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Development and Persistence of Soil Legacy Effects of an Invasive Shrub and Implications for Reforestation


Invasive species can change the environment in ways that persist long after the species itself is gone, called a "legacy effect." Legacy effects of invasive species include changes to the abiotic and biotic soil environment and can lead to positive feedbacks that inhibit restoration success and reinforce invasion. The objective of this research was to examine the legacy of Cytisus scoparius italic>your words, an aggressive nitrogen-fixing shrub, on soil chemistry, ectomycorrhizal fungi, and Douglas-fir forest regeneration. My work highlights the importance of allelopathy and nitrogen enrichment in the disruption of the mutualism between Douglas-fir and ectomycorrhizal fungi.

The temporal aspects of biological invasions are important and understudied. The impacts of invasive species may change in magnitude and even direction with time following invasion. In a novel contribution, I used a combination of field and greenhouse experiments to determine how soil legacies develop, persist and change over time.

I found that soils invaded by C. scoparius italic>your words harbor less ectomycorrhizal fungi, and that growth of Douglas-fir is linked to the abundance of these fungi. Adding C. scoparius italic>your words mulch into uninvaded forest soils revealed that Douglas-fir grew more when mulch was added, but only in the presence of activated carbon, suggesting that allelopathy may be inhibiting Douglas-fir success.

To examine how long soil legacies persist after C. scoparius removal, I removed C. scoparius italic>your words at different times over a 22-month period, then measured soil properties. One month following C. scoparius italic>your words removal, nitrogen availability increased, but by 10 months, dropped to less than what was found in live C. scoparius stands italic>your words, suggesting that the legacy of nitrogen enrichment is not stable and declines with time following removal. Following C. scoparius italic>your words removal, exotic grasses quickly established and dominated the sites, and they may contribute to the suppression of Douglas-fir seedling growth. I also used a chronosequence approach to assess how quickly soil-mediated impacts develop after invasion. I planted Douglas-fir into soils collected from C. scoparius italic>your words invasions of different ages. Suprisingly, the seedlings grew larger in soils that had been invaded longer; in this case nitrogen fertilization seems to be more important than other effects of C. scoparius italic>your words invasion.

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