Differential ecophysiological responses and resilience to heat wave events in four co-occurring temperate tree species

Extreme summer heat waves are known to induce foliar and stem mortality in temperate forest ecosystems, yet our mechanistic knowledge of physiological thresholds for damage is lacking. Current spatiotemporal simulations of forest growth responses to climate change fail to explain the variability between co-occurring tree species to climate extremes, indicating a need for new model frameworks that include mechanistic understanding of trait-specific responses. In this context, using manipulative heat wave (hw) experiments we investigated ecophysiological responses and physiological recovery in four co-occurring temperate tree species of the southeastern United States including three deciduous angiosperms: southern red oak (Quercus falcata Michx.), shumard oak (Q. shumardii Buckl.) and, tulip-poplar (Liriodendron tulipifera L.) and one evergreen conifer: eastern white pine (Pinus strobus L.). The objectives were to investigate inter-specific differences in ecophysiological responses to hw events to understand mechanistic differences in resilience that may be useful for future model development. Two-year old, well-irrigated potted saplings were exposed to progressively increasing extreme hw diurnal cycles followed by a recovery cycle, with peak midday air temperature increasing from 37C to a maximum of 51C on the third day of the hw. Plants were assessed for various photosynthetic and water use responses, chlorophyll fluorescence and photosystem-II (PSII) activity, leaf temperature and foliar pigments. Intense heat caused progressive down-regulation in net photosynthesis, but the stomata remained operational, which helped cool leaves through loss of latent heat. Even as whole plant transpiration increased for all species, the rate plateaued at higher hw events that allowed leaf temperature to exceed 45°C, well beyond the optimal range. A significant increase in non-photochemical quenching over the hw cycles was evident in all species though indications of both transient and chronic PSII damage were evident in the most heat sensitive species, pine and tulip poplar. The oaks, especially Q. falcata, showed greater thermotolerance than other species with a higher threshold for photodamage to PSII, rapid overnight recovery of photoinhibition and minimal heat-induced canopy necrosis. We conclude that these co-occurring tree species exhibit large variability in thermotolerance and in their capability to repair both transient and chronic photodamage. From this work, extreme heat induced damage to PSII within the leaf chloroplasts may be a mechanistic trait that can be used to project how different species respond to extreme weather events.