Litter accumulation in emergent marshes: investigating the effects of litter on wetland invertebrates and the processes controlling litter distribution and accumulation

摘要

Abundant emergent plant litter produced in the interior of dense emergent stands in wetlands may influence abiotic conditions and biota but litter accumulation and its effects are seldom studied. This dissertation investigated the direct and indirect influences of litter on the structure and distribution of invertebrate communities and examined the processes controlling litter accumulation through modeling. Within emergent cattail stands, the impact of litter on the invertebrate community was studied through transect studies and an interior litter manipulation study. Anoxia was present at high litter treatments, while moderate and low litter treatments showed prolonged hypoxia. Lemnid densities were much lower in the high litter treatment. Quantitative invertebrate samples at lower litter conditions in the interior were dominated by hypoxic-tolerant nektonic species. Activity traps showed the emergent interior experienced increased Hyalella azteca (Saussure) (Amphipoda) abundance with more individuals inhabiting the top portion of the water column. Vertical compression increased with increasing litter. Invertebrate abundance decreased and the invertebrate community was dominated by semi-aquatic species and very hypoxic-tolerant taxa under high litter conditions in the interior. Litter accumulation within emergent macrophyte marshes may significantly influence abiotic conditions and biota but the process of litter accumulation is rarely considered in emergent macrophyte studies. The process of litter accumulation includes a species\u27 spatial extent, its annual production, and its breakdown rate. Annual production and breakdown rate can be combined for emergent species to estimate accumulated litter using litter breakdown rates from litter bag studies or a mass balance approach. Litter bag breakdown rates consistently overestimated litter accumulation for Phragmites, Typha, and Scolochloa. Mass balance derived rates for Phragmites and Typha tracked observed values and the breakdown rates but the mass balance approach was not suitable for Scolochloa which breaks down quickly. Litter bag studies may not be suitable to predict litter accumulation while the mass balance approach can provide more reliable estimates of litter accumulation for emergent species with recalcitrant litter. The mass balance breakdown rates were then combined with a spatial distribution model that included both dominant and subdominant species and a production model. Subdominants were included to better predict emergent distributions and expansion in mixed stands following a drawdown. The combined models were used to predict Typha litter accumulation as Typha stands developed after a drawdown. In areas where Typha was classified as subdominant at the start of the model, Typha often became dominant as less flood-tolerant dominant emergent species died out. Predicted Typha and Scolochloa distributions had greater spatial extents than observed distributions prior to the drawdown. Phragmites distributions, however, were underestimated by the model and neither the observed nor the predicted distributions reached pre-drawdown coverage. Predicted Typha litter accumulation reached pre-disturbance levels 5-6 years after the drawdown. Litter mass increased quickly in areas of Typha expansion with moderate increases in areas of the wetland where stands were \u3e2 years old. According to the model, litter also persisted for several years in areas where Typha was extirpated. The plant distribution model illustrates the importance of including subdominants and understanding persistence when studying emergent expansion. The implications of Typha litter accumulation, its interaction with water levels, and its potential effects on wetlands are also discussed.