Quercus Humboldtii Bonpl. (Fagaceae)1

BIOTROPICA *(*): ***–*** **** 10.1111/j.1744-7429.2006.00217.x Genetic Variation in Fragmented Forest Stands of the Andean Oak Quercus humboldtii Bonpl. (Fagaceae)1 Juan F. Fernandez-M.´ 2 Laboratoire d’Ecologie, Systematique´ et Evolution, Batˆ 360, Universite´ Paris Sud XI, 91405 Orsay Cedex, France and Victoria L. Sork Department of Ecology and Evolutionary, The University of California Los Angeles, CA 90095 1606, U.S.A. ABSTRACT Quercus humboldtii is a montane forest dominant species in Colombia, which has experienced significant habitat loss. Using three microsatellite loci, we compared the genetic diversity of adults and seedlings in fragments of small and large size. Results show high genetic diversity, comparable to other temperate oak species (Ho = 0.813, He = 0.780, and f =−0.044). However, allelic richness reduction in seedlings of the most fragmented part of the landscape, suggested restricted gene flow and risk of future genetic bottlenecks, if larger tracts of forest disappear. RESUMEN Quercus humboldtii es una especie dominante de las montanas˜ colombianas que ha sufrido una importante perdida de habitat.´ Usando marcadores microsatelites,´ comparamos la diversidad genetica´ de adultos y plantulas´ en fragmentos pequenos˜ y grandes. Encontramos una alta diversidad genetica,´ comparable a las especies de robles de zonas templadas (Ho = 0.813, He = 0.780 y f =−0.044). Sin embargo, existe una reduccion´ en la riqueza alelica´ de las plantulas´ de la zona mas´ fragmentada, sugiriendo que deben conservarse grandes areas´ boscosas para evitar riesgos futuros de perdida´ de la diversidad genetica.´ Key words: Andes; Fagaceae; forest fragmentation; genetic diversity; microsatellites; Quercus humboldtii; South America. HABITAT FRAGMENTATION CAN THREATEN POPULATION VIABILITY adult populations of the Andean oak, which were established prior by altering multiple ecological and genetic processes (Young et al. to landscape fragmentation? (2) What does adult genetic structure 1986, Templeton et al. 1990, Saunders et al. 1991, Ledig 1992, indicate about historical gene flow? and (3) does genetic diversity in Ellstrand & Elam 1993, Frankham et al. 2002). Long-lived woody present day seedling cohorts in fragmented areas show less diversity perennials, in particular, are expected to be resilient to changes than seedlings in large forest stands? in genetic diversity due to ample gene flow and long generation times (Shapcott & Playford 1996, Soejima et al. 1998, White et al. 1999, Merwe et al. 2000, Hamrick 2004). However, if fragmented METHODS populations become isolated and gene flow is reduced, eventually these small populations will be at risk for loss of genetic diversity STUDY SPECIES.—Quercus humboldtii Bonpl. is the southernmost through genetic drift. species of oak in the Western hemisphere and belongs to the group Fragmentation of the forest is widespread in the northern of red oak subgenus Erythrobalanus (Nixon 1993). It is a medium- Andes, where the Colombian oak is a conspicuous element of the to-large size tree that is endemic to the northern Andes and possibly vegetation forming extensive forest stands of several kilometers long the Darien in Panama. Quercus humboldtii is a characteristic element in the Andean vegetation belt (Hooghiemstra & Sarmiento 1991). of primary Andean vegetation (Cuatrecasas 1958) found in moist Oak trees are now highly subdivided among pastures, crops, com- forests between 1500- and 3300-m elevation, forming stands of mercial timber, and native forests and virtually all its populations almost a monospecific canopy. Virtually all remnant populations of are in fragments of larger or lesser extent. A critical conservation the Andean oak can be considered fragments varying in size from concern is whether the seedlings that have established in these frag- a fraction of a hectare to probably 5000 ha for the largest tracts of ments are starting to show the isolating effects of fragmentation on forest because of intensive use in the past for construction, fire wood, gene flow, or whether gene flow among extant fragments is suffi- and clearing of the forest for cattle farming. They are considered cient to retain previous levels of genetic diversity. Specific research a vulnerable vegetation type (Fernandez-M´ 1993, Calderon´ 2001) questions we address are: (1) what is the overall genetic diversity of and the species is protected from timber harvesting since 1974 by the Colombian law. Like all oaks, it is a wind-pollinated, monoecious 1 Received 13 May 2005; revision accepted 15 February 2006. tree that produces acorns dispersed mostly by gravity. The oak 2 Corresponding author; e-mail: [email protected] woodpeckers do not store acorns or rely heavily upon them, but feed C 2006 The Author(s) 1 Journal compilation C 2006 by The Association for Tropical Biology and Conservation 2Fernandez-M.