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Thesis Format



Master of Science




Henry, Hugh A. L.


Climate warming and atmospheric nitrogen (N) deposition are expected to have strong effects on plant productivity in temperate ecosystems over the next century. However, short-term experiments may not adequately address their potential effects because of lags in changes in species composition. I added new plots to a pre-existing field experiment to compare the short-term (1-2 year; new plots) vs. long-term (14-15 year; old plots) effects of warming and N addition on plant productivity, relative species abundances, plant tissue N content, and litter decomposition. In 2020, N addition increased aboveground plant productivity most in old plots and only increased belowground biomass in the old N plots. In 2021, N addition effects did not differ among old and new plots. There were no significant treatment effects on forb species composition. Overall, non-native C3 grasses appear to impede additional long-term responses of plant productivity to global change by suppressing changes in species composition.

Summary for Lay Audience

Across the globe, industrial and agricultural activities have intensified over the last century, which have led to an increase in greenhouse gas emissions. These gasses are causing an acceleration in climate warming, and as a result, average annual air temperature is predicted to increase by 1.5 °C to 2 °C in the coming decades. Climate warming increases will also intensify most towards the poles. Moreover, advancements in agricultural during the twentieth century has led to the overuse of nitrogen (N) fertilizers in many northern temperate regions, causing increased rates of N pollution in rain and snow. This N pollution is predicted to continue rising over the next century, and to negatively impact ecosystems by promoting undesirable changes in plant species composition and by increasing N runoff from land to water. I conducted a field experiment to examine the effects of warming and N addition on plant species composition, plant growth and the breakdown of dead plant material. In particular, I wanted to compare whether there were differences between short-term (1-2 year) vs. long-term (14-15 year) effects on plants. Overall, fertilized plots exhibited increases in aboveground grass biomass (primarily Kentucky Bluegrass) and increased breakdown of grass litter. Warming effects were minor and only influenced the breakdown of Canada Thistle litter. Lastly, over the past 15 years, there were no changes in the abundances of non-grass species, and the plots remained largely dominated by the grasses (Kentucky Bluegrass and Smooth Brome grass) present at the beginning of the experiment. These grasses appear to impede additional long-term aboveground responses of plant biomass and species diversity to global change (warming and N addition).