1), 98–107. Turner, R.M., Brown, D.E., 1982. Sonoran desert scrubs. Desert Plants 4, 181–222. West, P., Nabhan, G.P., Johnson, R.R., Siminski, A., 2000a. Ancient ironwood forest as wildlife habitat. In: Nabhan, G.P., Behan, M. (Eds.), Desert Ironwood Primer. Arizona-Sonora Desert Museum, pp. 21–26. West, P., Nabhan, G.P., Suza´n, H., Monti, L., 2000b. Ironwood diversity study. In: Nabhan, G.P., Behan, M. (Eds.), Desert Ironwood Primer. Arizona-Sonora Desert Museum, pp. 46–61. Zar, J.H., 1984. Biostatistical Analysis. Prentice-Hall.
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Comparative population analysis of desert ironwood (Olneya tesota) in the Sonoran Desert ˜ iga-Tovar*, H. Suza´n-Azpiri B. Zun ´ uregui, C.P. 76230 Quere´taro, Qro., Mexico ´ noma de Quere´taro, Av. De las Ciencias s/n. Juriquilla, Delegacio ´n Santa Rosa Ja Facultad de Ciencias Naturales, Universidad Auto
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 October 2008 Received in revised form 31 July 2009 Accepted 4 August 2009 Available online 11 September 2009
Olneya tesota (ironwood) is a keystone tree species that modifies the structure and function of Sonoran desert ecosystems. Presently, extraction of trees and branches in the region constitutes a severe risk for ironwood populations. We analyzed the size structure, two damage indicators (damaged basal area or ‘‘DBA’’ and percents of damaged individuals or ‘‘PDI’’) for eleven ironwood populations in Sonora and Baja California, Me´xico. In four populations, we compared damage indicators obtained in 1992 and 2004. We found that in unexploited populations, small size categories (<500 cm2 in basal area) and juveniles were better represented. In contrast in exploited populations large size individuals were severely damaged and lacked recruitment. Populations on the coast of Sonora have greater damage (up to 50% of the individuals), while those in the Baja California Peninsula exhibited less damage (smaller than 20%), with high juvenile recruitment (up to 40%). Historical comparisons of DBA and PDI exhibited non significant differences between 1992 and 2004 for three populations, and only in Bahia Kino population (Sonora) did the PDI decreased significantly. One population surveyed in 1992 (Sonoyta) was completely removed. Even that the damage seems to be constant, the damage indicators remain high, affecting the ecosystems dynamics. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Damage indicators Desert legume trees Size structure Wood harvesting
1. Introduction Olneya tesota (Fabaceae) is an endemic arboreal legume tree of the Sonoran Desert. Commonly, it’s called ironwood due to the hardness of the wood. Numerous studies have demonstrated their cultural and ecological importance as a key species that modifies the structure and function of desert ecosystems (Suza´n et al., 1996; West et al., 2000a, West et al., 2000b). Ironwood is used frequently by various Native American groups including Pimas, Mayos, Yaquis and Comca´acs or Seris (Nabhan and Plotkin, 1994). Felger and Moser (1985) reported that ironwood is an important element in Comca´ac ceremonies. Monti et al. (2000) mentioned that the seeds are ground into flour, and some parts of the plant are used in traditional medicine. The wood is also used for tool manufacturing, farm implements, and in building houses (Monti et al., 2000; St. Antoine, 1994). The most important use of this tree is the manufacturing of wood figures. This activity began in the 1960´s within Comca´ac communities (St. Antoine, 1994). Because the figures made by the tribe were handmade, the production numbers were initially small.
* Corresponding author. Tel.: þ52 442 1921327x5325. ˜ iga-Tovar), E-mail addresses: [email protected] (B. Zun (H. Suza´n-Azpiri).