´ and Sork on insects, sap, and fruit year-round (Kattan 1988), which limits and dehydrated in zip-lock bags containing about 90 g of silica long-distance dispersal for the species. Pollen records show that gel. DNA extractions and molecular biology conditions, and data Quercus dispersed into the Colombian Andes at least 340,000 yr BP scoring are described in detail in Fernandez-M. et al. (2000). For from Central America (Hooghiemstra & Sarmiento 1991), which this study, we used three microsatellite loci: QpZAG58, QpZAG15, is relatively recent in comparison to North American modern oaks and QpZAG9. in Western United States, which have been there for approximately 20 million yr (Raven & Axelrod 1978). DATA ANALYSIS.—Number of alleles (A) per locus was determined by direct counting. The effective number of alleles (Ae) was estimated STUDY SITE.—Sampling sites were located in the northeastern Andes ◦ ◦ as the inverse of the expected homozygosity. Observed heterozy- of Colombia, (73 30 13 W, 5 43 14 N) near the towns of Villa gosity (H ), and expected heterozygosity (H ) followed formulas by de Leyva and Arcabuco, Department of Boyaca.´ These locations o e Pons and Chaouche (1995) that account for different sampling size. occurred between 2400 and 2700 m in a fragmented landscape that Inbreeding was estimated as f = 1–H /H for each locus. Stan- was once part of a continuous forest, as judged by several kilome- o e dard deviations of the previous parameters were obtained through ters of continuous oak fragments. The landscape contains multiple resampling individuals with replacement 1000 times. Allele counts stands that range in size from few scattered trees within pastures to are useful direct measures of genetic diversity with highly variable more than 4000 ha. Even this larger area has been subject to logging genetic markers but they are also very sensitive to unequal sample and some parts exhibit small trees that are probably resprouts after sizes (Petit et al. 1998). Therefore, allele count sampling curves logging. Smaller fragments are usually composed of few large trees were simulated for each site by resampling the data with 2, 3,..., surrounded by smaller trees, presumably originated from seeds from N individuals 1000 times for each sampling size. Resampling was these remnant trees. Aerial photographs and topographic maps from performed individually for adults and seedlings classified in two the 1960s show that present day fragments have remained virtually extreme groups of low population density (smaller fragments) and the same for at least 40 yr (Instituto Agust´ın Codazzi map C-241-6, normal population density (plots within the larger fragment). This 1963). Forest fragmentation probably began in the mid 1900s when method provides an estimate of the number of alleles with a 95% trees were cut for charcoal, firewood, and lumber, and rangeland ci for the smallest sample size and allows a comparison of all sam- for cattle, although human disturbance may be at least 500-yr old ples with that minimum sample size. All described analyses were (Molano 1990, Etter & van Wijngaarden 2000). performed using specific functions written in Matlab R11 by JF. SAMPLE DESIGN.—During 1998 and 1999, we selected forest frag- The genetic structure was described by means of AMOVA ments consisting of small stands of isolated trees and a very large analysis (Excoffier et al. 1992) whose results are summarized in the remnant stand of continuous forest of ca 1200 ha. Separation of the parameter . We used a hierarchical genetic structure model with large fragment to the set of smaller fragments was about 11 km with two types of landscapes relative to the total population (LT )and several intervening oak fragments of variable size. The small stands subpopulations (fragments or plots) within landscape type (SL). were located at varying intersite distances with the closest sites be- Significance of the values was tested by bootstrapping individuals ing 240 m apart and the farthest 1200 m, with average fragment and comparing the upper and lower 95 percentiles. We conducted separation of 598 m (SD = 340; see Fig. 1A). Within the large tract AMOVA analyses using the GeneticStudio software (available by of forests, we selected three plots located in the interior such that request at http://dyerlab.bio.vcu.edu/). the minimum intersite distance was 440 m, the maximum 1400 m, We tested for isolation by distance (IBD) by estimating pairwise and the average = 951 m (SD = 482; see Fig. 1B). Hereafter, we fragment/plot genetic distances using pairwise values of from will consider and refer to the small fragments as representative of the the abovementioned AMOVA analyses. The transformed pairwise “subdivided” or fragmented part of the population, and the plots values of = /(1 – ) were regressed against the logarithm of within the larger remnant stand as representative of the “continu- the pairwise physical distances separating each study site (Rousset ous” part of the landscape. 1997). The significance of the association was tested by computing To compare present day levels of genetic diversity with those of the Z statistic of the Mantel test (Smouse et al. 1986) and by established adults, we sampled both seedlings and adults within all reshuffling 1000 times the genetic distance matrix and recalculating plots.

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