Postdefoliation ecosystem carbon and water flux and canopy growth dynamics in sagebrush steppe bunchgrasses

Developing land-use practices that lead to sustainable net primary productivity in rangelands are important, but understanding their consequences to population and community processes is not often accounted for in basic ecosystem studies. Grazed and ungrazed upland ecosystems generally do not differ in net ecosystem CO(2) exchange (NEE), but the underlying mechanisms and the concurrent effects of defoliation to vegetative and reproductive biomass allocation are unclear. To address this, we measured evapotranspiration (ET), NEE and its constituent fluxes of ecosystem respiration (R(eco)) and gross ecosystem photosynthesis (GEP) with live canopy leaf area index (LAI(live); m(2) live leaf area m/(2) ground area) and aboveground leaf, culm and reproductive biomass in plots of clipped and unclipped squirreltail (Elymus elymoides) and bluebunch wheatgrass (Psuedoregnia spicata) growing in intact sagebrush steppe. Clipping reduced LAI(live) by 75%, but subsequent re-growth rates in clipped plots was similar to LAI(live) accumulation in unclipped plots. Concurrently, ET and NEE was similar between clipped and unclipped plots, with NEE primarily determined by GEP. GEP was initially lower in clipped plots, but then converged with unclipped GEP even as LAI(live) continued to increase in both treatments. GEP convergence was driven by higher whole-plant photosynthesis (GEP(live) = GEP/LAI(live)) in clipped plots. Ecosystem water use efficiency (GEP/ET) was reduced by 16% with clipping, due to two weeks of low GEP/ET immediately following defoliation, but GEP/ET converged before GEP levels did. Proportional reproductive biomass was higher in squirreltail (21.4% total biomass) than in bluebunch (0.5% total biomass), likely due to lower allocation to leaf and culm specific mass. Clipping reduced reproductive effort in squirreltail, in terms of total reproductive biomass (-56%), seed mass per unit leaf area (-64%), and seed mass per flowering head (-77%). We concluded defoliation increased canopy-level light penetration, facilitating rapid recovery of ecosystem fluxes, but that allocation to vegetative regrowth supporting this led to lower reproductive effort in these range grasses. Insights from studies such as this will be useful in formulating systems-based land management strategies aimed at maintaining annual productivity and long-term population and community goals in semiarid rangeland ecosystems.