Richard A. Hallett

Is Nitrogen Deposition Altering the Nitrogen Status of Northeastern Forests

Abstract Concern is resurfacing in the United States over the long-term effects of excess nitrogen (N) deposition and mobility in the environment. We present here a new synthesis of existing data sets for the northeastern United States, intended to answer a single question: Is N deposition altering the N status of forest ecosystems in this region? Surface water data suggest a significant increase in nitrate losses with N deposition. Soil data show an increase in nitrification with decreasing ratio of soil carbon to nitrogen (C:N) but weaker relationships between N deposition and soil C:N ratio or nitrification. Relationships between foliar chemistry and N deposition are no stronger than with gradients of climate and elevation. The differences in patterns for these three groups of indicators are explained by the degree of spatial and temporal integration represented by each sample type. The surface water data integrate more effectively over space than the foliar or soil data and therefore allow a more comprehensive view of N saturation. We conclude from these data that N deposition is altering N status in northeastern forests.

REGIONAL VARIATION IN FOLIAR CHEMISTRY AND N CYCLING AMONG FORESTS OF DIVERSE HISTORY AND COMPOSITION

Although understanding of nitrogen cycling and nitrification in forest ecosystems has improved greatly over the past several decades, our ability to characterize spatial patterns is still quite limited. A number of studies have shown linkages between canopy chemistry and N cycling, but few have considered the degree to which these trends can provide an indicator of forest N status across large, heterogeneous landscapes. In this study, we examined relationships among canopy chemistry, nitrogen cycling, and soil carbon:nitrogen ratios across 30 forested stands in the White Mountains of New Hampshire. Plots included a range of species (sugar maple, red maple, American beech, yellow birch, paper birch, red spruce, balsam fir, eastern hemlock) and were broadly grouped into two disturbance categories: those that were historically affected by intensive logging and/or fire and those that experienced minimal human disturbance. Across all plots, rates of net N mineralization and net nitrification were correlated wi...

FOREST BIOGEOCHEMISTRY AND PRIMARY PRODUCTION ALTERED BY NITROGEN SATURATION

Results from four intensive site-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.

DIRECT ESTIMATION OF ABOVEGROUND FOREST PRODUCTIVITY THROUGH HYPERSPECTRAL REMOTE SENSING OF CANOPY NITROGEN

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 rep- resents 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.

Forest ecosystem response to four years of chronic nitrate and sulfate additions at Bear Brooks Watershed, Maine, USA

Nitrogen deposition to forest ecosystems is of growing concern, as total N emissions to the atmosphere continue to increase globally. Potential negative effects on forests and surface waters include soil and water acidification, mobilization and leaching of aluminum and heavy metals, and nutrient imbalances in trees. In this paper we report the results of a chronic nitrogen amendment experiment at the Bear Brooks Watersheds in northern Maine (BBWM), and compare them with results from similar studies conducted in Massachusetts and Vermont. Treatments included low and high nitrogen (2000 and 4000 eq ha−1 year−1 as HNO3), low and high sulfur (2000 and 4000 eq ha−1 year−1 as H2SO4), and nitrogen plus sulfur (2000 eq ha−1 year−1 each), with three replicates per treatment. Initial net N mineralization rates were similar in all plots, and net nitrification rates varied between 4 and 9 kg ha−1 year−1 in the control plots over the 3 years of measurement (4–12% of net annual mineralization). In 1989, net N mineralization rates in treated plots were equal to or higher than control plot rates for all but the low S treatment while in 1990, measured rates for all treatments were lower than controls. Net nitrification increased in all but the control and low S plots by 1990, representing from 8 to 25% of net annual N mineralization across treatments in that year. Foliar N concentration in nitrogen treated plots was consistently higher than in the controls, and those differences generally increased with time. Differences in woody biomass increment and foliar litterfall were not statistically significant, although tree mortality did increase substantially in all but the low S treatment. In general, N leaching losses increased with increasing N additions. Nitrogen retention ranged from 93 to 97% of inputs in the control and N amended sites. Measurement of ecosystem pools shows that 70–92% of inputs to the N treated plots were retained in the soil pool, similar to estimates obtained by 15N analyses. Results from the external plots at Bear Brooks are similar to those from other nitrogen manipulation experiments at the Harvard Forest, MA and Mt. Ascutney, VT in several ways, but N retention was less than expected. We hypothesize that differences in previous land use history have had a greater effect on current N cycling rates than have differences in cumulative N deposition.

Sugar maple growth in relation to nutrition and stress in the northeastern United States

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.

