Effects of high atmospheric demand on the growing season productivity of semiarid ecosystems

Abstract:

Intermittent rain events drive pulses of carbon and water exchange in many arid and semiarid ecosystems. When precipitation temporarily alleviates soil moisture stress, reductions in gross ecosystem photosynthesis (GEP) in response to high vapor pressure deficit (VPD) can decrease net ecosystem productivity (NEP) during pulses. However, it is not well known if these pulse responses to compound extremes in soil moisture and VPD influence seasonal and annual productivity in semiarid ecosystems. Because pulses contribute strongly to seasonal productivity in these systems, we hypothesized that losses in GEP at high VPD during pulses reduce growing season NEP. Across four semiarid sites with varied vegetation type, we found that variation in growing season (July-September) mean VPD explained 27-56% of GEP variation and a 1 kPa increase in VPD was associated with a 1.38 to 3.31 µmol CO₂ m² s^-1 decrease in GEP. Averaged across sites, the relationship between GEP and VPD was linear and showed no evidence of saturation at high VPD. Because growing season GEP ranged from 1.25 to 3.41 µmol CO₂ m² s^-1 across sites, these findings illustrate the potential for rising atmospheric demand to reduce the carbon sink capacity of arid and semiarid ecosystems.