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Poleward plant migration as ice sheets retreated during the late Pleistocene and Holocene has been described in many temperate and boreal plant communities throughout the world (Davis, 1981; Huntley & Birks, 1983; Petit et al., 2003). These range shifts have been important in reshuffling species into the observed modern plant communities (Davis, 1981), and within species this reshuffling has often led to secondary contact of previously isolated populations and allopatric divergence of previously contiguous populations (Critchfield, 1984; Gugger et al., 2010). Poleward shifts have also had the effect of creating gradients in genetic diversity, where poleward (leading edge) populations are often less diverse than their equatorward (rear edge) counterparts because of the effect of successive dispersal bottlenecks during migration (Petit et al., 1997; Hewitt, 2000; Gugger et al., 2008). In principle, migration in any direction would have similar effects but, for example, equatorward migration of temperate species is difficult to observe because of the extinction of rear-edge populations during the most recent postglacial period. Thus little is known about the extent to which modern temperate and subtropical forests comprise combinations of species originating in response to the onset of Pleistocene glaciations.
Given that equatorward migration has been identified as an important process in plant biogeography at deeper geological scales (e.g. North American contribution to South American communities during Great American Biotic Interchange; Wallace, 1876), it seems likely that the effects of such migration during the Pleistocene may still be detectable. In particular, southward migration may have been important in the formation of modern plant communities in subtropical Mexico, where temperate and subtropical genera intermix. The southward Pleistocene migration hypothesis was proposed based on early interpretations of the limited fossil record in Mexico (Deevey, 1949; Dressler, 1954; Perry et al., 1998), but subsequent authors have favored an older Tertiary (primarily Miocene) origin for most temperate taxa in Mexico (Graham, 1999).
Disjunct temperate taxa in Mexico form a major part of the Madrean pine-oak (Pinus–Quercus) biodiversity hotspot, which covers the middle to high elevations of the Sierra Madre Occidental, Sierra Madre Oriental, Trans-Mexican Volcanic Belt and Sierra Madre del Sur (Fig. 1). This region contains nearly 4000 endemic plant species, of which at least 20 tree species or subspecies in Pinaceae are considered threatened (Conservation International; Norma Oficial Mexicana, 1994, 2001; Farjon & Page, 1999).
Figure 1. Map of Mexico with Madrean pine–oak region shown in light gray (Conservation International Foundation); Pseudotsuga range as approximated by location of herbarium samples (from GBIF) and shown as crosses; sample sites shown as black points numbered according to Table 1. States mentioned in the text are labeled with abbreviations: AZ, Arizona; Chi., Chihuahua; Coa., Coahuila; Dur., Durango; Hid., Hidalgo; N.L., Nuevo León; NM, New Mexico; Oax., Oaxaca; Que., Querétaro; Tla., Tlaxcala; TX, Texas.
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Yet, such rear edge populations may be an important reserve of genetic diversity in changing climates because they have often been long-isolated, display strong differentiation, and may exhibit local adaptation in response to strong selection and lack of gene flow (Hampe & Bairlein, 2000; Chang et al., 2004; Martin & McKay, 2004; Hampe & Petit, 2005; Parisod & Joost, 2010). The rear edge populations of temperate taxa in subtropical Mexico have likely expanded and contracted in response to many climate fluctuations, producing complex patterns of intraspecific and interspecific biodiversity (Pennington et al., 2000).
Douglas-fir (Pseudotsuga menziesii) is a wide-ranging, ecologically and economically important tree found from central Mexico to central British Columbia. Fossil and molecular evidence point to a northern North American or Asian Tertiary origin of Pseudotsuga (Hermann, 1985; Schorn, 1994; Gernandt & Liston, 1999), which suggests that the isolated Mexican ‘sky-island’ populations are the result of a past southward expansion. The fossil record suggests two alternative hypotheses: early Miocene (c. 20 million yr before present (Ma)) colonization when many other temperate taxa are thought to have arrived (Graham, 1999) or Pleistocene colonization in response to glaciation at high latitudes (Deevey, 1949; Dressler, 1954; Perry et al., 1998). A few putative Pseudotsuga pollen grains found in Miocene sediment from Chiapas could suggest Miocene colonization (Palacios-Chavez & Rzedowski, 1993); however, Pseudotsuga and Larix pollen cannot be distinguished (Barnosky, 1985). Alternatively, Pleistocene colonization is supported by the much later first appearance of fossil Pseudotsuga pollen in the southern Rocky Mountains in the early Pleistocene (Gray, 1961) or late Pleistocene (Martin, 1963) in southern Arizona. In Mexico, the limited Pleistocene fossil record contains no evidence of Douglas-fir (Brown, 1985; Gugger & Sugita, 2010).
