Hernando Rodríguez-Correa

HOW ARE OAKS DISTRIBUTED IN THE NEOTROPICS? A PERSPECTIVE FROM SPECIES TURNOVER, AREAS OF ENDEMISM, AND CLIMATIC NICHES

Premise of research. The most important diversity hot spot of genus Quercus (Fagaceae) in America is situated in southern Mexico. From this area down to the Colombian Andes, oak species diversity decreases considerably, but the pattern of species distribution and turnover has not been analyzed. This study aimed at determining geographical patterns of species turnover, species distribution, and endemism for Neotropical Quercus species. Methodology. Occurrence records for 58 oak species belonging to the Quercus and Lobatae sections were obtained. Patterns of species turnover were determined by comparing species composition among latitudinal/longitudinal units. Areas of endemism were determined using weighted networks. The potential distribution of oak species was determined using ecological niche models. Finally, a principal component analysis was used to identify changes in the oak species’ ecological niche across areas. Pivotal results. The species composition analysis indicated that the Tehuantepec Isthmus, the Nicaraguan Depression, and the Panamanian Isthmus represent species turnover points. Nine areas of endemism were recovered, distributed through mountainous ranges from Mexico to Costa Rica. Most of these areas were delimited by the species turnover points detected. Ecological niche modeling indicated that the turnover points represent areas with low climatic suitability for most oak species and represent discontinuities in the distribution of Quercus. Niche comparisons suggest niche differentiation among species distributed in different areas of endemism or on opposite sides of turnover points. Conclusions. The results indicate that the Tehuantepec Isthmus, the Nicaraguan Depression, and the Panamanian Isthmus have acted as important barriers to the dispersal of oak species, influencing species diversity, biogeographic patterns, and niche divergence.

Trees and the City: Diversity and Composition along a Neotropical Gradient of Urbanization

In this study we assessed tree species richness, density, and composition patterns along a gradient of urbanization of a megacity. Our results show that total, native, and exotic tree densities were highest in green areas where larger spaces are considered for greening purposes. Conversely, total, native, and exotic tree species richness were highest in land uses with intermediate levels of urban development (residential, residential-commercial areas). Not finding highest tree species richness in less developed urban areas suggests that cultural factors may shape the array of species that are planted within cities. Supporting this, tree composition analyses showed that green areas are comprised of different tree species when compared to the rest of the studied urban land uses. Thus, our results suggest that, to increase the ecological quality of cities, residents and managers should be encouraged to select a greater variety of trees to promote heterogeneous green areas.

Geographic structure of genetic and phenotypic variation in the hybrid zone between Quercus affinis and Q. laurina in Mexico

Analyzing the structure of hybrid zones is important for inferring their origin, dynamics and evolutionary significance. We examined the geographic structure of phenotypic and genetic variation in the contact zone between two Mexican red oaks, Quercus affinis and Q. laurina. A total of 105 individuals from seven populations were sampled along a 600-km latitudinal gradient representing the distribution area of the two species and their contact zone. Individuals were genotyped for nine nuclear and four chloroplast DNA microsatellite loci (ncSSR and cpSSR, respectively), and characterized for several leaf and acorn traits. The cpSSR data revealed extensive haplotype sharing among populations of the two species, while a Bayesian assignment analysis based on ncSSRs identified two main genetic groups, each corresponding to one of the species, and two populations in the contact zone showing evidence of admixture. The proportion of genetic ancestry in the populations was strongly associated with latitude and showed a pattern of variation with the shape of a narrow sigmoidal cline. The variation in three of the seven phenotypic traits was partially congruent with molecular variation, while the other traits did not conform to a geographic cline but instead were correlated with environmental variables. In conclusion, the hybrid zone between the two oak species has some of the characteristics of a tension zone, but heterogeneous variation across traits suggests differential introgression and the action of extrinsic selection.

