Defoliation increased above-ground productivity in a semi-arid grassland

Abstract

In light of the continuing debate regarding overcompensation we studied the responses of above -ground biomass in a high -elevation, semi -arid grassland to defoliation, defoliation history, and livestock grazing. The above -ground annual net primary productivity (ANPP) was measured over 2 years in one -hundred twenty, 1 -m2 plots that were exposed to single-and multi -year defoliation and grazing treatments. Plant communities showed an average increase in ANPP of 31 %-45% due to a single defoliation event.The most conservative estimate of average ANPP of defoliated subplots was 29.4 g In-2 greater than the non -defoliated controls. A history of defoliation, due to clipping or grazing, lessened the magnitude of the compensatory response, but above-ground overcompensation of biomass was still observed, ranging on average from 17% to 26 %. One dominant species, squirreltail grass [Elymus elymoides (Raf.) Swezeyi], accounted for nearly one -third of the community -level increases in ANPP. In contrast to above -ground patterns, below -ground root production of squirreltail did not increase in response to defoliation events.These results suggest that the above -ground production of highelevation, semi -arid grasslands in the American Southwest may be temporarily increased through certain grazing events, and may help explain shifts in species dominance in grasslands exposed to long -term grazing by livestock.Key Words: high -elevation grasslands, ANPP, overcompensation, clipping, root biomass, western wheatgrass, squirreltailThe hypothesis that certain plant species can respond to a defoliation event, such as grazing by an herbivore, in a manner that increases their reproductive fitness or productivity has recently attained greater acceptance in the scientific literature (Strauss and Agrawal 1999, Agrawal 2000). This response has been called "overcompensation" (Owen 1980, Owen and Wiegert 1976, 1981 to signify that production exceeds the level needed toWe gratefully acknowledge the assistance of S. Mezulis, L. Taylor, Z. Bayham, R. Rieder, G. Kendall, J. Chirco, J. White, T. Muñoz, and J. Battin in collecting these data. In addition we thank the Diablo Trust, Flying M Ranch, and the Peaks and Mormon Lake Districts of the Coconino National Forest for their participation in this collaborative research effort. This manuscript has benefited from the comments of two anonymous reviewers, the Sisk lab group, and specifically Brad McRae, Lisa Taylor, and Sharon Mezulis. This study has been supported through funding from Northern Arizona University, Merriam Powell Center for Environmental Research, and the Ecological Restoration Institute.Manuscript accepted 21 Feb. 04. ResumenA raíz del debate continuo respecto a la sobrecompensación estudiamos las respuestas de la biomasa aérea a la defoliación en una elevación alta de un pastizal semi -árido, al historial de defoliación y al apacentamiento por ganado. La productividad primaria neta anual aérea (ANPP) se midió en dos años en 120 parcelas de 1 In-2 que fueron expues...

Bluebunch Wheatgrass Response to Spring Defoliation on Foothill Rangeland

“…Clipping to # 6 cm, however, apparently removed so much current year's leaf area that plant growth was unable to benefit from the increased availability of light. Increased plant yield in response to defoliation (i.e., overcompensation) has been documented elsewhere on semiarid rangeland of North America, including shortgrass prairie in north-central Colorado (Williamson et al 1989;Varnamkhasti et al 1995) and mountain grasslands in north-central Arizona (Loeser et al 2004) and northern Wyoming (Frank et al 2002). Plant yield in the summer of 1999, however, was not increased on our foothill sagebrush steppe site in response to 9-cm clipping in April or May 1999.…”

Impact of Grazing Intensity during Drought in an Arizona Grassland

“…At our study site the grazing‐tolerant bunchgrass, E. elymoides , has expanded its range within the last century and currently occurs as a dominant species across large areas (M.R.R.L., unpublished data). We hypothesize that a century of livestock grazing, and in particular a multidecadal period of intensive grazing in the late 1800s, increased the relative composition of grazing‐tolerant species (Loeser et al 2004). Incomplete records from our site also suggest that community composition has been more stable during the last half of the twentieth century, leading up to the current period of rapid spread of exotic species, which our data suggest is being facilitated by recurrent drought.…”

Soil Carbon Pools in California’s Annual Grassland Ecosystems

“…Grazing management can affect soil C storage by altering above-and belowground plant C allocation (Holland et al 1992;Johnson and Matchett 2001), net primary productivity (NPP; Frank and McNaughton 1993;Turner et al 1993), and soil respiration (Kieft 1994;Bremer et al 1998;Cao et al 2004). Moderate grazing often increases aboveground NPP (ANPP;McNaughton 1985;Loeser et al 2004), and in perennial grasslands ANPP is frequently positively correlated with soil C pools when controlling for precipitation .…”