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0140-1963/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2009.08.004
Because of good market value, other craftsmen began to imitate the Comca´ac figures using industrialized methods that allowed the manufacture in larger volumes, with shorter production time and lower costs. This asymmetry in the production techniques lead to a reduction in the numbers of Comca´ac craftsmen (only fifteen in 1996; Durand, 1996), because the hand crafted products could not compete with the industrialized production of wood figures. In 2003, 250 workshops that elaborate ironwood figures were estimated to be active in Hermosillo city, Sonora. Durand (1996) points out that this industry requires approximately 5000 tons of wood annually, and that most of this wood was extracted illegally. The production of figures is one of the main reasons for the decrease of the ironwood populations in Sonora (Suza´n et al., 1999). In an evaluation of the damage caused by tree cutting Suza´n, Nabhan and Patten (1994) detected that in some populations 80% of the intermediate size plants were damaged. Nevertheless, there are other serious threats for this species, including extraction for firewood and charcoal, and the conversion of extensive desert areas into grasslands for livestock. These threats are compounded by the distorted population structure with limited presence of juveniles throughout all the ironwood distribution range, particularly in the sites undergoing wood extraction (Solı´s, 1997; Suza´n et al., 1997). Because ironwood is a keystone species, the decrease of natural populations set in risk the conservation of the Sonoran Desert (Nabhan and Plotkin, 1994). This species is considered a key
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(damaged area percentage and damaged individuals percentage) of ironwood populations in Baja California and Sonora, Mexico; b) describe and compare the population structure of ironwood in exploited and non-exploited sites, and; c) compare the damage indicators estimated in 1992 (Suza´n, 1994) with the more recent data obtained in this study for four populations (2004). 2. Methods 2.1. Study sites
Fig. 1. Sites of study and regional division of Sonora’s desert based on Shreve (1951) and modified by Turner and Brown (1982). Sites identification: (1) Comita´n (110.8416 W, 24.2511 N); (2) Bahı´a de los A´ngeles (113.6700 W, 28.9475 N); (3) San Felipe (114.9611 W, 30.906 N); (4) El Pinacate Biosphere Reserve (113.3597 W, 31.8964 N); (5) Sonoyta (112.8194 W, 31.8725 N); (6) Puerto Lobos (112.2474 W, 30.6040 N); (7) Puerto Libertad (112.3100 W, 29.6500 N); (8) Rancho Lobos (112.0398 W, 29.562 N); (9) Bahı´a Kino (111.4300 W, 28.900 N); (10) Pozo Hondo (110.9027 W, 29.9463 N); (11) Cuesta Blanca (110.8611 W, 29.6722 N).
modifier of the structure and function of this ecosystem (Mills et al., 1993), generating microhabitats that benefit many other species of the flora and fauna (Bu´rquez and Quintana, 1994; Nabhan and Suza´n, 1994; Suza´n et al., 1996; 2000). Ironwood and other legumes such as velvet mesquite (Prosopis velutina) under their canopies create ‘‘resources or fertile islands’’ where the biodiversity and the productivity are higher when compared with open sites (Bu´rquez and Quintana, 1994; Carrillo-Garcı´a et al., 1999;Tewksbury and Petrovich, 1994). This situation has multifactor origins, such as: the non-random dispersion of seeds, protection from sunlight, higher humidity and higher soil fertility, a reduction in the risk of freezing, and a general buffering of extreme environmental conditions (McAuliffe, 1984; Suza´n et al., 1994; Tewksbury et al., 1999). Nabhan and Plotkin (1994) indicated that more than 160 plant species were associated with ironwood, in particular the columnar cacti, Peniocereus striatus, Carnegia gigantea, Stenocereus thurberi and Lophocereus schotti. The distribution and densities of these cacti are correlated with the presence of ironwood nurse plants (Suza´n et al., 1994; Suza´n et al., 1996). West et al. (2000a), West et al. (2000b) reported that 424 species of fauna use ironwood for refuge, perching or nesting. In addition ironwood and mesquite also promote the growth of AM-fungi that enrich the soils beneath their canopies (Bashan et al., 2000). Intensive ironwood extraction and the shortage of information on the status of populations motivated to us to propose the following objectives: a) Estimate present damage indicators
