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Theses and Dissertations

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Browsing Theses and Dissertations by Subject "aboveground net primary productivity"

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    Effects of drought on the shortgrass steppe of the North American Great Plains
    (Colorado State University. Libraries, 2016) Cherwin, Karie Lynn, author; Knapp, Alan, advisor; Seastedt, Timothy, committee member; Kelly, Eugene, committee member; Blumenthal, Dana, committee member; Steingraeber, David, committee member
    Global climate models predict the frequency, intensity, and duration of extreme weather events, such as severe drought, will increase during the 21st century across many regions of the world, including the semi-arid grassland biomes of North America. Periods of water stress routinely occur in semi-arid grasslands, therefore, the consequences of even greater aridity on ecosystem services such as aboveground net primary production (ANPP) warrants considerable attention. Integrating complementary field studies that spanned a latitudinal gradient, this dissertation had three main objectives. The first was to assess the sensitivity of the shortgrass steppe (SGS), a semi-arid grassland that extends from northern Colorado to New Mexico and Texas, to varying degrees of drought. The second was to explore whether invasibility of the SGS increased following drought. And the third was to examine drought legacy effects on ecosystem functions of this semi-arid grassland after drought conditions subsided. Results from the study assessing drought sensitivity revealed variability in how these ecosystems responded to drought, ranging from no reductions in ANPP at the southern end of the latitudinal gradient to 51% reductions in ANPP at the northern end. I concluded that patterns of ANPP responses to drought across these grasslands were strongly related to rainfall event size. Specifically, when rainfall regimes were characterized by many small events, significant reductions in ANPP occurred whereas when rainfall regimes were dominated by fewer events that were larger in size reductions in ANPP did not occur. Results from the study exploring whether invasibility increased during the post-drought period showed that drought can create a "window of opportunity" for plant invasions – even in invasion resistant semi-arid grasslands – if rainfall is sufficient and soil nitrogen is elevated following the drought. This study found that invasibility increased in the northern end of the latitudinal gradient but not in the central region or the southern end, thus indicating that invasions are highly context-dependent. Finally, results from the study that examined drought legacy effects revealed that impacts of drought persisted into the post-drought year at two of the three sites in the SGS biome and that these were related to decreases in plant cover and increases in soil nitrogen. Results showed that when soil N is elevated post-drought, there may be a positive legacy effect, but when soil N is not higher following drought, a negative legacy effect will likely occur. Overall, these results indicate that the semi-arid SGS grasslands I studied were generally quite resilient to drought. However, when these ecosystems displayed drought sensitivity, invasion windows and legacy effects were evident.