Disentangling the impact of air temperature, vapor pressure deficit, and soil drought on photosynthesis, transpiration, and embolism

Heatwaves are becoming more frequent, with higher temperatures, drier air and reduced soil water availability. However, as increasing temperature, increasing VPD and soil drought are often coupled in nature, especially during heat waves, their isolated effects on tree physiology and, ultimately, mortality are not fully understood.

To disentangle the effects of these factors, we conducted a climate chamber experiment on nine tree species from different biogeographical backgrounds (three conifer species: Pinus sylvestris L., Pinus halepensis Mill. and Cupressus sempervirens L.; three temperate broadleaved species: Alnus cordata (Loisel.) Duby, Acer platanoides L. and Phillyrea angustifolia L.; three tropical broadleaved species: Terminalia microcarpa Decne., Syzygium jambos L. (Alston) and Trema orientale (L.) Blume). We exposed the trees to three different treatments which were imposed for two-day periods for a total of twelve days; (1) increasing temperature (20 to 40°C) with constant VPD (1.2 kPa), (2) constant temperature (35°C) with increasing VPD (1.2 to 4.7 kPa), and (3) increasing temperature (20 to 40°C) and VPD (1.2 to 6 kPa) but with constant vapor pressure (1.2 kPa). Each treatment was also divided into two groups: well-watered to field capacity (~35% soil moisture) and soil drought (~10% soil moisture). On the second day of each step, total water consumption, gas exchange (A_max, g_s, E), leaf temperature, and the maximum photochemical efficiency (Fv/Fm) were measured, and leaves were sampled for abscisic acid (ABA) analysis. The occurrence of stem and leaf embolism for conifer and broadleaf trees, respectively, was continuously monitored with the optical vulnerability method. G_min and P₅₀ were measured for all the tree species.

With this data set, we can study the isolated and combined effects of high temperature and VPD on plant gas exchange and xylem embolism, and how this response varies in plants with different biogeographic backgrounds and environmental adaptions. These findings help us better understand the underlying physiological drivers of globally rising tree mortality and improve models to better predict tree responses in a hotter and drier world.