Date of Award
Doctor of Philosophy
Ecology and Evolutionary Biology
Jake F. Weltzin
Dan Simberloff, Joanne Logan, Jim Drake, Michael Huston
Microstegium vimineum (Trin.) A. Camus, commonly known as Japanese grass, is a non-native plant of particular ecological concern in the United States due to its potential impact on native ecosystems, yet surprisingly little is known of its ecological requirements. In spite of its rapid expansion throughout its North American range, M. vimineum tends to occur in discrete patches on the landscape and is frequently found in disturbed understory habitats along roadbanks, floodplains, and nearby mesic forest. It is not found in many apparently suitable locations, which suggests that certain environmental factors, or interactions of factors, limit the distribution of this invasive grass. I used a combination of field and greenhouse experiments and a biogeographical survey to examine environmental factors that might cause the complex pattern of presence/absence and performance exhibited by this species.
The biogeographical survey was designed to (1) identify environmental factors that limit the distribution of this species to certain positions on the landscape (i.e., explain presence/absence), and (2) quantify the performance (i.e., height and biomass) of this species in response to environmental gradients. Within the study area, M. vimineum exhibited the broad environmental tolerance of many "weedy" species. Soil pH was the only environmental variable, among those measured, that was correlated with the presence of M. vimineum, whereas canopy openness and other species biomass are the most important variables that explain the performance of M. vimineum. This information can be used to predict habitats that are vulnerable to invasion by this species, prioritize the species as a land management concern, and guide the development of effective control strategies.
Light and water are important resources for all plants, and it has been suggested that these resources are mutually substitutable, at least within a certain range of resource levels. I conducted a greenhouse experiment to test a hypothesized light/water trade-off in M. vimineum, whereby light and water would be mutually substitutable and would have an interactive effect on plant growth. The experiment consisted of a randomized, 4 x 5 factorial design that included light levels (75%, 25%, 10%, 5%, and 2% sunlight) and soil moisture levels (40%, 30%, 20%, and 10%) that encompassed the ranges likely to be experienced by this species in a natural setting. Results demonstrate a light/water trade-off in M. vimineum in terms of root, shoot, and total biomass accumulation (light x water interaction, ppp=0.005). Shifts in biomass accumulation between roots and shoots does not appear to be the mechanism responsible for the trade-off (p>0.05), but stomatal conductance remains a plausible mechanism that should be tested in future research.
Within its introduced range, M. vimineum often occurs as extensive, dense patches with sharp boundaries and distinct gaps in cover. One example of this distributional pattern was observed relative to the native shrub Asimina triloba (pawpaw), whereby dense M. vimineum cover ended abruptly at the drip line of the A. triloba patch and was absent beneath the A. triloba canopy. I conducted field and greenhouse experiments to test several hypotheses regarding the causes of this observed pattern of M. vimineum distribution, including allelopathy, lack of seed dispersal, soil moisture limitations, and light limitations. I concluded that light reduction by the A. triloba canopy prevented establishment of M. vimineum beneath this shrub. In a complementary field shade experiment, a 60% reduction in ambient understory light reduced seedling survival, and the threshold for seedling survival is between 60% and 16% ambient understory light. These findings suggest that habitats in deep shade are less vulnerable to invasion by this non-native grass.
This research is the first explicit investigation of environmental constraints on the distribution of M. vimineum. I suggest that the complex pattern of this species' distribution and abundance in its introduced range is the result of interactions between resources, especially light and water, and seed dispersal limitations. The importance of maintaining undisturbed native vegetation is one of the conservation implications of these findings. My survey results are consistent with other reports that roads and trails are the most common habitat in which M. vimineum is found in its introduced range, and these linear gaps forested systems appear to provide a corridor for the spread of this non-native species. Forest fragmentations by development, including agriculture, increases the amount of edge habitat that favors the establishment of M. vimineum and increases the opportunities for its invasion into nearby intact forest. Comparative studies of this species in its native and introduced ranges and research on its ecological impacts are needed.
Cole, Patrice G., "Environmental Constraints on the Distribution of the Non-native Grass, Microstegium vimineum. " PhD diss., University of Tennessee, 2003.