Demography and potential extractive use of the liana palm, Desmoncus orthacanthos Martius (Arecaceae), in southern Quintana Roo, Mexico

Forest Ecology and Management 187 (2004) 3–18

Demography and potential extractive use of the liana palm, Desmoncus orthacanthos Martius (Arecaceae), in southern Quintana Roo, Mexico Sigfredo Escalantea,*, Carlos Montan˜ab, Roger Orellanaa a

Centro de Investigacio´n Cientı´fica de Yucata´n A. C. Apartado Postal 87, Cordemex, Me´rida, Yucata´n 97310, Mexico b Instituto de Ecologı´a A. C. Apartado Postal 63, Xalapa, Veracruz 91000, Mexico

Received 15 October 2002; received in revised form 15 December 2002; accepted 5 May 2003

Abstract In order to evaluate the extractive use potential of Desmoncus orthacanthos, various matrix models were employed to assess population structure and dynamics. To this end, populations of this liana palm were investigated in southern Quintana Roo, Mexico, at two localities showing contrasting fragmentation levels arising from different land use regimes. In one case continuous forest (CF) are maintained under forestry management, while in the other (fragmented forest, FF) isolated forest patches remain inside of a matrix of crop and livestock lands. Three forest conditions: mature forest (MF), young forest (YF) and forest edge (FE) were nested in each fragmentation level. Results showed that more plants were encountered under FF situations than under CF conditions; however, no differences were noted in exploitable shoots (i.e., total numbers and lengths) when both were compared. Under forest conditions, number and length of exploitable shoots did not differ between FE and MF populations, however these values were smallest in YF populations. Intrinsic population growth rates (l) were lower under FF than under CF, as reflected by periodic matrix values of 0.964 and 1.594, respectively. With respect to forest conditions, l for FE > l for YF > l for MF and periodic matrix values were 1.441, 1.193 and 1.075, respectively. Elasticity analyses for annual matrices showed that the demographic process having most influence on variations in l was (periodic matrix values) permanence (43– 63%), followed by growth and retrogression (15–22% and 14–23%, respectively); fecundity had the lowest influence in changes of l-values (8–13%). Analyses of the simulated extraction of shoots
5 m suggested that FE and CF populations might support harvest rates of 40% per year, since l > 1; however, simulation of harvest rates as low as 20% in other populations always resulted in l < 1. Interestingly, the simulated addition of nursery-raised juveniles resulted in a notable increase in l, especially for FE populations; simulations combining shoot extraction with the addition of juveniles suggested that high levels of harvest may be possible, even while maintaining l > 1; but only if sound forest management is practiced and agricultural fires are restricted. This work showed the value of matrix models for analyzing populations dynamics in establishing sustainable use strategies for D. orthacanthos as opposed to static evaluation of population numbers. # 2003 Elsevier B.V. All rights reserved. Keywords: Demography; Desmoncus; Extractive use; Lianas; Matrix models; Matrix transition elasticities; Non-timber forest products; Palms; Quintana Roo

*

Corresponding author.

0378-1127/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0378-1127(03)00228-7

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1. Introduction The presence of lianas is the most characteristic feature distinguishing tropical forests from their temperate counterparts. Structurally, lianas literally tie together the community, providing: (1) food and habitat for wildlife, (2) up to a third of the litter fall, (3) 18–24% of stems greater than 2.5 cm DBH found in Neotropical rain forests, and (4) approximately 20% of the species richness in local floras (Gentry, 1991). Wild lianas are non-timber, forest products (NTFP). As such, they provide a potential source for sustainable use of tropical forests, allowing the economic development of human communities, even within forested areas set aside for conservation (Goodland, 1987; Peters, 1994; Pearce and Moran, 1994). Rattan, a very important NTFP from which fine furniture is made, is obtained from the shoots of diverse species of liana palms found in the Old World. The global market for rattan furniture is on the order of 6.5–7 billion dollars ($US) annually (Prebble, 1997; Sastry, 2002). In the Neotropics some species of Desmoncus (which cannot be considered as true rattans from the botanical point of view) are currently used to make ropes, baskets, and rustic furniture. Many authors (Lundell, 1937; Schultes, 1940; Balick, 1979; Galeano, 1992; Henderson and Cha´ vez, 1993; Chinchilla, 1994; Quero, 1994; Henderson et al., 1995; Lorenzi et al., 1996; Belsky and Siebert, 1998) have indicated that Desmoncus may serve as substitutes for rattan. For example, in the regions of Iquitos in Peru (Henderson and Cha´ vez, 1993) and El Pete´ n in Guatemala (Chinchilla, 1994), Desmoncus species are currently being used to make fine furniture. In general, any plant species with a high population density, products that are ready for harvesting throughout the year, a suitable demand in the marketplace, and little adverse effects on plant and animal life can have an important extractive use (Belsky and Siebert, 1998). In this respect, species of Desmoncus also possess other favorable characteristics, including broad distributions, principally in secondary plant communities (Uhl and Dransfield, 1987; Quero, 1992b), and clonal growth allowing individual plants to be harvested without their elimination. Given these characteristics, and because of its potential local importance, various studies utilizing species within this genus have been realized. In

