Biomass Production and Net Ecosystem Exchange Following Defoliation in a Wet Sedge Community

Abstract

Riparian ecosystems provide many ecosystem services, including serving as an important forage resource for livestock grazing operations. We evaluated defoliation impacts on above- and belowground production, and net ecosystem exchange of CO2 (NEE), in a wet sedge (Carex nebrascensis Dewey)-dominated plant community. In June or 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 aboveground production calculated. Root ingrowth cores were used to estimate annual root production and root length density (RLD). A portable gas exchange system and plexiglass chamber were used to measure NEE in 2005. An elevated water table in 2005 vs. 2004 was associated with higher (P,0.001) season-long aboveground production (about double), but lower (P0.05) belowground production (about half). Total productivity did not differ between years, but below-:aboveground ratios were 33 higher in 2004 vs. 2005. RLD was not different between years
(P.0.05). Clipping reduced (P0.05) end-of-season aboveground standing crop by 33% to 73% depending on clipping month and year. Effects of clipping month on season-long aboveground production were inconsistent between years; June clipping decreased (P0.05) production (10%) in 2005 and July clipping decreased (P0.05) production (25%) in 2004. NEE for June-clipped plots recovered within 1 mo of clipping, whereas NEE for plots clipped in July remained below unclipped levels at the end of the growing season. Water table levels strongly influenced below-:aboveground ratios, although total production was relatively stable between years. Year effects overwhelmed clipping effects on season-long aboveground production. Defoliation after mid-summer did not allow recovery of photosynthetic capacity by the end of the growing season, suggesting the potential for long-term impact with regular late-season defoliation.