Carbon sequestration and nutrient cycling implications of the evergreen understory layer in Appalachian forests

Evergreen understory communities dominated by mountain laurel (Kalmia latifolia L.) and/or rosebay rhododendron (Rhododendron maximum L.) are an important but often overlooked component of Appalachian forests. In the dense thickets in which these species often occur, they have high carbon sequestration potential and play important roles in nutrient storage and cycling. We used allometric modeling of the aboveground biomass to quantify the importance of K. latifolia and R. maximum, relative to overstory tree species, in driving biogeochemical cycling in the Central Appalachian mountains. Carbon sequestration and nitrogen and phosphorus storage potentials were investigated by running 50-year simulations of the ecosystem accounting model NuCSS for two situations: forests comprising the canopy overstory layer with or without the evergreen understory layer. When simulating forests in several test watersheds based only on the composition and biomass of the overstory canopy, these forests contain between 1631 and 4825 kg/ha less in overall C content and 41–224 kg/ha less N content than if the evergreen understory layer is included. Additional N uptake by evergreen understory vegetation was estimated to amount to between 6 and 11 kg N ha−1 yr−1 at year 50 for the overstory-with-understory forest compared to the overstory-only forest. Vegetation pool nutrient storage was higher by 2–4% for N, and by 2–14% for P at year 50 when R. maximum and K. latifolia were included in the model. Aboveground standing biomass of R. maximum and K. latifolia accounted for only a modest portion of the C sequestered and N stored in the forest ecosystems at the watershed scale. In contrast, notably higher amounts of C and N were simulated as stored in the forest floor and soil pools when the understory was included. N storage predominated in the forest floor compared to the soil pool when a larger amount of R. maximum was present in a watershed, most likely due to the larger amounts of recalcitrant litter produced annually by this species compared to K. latifolia. In addition, storage of P in K. latifolia and R. maximum exceeded expectations compared to their watershed-scale standing biomass.