Guatemala, e.g., investigations have been carried out to: (1) diagnose the commercial production potential for local crafts (Chinchilla, 1992), (2) characterize the suitability of plant populations for developing such local craft industries (Chinchilla, 1994), and (3) determine the effect of timber exploitation on Desmoncus populations (Marmillod and Ga´ lvez, personal communication). In Belize, studies have been undertaken on (4) shoot growth and abundance as a function of light availability and harvest intensity (Siebert, 2000). In Brazil, efforts have included (5) developing a protocol to measure plant size and abundance in relation to forest structure (Troy et al., 1997). Biologically, in order to attain an extractive management use that sustains the resource base, or even increases it, a detailed knowledge of species life history and demographic behavior (e.g., population structure, rate of regeneration, number of harvestable stems per hectare) is essential (Sunderland and Dransfield, 2002). The response of demographic rates (e.g., birth, growth, and death rates) to environmental variability determines population dynamics in ecological time, as well as the selection of life history strategies in evolutionary time (Caswell, 1989). Population matrix models (extensively developed by Caswell, 1989) have been applied to many species of palms in order to describe population dynamics and structure, evaluate the relative importance of demographic processes on population growth, assess the effects of harvesting on demographic parameters, and develop management strategies for sustainable use (Pin˜ ero et al., 1984; Pinard and Putz, 1992; ´ lvarez-Buylla, 1995; Pinard, 1993; Olmsted and A Knudsen, 1995; Ratsirarson et al., 1996; Bernal, 1998). In the present study, the population dynamics and structure of Desmoncus orthacanthos was examined by means of matrix projection models for: (1) two contrasting conditions of forest fragmentation and management, and (2) three distinct forest conditions that differed in light availability and tree biomass. This analysis was undertaken for the purposes of comparing the effects of environmental conditions on population growth, simulating the effects of shoot harvesting on population growth, and suggesting appropriate management strategies and harvest intensities.

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2. Study area and methods 2.1. Study area This study was undertaken at two tropical forest locations (INEGI, 1981; Olmsted et al., 1999) in southern Quintana Roo, Mexico, separated by a distance of 144 km (Table 1). These sites differed in fragmentation level due to different land use regimes, one being subjected to forestry management and the other submitted to agricultural management, whereby the former (continuous forest, CF) is practically a matrix of continuous forest with scarce farm patches, while in the second (fragmented forest, FF) isolated forest patches remain inside of a matrix of crop and livestock lands. The floristic composition of both sites was very similar, with species such as Manilkara sapota, Nectandra salicifolia, Alseis yucatanensis, Guettarda combsii and Metopium brownei characterizing the canopies, and Pouteria unilocularis and Cryosophila stauracantha commonly found in the intermediate strata. Nevertheless, other features, such as those summarized in Table 1, readily distinguished these sites. Human communities were also actively exploiting Desmoncus at both study sites and seeking information on the conservation and management of these plant populations. Both sites have been periodically submitted to natural perturbations (see Brokaw and Walker, 1991, or Lodge and McDowell, 1991, for data on

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hurricanes), but of more common occurrence are human disturbances. On the site under forestry management regime, human perturbations have been limited to road opening, clearings, skid trails, and log landings for stockpiling extracted timber. Human perturbations at the site under agricultural management regime include the use of fires to clear land for planting crops and pasture grasses. Such intentionally set fires often escape control and negatively affect remaining forest areas. At each site, three forest conditions differing in perturbation level, tree biomass and light availability were chosen. These were mature forest (MF) without major disturbance in the last 50 years according to local dwellers, young forest (YF) severely burned 25 years ago, and forest edge-road (FE) heterogeneous forest patches beside rural roads, and its characteristics are detailed in Table 2. Twenty permanent plots (10 m  10 m per plot) were randomly laid out under each forest conditions at each site. Each December from 1997 to 2000, these plots were subjected to an annual census. 2.2. The plant Desmoncus is a Neotropical genus of liana palms distributed from southern Brazil and Bolivia to southern Veracruz, Mexico (Uhl and Dransfield, 1987). There is currently no consensus on the number of species comprising this genus. In the present study,

Table 1 Characterization of two localities in southern Quintana Roo, Mexico, where D. orthacanthos was sampleda Locality Latitude Longitude Geomorphology Vegetation Degree of fragmentation Mean annual temperature (8C) Yearly rainfall (mm) Soil type Land use Trees >5 cm DBH (individuals ha1) Basal area of trees (m2 ha1)

La Unio´ n (FF) 0

00

17857 58 N 888540 3900 W Hills, lowlands, lagoons Tropical forest High 26.0 1388 Gleysol Agriculture 1918.3a (114.9) 22.57a (1.75)

Noh Bec (CF) 198070 4500 N 888200 4500 W Plains, lowlands, lagoons Tropical forest Low 25.8 1496 Rendzina and gleysol Forest extraction 1831.7a (76.8) 30.30a (2.94)

a Climatic data are taken from the nearest meteorological station (Pucte´ for La Unio´ n and Limones for Noh Bec). Agricultural land use at La Unio´ n has involved crop and livestock production since 1980. Before that date, land was dedicated to forest product extraction. The mean number and basal area of trees >5 cm DBH (standard errors in parentheses) that were found among 60 square sampling plots per locality (0.01 ha per plot) are shown. Different letters associated with tree data indicate significant differences at P < 0:05.

