Long-term effects of pest-induced tree species change on carbon and nitrogen cycling in northeastern U.S. forests: A modeling analysis

Abstract

Invasive insects and pathogens can cause long-term changes in forest ecosystems by altering tree species composition, which can radically alter forest biogeochemistry. To examine how tree species change may alter long-term carbon (C) and nitrogen (N) cycling in northeastern U.S. forests, we developed a new forest ecosystem model, called Spe-CN, that allows species composition to shift over time. We simulated the effects of species change due to three invaders—beech bark disease (BBD), hemlock woolly adelgid (HWA), and sudden oak death (SOD)—on forest productivity, C storage, and N retention and loss over a 300-year period. The model predicted changes in C and N cycling rates and distribution between vegetation and soils after stands were invaded, with the magnitude, direction, and timing dependent on tree species identity. For a stand in which sugar maple (Acer saccharum Marsh.) replaced American beech (Fagus grandifolia Ehrh.) due to BBD, the model predicted a change from net C loss (−13% after 100years) to net C storage (+10% after 300years), as plant C gain (+36%) overtook C loss from soils (−11%) and downed wood (−24%). Following replacement of eastern hemlock (Tsuga canadensis (L.) Carr.) by yellow birch (Betula alleghaniensis Britt.) due to HWA, early loss of forest floor C (−28% after 100years) was exceeded by gain of plant and downed wood C after 145years; by 300years, total C differed little between invaded and un-invaded stands. Where red maple (Acer rubrum L.) replaced red oak (Quercus rubra L.) due to SOD, loss of plant and soil C generated net C loss (−29%) after 100years that continued thereafter. In contrast to C, for which patterns of storage and loss differed considerably among invasion scenarios, total N was ultimately lower following invasion across all three scenarios. Predicted nitrate leaching was also correspondingly higher in invaded vs. un-invaded stands (+0.3gm−2year−1 of N from nitrate), but the leaching increase lagged by nearly 100years following HWA invasion. Together, these results demonstrate that the effects of pest-induced tree species change on forest C and N cycling vary in magnitude, direction of effect, and timing of response following invasion, depending on the identity of the declining and replacing species, and that species-specific modeling can help elucidate this variation. Future predictions will need to account for tree species change to generate meaningful estimates of C and N storage and loss.