Stomatal behaviour and stem xylem traits are coordinated for woody plant species under exceptional drought conditions

Abstract

Isohydry (maintenance of plant water potential at the cost of carbon gain) and anisohydry (gas exchange maintenance at the cost of declining plant water status) make up two ends of a stomatal drought response strategy continuum. However, few studies have merged measures of stomatal regulation with xylem hydraulic safety strategies based on in situ field measurements. The goal of this study was to characterize the stomatal and xylem hydraulic safety strategies of woody species in the biodiverse Mediterranean-type ecosystem region of California. Measurements were conducted in situ when California was experiencing the most severe drought conditions in the past 1,200 years. We found coordination among stomatal, hydraulic, and standard leaf functional traits. For example, stem xylem vulnerability to cavitation (P ) was correlated with the water potential at stomatal closure (P ); more resistant species had a more negative water potential at stomatal closure. The degree of isohydry-anisohydry, defined at P -P , was correlated with the hydraulic safety margin across species; more isohydric species had a larger hydraulic safety margin. In addition, we report for the first time P values below -10 MPa. Measuring these traits in a biodiverse region under exceptional drought conditions contributes to our understanding of plant drought responses.

More than iso/anisohydry: Hydroscapes integrate plant water use and drought tolerance traits in 10 eucalypt species from contrasting climates

“…Consequently, all 10 species would be classified as being isohydric according to this definition. Although anisohydric behaviour with negative HSM ST has been reported in some species (Pivovaroff et al, ; Skelton et al, ), there is mounting evidence that stomatal closure occurs prior to the occurrence of embolism in many species (Choat, Brodribb, et al, ; Martin‐StPaul et al, ; Sperry et al, ). Considering that embolism recovery in woody plants may be limited (Choat, Nolf, et al, ; Creek, Blackman, Brodribb, Choat, & Tissue, ; Li, Blackman, Rymer, et al, ), the occurrence of “extreme” anisohydry, where stomata remain open beyond the point where significant cavitation occurs, may be rare.…”

“…Consequently, all 10 species would be classified as being isohydric according to this definition. Although anisohydric behaviour with negative HSM ST has been reported in some species (Pivovaroff et al, 2018;Skelton et al, 2015), there is mounting evidence that stomatal closure occurs prior to the occurrence of embolism in many species (Choat, Brodribb, et al and relative stomatal conductance (closed circle, pink band) responds to water potential in two species (Figure 2a,b) with contrasting Hydroscape area ( Figure 2c,d). Shaded bands represent confidence interval for stem and stomatal response to water potential and denote standard error for leaf response.…”

“…Species with isohydric behaviour adopt a conservative water use strategy, with the stomata closing early during drought, thereby maintaining relatively constant leaf water potential with declining soil water content. Anisohydric species, on the other hand, maintain stomata open despite decreasing leaf water potential as soil water availability declines (Garcia‐Forner et al, ; Meinzer et al, ; Pivovaroff, Cook, & Santiago, ). The revival of this terminology in recent years largely lies in its simplicity, nicely contrasting plant carbon and hydraulic mechanisms of plant response to drought.…”

“…Anisohydric species, on the other hand, maintain stomata open despite decreasing leaf water potential as soil water availability declines (Garcia-Forner et al, 2016;Meinzer et al, 2016;Pivovaroff, Cook, & Santiago, 2018). The revival of this terminology in recent years largely lies in its simplicity, nicely contrasting plant carbon and hydraulic mechanisms of plant response to drought.…”

Hydraulics of woody plants

“…Manrique‐Alba et al () compared stem radial growth between an isohydric and an anisohydric conifer species and found that the different strategies translate into alternative growth and water storage strategies under drought and heat. In another field study, Pivovaroff, Cook, and Santiago () showed that the degree of isohydry/anisohydry was correlated with the size of hydraulic safety margins across species in a Mediteranean‐type ecosystem in California. More isohydric species had larger hydraulic safety margins, implying that maintenance of a more conservative hydraulic strategy occurs at the expense of carbon gain.…”