# Effects of Grazer Exclusion on Carbon Cycling in Created Freshwater Wetlands
## 4 Discussion  
Exclusion of herbivores had large impacts on the plant community and thereby carbon cycling and storage, with substantially lower biomass and gross productivity where grazers have access. While carbon losses in the form of ecosystem respiration and decomposition were also higher in the absence of grazers, higher rates of net ecosystem exchange and increased soil C content suggest that overall carbon sequestration is increased when herbivores are excluded. This points toward grazer management as a potential tool for establishing robust vegetation communities in created wetlands and for accelerating the accumulation of soil C, a characteristic that often differentiates created from natural wetlands [7; 8; 54]. Plant communities were impacted by grazers throughout the growing season, suggesting that exclusion of megaherbivores over multiple years has a persistent influence on the wetlands. Caged plots were more diverse and consistently had higher plant cover and aboveground biomass. When converted to units of carbon, using the average of 42.5% C in plant tissue measured at the site (Table 2), peak growing season aboveground biomass after five growing seasons of grazer exclusion was 175 g C m\({}^{-2}\) compared to 75 g C m\({}^{-2}\) in control plots (Figure 6). The two to three fold increase in aboveground biomass and vegetation cover in caged plots is greater than the 25-60% increase in aboveground biomass in riparian wetlands protected from grazers reported by [PERSON] et al. [55] and the 50% increase in reed stem density observed by [PERSON] et al. [33] when geese were excluded from  
Figure 5: Relationship between peak growing season (July) aboveground biomass and (**A**) ecosystem respiration (ER) and (**B**) gross primary production (GPP). Regression analysis includes data from all plots and years in which both biomass and gas fluxes were measured (n = 31). wetland plots. Likewise, [PERSON] and [PERSON] [56] reported a significant decrease in above-ground biomass in their grazed treatments in salt marshes. The relative increase in cover in caged plots was greater than observed in earlier reported values from this experiment [21], suggesting that the impact of grazers is cumulative over time with continued access to young wetland sites. Further, the higher vegetation cover (but not biomass) in caged plots in spring 2018 suggests that the effects of grazer exclusion carry over year to year, and in created wetlands, which typically have young plant communities with lower stem density and belowground rhizomal development, limiting grazing may encourage the growth and establishment of stable and diverse plant communities [57]. Belowground biomass was higher than that measured by [PERSON] and [PERSON] [21], suggesting that as the wetland is maturing, belowground biomass is increasing and will likely continue to contribute carbon stocks to the soil. In contrast to aboveground biomass, belowground biomass was only slightly, and not significantly influenced by grazer access. [PERSON] and [PERSON] [21] previously observed significantly higher belowground biomass in these same caged plots, indicating that more intensive sampling may strengthen our observed trend. Unlike measurements of aboveground biomass, our measurements of belowground biomass only occurred once during the year (October), potentially missing some impacts of grazers on belowground processes. A more detailed sampling approach, including seasonal sampling and species-level identification of belowground biomass, would enable better linkages between aboveground responses to herbivory and belowground C storage. Shifts in vegetation dominance when grazers were excluded were consistent with the grazing preferences of waterforl at the site. In uncaged plots, semi-desirable species were frequently damaged by grazing, such as _S. latifolia_, resulting in lower biomass. In contrast, some plants in the caged plots were able to grow to maturity and may eventually become less palatable to grazers [58]. These species included _L. oryzaeides_ and _T.