High Vapor Pressure Deficit Decreases the Productivity and Water Use Efficiency of Rain‐Induced Pulses in Semiarid Ecosystems

Abstract

Intermittent rain events drive dynamic pulses of carbon and water exchange in many arid and semiarid ecosystems. Although soil moisture is known to control these pulses, the effect of atmospheric dryness on pulses is not well documented. Here we hypothesized that vapor pressure deficit (VPD) modulates net ecosystem production (NEP) and ecosystem-scale water use efficiency (WUE) during pulse events due to its effects on canopy stomatal conductance and evapotranspiration. We quantified relationships between VPD and carbon and water exchange during growing season rain events and tested their generality across four semiarid flux sites with varied vegetation in the southwest United States. Across grassland, shrubland, and savanna sites, we found that high VPD during pulses suppressed ecosystem photosynthesis and surface conductance to a greater degree than respiration or evapotranspiration, particularly when soil moisture was high. Thus, periods of high VPD were associated with a 13-64% reduction in NEP and an 11-25% decrease in WUE, relative to moderate VPD conditions. Sites dominated by shrubs with the C3 photosynthetic pathway were more sensitive to VPD than sites dominated by C4 grasses. We found that a 1 kPa increase in VPD reduced the average NEP of pulse events by 13-56%, which illustrates the potential for projected increases in atmospheric demand to reduce the net productivity of semiarid ecosystems. Plain Language Summary In many water-limited regions, summer storms provide water that drives brief periods of high ecosystem activity (photosynthesis, respiration, and evapotranspiration) called "pulse events". While many studies have focused on soil moisture as a control on pulse events, it is not well known how changes in the air's evaporative strength, or dryness, influence patterns of carbon and water exchange during pulses. We analyzed data from four semiarid sites in southern Arizona and found that air dryness modified the widely accepted pulse framework of carbon and water exchange for semiarid areas. Drier air led to larger reductions in photosynthesis than respiration and evapotranspiration, which decreased the overall productivity and water use efficiency of plants. These findings indicate the potential for drier air in a warming climate to reduce the ability of dryland plants to use water. Because pulses are important for the carbon balance of these systems, drier during these critical pulse periods could reduce how much carbon global drylands remove from the atmosphere. Incorporating the effects of air dryness on pulse patterns into models may help us better understand global change impacts on semiarid ecosystems.

Drivers and Environmental Impacts of Vegetation Greening in a Semi-Arid Region of Northwest China since 2000

“…This is presently not the case in this semi-arid region-at six of the seven low elevation sites, vegetation greenness was correlated more with precipitation than with VPD, which may be due to the prevalent isohydric ecosystems in this region. Roby et al [60] found that shrubs dominated by C3 photosynthetic pathways are more sensitive to VPD than grasses dominated by C4 pathways, which agrees with our results. Most of the vegetation in our study region are C3 plants, hence the vegetation greenness interannual variability is more sensitive to precipitation than to VPD.…”

Landscape‐Scale Plant Water Content and Carbon Flux Behavior Following Moisture Pulses: From Dryland to Mesic Environments

“…We suspect that the smaller Re changes here are due to evaluating pulse behavior during spring and summer months when soil microbes have already largely upregulated following dormancy during the winter months. We ultimately interpret the NEP increases to be due to primary production as seen in previous studies (Roby et al, 2020), though with caution considering uncertainties in partitioning GPP and Re and using nighttime carbon fluxes. Table 1 on each day after rain pulses.…”

“…Results repeated with FLUXNET‐partitioned GPP show nearly identical results (not shown). Furthermore, previous field studies show that R e tends to increase which would instead decrease NEP immediately after a rain event due to the “Birch effect” (Chen et al., 2009; Huxman, Cable, et al., 2004; Jenerette et al., 2008; Roby et al., 2020; Williams et al., 2009; Xu et al., 2004). Therefore, we interpret the frequent NEP increases as GPP upregulation.…”

“…Furthermore, with the streamlining and greater availability of eddy covariance towers, multiyear records of hourly ecosystem‐scale (100‐m scales) carbon flux measurements are also available across many land cover types (Pastorello, 2020). These measurements also have been used to reveal much about carbon pulse dynamics primarily in drylands (Huxman, Cable, et al., 2004; Kurc & Small, 2007; Potts et al., 2006; Roby et al., 2020). Therefore, these plant water content and carbon flux measurements show promise in providing complementary information to assess plant pulse behavior across climates and biomes, without process‐model assumptions.…”

Precipitation temporal repackaging into fewer, larger storms delayed seasonal timing of peak photosynthesis in a semi‐arid grassland

“…The mean annual temperature (2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) is approximately 19.0°C, with daytime maximum temperature often exceeding 35°C in June (Scott et al, 2009). The mean annual precipitation (2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) is 384 mm, about 50%-60% of which arrives during the summer growing season associated with the North American Monsoon (July-September) (Roby et al, 2020;Scott et al, 2008)…”