Demography of Bursera glabrifolia, a tropical tree used for folk woodcrafting in Southern Mexico: An evaluation of its management plan

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  Demografía de Bursera Glabrifolia
  Demography of   Bursera glabrifolia , a tropical tree usedfor folk woodcrafting in Southern Mexico: An evaluation of its management plan Mariana Herna´ndez-Apolinar a, *, Teresa Valverde a , Silvia Purata b a  Departamento de Ecologı´ a y Recursos Naturales, Facultad de Ciencias, Universidad Nacional Auto´ noma de Me´  xico,Ciudad Universitaria, 04510 Me´  xico, D.F., Me´  xico b  Instituto de Ecologı´ a, A.C. Antigua Carretera a Coatepec, km 2.5. Xalapa, 91000 Veracruz, Me´  xico Received 17 November 2004; received in revised form 25 October 2005; accepted 26 October 2005 Abstract Tropical dry forest in Mesoamerica has long been disturbed by the extraction of different forest products by local inhabitants.  Burseraglabrifolia  (white ‘‘copalillo’’) is a dry tropical forest tree extensively used in several communities of the State of Oaxaca, Mexico, for theelaboration of folk-art woodcarvings called ‘‘alebrijes’’. Historically, the extraction of   B. glabrifolia  had not been regulated; however, in October2002,thefirstmanagementplanforthisspecieswasimplementedinSanJuanBautistaJayacatla´n,basedonabundance,distributionanddasonomicdata. However, no demographic information is available on which to base a sounder management plan that would guarantee the ecologicalsustainability of this extractive activity. Thus, we studied the population dynamics of this species over a 2-year period in Jayacatla´n, where no treeextraction had taken place since 1998. We used stem expansion rates and observations on survival and fecundity to build size-based populationprojection matrices. The projected population growth rate values ( l ) were 1.14 and 1.04 for the 2001–2002 and the 2002–2003 periods,respectively. These values suggest that the  B. glabrifolia  population at Jayacatla´n shows a growing trend. Several harvesting scenarios weresimulated totheoretically assess the impact on population dynamicsof harvestingwhole treesand, inparticular,toascertain whether the extractionregime recommended bythe managementplan(8trees/ha/yr) issustainable. Forthispurpose,we usedanaveragepopulationmatrix wherespecificentriesweremodifiedtosimulatedifferentextractionlevels.Theresultssuggestthattheharvestingregimesestablishedinthemanagementplanfor  B. glabrifolia  are sustainable. However, resource managers should not exceed the amounts recommended by the management plan and shouldavoid extrapolating our results to other regions of the country where no demographic studies have been carried out. The need for long-termdemographic studies and their incorporation in ‘adaptive management plans’ is discussed. # 2005 Elsevier B.V. All rights reserved. Keywords:  Alebrije; Folk-art; Matrix models; Management plan; Sustainability; Tropical dry forest; White copalillo 1. Introduction Tropical forests are an important source of natural resourcesfor local inhabitants all over the world (Challenger, 1998).In particular, tropical dry forests in Mesoamerica have beenintensively exploited by local communities since Pre-Colum-bian times (Murphy and Lugo, 1995; Janzen, 1988; Maass,1995). This recurrent and growing disturbance pressureassociated to human activities has posed a severe threat totropical dry forests (Janzen, 1988; Murphy and Lugo, 1995;Gentry, 1995; Trejo and Dirzo, 2000). In the Mexican state of Oaxaca, one of the products obtained from tropical dry forestsis the wood of   Bursera glabrifolia , which is used for folk woodcrafting. The success and sustainability of this extractiveactivity depends on the application of management plans basedon sound demographic information, which is now lacking.In this study, we present a 2-year demographic analysis of apopulation of   B. glabrifolia  on which management guidelinesmay be based in the future.Tropical dry forests (also called seasonally dry tropicalforests) represent 42% of all tropical forests of the world(Murphy and Lugo, 1986). Awide array of plant communitiesthat share a number of structural and physiognomic featuresare generally regarded as examples of this vegetation type. Ecology and Management 223 (2006) 139–151* Corresponding author. Tel.: +52 55 56224912; fax: +52 55 56224828. E-mail address: (M. Herna´ndez-Apolinar).0378-1127/$ – see front matter # 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.foreco.2005.10.072  Some of these common features are a high diversity of lifeforms and a marked phenological response of most woodyelements to the drought season, typically lasting for severalmonths (Kalacska et al., 2004). In Mesoamerican tropical dryforests, the canopy is usually composed of deciduous trees of short stature (ranging from 5 to 15 m tall) as well as thornyshrubs and a number of xerophitic elements. These forests areassociated with subtropical areas with ahighly seasonal climatein relation to rainfall, i.e. the dry season may last for 7–9months, while the rainy season (during which ca. 70% of theyearly precipitation falls) is generally shorter (3–5 months—Bullock et al., 1995). Tropical dry forests occupy an importantproportion of the Mexican territory (11%); which house a largepart of the biodiversity of the country and show high floristicendemism(ca.40%—Rzedowski,1991;TrejoandDirzo,2000;Ceccon et al., 2003). This vegetation type is rapidly decliningworldwide. In Mesoamerica, the deforestation of theseecosystems has been taking place since pre-Columbian timesand continues today (Bye, 1995; Maass, 1995; Murphy andLugo, 1986, 1995; Janzen, 1988). The main cause of thisdecline has been land-use change towards cattle ranching andagriculture. However, it has also been recognized that selectivetimber extraction and harvesting of non-timber forest productshas played an important role in this respect, especially in thelast decades (Bye, 1995; Challenger, 1998).Only a few studies have evaluated the effects of harvestingon the population dynamics of dry tropical forest trees. Amongthem, the information available for  Amphipterygiumadstringes ,  Bursera aloexilon ,  Croton  sp.,  Crescentia  sp., Guaiacum  sp.,  Hermiagium excelsum ,  Leucena  sp., and Swietenia macrophylla , have shown that natural populationshave been dramatically affected by extraction (Bye, 1995;Gullison et al., 1996; Hughes, 1998; Grow and Schwartzman,2001; Hersch Martı´nez et al., 2004). The trees of the genus  Bursera  have been the targets of different extraction practices.In the central valleys of the Mexican state of Oaxaca, three  Bursera  species (  B. glabrifolia ,  B. submoniliformis , and  B.aloexilon ) are exploited by local inhabitants who fell wholetrees and use the wood to produce woodcarvings called‘‘alebrijes’’ (Fig. 1). These alebrijes usually evoke smallfantasy animals and are sold very successfully in local,national, and even international markets as an example of Mexican folk-art (Chibnik, 2003). Of the three  Bursera  speciesmentioned,  B. glabrifolia  (locally known as white ‘‘copalillo’’or ‘‘copal blanco’’) is by far the most frequently used foralebrije elaboration. Local artisans report that this preference isrelated to its softer and lighter wood, compared to that of theother species (Purata et al., 2004).Alebrijes are a relatively new folk-art product (Chibnik,2003).Originally (ca.40yearsago), thesefigureswere madebypeople from the villages of Arrazola and Tilcajete, near the cityof Oaxaca (Fig. 2). The wood was extracted from the tropicaldry forests surrounding these villages. However, in the last 30years the commercialization of alebrijes has expandeddramatically and people from many villages are now involvedin their production. As the demand increased,  B. glabrifolia populations have declined noticeably around Arrazola andTilcajete and now trees are being extracted from forest areas asfar as 100 km from these villages (Peters et al., 2003). So far,these extractive activities have not been regulated or designedthrough appropriate management plans. In recent years,Mexican government agencies are requiring resource managersto elaborate management plans for the forest products beingextracted in different parts of the country. The lack of suchmanagement plans for  B. glabrifolia  (and in fact for most of theproductsharvestedfromtropicalforestsinMexico)implies thatcurrent extraction is being carried out illegally (Peters et al.,2003).A management plan for  B. glabrifolia  was devised in 2000by professional ecologists, based on abundance, distributionand dasonomic data obtained in the region of Jayacatla´n, inOaxaca (Fig. 2—Brosi et al., 2000). A complete