Hydraulic traits are not robust predictors of tree species stem growth during a 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 hydraulicly vulnerable xylem and narrower safety margins would
grow less during drought. Q2. How do species successional association
relate to 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 and risk of
hydraulic failure experienced substantial declines during drought but this
pattern was 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.