Root Dynamics, Management Effects and Carbon Sequestration on Invaded and Native Semi-Arid Grasslands

Abstract

Root dynamics are a major component of terrestrial ecosystem processes but they have not been broadly studied, due to the difficulty of quantifying root production and mortality. The use of minirhizotrons facilitates belowground research, but sample intervals are controversial since long intervals miss roots that grow and die between the samples. Also, root responses to grassland management such as mowing are less well-known than shoot responses, even though as much as 83% of productivity in semiarid grasslands occurs belowground. Finally, the belowground dynamics of invasive species that change the vegetation of an area can be expected to alter the local carbon (C) cycle in ways. I investigated the optimum minirhizotron sample interval for measures of production and mortality in five contrasting ecosystems. Root production was significantly affected by sample interval, while root mortality was significantly affected by sample interval only in one ecosystem with large mortality. Overall, longer sample intervals tended to measure less root production and mortality than shorter ones, resulting in potential underestimation of ecosystem root dynamics. Further, I asked whether root responses reflect shoot responses to mowing and fertility in crested wheagtrass (Agropyron cristatum [L.] Gaerth) stands and native grasslands. Surprisingly, roots of both native and invaded grasslands were unaffected by mowing and N addition, despite significant changes in shoot mass in both vegetation types. Given that most production and competition in grasslands occurs belowground, this suggests that mowing may not be a successful tool for reducing crested wheatgrass root length. Additionally, I examined the effects of crested wheatgrass on the local C cycle through changes in tissue lignin content, CO2 evolution via decomposition, and soil water and C content. Crested wheatgrass had significantly lower root tissue lignin content, higher CO2 evolution from root decomposition, and did not change soil C content. These results suggest that the lack of increase in soil C content under crested wheatgrass stands is due to its lower lignin content and higher rates of decomposition. Overall, my results highlight the belowground dominance of an invasive species under different management treatments, as well as accounting for the inability of an invasive species to increase soil carbon in spite of increased production.