Fieldwork was carried out from February 2004 to May 2005 at eleven sites in the Sonoran Desert. In the Peninsula of Baja California three sites were selected: Comita´n (1), Bahia de Los Angeles (2), and San Felipe (3). In Sonora eight sites were selected: El Pinacate Biosphere Reserve (4), Sonoyta (5), Puerto Lobos (6), Puerto Libertad (7), Rancho Lobos (8), Bahia Kino (9), Pozo hondo (10) and Cuesta Blanca (11) (Fig. 1). The sites were located in the following vegetation subdivisions of Shreve (1951) for the Sonoran Desert (Fig. 1): Sites 1, 6, 7, 8 and 9 were placed in the Central Gulf Coast subdivision. This region receives an average precipitation between 100 and 250 mm annually, generally occurring at the end of the winter and in the middle of the summer, although occasionally there are several successive years without recorded rainfall. Sites 2, 3, 4 and 5 occur in the Lower Colorado Valley subdivision. This is the largest and most arid subdivision of the Sonoran Desert, characterized by its high temperatures and low precipitation (<250 mm). Sites 10 and 11 corresponded to the Plains of Sonora subdivision, which is the smallest and less diverse subdivision with an annual average precipitation that fluctuates between 250 and 375 mm. 2.2. Population structure and cutting damage In each study site, 15 circular parcels of 250 m2 were outlined according to the methodology proposed by McAuliffe (1990). In each parcel, we registered the number of ironwood trees, their height, canopy cover (calculated from two radii assuming ellipsoidal canopies), and base diameter. To evaluate the damage, the numbers of cut or dead branches were noted, as well as the number and diameter of branches with evidence of cutting. We compared the cover, height and basal area between sites using a multivariate analysis of variance (Manly, 1994), with a completely randomized with fixed effects model. Trees densities for all the sites were compared with a completely randomized analysis of variance. We measured the damaged basal area (DBA) in each site (obtained from the diameter of each stem and branches beneath 1.3 m with evidence of cutting with a Hagloff caliper), and we compared them by an analysis of covariance with total basal area as a covariate. We also compared the percentages of damaged and dead individuals (PDI) in each site by an analysis of variance of the arcsine transformed percentages with a randomized fixed effects design (Daniel, 2002; Zar, 1984). In order to compare densities among exploited and non-exploited sites we classified them according to the percent of damaged trees (<20% as unexploited, and >20% as exploited) and compared them with a completely randomized analysis of variance. Size structures were determined, in reference to diameter categories, using the basal area according to Suza´n (1994). Ten categories were considered: individuals smaller to 50 cm2, 50–200 cm2, 201–500 cm2, 501–800 cm2, 801–1100 cm2, 1101– 1400 cm2, 1401–1700 cm2, 1701–2000 cm2, 2001–2300 cm2 and finally those larger than 2300 cm2. Juvenile plants were those smaller or equal to one meter in height and five cm in diameter. The
˜iga-Tovar, H. Suza´n-Azpiri / Journal of Arid Environments 74 (2010) 173–178 B. Zun Table 1 Multivariate analysis of variance for height, canopy cover, basal area and density of ironwood adult individuals for all study sites. Data per site in mean, standard error. Sites numbers according to Fig. 1. Population (Sites) 1 2 3 4 5 6 7 8 9 10 11
Height (m)
Covering (m2)
Basal area (cm2)
Density per plot (250 m2)
3.5, 0.2 2.2, 0.1 2.9, 0.1 4.5, 0.2 3.6, 0.1 3.27, 0.2 4.4, 0.2 3.7, 0.1 3.8, 0.1 2.3, 0.1 3.0, 0.1
13.8, 4.3 14.5, 2.3 13.5, 3.2 47.0, 3.8 23.1,3.3 21.2, 4.8 16.8, 4.0 17.5, 2.1 23.0, 3.2 8.1, 2.0 13.1, 3.0
349.3, 303.3, 401.9, 1346.8, 527.0, 880.9, 770.4, 617.5, 908.1, 179.3, 317.1,
3.6, 1.8, 4.5, 2.4, 2.3, 2.2, 2.1, 2.9, 2.0, 5.7, 4.5,
142.0 77.9 16.6 125.8 108.0 156.9 130.6 69.0 105.6 85.6 99.2
0.4 0.1 0.9 0.3 0.2 0.4 0.3 0.2 0.1 0.3 0.2
Test
Value
F
Significance (P)
Wilks Lambda Pillai´s Trace Hotelling–Lawley Roy´s Max Root
0.612480 0.431642 0.561962 0.394099
9.1101 8.6384 9.5659 20.2567
