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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 100 years) 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 eastern hemlock (Tsuga canadensis Tsuga (L.) Carr.) by yellow birch (Betula alleghaniensis Britt.) due to HWA, early loss of forest floor C (-28% after 100 years) was exceeded by gain of plant and downed wood C after 145 years; by 300 years, total C differed little between invaded and un-invaded stands. Where red maple (A. rubrum L.) replaced red oak (Quercus rubra L.) due to SOD, loss of plant and soil C generated net C loss (-29%) after 100 years 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 (+0.3 g m-2 year-1 of N from nitrate), but the leaching increase lagged by nearly 100 years following HWA invasion. 3 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 speciesspecific 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.

Mosses play an integral role in the hydrologic regimes of ecosystems where they cover the soil surface, and thus affect biogeochemical cycling of elements influenced by soil oxidation-reduction (redox) reactions, including the plant growth-limiting nutrients, nitrogen and phosphorus (P). In rich fens where P often limits plant growth, we hypothesized that feedbacks between mosses and redox conditions would determine P availability to shallow-rooted forb species that constitute much of these wetlands' unusually high plant species diversity. In a moss removal experiment in three fens, forb tissue P and microbial P were greater while anion exchange membrane (AEM) resin P was lower where mosses occurred than where they were removed, suggesting both higher availability and greater demand for P in moss-covered soils. Coupled physicochemical and biological mechanisms drove moss effects on P cycling, ultimately through effects on soil oxygenation or reduction: higher redox potential underlying mosses corresponded to greater microbial activity, phosphatase enzyme activity, and colonization by arbuscular mycorrhizal fungi (AMF), all of which can promote greater P availability to plants. These more oxidized soils stimulated: (1) greater microbial activity and root vigor; (2) correspondingly greater P demand via microbial uptake, forb uptake, and iron (Fe)-P reactions; and (3) greater P supply through soil and root phosphatase activity and AMF colonization. This work demonstrates that mosses improve vascular plant P acquisition by alleviating stresses caused by reducing conditions that would otherwise prevail in shallow underlying soils, thus providing a mechanism by which mosses facilitate plant species diversity in rich fens.

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