Variation in overstory nitrogen uptake in a small, high-elevation southern Appalachian spruce-fir watershed

Abstract

High-elevation red spruce (Picea rubens Sarg.) -Fraser fir (Abies fraseri (Pursh) Poir.) forests of the southern Appalachians exhibit considerable spatial heterogeneity in structure, and possibly in N uptake, because of a combination of natural disturbances and heavy fir mortality caused by infestations of the exotic balsam woolly adelgid (Adelges piceae Ratz). The objectives of this study are to determine spatial variability in tree N uptake in a small highelevation catchment in the Great Smoky Mountains National Park, compare outcomes among calculation methods, and assess the influence of stand and landscape properties on N uptake. Tree N uptake is estimated for fifty 20 × 20 m plots in the Noland Divide Watershed (NDW). Components considered in the calculations are stem growth, foliage increment, and mortality of spruce, fir, and yellow birch (Betula alleghaniensis Britt.) from 1993 and 1998 stand inventories; throughfall N flux measured in summers 1998 and 1999; litterfall N return for 1 year in a subset of 12 plots; tissue N analyses; and atmospheric N deposition and root turnover estimates from the literature. Overstory N uptake varies spatially within NDW, with a CV of 9-41% depending on the calculation method. Variability among methods is even higher, with an almost 15-fold difference between the smallest and largest average overstory uptake estimate (5 vs. 74 kg·ha -1 ·year -1 ). Only 5 and 3 kg·ha -1 ·year -1 of N is sequestered in wood and foliar increment, respectively, while 36 kg·ha -1 of N returns annually as aboveground litterfall. Uptake and its components are correlated with measures of stand structure but not with elevation or aspect.Résumé : Des forêts de haute altitude d'épinette rouge (Picea rubens Sarg.) et de sapin Fraser (Abies fraseri (Pursh) Poir.) du Sud des Appalaches montrent une hétérogénéité spatiale considérable dans leur structure et probablement dans leur prélèvement en azote du fait d'une combinaison de perturbations naturelles et d'une mortalité importante du sapin causée par des infestations du puceron lanigère du sapin (Adelges piceae Ratz.). Les objectifs de cette étude consistaient à déterminer la variabilité spatiale du prélèvement en azote des arbres dans un petit bassin de haute altitude localisé dans le parc national des Great Smoky Mountains, comparer les résultats entre les méthodes d'estimation et évaluer l'influence des propriétés du peuplement et du paysage sur le prélèvement en azote. Le prélèvement en azote des arbres a été estimé pour 50 placettes de 20 × 20 m dans le bassin du Noland Divide (BND). Les éléments considé-rés dans les calculs sont : la croissance de la tige, la croissance foliaire et la mortalité de l'épinette, du sapin et du bouleau jaune (Betula alleghaniensis Britt.) à partir des inventaires de peuplement de 1993 et 1998; le flux d'azote dans le pluviolessivage mesuré pendant les étés 1998 et 1999; le retour en azote par la litière pour une année dans un sous-ensemble de 12 placettes; les analyses d'azote dans les tissus; les dépôts ...

Characterizing Episodic Stream Acidity During Stormflows in the Great Smoky Mountains National Park

“…However depending on various environmental factors, other ion contributions were also found to be important, such as nitrate and OA. Others have reported the potential of increased contribution of nitrate and OA to acidic episodes by insect defoliated forests (Eshleman et al 1998, Barker et al 2002. The GRSM has experienced die-off of Hemlock forests from the balsam woolly adelgid, and die-off patches are located in the Ramsey Prong (M2) watershed (personal communication, K. Johnson, GRSM, 2007).…”

“…It appears contributions of organic acids correlate with increased nitrate contributions, and such occurrences are dependent on season and watershed vegetation (Inamdar et al 2004;Wright et al 2006). Increased episode contribution of dissolved organics and nitrate has been observed in defoliated watersheds from insect infestation (Eshleman et al 1998; Barker et al 2002). Export of stream nitrate is complex, governed by several biogeochemical processes including forest uptake, nitrification and mineralization, soil saturation, and rapid periphyton uptake once nitrate enters a stream (Pauley et al 1996;Worrall et al 2003;Mulholland 2004).…”

Using Silviculture to Influence Carbon Sequestration in Southern Appalachian Spruce-Fir Forests

“…Trees in the NDW watershed are generally shorter than in other southern Appalachian (or nearby) spruce-fir forests [33]. To estimate tree height for red spruce, Fraser fir and yellow birch, we used site-specific allometric equations fit from the height-diameter data in Barker et al [33]. This resulted in at least three modeled individual tree heights for each species, on any given plot, the minimum necessary for SN-FVS to modify height growth to reflect local conditions.…”

“…The NDW is dominated by red spruce at lower elevations transitioning into Fraser fir at higher elevations with a component of yellow birch (Betula alleghaniensis Britton) and various other hardwoods distributed across the range of elevations. The NDW has not been impacted by logging or fire [32], but has been severely impacted by the BWA [33,34] and wind related events [15,18]. The soils are mainly Inceptisols, occasionally with spodic characteristics [35,36], are generally shallow, (<50 cm depth to bedrock) and have a silt loam to sandy loam texture [37].…”

“…SN-FVS has not been explicitly evaluated for spruce, fir or yellow birch; however, it has been validated and analyzed for other species [43][44][45] that displayed its ability to effectively simulate stand dynamics. Trees in the NDW watershed are generally shorter than in other southern Appalachian (or nearby) spruce-fir forests [33]. To estimate tree height for red spruce, Fraser fir and yellow birch, we used site-specific allometric equations fit from the height-diameter data in Barker et al [33].…”