description of  the ecological data used for the elaboration of this managementplan may be found in Peters et al. (2003) and Purata et al. (2004). The management plan is assumed to be sustainable;however, no demographic information is available in which tosupport this assumption. A sounder management plan based ona deeper knowledge of the population dynamics of this specieshas still to emerge.Population projection matrices have been used by plantecologist to address different ecological matters, from life-history evolution to issues in conservation biology and resourcemanagement (de Kroon et al., 1986; Pinard, 1993; Silvertownet al., 1993). In the last decade, these models have beensuccessfully applied to ponder over different resource manage-ment strategies in several plant species. The consequences of extractive activities may be evaluated through the analysis of the numerical changes that take place within populations, andthe identification of the most vulnerable life-cycle stages of thetarget population (Olmsted and Alvarez-Buylla, 1995; Ander-son and Putz, 2002; Ticktin, 2002). The advantages of population projection matrices for resource management arenow widely recognized and management plans for some plantspecies havebeen based on thistype ofmodels (SoehartonoandNewton, 2001; Anderson and Putz, 2002; Ticktin, 2002;Zuidema and Boot, 2002). In this paper, we present the resultsof a demographic study of a population of   B. glabrifolia  inwhich we used population projection matrices to address  M. Herna´ ndez-Apolinar et al./Forest Ecology and Management 223 (2006) 139–151 140Fig. 1. An example of an alebrije, a fantasy animal hand-carved using  B.glabrifolia  wood.  different ecological and management issues. In particular, ouraims were: (a) to describe the demographic behavior of apopulation of   B. glabrifolia  in Jayacatla´n to evaluate its currentstatus; (b) to identify the most vulnerable life-cycle stages onwhich to prevent extraction; (c) to use the matrix model totheoretically evaluate the sustainability of the harvestingregime recommended by the management plan by simulatingdifferent intensities of tree extraction. 2. Methods 2.1. The studied species In Mexico,  B. glabrifolia  trees are known as white copalillo(copal or copalillo blanco). This species belongs to theBurseraceaefamilyandisadioecioustreethatintheadultstagemay reach between 5 and 12 m in height. Its bark is persistent,which characterizes the Section Bullockia within the Burser-aceae, and is smooth, gray or gray-reddish in color (Rzedowskiet al., 2004). Leaves are composite and have a whitish aspectdue to the presence of pubescence on the abaxial surface; theyare shed during the dry season, which in the study area (seebelow) spans from November to May. Inflorescences arepaniculate, reaching a length of up to 10 cm and bearing four-merous unisexual flowers. Fruits are bivalve drupes with anovate shape (Toledo, 1982; Rzedowski et al., 2004). Each fruitgenerally contains a single seed (occasionally two).The distribution range of   B. glabrifolia  includes south-central Mexico and Central America. In Mexico, it is found ataltitudes of 500–2000 m, in tropical dry forests and dry  M. Herna´ ndez-Apolinar et al./Forest Ecology and Management 223 (2006) 139–151  141Fig. 2. Location of the study site within the Mexican state of Oaxaca.  temperate deciduous forest (Toledo, 1982). In our study site(Jayacatla´n, Oaxaca—see below), this species occupies analtitudinal range that spans from 500 to 1250 m; it is one of thedominant tree species of the tropical dry forests present in thisarea (Brosi et al., 2000). 2.2. The study site This study was carried out near the village of Jayacatla´n(17 8 25 0 24 00 N, 96 8 49 0 19 00 W), in the Mexican state of Oaxaca.This area has a warm and seasonally dry climate with a meanannual temperature of 21.7 8  C and an annual precipitation of 616 mm (Garcı´a, 1988). The summer rainy season spans fromJune to September, during which 78% of the yearlyprecipitation falls. Soils are generally shallow, calcareousand of diverse srcin. This area is set within the southern SierraMadre, which is a complex mountain range with a highecological andfloristicdiversity.Somevegetation typesthatarewell represented in the area are tropical dry forests, drytemperate forests, temperate deciduous forests, temperateevergreen forests, and anthropogenic grasslands and agrosys-tems. Our study was carried out in the tropical dry forest, whichin this municipality covers an area of ca. 5000 ha; in thisecosystem the vegetation is dominated by trees of mediumheight (ca. 6–12 m), with a high representation of theBurseraceae and Leguminosae, as well as other conspicuoustree species such as  Amphyterigium adstringens . Somecolumnar cacti are also present (e.g.  