Predicting plant vulnerability to drought in biodiverse regions using functional traits

Abstract

Attempts to understand mechanisms underlying plant mortality during drought have led to the emergence of a hydraulic framework describing distinct hydraulic strategies among coexisting species. This framework distinguishes species that rapidly decrease stomatal conductance (g s ), thereby maintaining high water potential (P x ; isohydric), from those species that maintain relatively high g s at low P x , thereby maintaining carbon assimilation, albeit at the cost of loss of hydraulic conductivity (anisohydric). This framework is yet to be tested in biodiverse communities, potentially due to a lack of standardized reference values upon which hydraulic strategies can be defined. We developed a system of quantifying hydraulic strategy using indices from vulnerability curves and stomatal dehydration response curves and tested it in a speciose community from South Africa's Cape Floristic Region. Degree of stomatal regulation over cavitation was defined as the margin between P x at stomatal closure (P g12 ) and P x at 50% loss of conductivity. To assess relationships between hydraulic strategy and mortality mechanisms, we developed proxies for carbon limitation and hydraulic failure using time since P g12 and loss of conductivity at minimum seasonal P x , respectively. Our approach captured continuous variation along an isohydry/anisohydry axis and showed that this variation was linearly related to xylem safety margin. Degree of isohydry/anisohydry was associated with contrasting predictions for mortality during drought. Merging stomatal regulation strategies that represent an index of water use behavior with xylem vulnerability facilitates a more comprehensive framework with which to characterize plant response to drought, thus opening up an avenue for predicting the response of diverse communities to future droughts. drought | functional traits | plant hydraulics | xylem vulnerability | stomatal response

The correlations and sequence of plant stomatal, hydraulic, and wilting responses to drought

“…The statistical independence of g S Ψ 50 and Ψ 95 and K leaf Ψ 50 is consistent with previous studies, showing wide species variation in the safety margins between stomatal closure and leaf hydraulic dysfunction (32), wherein species vary between "isohydry," which maintains high Ψ leaf and K leaf via early stomatal closure, and "anisohydry," which maintains gas exchange to low Ψ leaf at the expense of hydraulic function. The correlation between the stomatal traits and K stem Ψ 50 and Ψ 88 corroborates a previous metaanalysis of species from ecosystems worldwide (6), but contradicts two studies within specific ecosystems (10,33). Thus, the coordination of stomatal sensitivity with stem vulnerability across species appears to be related to their independent roles in drought tolerance rather than to coordinated function, with stomatal responses affecting carbon uptake during mild and moderate drought, and vulnerability affecting the ability of stems to survive strong drought (2, 15).…”

No growth stimulation of Canada’s boreal forest under half-century of combined warming and CO 2 fertilization

“…5), including the most abundant boreal trees such as Picea mariana, Pinus spp., and Populus tremuloides. Decreasing soil moisture leads to reductions in photosynthesis through stomatal closure, whereas extreme drought conditions lead to an increased risk of xylem cavitation (27)(28)(29)(30). In such circumstances, structural growth is killed for carbon investment in maintenance respiration and osmoprotection.…”

Are Northeastern U.S. forests vulnerable to extreme drought?

“…Isohydric species regulated water potential via stomatal closure (or reduced stomatal aperture) above a critical threshold while anisohydric species allow water potential to decline as the soil dries (Loewenstein and Pallardy 1998;Tardieu and Simonneau 1998), although the critical threshold was often ambiguous. More recently, studies have worked towards a quantitative description of the isohydric-anisohydric framework that required deriving parameters from empirical relationships of leaf water potential, stomatal conductance, and/or hydraulic conductance (Table 2, MartinezVilalta et al 2014;Klein 2014;Roman et al 2015;Skelton et al 2015;Garcia-Forner et al 2016;Meinzer et al 2016). An important finding of many of these studies is that species exhibit a continuum of isohydric-anisohydric behavior rather than a dichotomy.…”