Trees tolerate an extreme heatwave via sustained transpirational cooling and increased leaf thermal tolerance

Abstract

Heatwaves are likely to increase in frequency and intensity with climate change, which may impair tree function and forest C uptake. However, we have little information regarding the impact of extreme heatwaves on the physiological performance of large trees in the field. Here, we grew Eucalyptus parramattensis trees for 1 year with experimental warming (+3°C) in a field setting, until they were greater than 6 m tall. We withheld irrigation for 1 month to dry the surface soils and then implemented an extreme heatwave treatment of 4 consecutive days with air temperatures exceeding 43°C, while monitoring whole-canopy exchange of CO and H O, leaf temperatures, leaf thermal tolerance, and leaf and branch hydraulic status. The heatwave reduced midday canopy photosynthesis to near zero but transpiration persisted, maintaining canopy cooling. A standard photosynthetic model was unable to capture the observed decoupling between photosynthesis and transpiration at high temperatures, suggesting that climate models may underestimate a moderating feedback of vegetation on heatwave intensity. The heatwave also triggered a rapid increase in leaf thermal tolerance, such that leaf temperatures observed during the heatwave were maintained within the thermal limits of leaf function. All responses were equivalent for trees with a prior history of ambient and warmed (+3°C) temperatures, indicating that climate warming conferred no added tolerance of heatwaves expected in the future. This coordinated physiological response utilizing latent cooling and adjustment of thermal thresholds has implications for tree tolerance of future climate extremes as well as model predictions of future heatwave intensity at landscape and global scales.

Climate warming and tree carbon use efficiency in a whole‐tree 13CO2 tracer study

“…All chambers tracked ambient CO 2 concentration and were irrigated equally every 2 wk at half of the mean monthly rainfall measured over the past 30 yr. This particular warming experiment has been described previously in detail (Drake et al ., ) and is similar to previous warming experiments at this site with different tree species (Aspinwall et al ., ; Drake et al ., ; Crous et al ., ).…”

“…a). Each WTC measured the net exchange of CO 2 and H 2 O for an entire tree crown at 15‐min resolution while controlling air temperature ( T air ), relative humidity (RH), and atmospheric CO 2 concentration in the crown air space (Barton et al ., ; Drake et al ., , ). A vertical root‐exclusion barrier extended belowground for c .…”

Woody tissue photosynthesis reduces stem CO2 efflux by half and remains unaffected by drought stress in young Populus tremula trees

“…Higher E A in DRY and DRY LE trees compared with WET trees could be explained by enhanced maintenance respiration, as it completely determines respiratory costs in the absence of growth according to the growth and maintenance respiration paradigm (Amthor, ). Increased maintenance respiration in drought‐stressed trees might be partly caused by their higher stem temperature compared with WET trees, in which continued sap flow reduced stem temperature (Figure S2) via transpirational cooling (Drake et al, ). In any case, although lowered stem water status and cell turgor commonly downregulate maintenance metabolism, respiratory behaviour in mature nongrowing organs has been found to be variable under water shortage (Atkin & Macherel, ; reviewed by Flexas, Bota, Galmés, Medrano, & Ribas‐Carbó, ).…”

On the minimum leaf conductance: its role in models of plant water use, and ecological and environmental controls

“…). This decrease in g min is consistent with the drought response because, again, g min is reduced in leaves that are subjected to conditions that increase evaporative demand (VPD was higher in the elevated temperature treatment, see Drake et al ., ). The direction of this response is consistent with Duarte et al .…”

“…Minimum conductance ( g min ) measured with mass loss of detached leaves on Eucalyptus parramattensis grown in whole‐tree chambers. Trees were grown following ambient conditions or in an elevated temperature treatment (+ 3°C) (see Drake et al ., ). Measurements of g min were carried out at five different temperatures on replicate leaves.…”