Ecosystem sensitivity to variation in precipitation and defoliation across grasslands of Alberta
Creators: Amgaa, Batbaatar
Publisher: name: University of Alberta Library
Year: 2022

<p>Climate models project a greater likelihood of increases in precipitation variability and droughts over the Great Plains of North America. Drought’s interactions with other co-occurring factors, such as grazing, can lead to substantial and long-lasting changes to ecosystem goods and services. In this thesis, I examine variations in precipitation and defoliation to better understand how their interactive effects shape ecosystem structure and function. First, I test the effects of grazing on ecosystem sensitivity to precipitation using spatially extensive long-term records of aboveground net plant productivity (ANPP) and precipitation. I found that grazing increased ANPP sensitivity to interannual variability in precipitation, especially at arid grasslands, suggesting that the explanatory power of the precipitation-ANPP relationship may not hold for ecosystems subjected to two or more global change drivers. Second, I examine the commonly assumed but little tested hypothesis – that drought impacts are progressive through time. I do so using a factorial experiment crossing drought and defoliation that I conducted at seven northern temperate grasslands over four years. I found that multi-year drought led to greater changes to community composition than productivity, and effects did not compound through time. Shifts in species composition were driven by variation in the abundance of dominant species, which also likely resulted in stability in ANPP under drought. Third, I examine the sensitivity of both shoot and root biomass to the combined effects of drought and defoliation. This topic is important because root responses are often overlooked in ecosystem models. I found that ANPP increased under drought relative to the ambient treatment, and the combined effects of drought and defoliation reduced ANPP but had no impacts on root biomass. Moreover, using minirhizotrons, I further examine root length dynamics in response to drought and defoliation at two of my seven field sites. I found root length dynamics were affected by every factor manipulated or measured in this thesis. In general, I found decreases in length, production, and lifespan of roots with drought in the shallow soil depth in the absence of defoliation. Notably, I found that the two sites exhibited general convergence in many aspects of their root length dynamics when exposed to both drought and defoliation, but when differences existed, they occurred under the ambient condition or at the deep soil depth. Further, the mortality and lifespan of roots were the aspects of root length dynamics that caused the observed shifts in total root length resulting from drought and variation in defoliation. In sum, this thesis demonstrates that northern temperate grasslands are highly resistant to a reduction in water availability via stability among dominant species or root responses, particularly root length dynamics, but defoliation, regardless of what regime, makes these systems sensitive to drought. Further, my results emphasize that the combined effects of global change drivers on plant root and shoot responses must be included in ecosystem models and will more accurately project ecosystem sensitivity to future variable weather.</p>