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Item Relationship between spotted knapweed and indigenous plant assemblages and prediction of plant community response to picloram(Montana State University - Bozeman, College of Agriculture, 1999) Kedzie-Webb, SusanItem Directing succession by altering nutrient availability(Montana State University - Bozeman, College of Agriculture, 1999) Herron, Gretchen J.Item Biogeochemical and plant functional group response to long-term snow manipulation in a subalpine grassland(Montana State University - Bozeman, College of Agriculture, 2013) Holsinger, Jordan Paul; Chairperson, Graduate Committee: Jack BrookshireSnow represents an important control over plant communities in seasonally snow-covered ecosystems. It constrains the growing season and affects the availability of important resources including water, nitrogen (N) and phosphorus (P). Snow depth, distribution and duration have been affected by global climate change making it increasingly important to understand the effects of changing snow regimes on terrestrial ecosystems. Here we leverage a 43-year snow manipulation experiment to examine the effects of long-term changes in snow depth on plant community structure, resource availability and interactions therein in a common grassland type of the northern Rocky Mountains in western North America. Long-term experimental doubling and quadrupling of snowpack was associated with a significant shift in plant functional group distribution to a more forb rich community. Snow addition has resulted in a two to three-fold increase in forb to grass biomass ratios over time. Forbs consistently had greater N and P contents and lower nutrient use efficiencies compared to grasses. Forbs also displayed higher rates of net photosynthesis relative to grasses and sustained positive carbon (C) fixation rates late into the growing season after grasses had ceased. Though there is evidence that water exerts considerable control over ecosystem processes, increased snow depth did not have affect soil water availability through the growing season. However, snow depth was associated with significant differences in plant-available phosphate across the entire growing season with approximate 15% and 31% increases in pools of available P relative to ambient snowpack depth for doubled and quadrupled snowpacks respectively. Estimates of direct P inputs via dust and the ratio of available P to total P in the soil suggest that internal cycling was largely responsible for the observed differences in pools of available P. However, growing season net mineralization rates do not differ across treatments. This may suggest that winter processes make significant contributions to nutrient cycles. It is possible that the increased availability of P favors the shift to a forb-rich community under deeper snow because of their increased productivity under dry conditions and that the increased litter quality of forbs likewise promotes increased litter decomposition and mineralization, especially of P.Item Quantifying non-native plant impacts : Centaurea stoebe L. (spotted knapweed) and Bromus tectorum L. (downy brome) in sagebrush-grasslands of the greater Yellowstone ecosystem(Montana State University - Bozeman, College of Agriculture, 2012) Skurski, Tanya Christine; Chairperson, Graduate Committee: Bruce D. Maxwell; Lisa J. Rew (co-chair); Bruce D. Maxwell and Lisa J. Rew were co-authors of the article, 'Quantifying non-native plant impacts for natural areas management: a review of experimental research' in the journal 'Journal of applied ecology' which is contained within this thesis.; Lisa J. Rew and Bruce D. Maxwell were co-authors of the article, 'Mechanisms underlying non-native plant impacts: a review of recent experimental research' in the journal 'Biological invasions' which is contained within this thesis.; Bruce D. Maxwell and Lisa J. Rew were co-authors of the article, 'Ecological tradeoffs in non-native plant management' in the journal 'Biological conservation' which is contained within this thesis.; Lisa J. Rew and Bruce D. Maxwell were co-authors of the article, 'Abundance-impact relationships of non-native plants: an examination of Bromus tectorum L. in Southwest Montana sagebrush-grassland plant communities' in the journal 'Biological invasions' which is contained within this thesis.Non-native plant species (NIS) are considered a significant threat to individual species, communities, and ecosystems; thus, NIS management is mandated in most natural areas (i.e. non-crop). Controlling NIS in natural areas should, ideally, not only reduce target NIS abundance, but also benefit broader management objectives such as conserving native species and improving wildlife habitat. In this context, the benefits of NIS control must be weighed against the impacts of NIS themselves. This dissertation examines ecological impacts of NIS through a synthesis of previous experimental research and field-based manipulative experiments. In a review and synthesis of experimental research, we found that NIS caused significant impacts in approximately half of all experiments. Negative impacts were most frequent on community structure, followed in descending order by individual species, ecosystem properties, and ecosystem processes. Contrary to common assumptions, NIS typically caused impacts by modifying the abiotic environment rather than outcompeting native species for resources. NIS impacts were also examined through experiments conducted in sagebrush-grasslands of the GYE. The first study compared plant community impacts of Centaurea stoebe L. and common herbicide treatment for C. stoebe. The broadleaf herbicide, picloram, was highly effective at reducing C. stoebe, but also significantly reduced native forb cover and significantly increased non-native grass cover. Native forb cover increased with manual removal of C. stoebe, suggesting C. stoebe had been suppressing native forbs. However, there was an equivalent increase with no treatment. In these communities, C. stoebe appears to have a negligible effect on native forb and grass cover and richness. The final study examined plant community impacts of the non-native annual grass, Bromus tectorum L, and relationships between impacts and NIS abundance. In a four-year field experiment, we did not detect significant impacts of B. tectorum on native plant cover and richness. Environmental factors, particularly climate variability, are likely more important determinants of current vegetation patterns in these communities rather than the presence of B. tectorum. Overall, the research shows NIS often do not have significant detectable impacts on native species and communities, and that the negative non-target effects of herbicide treatments may outweigh the benefits of NIS control.