Photosynthesis and isoprene emission from trees along an urban–rural gradient in Texas

Abstract

Isoprene emission is an important mechanism for improving the thermotolerance of plant photosystems as temperatures increase. In this study, we measured photosynthesis and isoprene emission in trees along an urban-rural gradient that serves as a proxy for climate change, to understand daily and seasonal responses to changes in temperature and other environmental variables. Leaf-level gas exchange and basal isoprene emission of post oak (Quercus stellata) and sweet gum (Liquidambar styraciflua) were recorded at regular intervals over an entire growing season at urban, suburban, and rural sites in eastern Texas. In addition, the temperature and atmospheric carbon dioxide concentration experienced by leaves were experimentally manipulated in spring, early summer, and late summer. We found that trees experienced lower stomatal conductance and photosynthesis and higher isoprene emission, at the urban and suburban sites compared to the rural site. Path analysis indicated a daily positive effect of isoprene emission on photosynthesis, but unexpectedly, higher isoprene emission from urban trees was not associated with improved photosynthesis as temperatures increased during the growing season. Furthermore, urban trees experienced relatively higher isoprene emission at high CO2 concentrations, while isoprene emission was suppressed at the other sites. These results suggest that isoprene emission may be less beneficial in urban, and potentially future, environmental conditions, particularly if higher temperatures override the suppressive effects of high CO2 on isoprene emission. These are important considerations for modeling future biosphere-atmosphere interactions and for understanding tree physiological responses to climate change.

Increase in leaf temperature opens stomata and decouples net photosynthesis from stomatal conductance in Pinus taeda and Populus deltoides x nigra

“…However, these studies have often provided conflicting results. While some evidence suggested that g s increased with increasing temperature (Schulze et al , 1974; Lu et al , 2000; Mott and Peak, 2010), other studies found that temperature had no effect on stomata (Teskey et al , 1986; Sage and Sharkey, 1987; Cerasoli et al , 2014; von Caemmerer and Evans, 2015), or that increased temperature triggered stomatal closure (Weston and Bauerle, 2007; Lahr et al , 2015). One explanation for the conflicting results across these studies might be that the experiments were often conducted in uncontrolled environmental conditions in the field.…”

“…Previous studies have reported a complete range of responses to increased temperature, including stomatal opening (Schulze et al , 1974; Freeden and Sage, 1999; Lu et al , 2000; Mott and Peak, 2010), no significant response (Teskey et al , 1986; Sage and Sharkey, 1987; Cerasoli et al , 2014; von Caemmerer and Evans, 2015), and stomatal closure (Raven et al , 2005; Weston and Bauerle, 2007; Lahr et al , 2015). A peaked response with maximum g s at 20°C (Way et al , 2011) or more complex responses with one peak between 20 and 30°C and an additional increase at extremely high temperatures (Slot et al , 2016) have also been described.…”

Functional traits of urban trees: air pollution mitigation potential

“…BVOC emissions are expected to rise with higher temperatures and decrease with elevated CO 2 (Lahr et al . ). The latter effect depends on nutrient availability.…”

Stomatal conductance increases with rising temperature

“…7,8 Results of experiments that examined the direct dependence of stomatal conductance on temperature have not been consistent. Previous studies have reported a complete range of responses to increased temperature, including stomatal opening, [9][10][11][12] no significant response, [13][14][15][16] stomatal closure, [17][18][19] peaked response with maximum g s at temperatures optimal for photosynthesis 20 or more complex responses. 21 One possible explanation for these inconsistent results is that to isolate the direct effect of temperature on g s requires a well-controlled environment, particularly with respect to VPD, which is often hard to achieve.…”