The effect of drought stress on carbon assimilation and isoprene emission capacities of oak species in urban and rural areas of Texas

Isoprene released from vegetation affects regional and global air quality directly through chemical reactions generating ozone and secondary organic aerosol, and indirectly through as a sink for the OH radical, leading to increased methane lifetimes. The formation and emission of biogenic isoprene is controlled by the emitting species' physical growth environment, and altered by exposure to environmental stress. It has been hypothesized that a warmer and more drought-prone climate as a consequence of global warming may enhance biogenic isoprene emissions with mostly positive feedbacks to warming. However, the magnitudes of these actions and reactions in a warming climate are uncertain, as their drivers are not yet completely understood. To address these interactions, we initiated a field study in spring 2011 to characterize the sensitivity of carbon assimilation and isoprene emission capacity of dominant Texas oak species (Quercus falcata, Q. nigra and Q.stellata) to measured climatic and pollution gradients along an urban to rural transect from downtown Houston to Sam Houston National Forest. We hypothesize that the conditions in urban areas may serve as a proxy for future warmer, more polluted and drier conditions. Leaf level measurements were conducted on sun-exposed leaves under standard conditions (30 C and 1000 PAR units); temperature and CO2 response curves, and post-illumination isoprene emissions were evaluated. Our results show, that the early onset of exceptional drought (SPI of -2 to -3, leading to soil moistures of less than 10%) had a strong impact on the assimilation of all studied species, with site-specific differences in species' response. Q. falcata and Q. nigra were particularly sensitive to their local environment. Their net assimilation abruptly declined by 90% below their expected optimum rates already in May and remained low throughout the summer at the urban and suburban sites. In contrast, Q. stellata, a species native to Texas, was more resistant to stress, its assimilation declining by only 35% at the urban- and less at the suburban and rural sites. Isoprene emission however was less affected than expected by the severe drought stress. Emission rates were correlated with photosynthesis in the sensitive species, but the maximum decrease was only 50% in Q. falcata and 40% in Q. nigra. On the other hand, isoprene emission response to increasing temperatures in these two species indicated a higher sensitivity of isoprene synthase to temperatures above 40 C, as compared to the response of Q. stellata, in the severe phases of stress in the urban-suburban area. In contrast, at the forested site, higher total and seasonally more evenly distributed precipitation allowed for partial recovery of net assimilation after each rain event, while isoprene emission rates were not significantly affected in any of the studied species. Measured standard emission rates at this site ranged between 60-80 μg C g-1 h-1 in Q. stellata, 75-105 μg C g-1 h-1 in Q. falcata, and 65-85 μg C g-1 h-1 in Q. nigra throughout the summer season, mostly in line with literature values.