Graduate Studies

 

Date of Award

Spring 2024

Document Type

Dissertation

Department

Biological Sciences

First Advisor

Chad Brassil

Second Advisor

Johannes Knops

Abstract

The availability of nutrients plays an important role in shaping the composition, productivity and diversity of plant communities. Such effects are particularly well documented in grassland ecosystems, where both independent studies and collaborations such as the Nutrient Network consistently show increasing productivity and decreasing diversity with the addition of limiting nutrients. The specifics of this pattern, however, vary widely from site to site, and within a single site, they may shift with changes in environmental conditions and disturbance regimes. My research at Cedar Creek Ecosystem Science Reserve (Minnesota, USA), documents such a pattern through resampling of a long-term experiment on pine encroachment into old field grasslands. In burned plots, where pine encroachment is inhibited, nitrogen addition shifts the herbaceous plant community from one dominated by perennial C4 grasses with native forbs and legumes to one where invasive C3 grasses are abundant. In unburned plots subject to pine encroachment, however, invasive C3 grasses are abundant regardless of nitrogen treatment. This community pattern does not, however, extend to the ecosystem’s overall carbon and nitrogen dynamics. While several ecosystem carbon pools respond to fire, they do not respond to nitrogen addition or to fire-nitrogen interactions. Alongside such measures of species composition and resource cycling, plant functional traits may provide valuable insights into community dynamics. Such traits show considerable intraspecific variability, and understanding this variation is critical to their effective use. At Cedar Point Biological Station (Nebraska, USA), grassland plant species’ functional traits show considerable variation with nutrient addition. I found, however, that these traits, and in some cases the strength of their nutrient responses, are also dependent on sampling year. Species display more acquisitive trait values both with nitrogen addition and in high rainfall years, and for leaf dry matter content, these effects interact to produce an elevated nitrogen effect in high rainfall years. Through experimental rainfall manipulation, I found that these patterns can be partially explained by total rainfall but that a substantial component remains to be explained by other components of annual environmental variation.

Comments

Copyright 2024, George Wheeler. Used by permission

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