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Table 2 Characteristics of three forest site conditions where D. orthacanthos was sampleda Characteristics 1

Trees >5 cm DBH (individuals ha ) Basal area of trees (m2 ha1) Light availability at ground level Degree of canopy opening Degree of understory opening

Mature forest (MF)

Young forest (YF)

Forest edge (FE)

1938a (89) 35.5a (4.0) Low Closed Open

2227a (117) 25.4a (1.8) Medium Closed Closed

1460b (118) 18.4b (2.1) High Variable Variable

a Site characteristics include the mean number and basal area of trees >5 cm DBH (standard errors in parentheses) that were encountered in 40 square sampling plots per forest condition (0.01 ha per plot). Different letters indicate significant differences at P < 0:05.

we follow Henderson et al. (1995), who recognized only seven species and seven varieties. They further considered that all Mexican species were synonymous with D. orthacanthos. Desmoncus is common to the humid, lowland tropical forest, being principally found in open secondary vegetation and along river margins. However, individuals are rarely encountered in the understory of closed forest (Uhl and Dransfield, 1987; Quero, 1992a). In Mexico, Desmoncus is known from the southern states of Veracruz, Oaxaca, Tabasco, Chiapas, Campeche, and Quintana Roo (Quero, 1992b, 1994). All species of Desmoncus are easily recognized by their spines and climbing habit, as well as by leaf morphology. In the latter case, the leaf is especially noted for an elongated apex, or cirrus, whose pinnae include modified, hook-like acanthophylls. Additionally, these plants form dense aggregates of climbing shoots that arise from buds of rhizomes of very short internodes (Standley and Steyermark, 1958; Corner, 1966). Our field observations indicated that successive shoots emerged very close together from a given genet, and no evidence was found to suggest that shoots separated and developed independently afterwards. Only the larger plants produced fruit. In general, individuals bearing fruits had shoots >15 m in length and were found in high light conditions. Fruiting was asynchronous, occurring from July to January, with a peak production period between July and August. As long as incident solar radiation is low, individual plants can remain as seedlings or juveniles for many ´ lvarez-Buylla, 1995). years (Martı´nez-Ramos and A Branch and tree fall clearly contributes to the damage received by seedlings and juveniles (Clark and Clark, 1991). In addition, one or more undetermined phytophages are known to perforate and consume shoot

meristems in D. orthacanthos. This attack almost always slows growth and usually kills the ramets, although occasionally some shoots may sprout again. 2.3. Population structure and dynamics A genet was considered to be any plant that was clearly separated from other surrounding plants by a minimum distance of 1 m. In doubtful cases, separation was verified by confirming the absence of subterranean connections. A ramet was defined as an individual shoot. All genets rooted in each plot were marked. In each plot the total number of genets was recorded. In addition, the basal diameter at ground level for each group of ramets making up a genet was measured and the number of live ramets per genet was also tallied. The following data were recorded for each ramet: (1) the diameter at the midpoint of the first internode, (2) the length of the ramet as measured from its base to the apex of the distal leaf, and (3) the number of inflorescences or infructescences. Measurements of basal diameter were used to calculate the basal area for each genet (considered to be a circular form). The summed lengths of ramets per genet were utilized to estimate the total length of each genet. Shoots
5 m in length were used to calculate the length of the potential crop. To effect this calculation, reproductive ramets were not considered in the summation. Instead, the remaining, non-reproductive shoots
5 m were summed, and then 10% of this subtotal was deducted in order to account for the elimination of unusable apices. The resulting total was considered to be an estimate of exploitable shoot length. Annual fecundity was estimated directly by counting recruits.

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In the first annual census, the diameters of trees >5 cm DBH were measured in each plot. All data were recorded by the same observer using diametric tape, caliper, metric tape, and a 5 m telescoping rod. The initial difficulty encountered in measuring very long shoots was overcome by using two persons with poles to mark and measure each shoot, section-by-section. Various analyses were undertaken, all at two levels: (1) by fragmentation level, in which the three FF forest conditions data were averaged and compared against the average of the three CF forest conditions data; and (2) by forest condition, in which pair-wise comparisons were made among each of the averages of the MF, YF and FE plots recorded under the two fragmentation levels (FF and CF). Comparisons of ramet and genet densities (total and by size classes) were undertaken using generalized linear models based upon Poisson distributions and a hierarchical experimental design in which forest conditions were nested within fragmentation levels. Differences in basal areas and lengths were evaluated using this same experimental design, but declaring a normal distribution for the data, instead. Comparisons among forest conditions were made using the Tukey’s non-parametric, multiple comparison method (Zar, 1984). Differences between observed and stable size distributions were evaluated using w2 tests and the Keyfitz dissimilarity index (D, Bierzychudek, 1982) which expresses the magnitude of the difference between both distributions as a percentage, but only considers positive values in the calculation. 2.4. Matrix analyses For demographic analyses, genets were classified in four categories, according to size and morphology. These categories were established as follows: (1) Seedling; a recently germinated individual with no shoot, but having a bipartite or tetrapartite leaf. (2) Juvenile; an individual with one or more shoots <5 m in length. (3) Subadult; an individual with at least one shoot
5 m, but <15 m in length. (4) Adult; an individual with at least one shoot
15 m in length, and/or bearing flowers or fruits. It should be mentioned that craftspeople usually choose and utilize shoots >5 m in length (Chinchilla, 1994; Siebert, 2000).

