Hydrology in watersheds that are important to the supply of clean water can be negatively impacted by forest pathogen outbreaks.
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Invasive Species and Forest Pathogens
When the introduction or increased activity of invasive or pathogenic plant and animal species dramatically alters the structure and function of ecosystems, the benefits that those ecosystems provide to people are also affected.
Colonization by invasive species and outbreaks of forest pathogens can have important hydrological effects, principally by changing the structure and function of forest vegetation. For example, the widespread and continuing die-off of Eastern Hemlock (Tsuga canadensis) in the Appalachians due to the invasive Hemlock wooly adelgid (Adelges tsugae) can alter stream hydrology, temporarily increasing stormflow peaks and causing long-term decline in total stream yield.
Highly invasive plants, particularly when they become densely established, can change soil properties in ways that reduce nutrient and water availability for native plants. Soil microbial communities are also negatively affected by some invasive plants across large areas. These changes in soil structure and function can be long term, negatively affecting native plant communities and ecological restoration efforts.
Timber and nontimber forest products
When they affect large areas, outbreaks of insect pathogens on forest trees are one of the most important risk factors for harvested tree species. Large outbreaks can also change forest structure and composition in a way that reduces habitat quality for nontimber forest species, such as harvested understory herbs. Dense colonization of forested landscapes by invasive, non-native plants can negatively influence regeneration of native tree species, including those with economic value. Invasives can also alter the composition and structure of forests, potentially reducing the overall productivity of marketable timber and nontimber species. The negative effects of forest pathogens and invasives on marketable species may be exacerbated by climate change, forest fragmentation, fire, and other landscape processes, which may facilitate outbreaks or invasions.
Over the short term, forest primary productivity is often significantly reduced after a major tree pathogen outbreak, slowing carbon sequestration. A forest stand can switch from a carbon sink to a source under these conditions depending on outbreak severity, but this process can reverse over the long term as forest productivity recovers. The cumulative impacts of multiple outbreaks across a landscape can reduce carbon storage and sequestration at regional scales, but the overall, long-term results of these dynamics are not well understood. Forest pathogen outbreaks are affected by climate change, because they can become more likely as extreme heat and drought events become more frequent.
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When the introduction or presence of invasive plant or animal species alters the structure or function of an ecosystem, the services provided by the ecosystem are also affected.