Concern is resurfacing in the United States over the long-term effects of excess nitrogen (N) deposition

The concentration of nitrogen in foliage has been related to rates of net photosynthesis across a wide range of plant species and functional groups and thus represents a simple and biologically meaningful link between terrestrial cycles of carbon and nitrogen. Although foliar N is used by ecosystem models to predict rates of leaf‐level photosynthesis, it has rarely been examined as a direct scalar to stand‐level carbon gain. Establishment of such relationships would greatly simplify the nature of forest C and N linkages, enhancing our ability to derive estimates of forest productivity at landscape to regional scales. Here, we report on a highly predictive relationship between whole‐canopy nitrogen concentration and aboveground forest productivity in diverse forested stands of varying age and species composition across the 360 000‐ha White Mountain National Forest, New Hampshire, USA. We also demonstrate that hyperspectral remote sensing can be used to estimate foliar N concentration, and hence forest production across a large number of contiguous images. Together these data suggest that canopy‐level N concentration is an important correlate of productivity in these forested systems, and that imaging spectrometry of canopy N can provide direct estimates of forest productivity across large landscapes.

Mortality of sugar maple (Acer saccharum Marsh.) has reached unusually high levels across northern Pennsylvania since the early to mid-1980s. We evaluated the influence of glaciation, topographic position, foliage chemistry, defoliation history, and stand characteristics (species composition, structure, density) on the health of sugar maple in 43 stands at 19 sites on the northern Allegheny Plateau. Using percent dead sugar maple basal area as the measure of health, we found that all moderately to severely declining stands were on unglaciated summits, shoulders, or upper backslopes. Stands on glaciated sites and unglaciated lower topographic positions were not declining. The most important factors associated with sugar maple health were foliar levels of Mg and Mn and defoliation history. The lowest foliar Mg, highest foliar Mn, and highest number and severity of insect defoliations were associated with unglaciated summits, shoulders, and upper backslopes. Declining stands had less than -700 mg·kg -1 Mg and two or more moderate to severe defoliations in the past 10 years; both conditions were associated with moderately to severely declining stands. The decline disease of sugar maple seems to result from an interaction between Mg (and perhaps Mn) nutrition and stress caused by defoliation.Résumé : La mortalité de l'érable à sucre (Acer saccharum Marsh.) a atteint des proportions anormalement élevées partout dans le nord de la Pennsylvanie depuis le début ou le milieu des années 1980. Nous avons étudié l'influence de la glaciation, de la topographie, de la chimie foliaire, des défoliations passées, et des caractéristiques du peuplement (composition, structure et densité) sur la santé de l'érable à sucre dans 43 peuplements établis sur 19 sites dans la partie nord du plateau alléghanien. En utilisant le pourcentage de surface terrière que représentent les érables à sucre morts comme mesure de l'état de santé, nous avons trouvé que tous les peuplements où le dépérissement était modéré à sévère étaient situés sur les sommets, les contreforts, ou le haut des revers qui n'avaient pas subi la glaciation. Les peuplements situés sur des sites exposés à la glaciation ou des sites plus bas qui n'avaient pas subi la glaciation ne dépérissaient pas. Les facteurs les plus importants associés à la santé de l'érable à sucre étaient les concentrations foliaires de Mg et Mn et les défoliations passées. La plus faible concentration foliaire de Mg, la plus forte concentration foliaire de Mn, la sévérité la plus élevée et le plus grand nombre de défoliations d'insectes étaient associés aux sommets, aux contreforts, ou au haut des revers qui n'avaient pas subi la glaciation. Les peuplements dépérissants avaient moins d'environ 700 mg·kg -1 de Mg et avaient subi au moins deux défoliations modérées à sévères au cours des 10 dernières années. Ces deux facteurs étaient associés à des peuplements où le dépérissement était modéré à sévère. Le dépérissement de l'érable à sucre semble être le résultat d'une interaction entre la nutri...

Results from four intensive she-level manipulations and one extensive field survey in northern temperate and boreal forests show a consistent set of responses to chronic N additions. These include 1) initial and often large increase in net N mineralization followed by decreases, 2) increases in net nitrification. 3) increases in N concentration in foliage, and 4) decreased Mg:N and Ca:Al ratios, and declining tree growth and vigor in all evergreen stands. These results are synthesized into a set of proposed summary relationships that define the temporal pattern of responses of N-limited systems to N additions.

Sugar maple, Acer saccharum, decline disease is incited by multiple disturbance factors when imbalanced calcium (Ca), magnesium (Mg), and manganese (Mn) act as predisposing stressors. Our objective in this study was to determine whether factors affecting sugar maple health also affect growth as estimated by basal area increment (BAI). We used 76 northern hardwood stands in northern Pennsylvania, New York, Vermont, and New Hampshire, USA, and found that sugar maple growth was positively related to foliar concentrations of Ca and Mg and stand level estimates of sugar maple crown health during a high stress period from 1987 to 1996. Foliar nutrient threshold values for Ca, Mg, and Mn were used to analyze long-term BAI trends from 1937 to 1996. Significant (P < or = 0.05) nutrient threshold-by-time interactions indicate changing growth in relation to nutrition during this period. Healthy sugar maples sampled in the 1990s had decreased growth in the 1970s, 10-20 years in advance of the 1980s and 1990s decline episode in Pennsylvania. Even apparently healthy stands that had no defoliation, but had below-threshold amounts of Ca or Mg and above-threshold Mn (from foliage samples taken in the mid 1990s), had decreasing growth by the 1970s. Co-occurring black cherry, Prunus serotina, in a subset of the Pennsylvania and New York stands, showed opposite growth responses with greater growth in stands with below-threshold Ca and Mg compared with above-threshold stands. Sugar maple growing on sites with the highest concentrations of foliar Ca and Mg show a general increase in growth from 1937 to 1996 while other stands with lower Ca and Mg concentrations show a stable or decreasing growth trend. We conclude that acid deposition induced changes in soil nutrient status that crossed a threshold necessary to sustain sugar maple growth during the 1970s on some sites. While nutrition of these elements has not been considered in forest management decisions, our research shows species specific responses to Ca and Mg that may reduce health and growth of sugar maple or change species composition, if not addressed.