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In testing seeds from 19 individuals under greenhouse conditions (Escalante, unpublished data), it was determined that germination took place from 2 to 8 months after planting; that the average percent germination was 20:4%  6:4 S.E.; and that seeds were not viable for more than 1 year. For these reasons, it was assumed that a natural seed bank for this species was unlikely to exist, and that accordingly, there was no need to include a seed category in this study. Transition matrices (Lefkovitch, 1965) were constructed for each of the years 1998, 1999 and 2000. Also, an average matrix was constructed for each fragmentation level and forest condition during this 3-year period by averaging annual entries. Projection matrices (i.e., A ¼ faij g, where i; j ¼ 1; 2; . . . ; 4) were used to summarize the transitions, per unit time, of an average individual through each size class category, and operated over a vector (nt) representing the total number of individuals per size class per population at time t (Caswell, 1989). The dominant eigenvalue (l) of the transition matrix represented the finite rate of population growth, and the right (w) and left (v) eigenvectors represented, respectively, the stable size class frequency distribution and the reproductive values of each size class (Caswell, 1989). For each transition matrix an associatedPelasticity P matrix was calculated. Elasticity values eij ( eij ¼ 1) calculated as indicated by de Kroon et al. (1986) examined the change in l produced by a proportional change in each one of the coefficients of the matrix A. The elements of each elasticity matrix were summed for each one of four demographic processes (Silvertown et al., 1993, 1996): permanence (diagonal entries), growth (entries below the diagonal), retrogression (entries above the diagonal, except for the first row), and fecundity (entries in the first row, except for the first element). The elasticity of the four size classes categories were also summed by columns. Periodic matrix models have been utilized to incorporate between-year variation into the calculation of l and the elasticities (Caswell, 1989; Caswell and Trevisan, 1994) under the assumption that the observed inter-annual variability occurs cyclically. The population dynamics for a single cycle (comprising m units of time) was described by the relationship nðtþmÞ ¼ ½BðmÞ Bðm1Þ Bð1Þ nðtÞ ¼ Að1Þ nðtÞ , where A(1) is the periodic matrix and each matrix B corresponded to a

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phase in the cycle (Caswell and Trevisan, 1994). l and elasticity values were calculated according to Caswell and Trevisan (1994). For each transition matrix, bootstrap techniques (Caswell, 1989; Kalisz and McPeek, 1992; Golubov et al., 1999; Mandujano et al., 2001) were used to estimate the 95% confidence intervals for l. In the case of the annual matrices, the raw data (individuals) were resampled 1400 times to obtain 1400 transition matrices and calculate the corresponding 95% CI of l and elasticities. In the case of the average and periodic matrices the elements of the transition matrices were resampled. In each resampling iteration one element of the transition matrix was randomly selected and modified by 5% to calculate a new lvalue and a new elasticity matrix. This was repeated 1000 time to calculate the corresponding 95% CI. Statistical analyses were carried out using version 4 of the GLIM package (Royal Statistical Society, 1992), version 6.0 of the Statistica program (StatSoft, 1998), and version 2.3 of the Stage Coach software package (Cochran, 1992). 2.5. Simulations of shoot extraction and the addition of juveniles Various simulations were undertaken by altering the percentage values for various aij in the average and

periodic matrices. First, a simulated annual extraction of shoots
5 m was carried out for adult and subadult plants by decreasing the transitions a33, a43 and a44 in favor of retrogression. This simulation used harvest percentage rates that increased from 20 to 99%. Later, the addition of juveniles to the population was simulated in an amount that equaled the average number of seedlings recruited annually. This situation was simulated in order to evaluate the effect on l-values of incorporating nursery-reared juveniles into managed field populations.

3. Results 3.1. Structure Fig. 1 shows the population structures encountered for each of the two fragmentation levels and three forest conditions seen during the first year of observation. It is clear that populations were formed primarily of seedlings and juveniles, but that there were very few subadults and adults. The total number of genets was not different between populations under different fragmentation levels. However, analyses by size categories revealed that the number of juveniles was significantly greater for populations under FF (w2 1 d:f: ¼ 27:9; P  0:001).

Fig. 1. Size structure of D. orthacanthos populations found under different fragmentation levels and forest conditions in 1997. Size classes are: (1) seedlings, (2) juveniles, (3) subadults, (4) adults. Fragmentation levels are: fragmented forest (FF), continuous forest (CF). Forest conditions are: mature forest (MF), young forest (YF), forest edge (FE). SE bars are shown. Different letters indicate significant differences at P < 0:05.

S. Escalante et al. / Forest Ecology and Management 187 (2004) 3–18

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Table 3 Mean values (standard errors in parentheses) of basal area, total length, number of genets, and number of ramets per genet for D. orthacanthos populations (excluding seedlings) under different fragmentation levels and forest conditionsa Fragmentation level

2

1

Basal area (m ha ) Total length (m ha1) Number of genets (individuals ha1) Ramets/genet

Statistic

FF

CF

8.2a (1.5) 3519a (503) 657a (67)

4.4a (0.08) 2437a (504) 253b (19)

3.16a (0.63)

P

Forest condition MF

F1;4 ¼ 1:069 n.s. F1;4 ¼ 0:592 n.s. w21 ¼ 27:37 <0.001

2.11a (0.25) w21 ¼ 2:42

n.s.

Statistic

YF

FE

4.0ab (0.5) 3.7b (0.9) 2927a (392) 1417b (217) 640a (92) 458a (47) 1.65b (0.09)

P

1.56b (0.09)

11.2a (2.2) 4589a (918) 268b (40)

F4;114 ¼ 5:462 <0.001 F4;114 ¼ 4:337 <0.005 w24 ¼ 20:55 <0.001

4.69a (0.94) w24 ¼ 20:37

<0.005

a

Data are from the 1997 census. Fragmentation levels are: (FF) fragmented forest, (CF) continuous forest. Forest conditions are: (MF) mature forest, (YF) young forest, (FE) forest edge. Different letters indicate significant differences at P < 0:05.

