Thermophilization of adult and juvenile tree communities in the northern tropical Andes

Abstract

Climate change is expected to cause shifts in the composition of tropical montane forests towards increased relative abundances of species whose ranges were previously centered at lower, hotter elevations. To investigate this process of "thermophilization," we analyzed patterns of compositional change over the last decade using recensus data from a network of 16 adult and juvenile tree plots in the tropical forests of northern Andes Mountains and adjacent lowlands in northwestern Colombia. Analyses show evidence that tree species composition is strongly linked to temperature and that composition is changing directionally through time, potentially in response to climate change and increasing temperatures. Mean rates of thermophilization [thermal migration rate (TMR), °C ⋅ y(-1)] across all censuses were 0.011 °C ⋅ y(-1) (95% confidence interval = 0.002-0.022 °C ⋅ y(-1)) for adult trees and 0.027 °C ⋅ y(-1) (95% confidence interval = 0.009-0.050 °C ⋅ y(-1)) for juvenile trees. The fact that thermophilization is occurring in both the adult and juvenile trees and at rates consistent with concurrent warming supports the hypothesis that the observed compositional changes are part of a long-term process, such as global warming, and are not a response to any single episodic event. The observed changes in composition were driven primarily by patterns of tree mortality, indicating that the changes in composition are mostly via range retractions, rather than range shifts or expansions. These results all indicate that tropical forests are being strongly affected by climate change and suggest that many species will be at elevated risk for extinction as warming continues.

Keywords: climate change; conservation biogeography; dispersal modes; species migrations; thermal niches.

Fig. 1.

Mean CTSs of the 16 study plots versus (A) plot elevation (mm) and (B) plot mean annual temperature (°C). Points at the same elevation or temperature represent the estimated CTS values for adults (diamonds; DBH > 10 cm) and juveniles (triangles; DBH < 10 cm) from the same plot. Lines indicate linear regressions between plot elevation or temperature and CTS estimates for adults (dashed) and juveniles (solid).

Fig. 2.

The estimated community temperature (°C) and elevation (m) score anomalies of plots in the northern Andes of Colombia. (A) Large individuals (adults). (B) Small individuals (juveniles). Anomalies in temperature were calculated as the net difference between a plot’s CTS in the respective census and the plot’s initial CTS. Anomalies in elevation were assessed by assuming an adiabatic lapse rate of 5.5 °C⋅km⁻¹. Positive values represent an increase in basal area of taxa with distributions centered at lower, hotter elevations. Plot numbers correspond to the increasing ordinal position from lowlands to highlands along the elevational range covered by the plots (SI Appendix, Table S1). Red dashed lines with filled symbols indicate CTS anomalies of lowland plots (<1,100 m asl), and blue solid lines with open symbols indicate CTS anomalies of highland plots (>1,700 m asl).

Fig. 3.

Thermal and elevational migration rates (°C⋅y⁻¹ and m⋅y⁻¹, respectively) of plots and TMR resulting from just basal area growth (TMR_grow), just tree basal area recruitment (TMR_rec), and just tree basal area mortality (TMR_mort). (A) Large individuals (adults). (B) Small individuals (juveniles). For both stem size classes, the solid black line corresponds with the overall change in plot temperature and elevation (i.e., the annualized change between the initial and final plot censuses). In both panels, plots are ordered by increasing TMR_adult. Plot numbers correspond to the ordinal elevation of plots from lowlands to highlands (SI Appendix, Table S1).