ARS | Publication request: Soil moisture and ecosystem function responses of desert grassland varying in vegetative cover to a saturating precipitation pulse

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 1, 2011
Publication Date: May 1, 2012
Citation: Hamerlynck, E.P., Scott, R.L., Stone, J.J. 2012. Soil moisture and ecosystem function responses of desert grassland varying in vegetative cover to a saturating precipitation pulse. Ecohydrology. 5: 297-307. doi:10.1002/eco.214.

Interpretive Summary: Climate change models predict an increase in the intensity and frequency of extreme precipitation events. A critical ecological and hydrological feature that will determine the impacts of such events is vegetative cover, yet how grassland soil moisture and attendant ecosystem function will respond to large, saturating events, is largely unknown. This study showed that after application of experimental rainfall similar in total to a typical single summer monsoon growing season, net carbon balance (NEE) dynamics were nearly identical between high and low cover desert grassland plots. This was due to the fact that ecosystem respiration (Reco, or the carbon give out by the ecosystem) in low cover plots leveled off, while Reco in high cover plots continued to rise following the pulse. At the same time, gross ecosystem photosynthesis (GEP, or carbon uptake) rose in parallel between high- and low-cover plots, resulting in similar NEE. For 35 days after the large pulse, soil moisture dried down and wetted up in a similar manner between high and low cover plots, then later in the season high cover plots dried out much sooner than low cover plots, probably because of greater plant water use. These findings show that how plants balance aboveground and belowground growth varies with the degree of plant cover, and this has a strong impact on how the ecosystem responds to extreme precipitation events.

Technical Abstract: A critical linkage between hydrological and ecological processes is plant cover, yet the ecosystem-level responses of aridland systems of varying plant cover to extreme precipitation events, predicted to increase in number and severity in the future, has not been well studied. We tracked average rooting-depth volumetric soil water (q15cm), ecosystem evapotranspiration (ET), net ecosystem carbon dioxide exchange (NEE), ecosystem respiration (Reco), gross ecosystem photosynthesis (GEP), and leaf-level net carbon assimilation (Anet) and stomatal conductance to water vapor (gs) in high- (ca 50%) and low-cover (ca. 23%) desert grassland plots in response to a single experimental runoff generating rainfall representing typical total annual precipitation. For 35 d after this pulse, q15cm were 2.5% higher in high-cover compared to low-cover plots, and q15cm e-folding times during soil drying were identical between plots. After q15cm converged, dry-down rates were consistently longer in low-cover plots. Anet rose rapidly to maximum rates in low-cover plots (+4 d), with high-cover plots attaining similar maximum Anet at +7 d. Whole-plot evapotranspiration (ET) and leaf-level gs did not differ between plots. NEE did not significantly differ between high- and low-cover plots for 30 d following the pulse. This was due to: 1) higher Reco in high-cover plots, which steadily rose through the post-pulse period, while low-cover plot Reco leveled at +21 d, and 2) parallel increases in GEP between high-cover and low-cover plots. These findings suggest aboveground and belowground allocation by canopy dominants will strongly modulate processes controlling NEE and productivity in grassland systems under future climate conditions.