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Consequences and recovery after nutrient enrichment and herbivore reduction in the boreal forest understory

deKoning, P. K. 2011. M.Sc. thesis. University of British Columbia

Abstract

Atmospheric nitrogen deposition poses a serious threat toplant communities globally. Furthermore, nitrogen-induced shifts in plant community composition may create positive feedbacks via litter decomposition by the soil microbial community. These feedbacks could prevent the recovery of plant communities, even in the absence of further nutrient addition, because nutrient availability and cycling remain high. We investigated the role of nutrients and herbivores in regulating plant and soil microbial communities in the boreal forest understory in northwestern Canada. We used an experiment that began in 1990 where plots were fertilized, exclosed (herbivore reduction), or both, in a 2 x 2 factorial design. In early 2000, plots were divided in half; treatments were continued on one half, and discontinued on the other half. In 2009, we sampled plant community composition, along with carbon/nitrogen ratios and total phenolics in the plant tissue. Using phospholipid fatty-acid analyses and extracellular enzyme activity assays, we estimated the soil microbial community composition and activity. Lastly, we measured soil pH and chemistry. Overall, fertilization had significant impacts on the variables we measured; herbivore exclosures mostly had no detectable impacts. In fertilized plots, species richness declined and the plant community became dominated by Epilobium angustifolium and Mertensia paniculata. Total phenolics and the carbon/nitrogen in the plant tissue declined. The total microbial biomass declined, as did the ratio of fungi to bacteria, indicating a more bacteria-dominated food web in the soil. Extracellular enzymes involved in the breakdown of cellulose increased in activity, but those involved in the acquisition of nitrogen and phosphorus were mostly unaffected, except urease, which declined in activity. Soil pH declined significantly, and fertilizer increased the availability of many nutrients. In recovery plots, the results do not fit the predictions of a plant-soil feedback hypothesis. Instead, the system follows a "cascade of responses", where soil chemistry recovers first, then plant tissue chemistry, followed more slowly by plant community composition. After 10 years, the soil microbial community has yet to show significant signs of recovery. These results highlight the sensitivity of the boreal forest to nutrient enrichment and demonstrate that recovery of these ecosystems may take decades.