Comparing leaf temperature with canopy carbon and water fluxes during heat waves

Fluxes of carbon and water to and from the biosphere are mediated by leaf level physiological processes that are sensitive to temperature. However, while air temperatures will increase in the future, it is less well understood how leaf temperature will respond among species and ecosystems. In this experiment we compared the physiological response of an old-growth Douglas fir (Pseudotsuga menziesii) forest canopy (Wind River) and a ponderosa pine (Pinus ponderosa) canopy (Metolius) to a series of heatwaves during the 2015 growing season. Leaf temperature measured with a thermal infrared camera and net ecosystem exchange of carbon (NEE), latent and sensible heat fluxes measured by eddy-covariance flux towers were compared before, during and after five successive heatwaves in the Pacific Northwest, USA in 2015. The impact of the heatwaves on canopy temperature and carbon fluxes varied according to the time of year and was not consistent between the two sites. Overall, the Douglas fir forest was more sensitive to heatwaves than the ponderosa pine forest, with GPP early in the growing season reduced by each heatwave. Furthermore, three large heatwaves in June and July caused the Douglas fir forest to become a carbon source, whereas the ponderosa pine forest remained a carbon sink until the final heatwave at the end of July. The canopy of the ponderosa pine forest was also always closer to air temperature than Douglas fir. Douglas fir remained on average 3.3 - 5.9 °C warmer than air during all five heatwaves, whereas ponderosa pine started at an average daily maximum of 1.87 °C warmer than air and became progressively warmer for each subsequent heatwave, reaching an average daily maximum of 4.33 °C warmer than air by the end of July. The extremely elevated canopy temperatures of both forests also resulted in maximum leaf VPD values of 7.03 and 5.54 kPa for Douglas fir and ponderosa pine respectively, well above the values estimated from air temperature (5.65 and 4.50 kPa). Higher sensitivity of the Douglas fir forest to all five heatwaves may be explained by the higher daily maximum air temperatures compared to the ponderosa pine forest (seasonal maximum air temperatures of 37.2 and 33.0 °C respectively). Our study highlights the critical need for future work examining the consequences of extreme events on the growth and survival of Douglas fir.