Drier tropical forests are susceptible to functional changes in response to a long‐term drought

Abstract

Climatic changes have profound effects on the distribution of biodiversity, but untangling the links between climatic change and ecosystem functioning is challenging, particularly in high diversity systems such as tropical forests. Tropical forests may also show different responses to a changing climate, with baseline climatic conditions potentially inducing differences in the strength and timing of responses to droughts. Trait‐based approaches provide an opportunity to link functional composition, ecosystem function and environmental changes. We demonstrate the power of such approaches by presenting a novel analysis of long‐term responses of different tropical forest to climatic changes along a rainfall gradient. We explore how key ecosystem's biogeochemical properties have shifted over time as a consequence of multi‐decadal drying. Notably, we find that drier tropical forests have increased their deciduous species abundance and generally changed more functionally than forests growing in wetter conditions, suggesting an enhanced ability to adapt ecologically to a drying environment.

Long-term droughts may drive drier tropical forests towards increased functional, taxonomic and phylogenetic homogeneity

“…Yuan et al 38 have shown how increases in VPD can reduce vegetation growth as a result of changes in photosynthetic activity, and may also cause faster mortality during drought for tree seedlings 39 , which could thus affect ecosystem functioning. Our results and other recent work analysing functional trait shifts in tropical forests 4 evidence how climate may be acting on the filtering of species in such high water deficit communities, which are already under high climatic pressure and at the edge of their climatic suitability. We show that those communities are in general becoming functionally, taxonomically and phylogenetically more homogeneous than forest communities in areas less restricted by water availability, and thus they may be less resistant 40 to further changes in climatic conditions.…”

“…Some of the main drivers of such biodiversity decline are climate related: altered precipitation and temperature patterns, and extreme weather events 3 . In West Africa, a drying environment over the last decades has been associated with changes in forest composition, leaf phenology and communitylevel functional traits 4,5 , i.e. the intrinsic morphological/physiological characteristics of species.…”

“…Moreover, the question remains as whether tropical forests along climatic gradients show coordinated responses to climate change regarding their functional, taxonomic and phylogenetic facets of diversity. It has been recently shown that the plant traits composition in West African tropical forests has shifted more in drier than wetter forests, arguably as a result of a changing climate 4 . In such drier forests the abundance of deciduous species is increasing, which could be generating forest communities better adapted to a drying climate 5 .…”

Combining Contemporary and Paleoecological Perspectives for Estimating Forest Resilience

“…This would indicate low adaptive capacity to rapid changes in temperature and rainfall regimes, resulting in higher mortality rates for trees more at risk of embolism . Predictions for the tropics are that species dominance will shift to those with drought tolerant traits, with concomitant loss of evergreen species, particularly in sites exhibiting increasing drought duration (Aguirre-Gutiérrez et al, 2019;Bartlett et al, 2019). This transition is likely to entail large-scale die-off of the tallest trees in old-growth forest, resulting in a pulse in global carbon emissions .…”

Functional shifts in natural forests under environmental change over the last 65 years are faster in colder regions

“…If long-term global environmental change favours tree species with traits that are better adapted to new climate realities (while causing higher mortality for species with unfavoured traits), these differences in demographic changes may induce spatially divergent shifts in functional composition. However, exactly how temporal shifts in functional traits are associated with spatial gradients of the baseline climate have rarely been tested, with insights being limited only to a water availability gradient in tropical forests 7 . Yet, we are not cognizant of how the rate and directionality of compositional shifts are dependent on larger scale environmental contexts across regions and biomes, particularly as relates to baseline temperature.…”