The density of palms differed between forest conditions nested within fragmentation levels (w2 5 d:f: ¼ 11:22; P  0:05). The number of seedlings decreased from FE to YF to MF populations, whereas the number of juveniles and subadults showed a reversed trend (Fig. 1). There were a greater number of adult individuals in the FE populations, than in the MF and YF populations (w2 ¼ 5, d:f: ¼ 20:9; P  0:001), with averages of 35  9 S.E., 8  4 S.E., and 3  3 S.E. adults ha1, respectively. After excluding seedlings from the analysis, the total number of genets were found to be greater for FF populations than for CF populations (Table 3), and the total length of genets was less in YF populations than in FE and MF populations (Table 3); the number of genets was less in FE populations than in MF and YF populations; the basal areas was higher in FE, and

genets in FE populations had three times the number of ramets as genets in YF and MF populations (Table 3). The analysis of shoots
5 m in length (i.e., the exploitable part of the genets) showed no significant differences between fragmentation levels, but revealed significant differences among forest conditions (Table 4). The number of shoots was 3.4 and 2.6 times greater in FE and MF populations, respectively, than in YF populations. Shoot length was 5.4 and 2.7 times greater in FE and MF populations, respectively, than in YF populations. On the other hand, reproductive shoot length was much greater in FE populations than in MF and YF populations (45.9, 7.5 and 5.6% of the total length of exploitable shoots, respectively; also see Table 4). On a per plant basis, there were differences among forest conditions (Table 5). Exploitable shoots were

Table 4 Mean values ha1 (standard errors in parentheses) in D. orthacanthos populations (excluding seedlings) under different fragmentation levels and forest conditions for: the number of ramets
5 m, the total length of ramets
5 m, the total length of reproductive ramets, and the total length of potential cropa Fragmentation level FF Number of ramets
5 m 238.3a Total length of 2133.3a ramets
5 m (m) Total length of reproductive 595.8a ramets (m) Total length of 1383.8a potential crop (m) a

Statistic

P

CF (39.4) 183.3a (28.0) w21 ¼ 1:29 n.s. (440.0) 2009.7a (426.4) F1;4 ¼ 0:009 n.s. (227.0) 646.7a (302.4) F1;4 ¼ 0:004 n.s. (253.0) 1226.7a (179.9) F1;4 ¼ 0:076 n.s.

Forest condition MF

Statistic YF

P

FE

232.5a (31.3) 90.0b (22.6) 310.0a (56.8) w24 ¼ 18:57 <0.005 1841.3a (299.3) 682.0b (170.3) 3691.3a (784.5) F4;114 ¼ 5:062 <0.001 133.8b (76.1)

37.5b (37.5) 1692.5a (522.5) F4;114 ¼ 4:645 <0.0025

1536.8a (227.6) 580.1b (139.5) 1798.9a (356.2) F4;114 ¼ 3:67

<0.01

Data are from the 1997 census. Reproductive ramets were those bearing flowers, fruits, or >15 m long. Length of potential crop was calculated as the total length of the ramets
5 m, minus 10% (i.e., a value used to estimate the length of unusable apices), minus the length of the reproductive ramets. Fragmentation levels, forest conditions and letters are as in Table 3.

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Table 5 Mean individual values (standard errors in parentheses) of the lengths and diameters of ramets
5 m for D. orthacanthos populations found under different fragmentation levels and different forest conditionsa Fragmentation level

Sample size (N) Mean length of ramets
5 m (m) Mean diameter of ramets
5 m (mm) a

FF

CF

(143) 8.95a (0.37) 12.3a (0.2)

(110) 10.96a (0.62) 14.8a (0.3)

Statistic

P

Forest condition

F1;4 ¼ 0:947

n.s.

F1;4 ¼ 2:183

n.s.

MF

YF

FE

(93) 7.92b (0.30) 11.7b (0.3)

(30) 7.58b (0.45) 11.9b (0.6)

(124) 11.91a (0.60) 15.1a (0.2)

Statistic

P

F4;247 ¼ 10:405

<0.0005

F4;247 ¼ 34:250

<0.0005

Data are taken from 1997. Fragmentation levels, forest conditions and letters are as in Table 3.

much longer and with greater diameters in FE populations than in MF or YF populations. 3.2. Population dynamics Fig. 2 show the values of l obtained from projections of the annual, average, and periodic matrices. In the majority of cases, the observed frequency distribution of size classes differed from stable size class distribution obtained from the analyses of annual and average matrices. Only in the periodic matrix were no differences detected between observed and expected structures for MF and FF populations (w20:05;3 ¼ 2:91 and 0.44, Dð%Þ ¼ 6:0 and 1.3, respectively). For the annual transition matrices, strong oscillations in l were evident among fragmentation levels and between forest conditions, whereas results from the average and periodic matrices were fairly similar. Populations were clearly growing under CF; however, under FF the average matrix was indistinguishable from 1 and the periodic matrix was less than 1. Populations were found to be growing under all three forest conditions, but the l-values increased in magnitude from MF to YF to FE populations. Fig. 3 shows the elasticities obtained from the annual, average, and periodic matrices when considered by size category and demographic process. For demographic processes in the annual and average matrices, it was evident that permanence had the highest elasticities, both for forest conditions and fragmentation levels. This was followed in importance by growth and retrogression, while fecundity showed relatively little influence on changes in l. Permanence was also the most influential factor in the periodic

matrix, with values for growth, retrogression, and fecundity showing increased importance compared to the annual matrices. For size classes, in the average matrix, elasticity was found to be greatest for adults in YF, FE and CF, while the elasticity of juveniles was of secondary importance. On the other hand, elasticity was greatest for juveniles in the average matrix for MF and FF populations, followed in importance by subadults. Elasticities of the periodic matrix showed a similar pattern. Differences in elasticity values for the average and periodic matrices can be clearly appreciated by placing them within the demographic triangle proposed by Silvertown et al. (1993, 1996). In Fig. 4, it can be seen that all populations fall within the portion of the triangle occupied by woody tree species. Such species are characterized by variations in l principally associated with survival. Elasticities related to growth and fecundity increased from YF to MF and FE populations, whereas elasticities for survival decreased. This same tendency was also observed for fragmentation levels, such that for CF populations there was a small increase in elasticities for fecundity and growth compared to a larger increase for FF populations. 3.3. Harvest simulations Fig. 5 shows the effect on l of the simulated extraction of shoots
5 m and the simulated addition of nursery-reared juveniles. Using the average matrix, FE and CF populations supported a 40% simulated rate of shoot extraction, while maintaining l > 1. On the other hand, 20% harvest rates for the other populations always resulted in values of l < 1. The addition of juveniles increased l in all populations, but

