Numerous biotic and abiotic factors may affect the rate at which organic matter is decomposed in tidal wetlands, but the effects of living plant abundance, composition, or diversity on decay rates in these ecosystems has seldom been investigated. To better understand how emergent wetland plant communities may affect decomposition, we conducted manipulative experiments in field and greenhouse mesocosms to test the effects of plant presence and abundance on short-term rates of litter decay in subsurface litter bags. Additionally, we examined in situ rates of litter decay in seven Pacific coast tidal marshes from southern California to central Washington to observe how variation in plant cover, composition, and species richness was related to rates of litter loss.
In a controlled greenhouse study of the effects of plant presence-absence and salinity on litter loss rates, we found that plant presence increased sub-surface decomposition relative to the absence of plants, but only at the very lowest salinities in the experiment. At higher salinities, decomposition was suppressed in the presence of plants relative to their absence. In a similar field experiment, root and rhizome biomass of living plants was not correlated with litter decay rates. In situ rates of litter decomposition across the series of Pacific coast marshes varied by site, and by position in the tide frame (low versus high marsh). Total above-ground plant cover and species richness were positively correlated with decay rates. Plant composition was also weakly associated with litter decay rates. Litter decay was lower at higher cover of pickleweed (Salicornia pacifica) and higher in the presence of certain high marsh species such as Distichlis littoralis. Our findings suggest that plant community structure can impact tidal marsh litter decomposition. As climate change, invasive species, or other stressors change the composition of tidal marsh plant communities, rates of carbon re-mineralization via decomposition may also be impacted.