Does disjunct mean differentiated? Variability analysis of disjunct populations of the neotropical tree Anadenanthera colubrina var. cebil in phenotypic traits

Abstract

Anadenanthera colubrina var. cebil is the paradigmatic tree species of the Seasonally Dry Tropical Forest (SDTF) of central South America. The southernmost part of its range is divided into two disjunct nuclei: Misiones (Mi) and the Sub-Andean Piedmont (SAP), separated by some 800 km and with contrasting environmental conditions. This disjunct distribution provides a suitable experimental scenario to evaluate possible adaptive changes. We sampled 262 individuals of the species from 26 natural populations from both nuclei and two rare ones from the Humid Chaco. Fresh leaves and ripe fruits were collected from each tree, on which 13 foliar and seven reproductive morphological traits were measured. Differences between regions and variability between and within populations were analysed using a linear mixed factorial model. To determine the possible relationship between phenotypic variability of morphological traits and ecological environmental variables, a redundancy analysis (RDA) was performed considering climate data from the CHELSA database, the enhanced vegetation index, and main soil depth. The three regions differed in half of the traits studied, with the greatest contrast between SAP and Mi, while the Humid Chaco was most similar to the latter. Variability between and within populations was found in almost all traits studied, indicating possible adaptive effects in the past and the potential occurrence of adaptations in the future. The ‘tree’ factor showed greater variability than the ‘population’ factor for almost all traits and even explained more of the variance than the residuals of the model. The RDA showed a clear separation between SAP and Mi populations and identified isothermality, mean temperature of the driest quarter, seasonality of precipitation, and precipitation of the warmest quarter as associated with the variability of the traits considered. These results are evidence of adaptation and also of a common gene pool due to a shared evolutionary history of both SDTF nuclei. Genetic studies using molecular markers and common garden trials should be carried out to gain more reliable conclusions.