Genetic diversity of the rain tree (Albizia saman) in Colombian seasonally dry tropical forest for informing conservation and restoration interventions

Abstract

Albizia saman is a multipurpose tree species of seasonally dry tropical forests (SDTFs) of Mesoamerica and northern South America typically cultivated in silvopastoral and other agroforestry systems around the world, a trend that is bound to increase in light of multimillion hectare commitments for forest and landscape restoration. The effective conservation and sustainable use of A. saman requires detailed knowledge of its genetic diversity across its native distribution range of which surprisingly little is known to date. We assessed the genetic diversity and structure of A.saman across twelve representative locations of SDTF in Colombia, and how they may have been shaped by past climatic changes and human influence. We found four different genetic groups which may be the result of differentiation due to isolation of populations in preglacial times. The current distribution and mixture of genetic groups across STDF fragments we observed might be the result of range expansion of SDTFs during the last glacial period followed by range contraction during the Holocene and human-influenced movement of germplasm associated with cattle ranching. Despite the fragmented state of the presumed natural A. saman stands we sampled, we did not find any signs of inbreeding, suggesting that gene flow is not jeopardized in humanized landscapes. However, further research is needed to assess potential deleterious effects of fragmentation on progeny. Climate change is not expected to seriously threaten the in situ persistence of A. saman populations and might present opportunities for future range expansion. However, the sourcing of germplasm for tree planting activities needs to be aligned with the genetic affinity of reference populations across the distribution of Colombian SDTFs. We identify priority source populations for in situ conservation based on their high genetic diversity, lack or limited signs of admixture, and/or genetic uniqueness.

Keywords: agroforestry; climate change; microsatellites; paleodistribution; seed zones; suitability modeling.

Figure 1

Typical use of Albizia saman as shade and fodder tree in pasture land in Colombia

Figure 2

Global distribution of Albizia saman (red dots). Its native area is believed to be restricted to the region from southern Mexico to Colombia and Venezuela, but it has been introduced to tropical areas all around the world. The countries that are believed to be part of the native range of the species are shown in green, and the locations of the trees sampled in the current study are shown as yellow dots

Figure 3

Summary of the modeling procedure used for identifying suitable habitat for Albizia saman in Colombian seasonally dry tropical forests during different past present and future climate scenarios

Figure 4

Spatial variation of different genetic parameters of Albizia saman against a background of the species' modeled habitat suitability under current climate conditions (dark green), and the historical distribution of seasonally dry tropical forest in Colombia (light green): (a) allelic richness without sample bias correction; (b) allelic richness with bootstrap correction; (c) Shannon index with bootstrap correction; (d) expected heterozygosity with bootstrap correction; (e) observed heterozygosity with bootstrap correction; and (f) inbreeding coefficient with bootstrap correction

Figure 5

Distribution of genetic groups across sampled populations of Albizia saman against a background of the species' modeled habitat suitability under current climate conditions (dark green), and the historical distribution of seasonally dry tropical forest in Colombia (light green). Circles sizes are proportionate to the number of individuals

Figure 6

Modeled distribution of suitable habitat of A. saman during (a) the Last Glacial Maximum (LGM; ~21,000 BP) and (b) the mid‐Holocene (~6,000 BP), in combination with richness of locally common alleles (LCA)

Figure 7

Future (2050s) habitat suitability of A. saman showing all areas identified as suitable by at least 1 (a) and 15 (b) of 31 future climate model projections in combination with the distribution of allelic richness