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especially in FE populations (43% of increase) and CF populations (25% of increase). It can also be appreciated in Fig. 5 that the addition of juveniles allowed FE populations and populations under CF to maintain l > 1, even when simulated extraction rates were as high as 80%. In contrast, populations of the other two forest conditions and populations under FF showed l < 1 even for simulated extraction rates as low as 20 and 40%. Periodic matrices showed similar results to average matrices; however, the numerical effects on l were more pronounced. For example, simulations suggested that an 80% harvest rate could be sustainably supported by FE populations and by populations under CF. Nevertheless, a simulated harvest rate as low as 20% in the remaining populations always resulted in l < 1. Also, the addition of juveniles increased l by 60% in FE populations, 24% in CF populations, and 16% in YF populations. Yet, the effect of juvenile recruitment was minimal in MF and FF populations. For the periodic matrices, the combination of simulated extraction and juvenile recruitment resulted in the projection of greater sustainable harvests: 20% for MF populations, 60% for YF populations, and 99% for FE and CF populations. The various harvest simulations, as well as the simulated addition of juveniles, appeared to have no effect upon elasticities associated with demographic processes. With respect to the elasticities associated with size classes, only one tendency was observed. Specifically, as harvest rates increased (with or without the addition of juveniles), there was a tendency for elasticity values associated with juveniles to increase, while elasticities for other size categories decreased. 3.4. Comparisons with other palm populations

Fig. 2. l-Values and 95% confidence intervals for matrix projections of D. orthacanthos populations under different fragmentation levels and forest conditions. Results are shown for three annual matrices and the corresponding average and periodic matrices. (FF) fragmented forest, (CF) continuous forest, (MF) mature forest, (YF) young forest, (FE) forest edge. Values for l are shown below each bar to improve readability.

Chinchilla (1994) presented data for Desmoncus spp. taken from the Forestry Management Unit in San Miguel, San Andre´ s, Pete´ n, Guatemala, and Siebert (2000) reported data for D. orthacanthos at five different sites in Belize. Considering the geographic closeness between these regions and the sites used in the present study, it seems valid to make comparisons for descriptive purposes. Although units of measure and sampling methods varied between these studies, great similarity can be seen in the recommended number of exploitable shoots >5 m (Table 6). Such

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Fig. 3. Elasticity values (95% CI) for size classes and demographic process in D. orthacanthos populations under different forest conditions and fragmentation levels. Results are shown for three annual matrices and the corresponding average and periodic matrices. Size classes are: (1) seedlings, (2) juveniles, (3) subadults, (4) adults. Demographic process are: (L) permanence (stasis), (G) transition (i.e., growth) to other life cycle stages, (R) retrogression, (F) fecundity. Fragmentation levels and forest conditions are as in Fig. 1.

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similarities suggest that the extraction potential for this species is similar throughout the Yucatan biogeographic region (sensu Miranda, 1958).

4. Discussion 4.1. Population structure and dynamics

Fig. 4. The position in the ‘‘demographic triangle’’ (sensu Silvertown et al., 1993, 1996) of elasticities for D. orthacanthos populations under different fragmentation levels and forest conditions. Results are shown for average and periodic matrices after combining the annual matrices over three successive years. Fragmentation levels and forest conditions are as in Fig. 1; m ¼ average matrices, p ¼ periodic matrices.

Populations of D. orthacanthos essentially displayed two types of structure: one in which small individuals predominated (CF, FE, and YF populations) and the other in which individuals with intermediate shoots predominated (FF and MF populations). This structural flexibility has also been described for populations of light demanding trees, as well as for pioneer species ´ lvarez-Buylla, 1995). The (Martı´nez-Ramos and A predominance of juveniles and subadults in the MF

Fig. 5. l-Values associated with matrix projections for D. orthacanthos populations under different fragmentation levels and forest conditions, and simulating different extraction intensities and the addition of juveniles to each population. Results are shown for average and periodic matrices after combining the annual matrices over three successive years. Annual extraction rates of 20, 40, 60, 80 and 99% were simulated in adult and subadult palms for ramets >5 m. A letter ‘‘a’’ accompanying the extraction percentage on the abscissa indicates the annual addition of nursery-raised juveniles to the population in a number equal to the number of seedlings recruited into the population each year. Fragmentation levels and forest conditions are as in Fig. 1.