Do Nutrient Limitation Patterns Shift from Nitrogen Toward Phosphorus with Increasing Nitrogen Deposition Across the Northeastern United States

Atmospheric nitrogen (N) deposition is altering biogeochemical cycling in forests and interconnected lakes of the northeastern US, and may shift nutrient limitation from N toward other essential elements, such as phosphorus (P). Whether this shift is occurring relative to N deposition gradients across the northeastern US has not been investigated. We used datasets for the northeastern US and the Adirondack sub-region to evaluate whether P limitation is increasing where N deposition is high at two geographic scales, based on N:P mass ratios. Using a model-selection approach, we determined that foliar N for dominant tree species and lake dissolved inorganic N (DIN) increased coincident with increasing N deposition, independent of relationships between foliar N or lake DIN and precipitation or temperature. Foliar P also increased with N deposition across the northeastern US for seven of eight deciduous species, but changed less across the Adirondacks. Foliar N:P therefore declined at the highest levels of N deposition for most deciduous species across the region (remaining nearly constant for most conifers and increasing only for black cherry and hemlock), but increased across all species in the Adirondacks. Ratios between DIN and total P (DIN:TP) in lakes were unrelated to N deposition regionally but increased across the Adirondacks. Thus, nutrient limitation patterns shifted from N toward P for dominant trees, and further toward P for predominantly P-limited lakes, at the sub-regional but not regional scale. For the northeastern US overall, accumulated N deposition may be insufficient to drive nutrient limitation from N toward P; alternatively, elements other than P (for example, calcium, magnesium) may become limiting as N accumulates. The consistent Adirondack foliar and lake response could provide early indication of shifts toward P limitation within the northeastern US, and together with regional patterns, suggests that foliar chemistry could be a predictor of lake chemistry in the context of N deposition across the region.

Foliar chemistry linked to infestation and susceptibility to hemlock woolly adelgid (Homoptera: Adelgidae)

Abstract Hemlock woolly adelgid (Adelges tsugae Annand) is an invasive insect pest that is causing widespread mortality of eastern hemlock. However, some stands remain living more than a decade after infestation. To date, this has been attributed to site and climatic variables. This multi-tiered study examines the role foliar chemistry may play in A. tsugae success and subsequent hemlock decline. Comparisons of resistant and susceptible hemlock species indicate higher concentrations of P and lower concentrations of N in resistant species. On experimentally colonized hemlocks, the numbers of live sistens present after two A. tsugae generations was correlated with higher K and lower P concentrations. A regional T. canadensis monitoring effort showed that concentrations of Ca, K, N, and P were most strongly correlated with A. tsugae densities, which was the driving factor in hemlock decline. From the results of this study, we hypothesize that higher N and K concentrations may enhance hemlock palatability, thereby increasing A. tsugae population levels, whereas higher concentrations of Ca and P may deter more severe infestations. Foliar chemistry alone can explain over one-half of the variability in hemlock decline witnessed at 45 monitoring plots across the northeastern United States. Combining chemistry and traditional site factors, an 11-class decline rating could be predicted with 98% 1-class tolerance accuracy on an independent validation set. These results suggest that foliar chemistry may play a role in eastern hemlock susceptibility to A. tsugae infestation and should be included in risk assessment models.

Assessing hemlock decline using visible and near-infrared spectroscopy: indices comparison and algorithm development.

Near-infrared reflectance spectroscopy was evaluated for its effectiveness at predicting pre-visual decline in eastern hemlock trees. An ASD FieldSpec Pro FR field spectroradiometer measuring 2100 contiguous 1-nm-wide channels from 350 nm to 2500 nm was used to collect spectra from fresh hemlock foliage. Full spectrum partial least squares (PLS) regression equations and reduced stepwise linear regression equations were compared. The best decline predictive model was a 6-term linear regression equation (R2 = 0.71, RMSE = 0.591) based on: Carter Miller Stress Index (R694/R760), Derivative Chlorophyll Index (FD705/FD723), Normalized Difference Vegetation Index ((R800 – R680)/(R800 + R680)), R950, R1922, and FD1388. Accuracy assessment showed that this equation predicted an 11-class decline rating with a 1-class tolerance accuracy of 96% and differentiated healthy trees from those in very early decline with 72% accuracy. These results indicate that narrow-band sensors could be developed to detect very early stages of hemlock decline, before visual symptoms are apparent. This capability would enable land managers to identify early hemlock woolly adelgid infestations and monitor forest health over large areas of the landscape.

Nitrogen Dynamics in Forest Soils after Municipal Sludge Additions

Nitrogen dynamics were assessed for a forested site in southern New Hampshire during the first and second growing seasons after surface application of an aerobically-digested, limed, liquid municipal sludge. Sludge was applied in June 1989, at 3.3, 6.9, and 14.5 M ha-1 (or 199, 396, and 740 kg TKN ha-1). Elevated net N mineralization (in situ buried bags) occurred in the organic and upper mineral soil horizons during the first two months after sludge application, but was similar to control levels thereafter. Net N mineralization was negligible at 30 and 60 cm soil depths. Foliar N concentration increased with sludge loading rates. Concentrations of NH4+ and NO3- in soil leachate were low, except in early fall when microbial activity was still high and plant demand was low. In trenched subplots where vegetative uptake was eliminated, NO3- concentrations in soil leachate (60 cm) rose to between 15 and 35 mg N L-1 in the first year and remained high in the second year. Other studies reported higher magnitudes of NO3- leaching from treated plots. These studies and the findings reported here shown the characteristics of the sludge being applied to land are at least as important as the physical and chemical characteristics of the site to which they are to be applied.