Complex phylogeographic patterns indicate Central American origin of two widespread Mesoamerican Quercus (Fagaceae) species

The northern Neotropical region is characterized by a heterogeneous geological and climatic history. Recent studies have shown contrasting patterns regarding the role of geographic elements as barriers that could have determined phylogeographic structure in various species. Recently, the phylogeography and biogeography of Quercus species have been studied intensively, and the patterns observed so far suggest contrasting evolutionary histories for Neotropical species in comparison with their Holarctic relatives. The goal of this study was to describe the phylogeographic structure of two Neotropical oak species (Quercus insignis and Quercus sapotifolia) in the context of the geological and palaeoclimatic history of the northern Neotropics. Populations through the distribution range of both species were collected and characterized using nine chloroplast DNA microsatellite loci. Both oak species showed high levels of genetic diversity and strong phylogeographic structure. The distribution of genetic variation in Q. insignis suggested an influence of two major barriers, the Isthmus of Tehuantepec and the Nicaraguan Depression, while Q. sapotifolia exhibited a genetic structure defined by the heterogeneity of the Chortis highlands. The haplotype networks of both species indicated complex histories, suggesting that colonization from the Sierra Madre de Chiapas to central Mexico and from the north of the Nicaraguan Depression to the Costa Rican mountains may have occurred during different stages, and apparently more than one time. In conclusion, the phylogeographic structure of Neotropical oak species seems to be defined by a combination of geological and climatic events.

Contrasting Patterns of Population History and Seed-mediated Gene Flow in Two Endemic Costa Rican Oak Species

Lower Central America is an important area to study recent population history and diversification of Neotropical species due to its complex and dynamic geology and climate. Phylogeographic studies in this region are few in comparison with other regions and even less for tree species. The aim of the present study was to characterize the phylogeographic structure in 2 partially co-distributed endemic oak species (Quercus costaricensis and Q. bumelioides) of the Costa Rican mountains using chloroplast short sequence repeats (cpSSRs), and to test for the effect of geological and palaeoclimatic processes on their population history. Genetic diversity and structure, haplotype networks, patterns of seed-mediated gene flow and historical demography were estimated for both species. Results suggested contrasting patterns. Quercus costaricensis exhibited high values of genetic diversity, a marked phylogeographic structure, a north-to-south genetic diversity gradient and evidence of a demographic expansion during the Quaternary. Quercus bumelioides did not show significant genetic structure and the haplotype network and historical demography estimates suggested a recent population expansion probably during the Pleistocene-Holocene transition. The phylogeographic structure of Q. costaricensis seems to be related to Pleistocene altitudinal migration due to its higher altitudinal distribution. Meanwhile, historical seed-mediated gene flow through the lower altitudinal distribution of Q. bumelioides may have promoted the homogenization of genetic variation. Population expansion and stable availability of suitable climatic areas in both species probably indicate that palaeoclimatic changes promoted downwards altitudinal migration and formation of continuous forests allowing oak species to expand their distribution into the Panamanian mountains during glacial stages.

Landscape genomics provides evidence of climate-associated genetic variation in Mexican populations of Quercus rugosa

Local adaptation is a critical evolutionary process that allows plants to grow better in their local compared to non‐native habitat and results in species‐wide geographic patterns of adaptive genetic variation. For forest tree species with a long generation time, this spatial genetic heterogeneity can shape the ability of trees to respond to rapid climate change. Here, we identify genomic variation that may confer local environmental adaptations and then predict the extent of adaptive mismatch under future climate as a tool for forest restoration or management of the widely distributed high‐elevation oak species Quercus rugosa in Mexico. Using genotyping by sequencing, we identified 5,354 single nucleotide polymorphisms (SNPs) genotyped from 103 individuals across 17 sites in the Trans‐Mexican Volcanic Belt, and, after controlling for neutral genetic structure, we detected 74 FST outlier SNPs and 97 SNPs associated with climate variation. Then, we deployed a nonlinear multivariate model, Gradient Forests, to map turnover in allele frequencies along environmental gradients and predict areas most sensitive to climate change. We found that spatial patterns of genetic variation were most strongly associated with precipitation seasonality and geographic distance. We identified regions of contemporary genetic and climatic similarities and predicted regions where future populations of Q. rugosa might be at risk due to high expected rate of climate change. Our findings provide preliminary details for future management strategies of Q. rugosa in Mexico and also illustrate how a landscape genomic approach can provide a useful tool for conservation and resource management strategies.