Invasive insects and pathogens can cause long-term changes in forest ecosystems by altering tree species composition. Because tree species vary in their influence on carbon (C) and nitrogen (N) cycles, pest-induced species changes can radically alter the biogeochemistry of a forest. To examine how tree species change may alter long-term C and N cycling in northeastern U.S. forests, we developed a new forest ecosystem model, Spe-CN, that allows species composition to shift over time. We simulated the effects of change in dominant tree species due to three invaders on forest productivity, C storage, and N retention and loss over a 300-year period. Specifically, we simulated stands in which sugar maple (Acer saccharum) replaced American beech (Fagus grandifolia) following beech bark disease (BBD) invasion; yellow birch (Betula alleghaniensis) replaced eastern hemlock (Tsuga canadensis) following hemlock woolly adelgid (HWA) invasion; and red maple (A. rubrum) replaced red oak (Quercus rubra) following invasion by the organism that causes sudden oak death (SOD). In the northeastern U.S., BBD is already distributed throughout the region, HWA is spreading currently, and SOD has not yet reached the area.
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 replaced beech due to BBD, net C loss (-13% after 100 years) shifted to net C storage (+10% after 300 years) as plant C gain (+36%) overtook C loss from soils (-11%) and downed wood (-24%). Following replacement of hemlock by birch due to HWA, early forest floor C loss (-28% after 100 years) was exceeded by plant and downed wood C gain after 145 years; by 300 years, total C differed little between invaded and un-invaded stands. Where red maple replaced oak due to SOD, loss of plant and soil C generated net C loss (-29%) after 100 years that continued thereafter. In contrast to C, total N was ultimately lower with invasion across all three scenarios, and predicted nitrate leaching correspondingly higher (+0.3 g nitrate-N m-2 year-1), but the increase lagged by nearly 100 years following HWA invasion. Future predictions will need to account for tree species change to generate meaningful estimates of C and N storage and loss.