| Introduction | Plant
Physiological Responses to Nitrogen Saturation
In nitrogen-limited systems, increased nitrogen availability will initially
result in increased productivity, a fertilizer-effect. However,
as nitrogen saturation progresses, primary productivity decreases due to
aluminum toxicity and nutrient imbalances (figure 1). These biogeochemical
changes can result in tree mortality due to increased succeptiblity
to stress (e.g. frost, pathogens), or directly from aluminum toxicity.
Acid deposition has been implicated in widespread declines of German spruce
and fir trees (Schulze, 1989) and in northeastern
United States spruce forests (Shortle and Smith, 1988;
Aber et al., 1989; McNulty et al., 1996).
Hypothesized physiological response of temperate forest ecosystems to long-term nitrogen additions (modified from Aber et al 1998).
In the early stages of nitrogen saturation, forest productivity and growth are stimulated by nitrogen inputs. Increased nitrogen availability results in greater leaf nitrogen contents and increased fine root nitrogen concentrations. For example, Magill et al (1997) found a mean increase of 25% in percent foliar nitrogen content of four species following five years of nitrogen fertilization (figure 2). There is a strong relationship (that holds across diverse ecosystems) between photosynthetic capacity and leaf nitrogen (Field and Mooney, 1996), due to a greater concentration of enzymes and pigments used in photosynthesis. Increases in photosynthesis lead directly to increased productivity and growth. Increases in fine root nitrogen have also been observed (Magill et al., 1997).
In the later stages of nitrogen saturation, productivity is expected
to decrease, and mortality may increase. This is partly due to nutrient
imbalances that result from the overwhelming availability of nitrogen and
other biogeochemical changes. The health
of plants is affected primarily by the relative concentrations of nutrients
as opposed to their absolute abundances (van Dijk and
Roelofs, 1988). As labile Al increases due to soil acidification,
the Ca:Al and Mg:Al ratios decrease. This is partly due to the higher affinity
of Al during pasive uptake by roots (Shortle and Smith,
1988). These relative nutrient proportions have been correlated
with declining Spruce populations in Europe (Schulze,
1989) and the United States. For example, McNulty and others
(1996) found a significant relationship between
foliar Ca:Al and net spruce growth in paired N fertilization plots (figure
3).
Figure 3. Relationship between foliar Ca:Al molar
ratios from red spruce trees (four paired N addition treatments, and one
paired control) on Mt. Acustney, Vermont (from McNulty
et al 1996).
Calcium has a number of important roles for plant functioning. For example, Ca is incoporated into new sapwood from cambial growth. Decreased Ca avaiability will restrict functioning sapwood, which would result in decreasing crown density. Magnesium is also an important element in plant enzymes, particularly chlorophyll. Limitations on chlorphyll production will neccesarily limit photosynthesis. Declining trees are often identified by the yellowing of leaves. This is believed to occur due to retranslocation of nutrients such as Ca and Mg during periods of new growth. Since the new growth is further stimulated by high nitrogen levels, the plant reallocates the neccesary Ca and Mg from leaves, regardless of its relative availability (Schulze, 1989). |
