The carbon balance of grazed and non‐grazed Spartina anglica saltmarshes at Skallingen, Denmark

Abstract

1 Canopy‐level CO2 exchange and biomass were measured monthly for 9 consecutive months during 1990 on grazed and non‐grazed portions of a Danish salt marsh dominated by Spartina anglica. The empirical measurements were used to calibrate a model of canopy photosynthesis and soil respiration that was subsequently integrated to provide estimates of annual net ecosystem production (NEP). 2 Standing total and green biomass on the non‐grazed site reached maxima during September of 1.9 ± 0.2 and 1.1 ± 0.2 kg m–2 ( ± 1 SD, n = 3), respectively. Corresponding maxima (0.8 ± 0.1 and 0.5 ± 0.05 kg m–2), were achieved during July at the grazed site. 3 Sediment organic matter as a percentage of dry weight was 31.6% and 5% at the non‐grazed and grazed sites, respectively. Accounting for the higher sediment bulk density at the grazed site meant that concentrations of organic carbon were 0.027 and 0.021 g cm–3 at non‐grazed and grazed sites, respectively. Thus, the fraction of NEP removed by grazing animals significantly reduced the accumulation of sediment organic matter. 4 Estimate of NEP varied between 17 and 38 mol m–2 year–1 of carbon at the grazed site and between 5 and 38 mol m–2 year–1 at the non‐grazed site, depending on assumptions about canopy geometry. Soil respiration, 36 mol m–2 year–1, did not differ between sites. Canopy respiration was less (21 mol m–2 year–1) at the grazed site than at the non‐grazed site (28 mol m–2 year–1), and accounted for 22–29% and 28–41% of gross photosynthesis at grazed and non‐grazed sites, respectively. The decrease in canopy respiration compensated the grazed canopy for losses to herbivores. 5 A canopy photosynthesis model was used to calculate a theoretical envelope of canopy biomass, termed the production window, inside of which NEP is positive and outside of which NEP is negative. The production window is bisected by the optimum canopy biomass, which is the theoretical biomass at which NEP is maximized. The seasonality of NEP is determined by the trajectory of actual canopy biomass through the production window. It was shown that canopy biomass on the grazed site was close to the theoretical optimum biomass late in the season, while the non‐grazed canopy biomass was supraoptimal.