Survival, growth, and functional traits of tropical wet forest tree seedlings across an experimental soil moisture gradient in Puerto Rico.

Droughts are predicted to become more frequent and intense in many tropical regions, which may cause shifts in plant community composition. Especially in diverse tropical communities, understanding how traits mediate demographic responses to drought can help provide insight into the effects of climate change on these ecosystems. To understand tropical tree responses to reduced soil moisture, we grew seedlings of eight species across an experimental soil moisture gradient at the Luquillo Experimental Forest, Puerto Rico. We quantified survival and growth over an 8-month period and characterized demographic responses in terms of tolerance to low soil moisture-defined as survival and growth rates under low soil moisture conditions-and sensitivity to variation in soil moisture-defined as more pronounced changes in demographic rates across the observed range of soil moisture. We then compared demographic responses with interspecific variation in a suite of 11 (root, stem, and leaf) functional traits, measured on individuals that survived the experiment. Lower soil moisture was associated with reduced survival and growth but traits mediated species-specific responses. Species with relatively conservative traits (e.g., high leaf mass per area), had higher survival at low soil moisture whereas species with more extensive root systems were more sensitive to soil moisture, in that they exhibited more pronounced changes in growth across the experimental soil moisture gradient. Our results suggest that increasing drought will favor species with more conservative traits that confer greater survival in low soil moisture conditions.

A functional perspective on the factors underpinning biomass-bound carbon stocks in coastal macrophyte communities.

Coastal ecosystems have received international interest for their possible role in climate change mitigation, highlighting the importance of being able to assess and predict how changes in habitat distributions and their associated communities may impact the greenhouse gas sink potential of these vegetated seascapes. Importantly, the range and diversity of macrophytes within the vegetated seascape have different capacities to store C within their biomass and potentially sequester C depending on their functional trait characteristics. To bridge the present knowledge gaps in linking macrophyte traits to C storage in tissue, we (1) quantified biomass-bound C stocks within diverse macrophyte communities, separately for soft and hard bottom habitats and (2) explored the links between various traits of both vascular plants and macroalgae and their respective biomass-bound C stocks using structural equation modeling (SEM). We conducted a field survey where we sampled 6 soft bottom locations dominated by aquatic vascular plants and 6 hard bottom locations dominated by the brown algae Fucus vesiculosus in the Finnish archipelago. Macrophyte carbon stocks of hard bottom locations were an order of magnitude higher than those found in soft bottom locations. Biodiversity was associated with aquatic plant carbon stocks through mass ratio effects, highlighting that carbon stocks were positively influenced by the dominance of species with more acquisitive resource strategies, whereas age was the main driver of carbon in the mono-specific macroalgal communities. Overall, our results demonstrate that habitat type and dominating life-history strategies influenced the size of the organism-bound carbon stocks. Moreover, we showed the importance of accounting for the diversity of different traits to determine the drivers underpinning carbon storage in heterogenous seascapes composed of macrophyte communities with high functional diversity.

Water-limited environments affect the association between functional diversity and forest productivity.

The link between biodiversity and ecosystem function can depend on environmental conditions. This contingency can impede our ability to predict how biodiversity-ecosystem function (BEF) relationships will respond to future environmental change, causing a clear need to explore the processes underlying shifts in BEF relationships across large spatial scales and broad environmental gradients. We compiled a dataset on five functional traits (maximum height, wood density, specific leaf area [SLA], seed size, and xylem vulnerability to embolism [P50]), covering 78%-90% of the tree species in the National Forest Inventory from Italy, to test (i) how a water limitation gradient shapes the functional composition and diversity of forests, (ii) how functional composition and diversity of trees relate to forest annual increment via mass ratio and complementarity effects, and (iii) how the relationship between functional diversity and annual increment varies between Mediterranean and temperate climate regions. Functional composition varied with water limitation; tree communities tended to have more conservative traits in sites with higher levels of water limitation. The response of functional diversity differed among traits and climatic regions but among temperate forest plots, we found a consistent increase of functional diversity with water limitation. Tree diversity was positively associated with annual increment of Italian forests through a combination of mass ratio and niche complementarity effects, but the relative importance of these effects depended on the trait and range of climate considered. Specifically, niche complementarity effects were more strongly associated with annual increment in the Mediterranean compared to temperate forests. Synthesis: Overall, our results suggest that biodiversity mediates forest annual increment under water-limited conditions by promoting beneficial interactions between species and complementarity in resource use. Our work highlights the importance of conserving functional diversity for future forest management to maintain forest annual increment under the expected increase in intensity and frequency of drought.