Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2012
Publication Date: 7/1/2012
Citation: Boyd, C.S., Svejcar, A.J. 2012. Biomass production and net ecosystem exchange following defoliation in a wet sedge community. Rangeland Ecology and Management. 65(4):394-400.
Interpretive Summary: Sedges play crucial ecological roles in maintaining the integrity of grazed riparian ecosystems, however, little is known about the production (above- and below-ground) and ecophysiological responses of wet sedge species to defoliation. We evaluated above- and below-ground growth patterns and net ecosystem exchange of CO2 (NEE) for clipped wet sedge communities. Our results suggest that clipping to 10 cm stubble height has minimal impact on above- or below-ground production as compared to the influence of yearly fluxes in water availability, but recovery of NEE to undefoliated levels is compromised with late season (July) clipping. These results suggest moderate grazing will not compromise within-year sedge production, but that long-term sustainability of grazing could be impacted by resource availability conditions that limit recovery of NEE.
Technical Abstract: Riparian ecosystems provide a multitude of ecosystem services, maintenance of which is tied to sustainable management of stream-side plant communities that provide important forage resources for livestock grazing operations. The objectives of this study were to evaluate above- and below-ground growth patterns, net ecosystem exchange of CO2 (NEE), and impacts of defoliation timing on a wet sedge (Carex nebrascensis Dewey.)-dominated plant community. We used a randomized block design with 4 replications along a small (< 2m width) stream in eastern Oregon. In June and July of 2004 – 2005, experimental plots were clipped to 10 cm stubble height and paired control plots left unclipped. All plots were clipped to 2.5 cm in mid-September and end-of-season and season-long above-ground production were calculated. Root ingrowth cores were used to estimate annual root production and root length density (RLD). A portable gas exchange system and 40 x 50 x 80 cm plexiglass chambers were used to measure NEE in 2005. An elevated water table in 2005 compared to 2004 resulted in higher (p < 0.001) season-long above-ground production (about double), but much lower (P < 0.05) below-ground production (about half). The net result was that total productivity was not statistically different between years, but below:above-ground ratios were 3x higher in 2004 compared to 2005. In contrast to biomass, RLD was not different between years (p > 0.05). Clipping reduced (p < 0.05) end-of-season above-ground standing crop by 33 to 73% depending on clipping month and year. Effects of clipping month on season-long above-ground production were inconsistent between years. June clipping decreased (p < 0.05) production (-10%) in 2005 and July clipping decreased production (-25%) in 2004. June-clipped plots recovered photosynthetic capacity within one month of clipping whereas NEE for plots clipped in July remained below unclipped levels at the end of the growing season. Water table levels appears to strongly influence above-ground/below-ground ratios in wet sedge communities, although total production was relatively stable during the two years we studied. Year effects overwhelmed clipping effects in terms of season-long above-ground production. Defoliation after mid-summer will not allow wet sedge communities to recover photosynthetic capacity by the end of the growing season, suggesting the potential for long-term impact with regular late-season defoliation.