Pachycereus weberi ,  Myrtillocactus geometrizans ,  Eschontria chiotilla ,  Stenocactusrectispinus  Schmoll.), as well as a number of thorny shrubs(  Acacia  sp.,  Mimosa  sp.), herbs, epiphytes ( Tillandsia  sp.,  Bromelia  sp.) and vines (lianas).During the 1990s the tropical dry forest of Jayacatla´n wasaffected by the unrestricted extraction of   B. glabrifolia . In 1998the extraction was stopped because local people becameconcerned in relation to the sustainability of this practice. Theywere interested in developing a management plan based onbiological information on which to design sustainableextractive regimes. This management plan was developed byecologists and forest managers (Brosi et al., 2000; Peters et al.,2003), based on tree growth rates and other biologicalattributes; it recommends a maximum extraction rate of 8trees/ha/yr (i.e. two trees in the 10–20 cm DBH category, fourtrees in the 20–30 cm category, and two trees in the  > 30 cmcategory),which addsupto4.3 m 3 ofwoodcarving materialperhectare. According to the data gathered by the managementplanners, these are the amounts that may be naturallyregenerated by the population, thus maintaining populationstructure and abundance relatively unaltered. 2.3. Field work  This study was carried out from the summer of 2001 to thesummer of 2003. First, a sample of   B. glabrifolia  individualswasselectedwhichwouldbemarkedandmonitoredfor2years.To select this sample, we analyzed the distribution datagathered by Brosi et al. (2000) and located five 0.5 ha plots(within the 5000 ha of dry topical forest owned by theJayacatla´n municipality) with a high density of   B. glabrifolia .These plots were separated from each other by a distance thatranged from 50 m to 5 km. All  B. glabrifolia  individualslocated within these plots ( n  = 816) were marked, mapped andmonitored for 2 years. Trees were classified in five categoriesbased on both height and DBH (diameter at breast height—Table 1). For convenience, we used the same size categoriesdefined in the management plan referred to above (Brosi et al.,2000). The height or DBH of each tree was measured in thesummer of 2001–2003, using ordinary measuring tapes, andeach year the death of previously recorded individuals wasnoted. From these data we described population structure andcalculated size-specific survivorship from 1 year to the next.To calculate accurate growth rates, we performed a moredetailed diameter measurement in a sample of 67 trees (40 insize-category 3; 20 in size-category 4; 7 in size-category 5 for  M. Herna´ ndez-Apolinar et al./Forest Ecology and Management 223 (2006) 139–151 142Table 1Size categories used to describe the population dynamics of   B. glabrifolia,  and mean individual growth rate (average increase in diameter from 1 year to the next, incm)  S.D. for each category, using different measuring techniquesStage Size intervals for categories Size-category Measuring techniqueCaliper Dye Dendrometer2001–2002Seedlings  < 1 1 0.22  0.28 (336)Juveniles 1–10 2 0.29  0.42 (193)Adult 1 10–20 3 0.61  0.49 (40) 0.06  0.11 (33)Adult 2 20–30 4 1.14  0.70 (20) 0.06  0.05 (18)Adult 3  > 30 5 1.33  0.50 (7) 0.06  0.07 (6)2002–2003Seedlings  < 1 1 0.20  0.24 (225)Juveniles 1–10 2 0.26  0.36 (206)Adult 1 10–20 3 1.16  1.01 (37) 0.15  0.18 (38)Adult 2 20–30 4 1.08  1.02 (23) 0.17  0.09 (22)Adult 3  > 30 5 1.33  1.13 (6) 0.18  0.09 (5)Sample sizes (the same trees were used for the dendrometer and dye techniques; the sample sizes were slightly different for each technique mainly as a result of theloss of a few dendrometers due to burglary, and because no dye could be injected to a few trees to which dendrometers were adjusted) are given in parenthesis besideaverage values. In the two smallest size categories, diameter was measured at the trunk base. For the three larger size categories, diameter values refer to DBH.
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