Climate change is hypothesized to cause shifts in species’ distributions along environmental gradients, potentially leading to novel species assemblages. Recent studies in the Andes indicate that tropical and subtropical tree communities are experiencing directional shifts in composition towards greater abundances of taxa with warmer mean ranges, a pattern known as thermophilization. However, the degree to which thermophilization reflects increased mortality of cold-adapted species, growth or recruitment of warm-adapted species, or a combination of both mechanisms is unknown. Using 38 years of permanent forest-plot data on the growth, mortality, and recruitment of >66,000 trees and >2,500 tree species along a 3,500 m elevational gradient in Peru and Bolivia, we tested (1) whether patterns of thermophilization differ between two contrasting Neotropical regions: the Andes (high-elevation communities) and the Amazon (low elevation communities), and (2) the relative contributions of tree growth, mortality, and recruitment to thermophilization in the Andes and Amazon.
Results/Conclusions
Thermophilization rates were higher and more variable among forest plots in the Andes compared to the Amazon. For individual forest plots, thermophilization rates ranged from -0.032 to 0.054 °C yr-1 across years. Across all plots, mean thermophilization rates were slightly positive when measured by changes in numbers of individual trees (mean ± 95 % CI = 0.003 °C yr-1 ± -0.0003 to +0.0031 °C yr-1) or changes in tree basal area (0.0001 °C yr-1 ± -0.0031 to +0.0033 °C yr-1). Thermophilization was weak to nonexistent in the Amazon (mean = -0.0002 °C yr-1 to -0.0001 °C yr-1). In contrast, thermophilization in the Andes was generally positive and more variable among forest plots (mean = +0.004 °C yr-1 to +0.0002 °C yr-1). Moreover, thermophilization in the Andes was primarily driven by tree mortality, whereas tree growth and recruitment contributed less. Given this high variability and the fact that observed responses are generally slower than concurrent warming rates, many Andean and Amazonian tree species are likely to be out of equilibrium with climate.