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Table 6 Abundance of Desmoncus plants (individuals ha1) at: (1) the Unidad de Manejo Forestal San Miguel in San Andre´ s, Pete´ n, Guatemala (Chinchilla, 1994), (2) the central region of Belize (Siebert, 2000), and (3) the La Unio´ n and Noh Bec localities in the Mexican state of Quintana Roo (this study)a

Size class 1 Length (m) Size class 2 Length (m) Size class 3 Length (m) Size class 4 Length (m) Total number of genets Total number of ramets >5 m Total length of ramets >5 m (m) Total length of potential crop (m) Number of samples Area of each sample (ha)

Pete´ n

Belice

La Unio´ n (FF)

Noh Bec (CF)

237 <0.5 91 0.5a 5 46 >5, with sprouts 46 >5, without sprouts 420 183 1779 (327) 1334 12 0.25

57 (27) <1 30 (8) 1a 5

493 (142) Seedlings 542 (63) <5 95 (16)
5<15 20 (6)
15 1150 (157) 238 (39) 2133 (440) 1384 (253) 60 0.01

505 (146) Seedlings 160 (19) <5 80 (11)
5<15 10 (5)
15 755 (145) 183 (28) 2010 (426) 1227 (180) 60 0.01

51 (8) >5 138 (33) 203 (86) 1927 (773) 1445 (580) 5 0.25

a

The first four rows show the number of genets by size categories (standard errors in parentheses, when known). The fifth row shows the total number of genets, summed across all size classes. Size classes differed between authors, but were generally defined as the length of the longest ramet (see value appearing below the number of genets in each case). The last five rows are dedicated to the total number of ramets >5 m, the total length of ramets >5 m, the total length of potential crop, the number of samples, and the area of each sample. Total length of potential crop was estimated as 75% of the total length of ramets >5 m (i.e., 25% of the total length of ramets >5 m was not considered to be potential crop because it corresponded to reproductive ramets and unusable apices).

populations suggested strong limitations on the establishment and development of seedlings. The inverted-J structure observed in FE populations, on the other hand, indicated better conditions for the recruitment of individuals. The predominance of juvenile-sized palms under FF suggested that the intensity and frequency of human perturbations played an important role in limiting the establishment of new individuals and in causing larger individuals to regress to smaller sizes. The basal areas and lengths of genets did not differ between fragmentation levels, but the density of genets was greater in FF populations than in CF populations. This result might be explained by historical factors related to perturbation, as well as to the greater degree of fragmentation caused by agricultural management. However, fragmentation levels did not differ with respect to the availability (number and total length) of exploitable shoots, or with respect to the diameter and length of exploitable shoots. For forest conditions, the basal area and total length of genets, as well as the length of potential crop, were greatest in FE populations, intermediate in MF populations, and least in YF populations. Adding to these

considerations the fact that greater diameters and lengths of exploitable shoots were also observed in FE populations, it seems likely that sustainable management efforts towards to produce liana shoots will benefit most from a focus on FE populations. Nevertheless, a substantial portion of exploitable length in FE populations also corresponded to reproductive shoots, and this may limit the desirability of working with FE populations. However, even maintaining the reproductive shoots for fruiting and propagule dispersion, and subtracting 10% of the total shoot length corresponding to unusable apices, the resulting exploitable shoot length is still three time greater in FE populations than in YF populations, and practically equal in FE and MF populations. Values of l obtained for 10 other palm species (based on annual or average matrix models) indicate that the majority of species grow very slowly or are found very near numerical equilibrium, even though some grow at annual rates between 10 and 20% (Table 7). In this study, annual and average matrix projections for D. orthacanthos produced similar results to those cited above, whereas the periodic matrix contrasted strongly, projecting a growth rate

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Table 7 l-Values reported for the matrix analyses of several palm species: (a) projection from an annual matrix, (b) projection from an average matrix, and (c) projection from a periodic matrix Species

l

Reference

Podococcus barteri (a) Astrocaryum mexicanum (b) Chamaedorea tepejilote (?) Pseudophoenix sargentii (a) I. deltoidea (?) Reinhardtia gracilis (a) C. readii (b) T. radiata (b) N. decaryi (b) P. seemannii (a) Chamaedorea elatior (b) D. orthacanthos (a) D. orthacanthos (b) D. orthacanthos (c)

1.0125 0.9932–1.0399 0.9699–1.1232 0.9971–1.2220 0.9894–1.0166 1.0123–1.0396 1.0549 1.0925–1.1492 1.0670–1.1610 1.0100–1.0660 1.0440 0.9120–1.2655 0.9914–1.1698 0.9639–1.5945

Bullock (1980) Pin˜ ero et al. (1984) Oyama (1987) Dura´ n, 1992 Pinard (1993) Mendoza (1994) ´ lvarez-Buylla (1995) Olmsted and A ´ lvarez-Buylla (1995) Olmsted and A Ratsirarson et al. (1996) Bernal (1998) Luna (1999) This study This study This study

of up to 59%. None of the studies mentioned above, however, reported periodic projections. Our average and periodic projections showed values of l that were substantially different for the same populations. Such results did not occur in other studies estimating both projections (e.g., Foster and Marks, 1987; Golubov et al., 1999; Mandujano et al., 2001). This finding suggests that population growth in D. orthacanthos may be strongly influenced by the temporal order of favorable and unfavorable years (i.e., the sequence of wet or dry years, or the sequence of years where environmental disturbances such as hurricanes or fires occurs). Projections of annual matrices for each population exhibited strong oscillations in population growth rates, a finding also reported for many herbaceous species (Bierzychudek, 1982; Kalisz and McPeek, 1992; Svensson et al., 1993; Horvitz and Schemske, 1995), succulents (Mandujano et al., 2001), trees (Golubov et al., 1999), and some palms (Dura´ n, 1992; Mendoza, 1994). Such findings probably indicate the differential effects of environmental conditions attributable to different years or populations, while projections based upon average and periodic matrices integrated this heterogeneity in the analysis. According to periodic matrix projections, populations associated with the three forest conditions were all increasing. However, the best conditions noted for population growth were associated with FE populations. With respect to fragmentation levels, FF popu-