<0.0001 <0.0001 <0.0001 <0.0001
relative frequency of healthy, damaged or dead plants was determined for each size category. All statistical analysis was performed with JMP version 5.0.1. (SAS, 2002). 2.3. Historical comparison of cut damage We analyzed four populations (4, 5, 7 and 9) previously monitored in 1992 by Suza´n (1994), comparing the percentage of dead or damaged trees and the percentage damaged basal area between the data collected in 1992 and those of 2004 with an analysis of variance of a single factor (Daniel, 2002). 3. Results 3.1. Population structure and damage Trees in the populations differed significantly in mean height, cover and basal area according to the multivariate analysis of variance (Table 1). Therefore, differences in the size of individual trees among the populations were the result of two distinct patterns of growth. In the first pattern, the individuals had a predominantly bushy pattern. These plants have many branches from the base, with thin trunks generally smaller than three meters tall. The populations
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displaying these characteristics were 1, 2, 3, 10 and 11. The second pattern had trees with distinct arboreal growth, with one or two well defined trunks, and generally reaching height over four meters. This pattern was observed in populations 4, 5, 6, 7, 8, and 9. The analysis of variance illustrates a significant difference in the density among populations P(F ¼ 10j10,160 d.f.) < 0.001. Highest densities were observed in sites 3, 10 and 11 with an average density of 4.56, 5.7 and 4.5 plants per plot (250 m2) respectively (Table1). Sites 2 and 9 had low densities, with an average of 1.85 and 2 plants per plot. The remaining sites (1, 4, 5, 6, 7 and 8) had an average density between 2 and 3.6 plants per plot. The comparison of densities among exploited and non-exploited sites showed significant differences P(F ¼ 27.69j1,169 d.f.) < 0.001. The average mean in exploited and non-exploited sites were 2.4 and 3.9 plants per plot respectively, converting to 96 and 156 plants per hectare, respectively. There was a wide range of values for the damage to the basal area of ironwood trees among sites. Sites 10 and 11 did not present any evidence of cutting (Fig. 2a) and differed significantly from the remaining sites. With the rest of the sites, significant differences were found (Fig. 2a). Sites 2 and 3 had a significantly smaller DBA than the rest of the sites with P(F ¼ 5.62j8,1134 d.f.) < 0.001. The populations with smaller percentages of DBA were sites 2 and 3 with a mean of 6.42% and 3.3%, respectively. Sites with the highest percentage of DBA were located in the Central Coast of California Gulf (sites 8 and 9) with a DBA of 26% and 31.54% each. Populations with an intermediate range of damage were found in sites 6, 7, and 4 with DBA of 18%, 21.2%, and 25%, respectively (Fig. 2a). The percentage of damaged plants (PDI) showed significant differences for nine exploited populations (P (F ¼ 3.28j8,133 d.f.) ¼ 0.0019). The sites with lower PDI were 2 and 3 with 15% and 14% respectively. The sites with higher PDI were 4, 8, and 9 with 49%, 47.5%, and 44.6%, respectively (Fig. 2b). Two groups with contrasting characteristics were observed (Table 2). The first group consisting of sites 1, 2, 3, 10 and 11 which were not subjected to commercial extraction. The dominant growth forms were bush with a basal area less than 500 cm2 (Figs. 3a, 3d). The sites with the highest percentages of juveniles plants were site 11 (15.5%), site 10 (29%) and site 2 (44.5%). The populations located in the Central Coast of the California Gulf extending to the Mexico–USA border (sites 4, 5, 6, 7, 8 and 9) made up the second group. These sites have endured high wood extraction. The size distributions differ greatly from the first group (Table 2). The intermediate categories, between 500 and 1400 cm2 of basal area, were best represented (Figs. 3b and 3c). It is noteworthy that no juvenile plants were observed within the sites. Within these populations, the largest plants with basal areas
Fig. 2. 95% Confidence intervals of the arcsine transformed percents of basal damaged area (a), and damaged and dead trees (b) per plot (250 m2). Site numbers according to Fig. 1. Sites with the same letter do not have significant differences.
