Hydraulic traits are not robust predictors of tree species stem growth during a severe drought in a wet tropical forest

Severe droughts have led to lower plant growth and high mortality in many ecosystems worldwide, including tropical forests. Drought vulnerability differs among species, but there is limited consensus on the nature and degree of this variation in tropical forest communities. Understanding species-level vulnerability to drought requires examination of hydraulic traits since these reflect the different strategies species employ for surviving drought. Here, we examined hydraulic traits and growth reductions during a severe drought for 12 common woody species in a wet tropical forest community in Puerto Rico to ask: Q1. To what extent can hydraulic traits predict growth declines during drought? We expected that species with more hydraulically vulnerable xylem and narrower safety margins (SM_P50) would grow less during drought. Q2. How does species successional association relate to the levels of vulnerability to drought and hydraulic strategies? We predicted that early- and mid-successional species would exhibit more acquisitive strategies, making them more susceptible to drought than shade-tolerant species. Q3. What are the different hydraulic strategies employed by species and are there trade-offs between drought avoidance and drought tolerance? We anticipated that species with greater water storage capacity would have leaves that lose turgor at higher xylem water potential and be less resistant to embolism forming in their xylem (P₅₀). We found a large range of variation in hydraulic traits across species; however, they did not closely capture the magnitude of growth declines during drought. Among larger trees (≥10 cm diameter at breast height—DBH), some tree species with high xylem embolism vulnerability (P₅₀) and risk of hydraulic failure (SM_P50) experienced substantial growth declines during drought, but this pattern was not consistent across species. We found a trade-off among species between drought avoidance (capacitance) and drought tolerating (P₅₀) in this tropical forest community. Hydraulic strategies did not align with successional associations. Instead, some of the more drought-vulnerable species were shade-tolerant dominants in the community, suggesting that a drying climate could lead to shifts in long-term forest composition and function in Puerto Rico and the Caribbean. Read the free Plain Language Summary for this article on the Journal blog.