lations were found to be decreasing in numbers. Therefore, these populations would not be recommended to be used for shoot extraction. On the other hand, CF populations had high growth rates and can be recommended for shoot exploitation. Apparently, the recurrence and magnitude of agricultural perturbation and/or fragmentation events favored the greater densities and sizes of D. orthacanthos plants seen in FF populations. However, these factors were probably also driving concomitant decreases in the population growth rate, or at least maintaining them at or very near equilibrium. Meanwhile, moderate levels of perturbation with no fragmentation broadly favored population growth rates under CF. It is relevant to emphasize the value of matrix techniques for analyzing population dynamics, especially in establishing sustainable use strategies for resource management. In the case of D. orthacanthos, recommendations might have been very different if they had been based upon a static evaluation of the populations numbers (Tables 3 and 4). Elasticity analyses of the annual matrices showed that the contribution of size categories to variation in l oscillated more strongly in populations with smaller growth rates (i.e., FF, MF, and YF populations) than in populations with higher growth rates (i.e., CF and FE populations). Average and periodic projections produced comparable elasticity values. They also more precisely defined

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the relative importance of each size category on l-value changes. Specifically, in populations with low growth rates, greater elasticity was associated with juveniles (FF and MF populations) and adults (YF populations). In the populations with higher growth rates, elasticity was partitioned more uniformly among the four categories. With respect to elasticities associated with demographic process, the annual and average matrices showed similar patterns in all populations. However, with regard to periodic matrices all populations showed decreases in elasticities associated with permanence, and increases in elasticities associated with growth, retrogression, and fecundity. 4.2. Simulations Matrix simulations effected in other palm species indicate that sustainable harvest rates depend upon the plant species involved and the portion harvested. For example, Bernal (1998) reported that Phytelephas seemannii tolerated a seed harvesting intensity of up to 86% before l decreased below equilibrium levels, and Ratsirarson et al. (1996) pointed out that harvesting seeds at less than a 95% intensity did not significantly alter l in Neodypsis decaryi. In another ´ lvarez-Buylla (1995) sugexample, Olmsted and A gested that up to 40 adults ha1 of Thrinax radiata, but no adults of Coccothrinax readii, could be removed annually. Although these yields increased upon extending the between harvest intervals, the authors advised caution in interpreting their results because they did not consider between-year variability. Finally, taking into account the elasticities of the different size classes, Pinard (1993) considered that harvest rates for Iriartea deltoidea adults were sustainable because the current practice of harvesting only the largest individuals resulted in a marginal effect on l. On the contrary, the harvest of palms of smaller sizes might produce drastic decreases the population growth rate. In our case, results indicated that sustainable harvest rates for shoots
5 m would vary with environment conditions in a way similar to population growth rates (i.e., harvest rates would be greater for environments with no light limitations and moderate levels of perturbation, and would be increased substantially by transplanting nursery-reared juveniles into the populations).

Siebert (2000) found that production and growth of new ramets in D. orthacanthos were significantly greater in high luminosity environments than in environments with low light availability. These values were also higher in plants subjected to harvest than in nonharvested plants.

5. Conclusions This study demonstrates that sustainable extraction of D. orthacanthos is possible. It also shows that matrix analytical methods, when applied population demographic investigations, constitute an excellent tool for elaborating management plans in a context of sustainability. The ideal environment for the use D. orthacanthos appears to be a mosaic of slightly perturbed areas located within a matrix of forest conditions that are represented by distinct successional stages. Clearings and forest edges with high light availability and sufficient trees for support are particularly desirable. Nevertheless, sustainability in the use of this species is likely to be difficult under the severely disturbed and fragmented conditions characteristic of traditional agricultural practices (including the use of fire) in tropical forests. In this study, the comparison of population demography under various fragmentation levels and forest environment conditions showed that sustainable extraction of D. orthacanthos is feasible, given a managed forest situation. In particular, matrix simulations showed that it is theoretically possible to increase harvest rates while maintaining l > 1, provided that juvenile nursery-raised are introduced into the population. If other forest management practices (e.g., allowing fructification and natural seed dispersion; rotating plots or individuals so that each plant can be harvested every 2 years; pruning dead or dying shoots to stimulate the productions of new growth; opening clearings or pruning branches of other species in order to allow access to the canopy; establishing evaluation procedures and adjusting harvests; selecting germplasm for plantings from mother plants showing the greatest vigor and growth rates; etc.) were implemented, it is likely that the amount of the exploitable resource might be increased.

S. Escalante et al. / Forest Ecology and Management 187 (2004) 3–18

The sustainable potential of D. orthacanthos is strongly suggested by this study. However, its extractive use implies the appropriate maintenance and management of forests in order to provide better yields. In addition, the economic value of this species might be equally extended to all tropical forest regions where Desmoncus species are found, and not remain restricted to just this study area.

Acknowledgements We acknowledge the funding of CONABIO (Comisio´ n Nacional para el Conocimiento y Uso de la Biodiversidad) and the MacArthur Foundation through the FB504/M066/97 grant. Similarly, we thank CONACYT (Consejo Nacional de Ciencia y Tecnologı´a) for a doctoral scholarship (#95016) awarded to S. Escalante to conduct his doctoral studies at the Instituto de Ecologı´a A. C. Two anonymous reviewers provided constructive comments on the manuscript. Ernesto Vega is acknowledged for his help with the bootstrap analysis and Wilberth Canche´ , Alberto Matı´as, and Juan Cruz for their assistance in the field.

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