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Table 2 Comparisons of arboreal and bush growing types. Study sites from Study sites
1, 2, 3, 10 and 11
4, 5, 6, 7, 8 and 9
Geographical location
Baja California Peninsula Hermosillo Plains Predominantly bush-shape Minor diametrical types < 500 cm2 basal area Dead or damaged plants: 0–15% Damaged basal area: 0–18% No-commercial exploitation, occasionally firewood domestic consume Percentages between 16 and 40 juveniles
Principally located at Central Gulf Coast and Lower Colorado Valley
Growth patterns Size types Damage indices Ironwood use history Recruitment evidence
greater than 2000 cm2 and heights between 4 and 7 m were found. Site 4 stands out where the majority of plants with basal areas of more than 1700 cm2 represent 35% of the population (Fig. 3b). 3.2. Historical comparison of damage indices Damaged basal area percent (DBA) and percentage of damaged individuals (PDI) for site 4 (El Pinacate Biosphere Reserve) did not show significant differences between 1992 and 2004 (P(F ¼ 3.13j 1,19 df) > 0.09 for DBA; P(F ¼ 4.97j 1,19 df) > 0.09 for PDI). However, the DBA mean for Pinacate decreased from 1992 to 2004 (33.5, 24.98, respectively). Similar trend occurs for PDI (73.33 and 49.22 respectively). Site 7 (Puerto Libertad) had non significant differences for DBA (P(F ¼ 1.79j 1,19 df) > 0.19), and PDI (P(F ¼ 0.531j 1.19 df) > 0.74). Both indices remain with low values, in 1992, 8.0 for DBA and 23.91 for PDI; in 2004, 21.22 for DBA and 35.80 for PDI.
Predominant tree-shape Intermedium types, between 500 and 1400 cm2 de basal area Dead or damaged plants: 30–49% Damaged basal area: 11–32% Wood extraction for commercial purposes: coil, firewood, handicraft manufacturing Almost null
In site 9 (Bahia Kino), non significant differences in the percentage of DBA were detected (P(F ¼ 0.09j 1,36 df) > 0.76). This region was the origin of ironwood exploitation for handcrafts, and in addition is the closest site to the city of Hermosillo, therefore, exhibited bigger damage indicators. Nonetheless, there was a significant difference in the PDI that had diminished from 76.2% in 1992 to 44.6% in 2004 (P(F ¼ 4.97j 1,36 df) < 0.0009). Finally, the site 5 studied by Suza´n (1994) was completely removed and the area was totally devoid of plant covering. 4. Discussion 4.1. Population structure and cutting damage estimation Ironwood commercial exploitation for artisan crafts manufacturing began in Bahı´a Kino (site 9), around 1960 (St. Antoine, 1994). During 1990 it was evident that the wood extraction
Fig. 3. Relative frequencies of ironwood basal areas for sites 3 (a); 4 (b); 9 (c) and 10 (d). Healthy trees are those without any damage.
