Compositional shifts in Costa Rican forests due to climate‐driven species migrations

Abstract

Species are predicted to shift their distributions upslope or poleward in response to global warming. This prediction is supported by a growing number of studies documenting species migrations in temperate systems but remains poorly tested for tropical species, and especially for tropical plant species. We analyzed changes in tree species composition in a network of 10 annually censused 1-ha plots spanning an altitudinal gradient of 70-2800 m elevation in Costa Rica. Specifically, we combined plot data with herbarium records (accessed through GBIF) to test if the plots' community temperature scores (CTS, average thermal mean of constituent species weighted by basal area) have increased over the past decade as is predicted by climate-driven species migrations. In addition, we quantified the contributions of stem growth, recruitment, and mortality to the observed patterns. Supporting our a priori hypothesis of upward species migrations, we found that there have been consistent directional shifts in the composition of the plots, such that the relative abundance of lowland species, and hence CTS, increased in 90% of plots. The rate of the observed compositional shifts corresponds to a mean thermal migration rate (TMR) of 0.0065 °C yr(-1) (95% CI = 0.0005-0.0132 °C yr(-1) ). While the overall TMR is slower than predicted based on concurrent regional warming of 0.0167 °C yr(-1) , migrations were on pace with warming in 4 of the 10 plots. The observed shifts in composition were driven primarily by mortality events (i.e., the disproportionate death of highland vs. lowland species), suggesting that individuals of many tropical tree species will not be able to tolerate future warming and thus their persistence in the face of climate change will depend on successful migrations. Unfortunately, in Costa Rica and elsewhere, land area inevitably decreases at higher elevations; hence, even species that are able to migrate successfully will face heightened risks of extinction.

Strong upslope shifts in Chimborazo's vegetation over two centuries since Humboldt

“…The substantial upward changes in the overall limit to plant growth, distribution of major vegetation zones, and upper boundaries of many species ranges show that plants are strongly responding to global warming, even within the tropics (7,8,16). Comparison of Humboldt's pioneering Chimborazo vegetation survey with our resurvey thus corroborates evidence for the sensitivity of tropical montane vegetation to climate change from modeling (12) and paleoecological studies (13), highlighting that dramatic shifts can occur at relatively short, contemporary time scales.…”

“…However, most studies stem from high-latitude areas and are generally restricted to dynamics across the past few decades (10). To our knowledge, only three previous resurveys have studied range shifts of tropical plant taxa, all at <4,000 m in elevation (7,8,11). Modeling (12) and paleoecological studies (13) suggest that tropical montane vegetation should be highly sensitive to climate change.…”

“…Andes | climate change | land use change | range shifts | tropical biodiversity T he biological impacts of ongoing climate change (1) are already apparent in species' poleward and upslope range shifts and earlier spring events (2)(3)(4)(5)(6)(7)(8)(9). However, most studies stem from high-latitude areas and are generally restricted to dynamics across the past few decades (10).…”

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

“…Studies from tropical forests are extremely sparse (1,5), but the available evidence suggests that many tropical plant species are "migrating" upward (6)(7)(8). Accordingly, the composition of tropical montane forests is changing toward increased relative abundances of species whose ranges were previously centered at lower, hotter elevations (6,8).…”

“…Accordingly, the composition of tropical montane forests is changing toward increased relative abundances of species whose ranges were previously centered at lower, hotter elevations (6,8). However, this process of "thermophilization" (9) is generally occurring at velocities slower than concurrent regional temperature increases (6,8). Furthermore, in at least one tropical site (Volcan Barva, Costa Rica), the observed shifts in tree species composition were mostly a result of increased mortality of species with ranges centered at higher, cooler elevations (8), indicating that species migrations and associated compositional shifts are occurring mostly via range retractions (1).…”