Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2016
Publication Date: 7/1/2016
Publication URL: http://handle.nal.usda.gov/10113/62975
Citation: Hamerlynck, E.P., Sheley, R.L., Davies, K.W., Svejcar, A.J. 2016. Postdefoliation ecosystem carbon and water flux and canopy growth dynamics in sagebrush steppe bunchgrass. Ecosphere. 7(7):1-21. doi: 10.1002/ecs2.1376.
Interpretive Summary: Land use managers constantly develop and improve practices that maintain rangeland productivity. We studied the effects of defoliation (simulated grazing) on photosynthesis, water use and plant productivity. Clipped plants recovered photosynthetic capacity quickly after defoliation, but new growth was allocated more to leaves and less to reproductive output. This led us to conclude that while net primary productivity may recover quickly, seed production for recruitment of new individuals may require periodic rest from defoliation, and that land use managers should weigh the net effect of practices that may be sustainable for forage production against their possible effects to plant reproduction.
Technical Abstract: 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 CO2 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 (Reco) and gross ecosystem photosynthesis (GEP) with live canopy leaf area index (LAIlive; m2 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 LAIlive by 75%, but subsequent re-growth rates in clipped plots was similar to LAIlive 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 LAIlive continued to increase in both treatments. GEP convergence was driven by higher whole-plant photosynthesis (GEPlive = GEP/LAIlive) 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 enhanced canopy-level light penetration, not increasing LAIlive, led to rapid recovery of ecosystem flux functionality, but that vegetative regrowth supporting high GEPlive reduced overall reproductive effort in these range grasses. We believe studies such as this provide the detailed, integrated results needed for systems-based land-use strategies to optimize annual productivity and attain long-term population and community goals in semi-arid rangeland ecosystems.