Impacts of climate change-induced increased rainfall variability on invasive plant species and ecosystem processes
Continuing changes in climate, nitrogen availability, and biological invasions are projected to alter ecosystem structure and function with consequences for global biodiversity. As atmospheric temperatures rise, rainfall is projected to occur in larger events separated by prolonged dry periods and drought in many regions of the world. By altering the timing and availability of soil resources, this increase in rainfall variability along with intensifying anthropogenic nitrogen deposition resulting from the combustion of fossil fuels can strongly influence plant performance and ecosystem processes. These changes in soil resource availability may also either enhance or inhibit the ability of exotic plant species to dominate and degrade terrestrial ecosystems. Here, I examine the relationships among plant community composition, exotic plant invasions, and ecosystem processes and how they are influenced by increased rainfall variability and anthropogenic nitrogen enrichment. ^ Much of this dissertation considers work done as part of the Prairie Invasion and Climate Experiment (PRICLE). PRICLE ran for three years starting in 2012 and consisted of 12 plots established in a functionally diverse tallgrass prairie restoration initially co-dominated by Solidago canadensis and Schizachyrium scoparium. These plots were exposed to a factorial combination of increased rainfall variability and nitrogen addition in three replicate blocks. Increased rainfall variability was achieved using partial rain-out shelters that intercepted and excluded half of all ambient rainfall during the growing season (May-October). The volume of water excluded by shelters was calculated and applied to sheltered plots every 30 days over that period, thus consolidating half of the rainfall into large pulses while maintaining total cumulative seasonal rainfall. Nitrogen addition was performed using a slow-release polymer-coated urea fertilizer that was applied during the first week of June each year. To examine the impacts of these global change factors on ecosystem structure and function, I measured and analyzed aboveground plant productivity and associated carbon and nitrogen pools, herbaceous litter decomposition, and woody plant encroachment and phenology. ^ The impacts of increased rainfall variability and nitrogen addition on biogeochemical cycles were largely mediated by plant functional type-specific responses of productivity. Increased rainfall variability provided punctuated relief from an extreme drought during 2012 and continued to enhance soil water availability throughout the remainder of the experiment. These wetter conditions synergistically favored the production of clonal forbs at the expense of previously co-dominant C4 grasses and sub-dominant nitrogen fixers with nitrogen addition. The loss of nitrogen fixers under increased rainfall variability led to smaller aboveground nitrogen pools, but the expansion of clonal forbs increased aboveground carbon pools under increased rainfall variability in 2012 and under nitrogen addition in 2013. Increased rainfall variability and nitrogen addition also accelerated the decomposition and release of nitrogen from S. canadensis litter by enhancing the susceptibility of litter to decay and also by providing more favorable conditions for decomposition. S. scoparium litter was more resistant to decay when produced under increased rainfall variability, but more susceptible to decay when produced under nitrogen addition. However, these changes in decay rates were relatively minor in comparison to the magnitude of difference in decay rates between the two species. Thus, the indirect impacts of increased rainfall variability and nitrogen deposition on biogeochemical cycles via changes in plant community composition and plant productivity are likely more important than their direct impacts on litter decomposition and nitrogen loss. ^ Some invasive tree species may also become more successful under increased rainfall variability. Increased rainfall variability stimulated growth of exotic Lonicera maackii seedlings. However, nitrogen addition negated this benefit, likely as a result of increased competition from clonal forbs. Additionally, the wetter conditions associated with increased rainfall variability extended autumnal phenology of both L. maackii and native Quercus palustris and could potentially lead to even greater growth in following years. In an experiment independent of PRICLE, I considered the decomposition of these two species, as well as three additional native and three additional invasive tree species in native-invasive litter mixtures. There, I found invasive species to shift the release of nitrogen from litter mixtures to better coincide with plant demand for nitrogen and that this effect was more prevalent as invasive species made up a greater proportion of the mixture. Thus, the increased growth of L. maackii under increased rainfall variability will likely also improve the synchrony between plant demand for nitrogen and its release from the litter layer.^ Taken together, these findings suggest some species can strongly exploit global change through niche construction. Both increased rainfall variability and nitrogen addition favored S. canadensis and accelerated nutrient cycling via litter decay. Thus, S. canadensis may benefit from a positive feedback under global change in which S. canadensis further alters environmental conditions to favor its own dominance. Similarly, L. maackii was stimulated under increased rainfall variability and greater nitrogen availability and was also shown to improve the synchrony of plant nitrogen demand and litter nitrogen release. Therefore, both S. canadensis and L. maackii illustrate the propensity for increased rainfall variability and nitrogen addition to strongly alter ecosystem structure and function in a way that facilitates self-reinforcing niche construction mechanisms.^
Jeffrey S. Dukes, Purdue University.
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