Shifts in plant functional groups along an aridity gradient in a tropical dry forest.

Increasing aridity associated with climate change may lead to the crossing of critical ecosystem thresholds in drylands, compromising ecosystem services for millions of people. In this context, finding tools to detect at early stages the effects of increasing aridity on ecosystems is extremely urgent to avoid irreversible damage. Here, we assess shifts in plant community functional structure along a spatial aridity gradient in tropical dryland (Brazilian Caatinga), to select the most appropriate plant functional groups as ecological indicators likely useful to predict temporal ecosystem trajectories in response to aridity. We identified seven plant functional groups based on 13 functional traits associated with plant establishment, defense, regeneration, and dispersal, whose relative abundances changed, linearly and non-linearly, with increasing aridity, showing either increasing or decreasing trends. Of particular importance is the increase in abundance of plants with high chemical defense and Crassulacean Acid Metabolism (CAM) photosynthetic pathway, with increasing aridity. We propose the use of these functional groups as early warning indicators to detect aridity impacts on these dryland ecosystems and shifts in ecosystem functioning. This information can also be used in the elaboration of mitigation and ecological restoration measures to prevent and revert current and future climate change impacts on tropical dry forests.

How Do Taxonomic and Functional Diversity Metrics Change Along an Aridity Gradient in a Tropical Dry Forest?

Ecological indicators based on biodiversity metrics are valuable and cost-effective tools to quantify, track and understand the effects of climate change on ecosystems. Studying changes in these indicators along climatic gradients in space is a common approach to infer about potential impacts of climate change over time, overcoming the limitations of lack of sufficiently long time-series data. Here, we studied the response of complementary biodiversity metrics in plants: taxonomic diversity (species richness and Simpson index) and functional diversity (diversity and redundancy) in 113 sampling sites along a spatial aridity gradient (from 0.27 to 0.69 of aridity index-AI) of 700 km in a Tropical dry forest. We found different responses of taxonomic and functional diversity metrics to aridity. Species diversity showed a hump-shaped curve peaking at intermediate levels of aridity between 0.38 and 0.52 AI as an ecotone, probably because it is where most species, from both drier and more mesic environments, still find conditions to co-exist. Functional diversity showed a positive linear relation with increasing aridity, suggesting higher aridity favors drought-adapted species with diverse functional traits. In contrast, redundancy showed a negative linear relation with increasing aridity, indicating that drier sites have few species sharing the same functional traits and resource acquisition strategies. Thus, despite the increase in functional diversity toward drier sites, these communities are less resilient since they are composed of a small number of plant species with unique functions, increasing the chances that the loss of one of such "key species" could lead to the loss of key ecosystem functions. These findings show that the integration of complementary taxonomic and functional diversity metrics, beyond the individual response of each one, is essential for reliably tracking the impacts of climate change on ecosystems. This work also provides support to the use of these biodiversity metrics as ecological indicators of the potential impact of climate change on drylands over time.

The response of plant functional traits to aridity in a tropical dry forest.

Drylands are experiencing an overall increase in aridity that is predicted to intensify in the future due to climate change. This may cause changes in the structure and functioning of dryland ecosystems, affecting ecosystem services and human well-being. Therefore, detecting early signs of ecosystem change before irreversible damage takes place is important. Thus, here we used a space-for-time substitution approach to study the response of the plant community to aridity in a Tropical dry forest (Caatinga, Brazil), and infer potential consequences of climate change. We assessed plant functional structure using the community weighted mean (CWM) and functional diversity, measured through functional dispersion (FDis), along a 700 km climatic gradient. We studied 13 functional traits, reflecting strategies associated with establishment, defense, regeneration, and dispersal of the most abundant 48 plant species in 113 sampling sites. Spearman correlations were used to test the relation between aridity and single-trait functional metrics. Aridity was a major environmental filter of the plant community functional structure. We found a higher abundance of species with deciduous leaves, zoochorous dispersal, fleshy fruits, chemical defense exudation and spinescence, and crassulacean acid metabolism towards more arid sites, at the expense of species with evergreen and thicker leaves, autochory dispersal, and shrub growth-form. The FDis of leaf type and thickness decreased with aridity, whereas FDis of fruit type, photosynthetic pathway, and defense strategies increased. Our findings provide functional indicators to early detect climate change impacts on Caatinga structure and functioning, to timely adopt preventive measures (e.g. conservation of forest remnants) and restoration actions (e.g. introduction of species with specific functional traits) in this threatened and unique ecosystem.