Invasive pests alter long-term carbon and nitrogen dynamics in northeastern U.S. forests
Forests of the U.S. have been subject to repeated invasions of destructive insects and diseases imported from other continents. Like other disturbances, these pests can produce short-term ecosystem effects due to tree mortality, but unlike other disturbances, they often target individual species and therefore can cause long-term species change in the forest. Because tree species vary in their influence on carbon (C) and nitrogen (N) cycles, pest-induced species change can radically alter the biogeochemistry of a forest. In this paper we use a modeling approach to examine how pest-induced species change may alter C and N cycling in forests of the northeastern U.S. We describe a new forest ecosystem model, Spe-CN, that distinguishes individual tree species and allows species composition to shift over the course of the model run, to simulate long-term ecosystem effects of species replacements due to invasive forest pests.
Preliminary estimates indicate that the mortality of eastern hemlock (Tsuga canadensis) by hemlock woolly adelgid and its replacement by deciduous species such as black birch (Betula lenta) will reduce forest floor C stocks by more than 50% and increase N leaching by more than 5-fold, but increase productivity by nearly 2-fold as the birch becomes established. Decline of American beech (Fagus grandifolia) from beech bark disease and its replacement by sugar maple (Acer saccharum) is predicted to decrease soil C storage by more than 20% and increase N leaching from the ecosystem by up to 80%. Ecosystem process responses to other invasive pests affecting the northeastern U.S. will also be discussed. The magnitude of these species-specific effects on C and N cycling is in many cases larger than direct effects expected from changes in climate and atmospheric N deposition, indicating that species change must be included in models that predict forest ecosystem function under future environmental conditions.