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

Abstract

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 (SMP50) 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 (SMP50) 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.

The interplay of drought and hurricanes on tree recovery: insights from dynamic and weak functional responses

“…For instance, while wood density has been found to predict tree performance and damage after hurricanes Hugo (1989) and Georges (1998) in the studied forest, wood density was not a predictor of tree damage or survival after hurricanes in 2017 [60]. Additionally, while hydraulic functional traits have been found to predict tree demography in other forests [92,93], hydraulic and morphological traits were poor predictors of tree growth during ENSO 2015 in this same forest [94,95], which agrees with the trends found in this study. Furthermore, factors like neighbourhood effects, the extent of damage of neighbouring trees, topographic position and exposure to disturbance, among others, could also have played a role in weakening the relationships between traits and growth [14,96].…”

Hydraulic variability of tropical forests is largely independent of water availability

“…For example, studies conducted at two sites with contrasting environmental conditions in Puerto Rico found large site-level variation in P 50 and HSMs. The first site which is largely aseasonal with high rainfall and low water deficit (MAP = 3086 mm year −1 , CWD = 202, MCWD = 0, SI = 0.04), had a range in P 50 values of −4.40 to −0.86 MPa and HSMs of −0.44 to 2.60 MPa (Smith-Martin et al, 2023;Smith-Martin, Muscarella, et al, 2022). The second site in a seasonal tropical dry forest (MAP = 1265, CWD = 432, MCWD = 267, SI = 0.18), had P 50 values between −10.98 to −1.25 MPa and HSMs between 0.13 to 6.81 MPa (Vargas et al, 2021), showing that although some species in this dry forest had higher drought-tolerant traits than those found in the much wetter Puerto Rican forest, there were also species with similar levels of drought tolerance traits despite the large difference in water availability between the sites.…”

“…drought tolerance and drought avoidance). Studies have found that in some wet environments, there is also a range of drought tolerance among species based on their hydraulic traits (Oliveira et al, 2019;Santiago et al, 2018;Smith-Martin et al, 2023;Smith-Martin, Jansen, et al, 2022;Smith-Martin, Muscarella, et al, 2022;Ziegler et al, 2019). This variation in drought tolerance across species in these wet environments, where there is less environmental pressure to withstand aridity, and where we would expect all species would be uniformly drought vulnerable, could be the product of historic climatic conditions, the biogeographical origin of the species, or reflect other past or present environmental factors (Baker et al, 2020;Blackman et al, 2012;Dick & Pennington, 2019).…”

“…However, this study only included 59 tropical tree species from mostly wet environments (>2400 mm mean annual rainfall) as data from tropical forests were limited at the time. Several recent studies have observed a wide range in P 50 values within wet (Barros et al, 2019;Fontes et al, 2020;Oliveira et al, 2019;Santiago et al, 2018;Smith-Martin et al, 2023;Smith-Martin, Jansen, et al, 2022;Smith-Martin, Muscarella, et al, 2022;Ziegler et al, 2019) and dry (Barros et al, 2019;Powers et al, 2020;Smith-Martin, Jansen, et al, 2022;Vargas et al, 2021) tropical forest sites and a growing number of studies have found that high HSMs (>1 MPa) are common within wet and dry tropical forest sites (Barros et al, 2019;Martin-StPaul et al, 2017;Powers et al, 2020;Smith-Martin et al, 2023;Smith-Martin, Jansen, et al, 2022;Smith-Martin, Muscarella, et al, 2022;Vargas et al, 2021;Ziegler et al, 2019).…”

Upscaling the effect of traits in response to drought: The relative importance of safety–efficiency and acquisitive–conservation functional axes

“…However, further research should evaluate the generality for these findings for other tropical dry forests by examining, for example, if the role of the conservative-acquisitive axis is always dominant over the safety-efficiency one. To our knowledge, in two tropical dry forests in the Neotropics, hydraulic traits showed mixed effects on tree mortality (Powers et al, 2020;Smith-Martin et al, 2022), leaving open the question of whether these traits can be directly scaled up to the community-level in these communities.…”