| Introduction | Community
Responses to Nitrogen Saturation
Differences in species response to nitrogen saturation may result in altered community structures. Species may differ in their tolerance of acidic soil, reactions to Al toxicity and in their demand for nitrogen. As some species decline from nitrogen deposition (e.g. spruce, beech) other species may increase (e.g. maple). Complex models of species interactions and biogeochemical changes need to be developed in order to predict community effects of nitrogen saturation. The ecosystem effects of nitrogen saturation may result in tree mortality, and some species may die at a faster rate than others. Also, the recruitment of new trees may be affected by nitrogen saturation in different ways for different species. Many studies have suggested that species may differ in their response to nitrogen saturation. For example, Lovett and Rueth (1999) examined nitrogen transformations (e.g. mineralization and nitrification) along a nitrogen deposition gradient and found strongly contrasting responses between beech (Fagus grandifolia) and maple (Acer rubrum) stands. Potential net mineralization and potential net nitrification increased with estimated nitrogen deposition in maple stands, but showed no correlation in beech stands (figures 1 and 2). Other studies by McNulty et al (1996) have suggested a shifting community structure between decidious trees species and evergreens. They found evidence that in plots with high mortality, decidious trees were regenerating much faster than spruce or fir. This and other evidence suggests that nitrogen saturation may cause a shift in community structure towards increased importance of birch and maple, and a spruce-fir decline in New England. They also suggest that, unless base cations are depleted, a shift in species composition will allow productivity of the forest to recover, and perhaps surpass pre-N-fertilization levels (figure 3).
Figure 3. Conceptual curve of changes in N availability and forest growth with continued N additions to high elevation spruce-fir forests (from McNulty et al 1996). Magill et al (1997) also found differing responses in NPP for pine and hardwoods stands following five years of treatment (Table I). In their study, pine stands increased NPP with a low nitrogen treatment, but decreased at high N fertilization, while hardwood stand NPP increased under both treatments. Table I. Aboveground annual NPP (in Mg ha-1
yr-1) in chronic-N-addition plots at Harvard Forest. Data
are 5-year averages for the years 1989 through 1993 (from Magill et al
1997).
In summary, these results suggest that the response of ecosystems to nitrogen saturation may depend on species composition, and that community structure may change as a result of nitrogen saturation. |
