Disentangling the environmental heterogeneity, floristic distinctiveness and current threats of tropical dry forests in Colombia

Abstract

Tropical dry forests (TDFs) have been defined as a single biome occurring mostly in the lowlands where there is a marked period of drought during the year. In the Neotropics, dry forests occur across contrasting biogeographical regions that contain high beta diversity and endemism, but also strong anthropogenic pressures that threaten their biodiversity and ecological integrity. In Colombia, TDFs occur across six regions with contrasting soils, climate, and anthropogenic pressures, therefore being ideal for studying how these variables relate to dry forest species composition, successional stage and conservation status. Here, we explore the variation in climate and soil conditions, floristic composition, forest fragment size and shape, successional stage and anthropogenic pressures in 571 dry forest fragments across Colombia. We found that TDFs should not be classified solely on rainfall seasonality, as high variation in precipitation and temperature were correlated with soil characteristics. In fact, based on environmental factors and floristic composition, the dry forests of Colombia are clustered in three distinctive groups, with high species turnover across and within regions, as reported for other TDF regions of the Neotropics. Widely distributed TDF species were found to be generalists favored by forest disturbance and the early successional stages of dry forests. On the other hand, TDF fragments were not only small in size, but highly irregular in shape in all regions, and comprising mostly early and intermediate successional stages, with very little mature forest left at the national level. At all sites, we detected at least seven anthropogenic disturbances with agriculture, cattle ranching and human infrastructure being the most pressing disturbances throughout the country. Thus, although environmental factors and floristic composition of dry forests vary across regions at the national level, dry forests are equally threatened by deforestation, degradation and anthropogenic pressures all over the country, making TDFs a top priority for conservation in Colombia.

Climate severity and land‐cover transformation determine plant community attributes in Colombian dry forests

“…In particular, water deficit, the main filter shaping TDF species diversity and structure (Neves, Dexter, Pennington, Bueno & Oliveira‐Filho, ; Pennington et al., ), may result from the combination of different climatic variables, and not just from rainfall regimes (González‐M. et al., ). For example, at the Colorados site, water deficit was driven by high solar radiation, despite a high total annual rainfall compared to other studied sites.…”

“…Isolation increases dispersal limitation and is likely to be accompanied by local extinctions, thereby reducing diversity Hooke, Martín-Duque & Pedraza, 2012;Pimm, 1998). In TDF plots, 0 D, 1 D, and BA were strongly negatively related to land-cover transformation, suggesting that agriculture, cattle ranching and the historic exploitation of hardwood species, or large trees, for fuelwood may have affected these forest attributes (Blackie et al, 2014;González-M. et al, 2018). Second, land-cover transformation may strengthen climate severity due to edge effects, which change microclimatic conditions within forest patches (Pimm, 1998).…”

“…Only factors that effectively impact dry forests should be considered constituents of this concept. Because TDF occur along a wide range of abiotic conditions (González-M. et al, 2018), evaluating the relative importance of these three key environmental factors on plant community assembly would provide critical insights into how harshness shapes TDF. For instance, at the community level, harder conditions would result in lower plant diversity and in changes of forest structure, moving towards characteristics that allow plants to survive under stressful environmental conditions.…”

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

“…On the other hand, a dispersal route across the Andes, connecting the Cauca and Magdalena river valleys might also have existed at one point in prehistory. Cross‐Andean dispersal has been documented previously for SDTF species (Quintana, Pennington, Ulloa, & Balslev, ), and the Colombian interAndean valleys are floristically more similar than other SDTF regions (González‐M et al, ). Bocanegra‐González et al () reported on a similar genetic pattern for Ceiba pentandra as we found for A. saman alluding to the possibility of cross‐Andean dispersal, especially because it is less likely that the trees they sampled there established through human intervention.…”

“…Today, STDF is one of the most threatened ecosystems worldwide (Janzen,1988;Miles et al, 2006). In Colombia, less than 8% of the original STDF cover remains, occurring in a highly fragmented state González-M et al, 2018). Particularly in predominantly outcrossing species such as A. saman, fragmentation of populations is known to negatively affect the reproduction, gene flow, and genetic diversity of tree populations, resulting in increased risk of inbreeding depression in progeny and loss of genetic diversity and fitness due to low numbers of mating partners and low pollen diversity (Aguilar, Ashworth, Galetto, & Aizen, 2006;Aguilar, Quesada, Ashworth, Herrerias-diego, & Lobo, 2008;Lowe, Boshier, Ward, Bacles, & Navarro, 2005).…”