Periodic heat waves are known to induce foliar and stem mortality in temperate forest ecosystems, yet our knowledge of physiological thresholds for damage is lacking. Current land surface models perform poorly in simulating ecological impacts of extreme heat events on forest ecosystems and the potential for new mechanistic representation is constrained by the paucity of data on tree responses during and after such extreme events. In this context, by manipulative heat-wave experiments, we are investigating both instantaneous ecophysiological responses and carryover effects (physiological recovery and phenology) in a range of co-occurring temperate tree species. In 2016, we investigated 13 tree species including evergreen conifers and deciduous angiosperms, initially maintained within controlled-growth chambers where photoperiod, air temperature (Tmax ~30°C), relative humidity and VPD conditions were diurnally regulated to mimic native seasonal summer conditions. The plants were then transferred in batches to an extreme growth chamber and were exposed to progressively increasing severe heat regimes (Tmax+5, +15, +17 and +19°C over a period of 6 days). At each thermal regime, plants were assessed for instantaneous photosynthetic and water use responses including, leaf gas exchange, dark respiration, photosystem-II efficiency, leaf temperature, pigments, whole plant water uptake and leaf relative water content.
During extreme heat wave events, net photosynthesis was substantially reduced in most of species (35 to 88% reduction, P<0.05), however, fewer than one third of the plants illustrated reductions in stomatal conductance. An initial increase (P<0.05) in whole plant water uptake was evident in most of species, however plant water uptake plateaued at higher heat wave events possibly indicating a hydraulic bottleneck that could not fulfill foliar water demand. Instantaneous chlorophyll a fluorescence indicated photosystem-II (PS-II) down-regulation as one of dominant factors for the loss in net photosynthesis, whereas the effect on dark respiration was modest. The dark-adapted fluorescence traits including PS-II quantum efficiency (Fv/Fm) and fast kinetics (OJIP) were significantly affected by the treatment exhibiting loss in photo-regulatory functions and photodamage. Loss in PS-II efficiency was more apparent in species like black cottonwood, yellow poplar, hickories, walnut and conifers, whereas the oaks maintained comparatively better PS-II functioning. Further analysis of photosynthetic pigments, leaf temperature and non-structural carbohydrates are ongoing. The post-treatment physiological recovery will be investigated in 2017 to separate temperature acclimation responses from permanent damage.