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ARS Home » Plains Area » Fort Collins, Colorado » Center for Agricultural Resources Research » Rangeland Resources & Systems Research » Research » Publications at this Location » Publication #319992
Title: Grazing intensity differentially regulates ANPP response to precipitation in North American semiarid grasslands

Author
item IRISARRI, J. GONZALO - Consejo Nacional De Investigaciones Científicas Y Técnicas(CONICET)
item Derner, Justin
item Porensky, Lauren
item Augustine, David
item Reeves, Justin
item Mueller, Kevin

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2015
Publication Date: 7/9/2016
Citation: Irisarri, J.N., Derner, J.D., Porensky, L.M., Augustine, D.J., Reeves, J.L., Mueller, K.E. 2016. Grazing intensity differentially regulates ANPP response to precipitation in North American semiarid grasslands. Ecological Applications. 26(5):1370-1380.

Interpretive Summary: Long-term data from grazing studies in two rangeland ecosystems (shortgrass steppe (SGS) and northern mixed-grass prairie (NMP)) provides the opportunity to evaluate the influence of different levels of grazing (none, light, moderate and heavy) on how precipitation effects forage production. As the level of grazing is increased in both rangeland ecosystems, forage production decreases from cool-season grasses and grasslike plants (sedges for example), whereas forage production from warm-season grasses increases. Doubling of the level of grazing in both rangeland ecosystems reduced forage production by about one-quarter. Changes in the composition of plant functional groups impacted the precipitation use efficiency at both rangeland ecosystems as this response was enhanced with higher abundance of cool-season grasses and grasslike plants. This study demonstrates how grazing can induce changes to both forage production and precipitation use efficiency through shifting the composition of plant functional groups which have different physiological responses.

Technical Abstract: Grazing intensity elicits changes in the composition of plant functional groups in both short-grass steppe (SGS) and northern mixed-grass prairie (NMP) in North America. How these grazing intensity-induced changes control aboveground net primary production (ANPP) responses to precipitation remains a central open question, especially in light of predicted climate changes. Here, we evaluated effects of four levels (none, light, moderate and heavy) of long-term (>30 yrs) grazing intensity in SGS and NMP on 1) ANPP, 2) precipitation use efficiency (PUE, ANPP:precipitation) and 3) precipitation marginal response (PMR, slope of a linear regression model between ANPP and precipitation). Spring (April-June) precipitation, the primary determinant of ANPP, was only 12% higher in NMP than in SGS, yet both ANPP and PUE were 25% higher. Doubling grazing intensity in SGS and nearly doubling it in NMP reduced ANPP and PUE by only 24% and 33%, respectively. Increased grazing intensity reduced C3 graminoid biomass and increased C4 grass biomass in both grasslands. Functional group shifts impacted PUE, as PUE was positively associated with the relative abundance of C3 species and negatively with C4 species across both sites. At the community level, PMR was similar between grasslands and was not affected by grazing intensity. However, ungrazed PMR of C3 graminoids was eight-fold higher in SGS than under any grazed level. In NMP, C3 PMR was only reduced under heavy grazing intensity. This study advances prior work by 1) examining the consequences of a range of grazing intensities (more than simply grazed vs. ungrazed), and 2) examining how grazing-induced changes in ANPP and PUE are related both to shifts in functional group composition and physiological responses within each functional group. Knowledge of the ecological consequences of grazing intensity provides policy makers with information for decision making in the face of climate change. In this sense, in SGS, moderate grazing (the recommended rate) would sequester the same amount of aboveground carbon as light grazing because ANPP was nearly the same. For the NMP, reductions in grazing intensity from moderate to light intensity would increase the amount of aboveground carbon sequestrated by 25% because of increased ANPP.