Plant litter effects on nutrient mineralization and vegetation dynamics in sage-steppe communities with differing levels of annual grass invasion

Background/Question/Methods

Plant litter is an integral part of ecosystem nutrient cycling, which directly impacts vegetation dynamics. The net effects of litter are largely determined by the quantity and source material of the litter. Invasion of sage-steppe communities by annual grasses (e.g., Bromus tectorum) has caused dramatic changes in plant litter inputs, which may facilitate or inhibit further invasion. Many studies have completely removed all above-ground litter to identify its impacts on ecosystem processes, but 100% elimination of litter is not ecologically relevant. We experimentally manipulated a gradient of spring litter removal treatments for 3 years in three plant community types with differing levels of invasion (highly invaded, mixed and native) to determine the role of above-ground litter on nitrogen mineralization and vegetation dynamics.

Results/Conclusions

The removal of spring litter had relatively few effects on mineralization rates, with the primary exception of increasing mineralization rates with more than 50% removal of litter in winter 2009. In contrast, path analysis showed that the original biomass of spring litter (prior to removal) was more related to nutrient mineralization, indicating strong legacy effects of litter on soil processes. The relationships between spring litter biomass and mineralization rates were generally negative in highly invaded communities and positive in native communities. Removal of spring litter had relatively strong effects on vegetation dynamics, particularly in the mixed communities, although the impacts differed among years. During the warm and dry year (2010), removal of more than 50% of litter had negative effects on annual grass production; in contrast, the same removal treatments during the cool and wet year (2011) had negative effects on perennial grasses, causing a shift in the relative abundances between native and invasive grasses. Structural equation modeling revealed a negative feedback between annual grass litter and annual grass production, which was mediated through soil nutrients. In contrast, native litter had positive effects on annual grass production, which could facilitate invasion into native and mixed communities. Overall, we show that a minimum of 50% removal is necessary to elicit vegetation and soil responses, but also that litter biomass has strong legacy effects even after removal. The impacts of litter biomass and its removal were highly dependent on the litter source material, the community type and the year-to-year variation in weather conditions, which reveals the complex role of litter on ecosystem processes, but also provides a framework for manipulating litter to managing invasive grasses in sage-steppe communities.