˜iga-Tovar, H. Suza´n-Azpiri / Journal of Arid Environments 74 (2010) 173–178 B. Zun
had extended as far as Sonoyta (site 5) located at the Mexico–US border (Suza´n, 1994). In this study, we observed that a large portion ˜ asco of the coastal region of Sonora, from Bahı´a Kino to Puerto Pen including the El Pinacate Biosphere Reserve (site 4), exhibited ironwood populations with evidence of wood extraction damage. Two conditions appear to exist concerning the population structure and wood extraction: 1) damage due to harvesting for firewood, and 2) damage as the result of wood extraction. The first condition occurs in the Baja California Peninsula and in the Plains of Sonora region where populations are not harvested for commercial use of the wood (Fig. 1). In this condition, ironwood grows predominantly as a bush, and does not reach sufficient sizes to provide adequate wood for the use by commercial craftsmen. Damage to trees in Baja California (sites 1, 2, and 3) were relatively low, associated to firewood collected for domestic purposes for rural inhabitants. The second condition was detected at sites 4, 5, 6, 7, 8 and 9, and exhibited high percentages of damage due to wood extraction for artisan crafts manufacturing and charcoal production, and significantly lower densities of recruits. The different conditions related to wood extraction coincide with remarkable differences in plant population structures. In areas where trees are used for wood sculpture whole trees and large branches are extracted and individuals in the minor size categories are absent or poorly represented. At these sites, intermediate sizes are the most frequently observed. In contrast, in areas where the trees are subjected to non commercial exploitation (firewood) the individuals are not extracted but have small basal areas (<500 cm2) and bush like architecture and greater overall densities. Therefore, the first group of sites has trees with bigger basal area, greater height and covering, whereas the second group has smaller basal area and height, and higher recruitment. Suza´n et al. (1997) reported two growth patterns, the first with individuals growing in uplands with bush architecture and the second with trees growing in washes or ephemeral streams with arboreal growth morphology and more size classes, and the differences in canopy architecture and population structure were related to water availability. The greatest frequencies of dead plants were in intermediate size categories (basal areas less to 1100 cm2). Suza´n et al. (1997) reports similar findings indicating that bigger size trees are more resilient and survive cutting damage. Moreover, these authors describe that allometric proportions change among healthy and damaged plants, modifying from arboreal to bush-type architecture. This situation was also observed in this study and as a consequence, results in a decreased area shadowed by ironwood canopies, which undoubtedly must have an effect, not yet studied, on the microenvironment available for the species associated to this keystone species. Further studies are needed on the root systems of ironwood and its relationship with soil water availability and fertility under damaged and healthy trees. Considering that ironwood is a highly interactive keystone species (Nabhan and Plotkin, 1994; Suza´n et al., 1996), the reduction of canopy shading area may have other important negative effects on the ecosystem. The 160 plant and 424 species of fauna associated with ironwood (West et al., 2000a; West et al., 2000b) might be affected in several ways. For example, phenological disruption patterns, changes in the species demography or geographical distribution. This is particularly important in the case of various cactaceae species that depend on ironwood as a nurse plant. A case in point is P. striatus, a rare species strongly associated with ironwood (Suza´n et al., 1996). The diminishing ironwood population and its lessened role as a shade provider could produce important changes in the biodiversity that might lead to degradation or even to simplification of Sonoran Desert ecosystem.
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4.2. Historical comparison The comparison between 1992 and 2004 indicate different tendencies for the four populations studied. El Pinacate Biosphere Reserve (site 4), did not show any changes in the damage indices, and a generalized lack of recruitment in the region. These results indicate that the designation as a protected natural area has been only partially effective for the non increase in damage indicators. For Puerto Libertad (site 7), no increments in the damage indices were registered, nevertheless, in this region, inhabitants mentioned that there had been an increase of illegal wood extraction, especially in areas far from highways. In population 9 (Bahı´a Kino) a decrease in the percentage of damaged trees of appropriate size for extraction was detected. However, for the period of this study we observed areas subjected to mesquite and ironwood charcoal extraction (Bahia Kino and Sonoyta) where stumps were removed. Whole tree extraction may lead to an underestimation of damaged individuals in presented data. Suza´n et al. (1997) observed that damage to populations in Bahı´a Kino was less than the other three sites studied in 1992. Therefore, data from both surveys (1992, 2004) suggest that illegal wood extraction has moved from its original exploitation region (Bahia Kino) to other areas of ironwood distribution. The situation in Sonoyta (site 5), demonstrate the continued harvesting pressure on ironwood populations. This population sampled in 1994 had higher damage in the first survey, and in the 2004 the population was completely removed, despite the location as a buffer zone between two protected areas: El Pinacate Biosphere Reserve in Me´xico and Organ Pipe Cactus National Monument in United States. These facts clearly illustrate the reasons why action must be promoted to preserve and restore this biological corridor that connects both protected areas. 