Does the shift to higher capacities for isoprene emission at extreme temperatures in some oak species reflect acclimation to extreme drought and high temperature conditions?

Isoprene emission from the vegetation accounts for a substantial fraction of assimilated carbon and is an important biosphere-atmosphere interaction, being a key constraint to the chemical properties of the troposphere. Isoprene synthesis is controlled by isoprene synthase and is connected to photosynthesis through production of precursor metabolites, while subsequent emission is affected by the physical environment and is altered by exposure to environmental stress. An intricate, yet not completely understood balance exists between the impact of isoprene on the atmosphere and the feedback of global change on the eco-physiology of vegetation that defines future emission potentials, affecting to a yet unknown extent future isoprene emissions. To address the nature of these interactions, we set up a field study that characterizes the effects of climate and pollution gradients on carbon assimilation and isoprene emission fluxes of dominant oak tree species in Texas. We followed selected oak species throughout their growth season during the exceptional drought in 2011 and following recovery during 2012 and 2013, by comparing tree response to their growth environment at three selected sites along an urban to rural transect from downtown Houston to the Sam Houston National Forest. As urban areas are warmer, more polluted and often drier than rural regions, they are used in our study as an environment that mimics conditions expected from global climate change. Our results revealed significant differences in the drought stress response of the investigated oak species, Quercus nigra and Q.stellata. The early onset of drought in 2011 affected mostly the urban trees: the assimilation of Q. nigra decreased by 90% below optimum already in the beginning of the season, while only by 35% in the more resistant Q. stellata. The extreme drought uncoupled isoprene emission from photosynthesis: though correlated with photosynthesis, emission rates were less affected, with a maximum decrease of 40% in the sensitive species. As opposed to 2011, the above average precipitation in the first months of 2012, allowed for recovery in both studied species. Photosynthesis rates were maintained at optimum levels throughout the summer of 2012, while standard isoprene emission rates completely recovered in the resistant Q.stellata. Photosynthesis and isoprene emission of the sensitive Q. nigra recovered only partially. Isoprene emission response to increasing temperatures in Q. stellata indicated a shift to higher capacities for isoprene emission at extreme temperatures, exceeding current model predictions during all three years, possibly reflecting an adaptation to the local climate. Additionally, in 2012 and 2013 we recorded a further shift of 3-5°C in the optimum temperature for isoprene emission in this species. We hypothesize, that these responses are due to the evolution of a more thermo-tolerant isoprene synthase enzyme in this species. For comparison, the sensitive species' emissions decreased above 40°C, as predicted by models.