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.