Multi-decadal trends of low clouds at the Tropical Montane Cloud Forests

Abstract

Clouds are critical to the biodiversity and function of Tropical Montane Cloud Forests (TMCF) as they control water regimes and sunlight that can be perceived by plants. These ecosystems provide a key role in ecosystem services to humanity and are considered hotspots of endemism, given that the number of species is restricted to their microclimates. The cloudiness in these ecosystems is projected to decline owing to global warming, but recent temporal trends remain unclear. Here, we evaluated trends in low-cloud fractions (CF) (e.g., the proportion of an area covered by low-cloud) and other Essential Climatic Variables (ECV) (e.g., surface temperature, pressure, soil moisture, and precipitation) for 521 sites worldwide with TMFCs from 1997 to 2020. We hypothesize that recent traces of global warming over the last few decades have led to decreases in CF on TMCFs. The previous was also assessed globally and among biogeographic realms to identify regional trends. We calculated trends by aggregating hourly observations from ERA5 reanalysis and CHIRPS into annual averages and then using linear regressions to calculate slopes (i.e., rate of change) $(\Delta, year{^−1}). Our results suggest that CF trends at the TMCFs range between $−64.7×10{^−4} \space and \space 51.4×10{^−4} \space CF \space year{^−1}$, revealing that 70 % of the assessed sites have experienced reductions in CF. Declines in low-clouds in these ecosystems are 253 % more severe than tropical landmasses when peak values of density distribution are compared $(TMCFs: −7.8×{10^−4}$ CF year−1; tropical landmasses $−2.3×10{^−4} \space CF \space year{^−1})$. Despite this, CF trends differ among biogeographic realms, as those TMCFs from the Neotropics and Indomalayan realms have the most pronounced declines. Decreases in CF were also associated with increases in surface temperature and pressure and decreases in soil moisture, revealing that the TMCF's climate is changing to warmer environments. These climatic shifts may represent imprints of global change on TMCFs, highlighting a current threat to species and essential ecosystem services that these ecosystems provide.