Abstract. Human activities have clearly caused dramatic alterations of the terrestrial nitrogen cycle, and analyses of the extent and effects of such changes are now common in the scientific literature. However, any attempt to evaluate N cycling processes within ecosystems, as well as anthropogenic influences on the N cycle, requires an understanding of the magnitude of inputs via biological nitrogen fixation (BNF). Although there have been many studies addressing the microbiology, physiology, and magnitude of N fixation at local scales, there are very few estimates of BNF over large scales. We utilized >100 preexisting published estimates of BNF to generate biome-and global-level estimates of biological N fixation. We also used net primary productivity (NPP) and evapotranspiration (ET) estimates from the Century terrestrial ecosystem model to examine global relationships between these variables and BNF as well as to compare observed and Century-modeled BNF. Our data-based estimates showed a strong positive relationship between ecosystem ET and BNF, and our analyses suggest that while the model's simple relationships for BNF predict broad scale patterns, they do not capture much of the variability or magnitude of published rates. Patterns of BNF were also similar to patterns of ecosystem NPP. Our "best estimate" of potential nitrogen fixation by natural ecosystems is -195 Tg N yr -•, with a range of 100-290 Tg N yr '•. Although these estimates do not account for the decrease in natural N fixation due to cultivation, this would not dramatically alter our estimate, as the greatest reductions in area have occurred in systems characterized by relatively low rates of N fixation (e.g., grasslands). Although our estimate of BNF in natural ecosystems is similar to previously published estimates of terrestrial BNF, we believe that this study provides a more documented, constrained estimate of this important flux.

We examined interspecific and intraspecific variation in tree seedling survival as a function of allocation to carbohydrate reserves and structural root biomass. We predicted that allocation to carbohydrate reserves would vary as a function of the phenology of shoot growth, because of a hypothesized tradeoff between aboveground growth and carbohydrate storage. Intraspecific variation in levels of carbohydrate reserves was induced through experimental defoliation of naturally occurring, 2-year-old seedlings of four northeastern tree species -Acer rubrum, A. saccharum, Quercus rubra, and Prunus serotina- with shoot growth strategies that ranged from highly determinate to indeterminate. Allocation to root structural biomass varied among species and as a function of light, but did not respond to the defoliation treatments. Allocation to carbohydrate reserves varied among species, and the two species with the most determinate shoot growth patterns had the highest total mass of carbohydrate reserves, but not the highest concentrations. Both the total mass and concentrations of carbohydrate reserves were significantly reduced by defoliation. Seedling survival during the year following the defoliation treatments did not vary among species, but did vary dramatically in response to defoliation. In general, there was an approximately linear relationship between carbohydrate reserves and subsequent survival, but no clear relationship between allocation to root structural biomass and subsequent survival. Because of the disproportionate amounts of reserves stored in roots, we would have erroneously concluded that allocation to roots was significantly and positively related to seedling survival if we had failed to distinguish between reserves and structural biomass in roots.

Studies of wind disturbance regimes have been hampered by the lack of methods to quantify variation in both storm severity and the responses of tree species to winds of varying intensity. In this paper, we report the development of a new, empirical method of simultaneously estimating both local storm severity and the parameters of functions that define species-specific variation in susceptibility to windthrow as a function of storm severity and tree size. We test the method using data collected following a storm that struck the western Adirondack Mountains of New York in 1995. For intermediate-sized stems (e.g., 40 cm DBH), black cherry (Prunus serotina Ehrh.) and red spruce (Picea rubens Sarg.) showed the highest rates of windthrow across virtually all levels of storm severity, while yellow birch (Betula alleghaniensis Britt.) and sugar maple (Acer saccharum Marsh.) had the lowest rates of windthrow. For much of the range of storm severity, the probability of windthrow for the most susceptible species was at least twice as high as for the least susceptible species. Three of the species, yellow birch, red spruce, and beech (Fagus grandifolia Ehrh.), had significantly lower probability of windthrow at a given storm severity in old-growth stands than in second-growth stands. Our results suggest that the distinctive abundance of these three species in old-growth forests of the Adirondacks is due, at least in part, to their ability to survive the intermediate-scale disturbance events that appear to dominate the natural disturbance regime in this region.Résumé : L'étude des régimes de perturbation par le vent a été limitée par le manque de méthodes permettant de quantifier la variation dans la sévérité des tempêtes et la réaction des espèces arborescentes à des vents de différentes intensités. Dans cet article, les auteurs font état du développement d'une nouvelle méthode empirique permettant d'estimer simultanément la sévérité locale d'une tempête et les paramètres définissant les variations spécifiques de susceptibilité au chablis en fonction de la sévérité de la tempête et de la dimension des arbres. La méthode a été testée à l'aide de données recueillies après une tempête qui a frappé la partie ouest des monts Adirondacks, dans l'État de New York en 1995. Pour les tiges de dimension intermédiaire (i.e. dhp de 40 cm), le cerisier tardif (Prunus serotina Ehrh.) et l'épinette rouge (Picea rubens Sarg.) présentaient les plus forts taux de chablis pour à peu près tous les niveaux de sévérité de tempête, tandis que le bouleau jaune (Betula alleghaniensis Britt.) et l'érable à sucre (Acer saccharum Marsh.) avaient les plus faibles taux de chablis. Pour une bonne partie de la gamme de sévérité de tempête, la probabilité de chablis des essences les plus susceptibles était au moins le double de celle des essences les moins susceptibles. Pour une sévérité donnée de tempête, trois des espèces : le bouleau jaune, l'épinette rouge et le hêtre (Fagus grandifolia Ehrh.), présentaient des probabilités plus faibles de chablis dans ...

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