Understanding How Plants Respond to Coexisting Soil and Air Dryness in Global Simulations
The vapor pressure deficit (VPD), the difference between the amount of moisture in the air and how much moisture the saturated air can hold, has risen over recent decades and is projected to further increase in the future with global warming. While numerous studies have highlighted ecosystem sensitivities to soil water availability, it is critically important to understand how plant physiology responds to VPD. This work used the Energy Exascale Earth System Model (E3SM) and statistical analysis to study the simulated daily canopy conductance (Gc), how easily water can transfer from plants to the atmosphere and its dependence on soil water and VPD. The results show that soil water stress has a stronger influence on Gc, a key variable in controlling the exchange of water vapor and carbon dioxide (CO2) between plants and the atmosphere, than VPD in the growing season.
E3SM simulations show that soil water stress limits of Gc more than VPD, except in grasslands. Plant hydraulics processes further accentuate the role of soil water stress in limiting Gc by increasing soil wetness and evapotranspiration, leading to lower VPD. This study also showed that elevated CO2 reduces Gc by increasing plant water use efficiency, mitigating soil water stress on Gc. The combined E3SM and statistical analysis framework introduced here is particularly useful for evaluating and improving model performance against experimental data by identifying which drivers Gc is most sensitive to. The method may also be practical for studying future plant response to extreme environmental conditions when increasing CO2 levels affect Gc.
Plants close the pores on their leaves in a process known as stomatal closure to prevent water loss, as excessive water loss can harm or kill them. It is unclear whether air or soil dryness plays a larger role when plants close their pores, especially when both conditions are present. As air and soil dryness may change differently in different regions under global warming, understanding their relative influence on stomatal closure is important for understanding plant response to rising temperatures and CO2 levels. The team applied a simple model to identify how much each kind of dryness contributes to stomatal closure as simulated by an Earth system model. The results showed a high probability that soil dryness dominates stomatal closure when both soil and air dryness are present during both present-day conditions and hypothetical future conditions with quadrupled CO2 concentrations. The team also found that including plant hydraulics, both water movement in the soil and water transport within the plant, in E3SM can increase the relative influence of soil dryness on stomatal closure because the plant physiological response reduces air dryness.