Global change‐type drought‐induced tree mortality: vapor pressure deficit is more important than temperature per se in causing decline in tree health

Abstract

Drought-induced tree mortality is occurring across all forested continents and is expected to increase worldwide during the coming century. Regional-scale forest die-off influences terrestrial albedo, carbon and water budgets, and landsurface energy partitioning. Although increased temperatures during drought are widely identified as a critical contributor to exacerbated tree mortality associated with "global-change-type drought", corresponding changes in vapor pressure deficit (D) have rarely been considered explicitly and have not been disaggregated from that of temperature per se. Here, we apply a detailed mechanistic soil-plant-atmosphere model to examine the impacts of drought, increased air temperature (+2°C or +5°C), and increased vapor pressure deficit (D; +1 kPa or +2.5 kPa), singly and in combination, on net primary productivity (NPP) and transpiration and forest responses, especially soil moisture content, leaf water potential, and stomatal conductance. We show that increased D exerts a larger detrimental effect on transpiration and NPP, than increased temperature alone, with or without the imposition of a 3-month drought. Combined with drought, the effect of increased D on NPP was substantially larger than that of drought plus increased temperature. Thus, the number of days when NPP was zero across the 2-year simulation was 13 or 14 days in the control and increased temperature scenarios, but increased to approximately 200 days when D was increased. Drought alone increased the number of days of zero NPP to 88, but drought plus increased temperature did not increase the number of days. In contrast, drought and increased D resulted in the number of days when NPP = 0 increasing to 235 (+1 kPa) or 304 days (+2.5 kPa). We conclude that correct identification of the causes of global change-type mortality events requires explicit consideration of the influence of D as well as its interaction with drought and temperature.

Groundwater-dependent ecosystems: recent insights from satellite and field-based studies

“…The most revolutionary recent development has, per- haps, been the application of remote sensing techniques to identify the location of GDEs but also to reveal key features of their functional behaviour. Increasing frequencies, spatial and temporal extent and severity of drought and resulting drought-induced mortality of forests have been recorded extensively (Dai, 2011;Eamus et al, 2013) in the past two decades. Climate-changeinduced changes in rainfall distribution and amounts pose a new stress to both groundwater resources and associated GDEs.…”

The Influence of Monsoon Climate on Latewood Growth of Southwestern Ponderosa Pine

“…A measure of atmospheric water demand, VPD is the driving force for transpiration and influences plant water potential, stomatal conductance, and photosynthesis. Increased VPD, when coupled with limited soil moisture, reduces stomatal conductance and photosynthesis, and in the extreme can drive hydraulic failure, cessation of growth, and depletion of carbohydrates [2,[44][45][46]. Our earlier research suggests that winter precipitation supplies the water used by ponderosa pine for latewood growth [24].…”

Response of water use efficiency to summer drought in a boreal Scots pine forest in Finland

“…The dominant reason is that low soil moisture leads to stomatal regulation of the plants, which limits plant carbon assimilation and transpiration. The decreased ET due to soil moisture drought may increase atmospheric VPD, which could in turn intensify stomatal closure (Eamus et al, 2013;Jarvis, 1976). Moreover, the GPP and ET were decoupled and EWUE decreased due to the soil moisture drought.…”