Mexican Douglas-fir is presently geographically isolated from USA populations by large deserts. To varying degrees Mexican populations are ecologically (Vargas-Hernández et al., 2004; Acevedo-Rodríguez et al., 2006), morphologically (Reyes-Hernández et al., 2005, 2006) and genetically (Li & Adams, 1989) distinct from those in the USA and Canada. Consequently, Mexican Douglas-fir populations have been classified as multiple separate species (Flous, 1934a,b; Martínez, 1949), a separate variety (Reyes-Hernández et al., 2006; Earle, 2009) or part of the Rocky Mountain variety of Douglas-fir (P. menziesii var. glauca), whose northern limit extends into central British Columbia (Little, 1952; Hermann & Lavender, 1990). Compared with northern Mexican populations, central Mexican populations are morphologically and phenologically more distinct from USA populations (Reyes-Hernández et al., 2005, 2006; Acevedo-Rodríguez et al., 2006).
Mexican Douglas-fir is listed as ‘subject to special protection’ (Norma Oficial Mexicana 1994, 2001) because all the Mexican Douglas-fir populations are small and fragmented, ranging from a few dozen to a few thousand individuals (Mápula-Larreta et al., 2007; Velasco-García et al., 2007), and a number of studies suggest low fertility and seedling recruitment rates because of inbreeding depression (Vargas-Hernández et al., 2004; Mápula-Larreta et al., 2007; Velasco-García et al., 2007). Within Mexico, the lowest fertility and seedling recruitment rates and highest inbreeding rates were found in central populations, which are among the smallest and most isolated in Mexico (Juárez-Agis et al., 2006; Mápula-Larreta et al., 2007; Velasco-García et al., 2007; Cruz-Nicolás et al., 2008). Leaf and cone morphology (Reyes-Hernández et al., 2005, 2006) and bud phenology (Acevedo-Rodríguez et al., 2006) are also least variable in central Mexican populations compared with northern Mexican populations. Therefore, we expect genetic diversity to be positively correlated with latitude, consistent with the signature of dispersal bottlenecks during southward migration and because latitude is correlated with morphological and phenological diversity, fertility rates, and population size.
Here, we investigate mitochondrial (mtDNA) and chloroplast DNA (cpDNA) sequence and cpDNA microsatellite (cpSSR) variation in 11 populations throughout Mexico to test Miocene vs Pleistocene southward migration hypotheses to explain the origins of temperate Douglas-fir in Mexico and test the association of geographic patterns of molecular variation and population size changes with Pleistocene climate history using ecological niche models. We assess the implications of these results for the taxonomic status of Mexican populations and for conservation strategies in an understudied biodiversity hotspot.
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Fig. S1 Posterior distributions for divergence time parameter (t) for the run with the highest effective sample size (ESS) under the full IMa model and a reduced model, where m1 = m2 = 0.
Table S1 Sampling site information and haplotype frequencies for each marker and each population
Table S2 Mitotype and chlorotype definitions in terms of V7, nad7i1, rps7-trnL and rps15-psaC haplotypes reported to GenBank (accessions in parentheses)
Table S3 Definitions of chloroplast DNA microsatellite (cpSSR) haplotypes based on fragment lengths of each cpSSR marker and binary coding used to calculate FS (Table 1)
Table S4 SAMOVA tables for groupings that gave the highest FCT for mitochondrial DNA (mtDNA) and chloroplast DNA (cpDNA) sequence data
Table S5 Training gain values when climatic variables were used in isolation for each model
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