Genetic diversity of Astronium graveolens Jacq. in Colombian seasonally dry tropical forest: support for the dry forest refugia hypothesis?

Seasonally Dry Tropical Forest (SDTF) is one of the most threatened ecosystems worldwide and in Colombia it is considered critically endangered according to the national red list of ecosystems. Ongoing processes of fragmentation and degradation of SDTFs are expected to be accompanied by a loss of genetic diversity of the tree populations contained in it. Identifying remaining healthy tree populations is key for the implementation of conservation and restoration programs that embrace genetic diversity to enhance the capacity of populations to adapt to environmental changes. We studied the genetic diversity and structure of Astronium graveolens, one of the most ubiquitous species in Colombian SDTFs, across ten representative populations. We assess the relevance of the dry forest refugia hypothesis (DFRH) for explaining the observed genetic structures. We identified three genetic clusters which may have originated through prolonged isolation of populations. The most diverse cluster ranges from the Caribbean region to the Chicamocha river canyon which may reflect their connectedness during the last glacial period. A second cluster likely originated through isolation in a small area enclosed in the Caribbean region. The third cluster groups populations from the Cauca and Magdalena river valleys, suggesting cross-Andean expansion of populations at one point in time, thus corroborating similar findings for other species of SDTFs. Our results add to a growing number of studies providing support for the DFRH. All the populations sampled showed heterozygosity scores close to the Hardy-Weinberg expectations, suggesting populations might be relatively resistant to habitat fragmentation. However, our suitability modelling results suggest that climate change might imperil numerous areas in SDTFs where the species occurs today, including some that hold unique genetic diversity.