4.3. Considerations for the conservation of ironwood populations Illegal ironwood extraction for artisan crafts and coal production are not the only activities that threaten this species. Unfortunately, the extensive conversion of lands for urban development and agricultural expansion are also significant problems (Suza´n, 1994). The most serious problem of habitat transformation has occurred as a direct result of plant removal to establish pastures for livestock. In the central and south portion of Sonora’s desert this has been the case as major habitat conversion for the introduction of buffel grass (Cenchrus ciliaris) on more than 400,000 hectares, that has threatened many native plants in Sonora (Bu´rquez and Quintana, 1994). Livestock-farming activities also affect ironwood populations because the removal of plant coverage and overgrazing inhibits species recruitment. Tewksbury and Petrovich (1994) pointed out that Tiburo´n Island, a protected area where cattle was excluded, had higher proportion of juveniles compared to the areas in Bahia Kino subjected to grazing. The fact that Mexican authorities have granted ironwood legal special protection (NOM-059-ECOL-2001) does not seem to be enough to secure the long term survival of its populations. Other leguminous species in the Sonora desert are also being affected by human activities. A case in point highlights mesquite (P. velutina and P. glandulosa), severely damaged for charcoal extraction exhibiting important population decreases in different regions in Mexico, affecting specially to those inhabiting the Sonoran Desert (Leo´n de la Luz et al., 2005; Suza´n et al., 1999). Therefore, the implementation of viable management plans for keystone species such as ironwood or mesquite is a crucial activity for governmental managers. The lack of accurate management programs for desert species is a common denominator in many ecosystems. As an example Lemeninh et al. (2007) pointed out the serious risks that
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Boswelli papyfera populations under commercial extraction faces in the arid zones of Ethiopia as a result of the absence of a management plan. To enact effective conservation of ironwood populations, four important strategies must be considered: 1) It is necessary to create environmental consciousness among tourists and locals who are the main buyers of ironwood figures and charcoal; 2) It is fundamental that governmental agencies impose effective restrictions that decrease the damage of natural populations; 3) It is essential an intensive strategy of environmental education, to decrease the illegal clearing of these lands for establishment of pastures, and improve range management in the region including restoration techniques and practices; and 4) A long term evaluation of the population structure and dynamics in protected and unprotected areas is needed. Ironwood is a long-lived species that contributes to ecological processes that may not be obvious on a human time scale, but evident only in the time span in which this species lives. Acknowledgments This work was financed by the CONACYT grant CO1-2002-273. Dr. Robert Jones from UAQ helped in editing the text. References Bashan, Y., Davis, E.A., Carrillo-Garcia, A., Linderman, R.G., 2000. Assessment of VA mycorrhizal inoculum potential in relation to the establishment of cactus seedlings under mesquite nurse-trees in the Sonoran desert. Applied Soil Ecology 14, 165–176. Bu´rquez, A., Quintana, M.A., 1994. Islands of diversity: ironwood ecology and the richness of perennials in a Sonoran Desert Biological Preserve. In: Nabhan, G., Carr, J.L. (Eds.), Ironwood: An Ecological and Cultural Keystone on the Sonoran Desert. Conservation International, Washington D.C., pp. 9–27. Carrillo-Garcı´a, A.J., Leo´n de la Luz, L., Bashan, Y., Bethlenfalvay, G.J., 1999. Nurse plants, mycorrhizae and plant establishment in a disturbed area of the Sonoran Desert. Restoration Ecology 7 (4), 321–335. Daniel, W., 2002. Bioestadı´stica, fourth ed. Limusa Wiley, Me´xico. Durand, L.,1996. El palo fierro, especie clave del Desierto de Sonora. Ciencias 43, 24–26. Felger, R.S., Moser, M.B., 1985. People of the Desert and Sea: Ethnobotany of the Seri Indians. University of Arizona Press. Lemeninh, M., Feleke, S., Tadesse, W., 2007. Constraints to smallholders production of frankincense in Metema district, north-western Ethiopia. Journal of Arid Environments 71, 393–403. Leo´n de la Luz, L., Domı´nguez Cadena, R., Dı´az Castro, S.C., 2005. Evaluacio´n del peso ˜ o a partir de variables dimensionales en dos especies de mezquite Prodel len sopis articulata S. Watson y P. palmeri S. Watson, en Baja California Sur, Me´xico. Acta Botanica Mexicana 72, 17–32.
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