Diversity patterns, environmental drivers and changes in vegetation composition in dry inter-Andean valleys

Aims We studied diversity, patterns of endemism and turnover of vegetation composition in dry inter-Andean valleys (DIAVs) where little is known about the influence of the abiotic drivers controlling plant species composition and occurrences, and the life forms that contribute most to α- and β-diversity, respectively. This study was focused on DIAVs located in the highlands (800–2800 m) around the Equator (1°N–5°S). The following questions were addressed: (i) what differences exist between endemic and non-endemic species in terms of species number, frequency and abundance? (ii) are patterns of α- and β-diversity correlated with latitude? (iii) what are the major environmental drivers controlling spatial patterns in species composition and occurrence? Methods We established 63 transects of 5 × 100 m in areas with DIAV vegetation, impacted as little as possible by human activities. In each transect, all mature trees and shrubs were identified and counted to determine their density. The coverage of terrestrial herbs was estimated. Generalized additive models were used to quantify the relationship between α- and β-diversity with latitude. To record α-diversity, we used the exponential Shannon index. The Sørensen index was used to measure β-diversity or species turnover. We used canonical correspondence analysis to determine species composition and generalized linear mixed models to quantify simultaneously the determinants of species occurrence across species and sites. The models were evaluated using the Akaike information criterion. All analyses were run separately for trees, herbs and shrubs. Important Findings Overall, we found that shrubs was the most diverse life form, constituting 43% (12% endemic) of the 313 species, followed by herbs (36%; 11.5% endemic) and trees (20%; 5% endemic). Trees comprise mostly species with low abundance (e.g. <3 individuals ha⁻¹), which was also true for herbs, whereas shrubs showed a larger number of abundant species (>30 individuals ha⁻¹), including four endemic species. Most of the endemic species were locally rare, and most of them were restricted to southern valleys, suggesting low migration rates due to local barriers. We identified seven species distribution patterns that gave rise to high β-diversity in the dry inter-Andean valleys. The β-diversity of trees was the lowest. Herbs had the highest β-diversity, which increased steadily with geographic distance. Our results further highlight the influence of disturbance, water availability and low temperature on plant species composition and occurrence. We also found significant, contrasting patterns in responses to environmental drivers, when analyzing our data separately by life form. Our results show that analyzing diversity patterns separately by life form can shed new light on the mechanisms that have generated present-day patterns of plant diversity in DIAVs.