STUDENT INSTRUCTIONS

Support a Statement

Read the explanation below about the newly recognized phenomenon called nitrogen saturation and a study by Peterjohn et al. (1999) on Virginia forests. Then, with this information plus evidence in and your interpretation of Figs. 2A and 2B, write an essay in which you support the following statement from the Peterjohn study.

Not all forests in West Virginia are negatively affected by high concentrations of nitrogen in precipitation. Differences in symptoms of nitrogen saturation are related to aspect (e.g. south or east-facing slopes).

Develop your argument in support of this statement logically — in a logical order. This means that you should 1) introduce the focus and intent of your essay, 2) give the general ecology reader some background information about nitrogen saturation — what it is and why it is important to study, 3) describe Peterjohn et al.'s study — the question(s) they were asking and their overall approach, 4) describe their findings in Figures 2A and 2B, and 5) explain the significance of these findings to the overall issue of nitrogen saturation in forests. Word limit is 750 words.

Nitrogen Saturation

The phenomenon called nitrogen saturation of forests occurs when the availability of inorganic nitrogen (ammonium and nitrate) exceeds demand by plants and soil microbes. It is a response to human caused increases in nitrogen deposition — specifically oxides of nitrogen (nitrate and NOx) mainly from fossil fuel emissions and also ammonium from production and use of fertilizers. With increasing recognition of this problem since the 1980's ecologists have focused more effort on the capacity of forests to retain high inputs of nitrogen and also effects of excess nitrogen on forest vegetation. In addition, some of these studies target effects of high nitrogen inputs to streams, rivers, and lakes draining the affected forests.

Study by Peterjohn et al. 1999.

The Figures 2A and 2B are from study by Peterjohn et al. (1999) that focuses on forests in the Appalachian mountains because high rates of nitrogen (N) deposition there makes these forests especially susceptible to N saturation. The effects have been well documented in Watershed 4 in the Fernow Experimental Forest, West Virginia. Some parts of this forest showed clear evidence of N saturation (e.g. high concentrations of nitrogen in streams). However, Peterjohn and colleagues were surprised to find that part of the watershed — specifically south facing slopes — were not saturated with N because they responded positively, not negatively, to experimental addition of nitrogen fertilizers. To better understand the phenomenon of N saturation, they investigated this N/S slope difference further.

Peterjohn et al. used root ingrowth to measure response of trees to N fertilization. They added ammonium and phosphate as fertilizer to soil cores which they put in the ground in four 35m transects. Every 5m they buried 3 cores (for 3 levels of treatment — high, medium, and low), and after 5 months they measured root growth into the cores. These are the data in Figure 2A.

They also used instruments called lysimeters to measure concentration of the ion nitrate (NO₃^-) leaching down into the soil. Lysimeters are thin tubes placed vertically into the ground with a porous cup on the bottom; water in the soil can move into the little cup through tiny pores and nitrate is dissolved in that water. These data are in Figure 2B.

Peterjohn et al. also found that tree species composition was different on the south and east facing slopes. Acer saccharum (sugar maple) and Prunus serotina (black cherry) dominated the east facing slopes while on the south slopes. Nyssa sylvatica (black tupelo) and Fagus grandifolio (beech) were the dominant trees.

The researchers conclude the paper with the following: "If future studies support the hypothesis that species composition is a good indicator of a forest's susceptibility to N saturation, then community composition may account for a significant portion of the unexplained variability in the response of forested watersheds to similar levels of elevated N deposition … It would also suggest that management practices, or natural changes, which favor certain species might delay or accelerate the onset of N saturation and the potentially negative changes associated with this process."

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Literature Cited

• Peterjohn, W. T., C. J. Foster, M. J. Christ, and M. B. Adams. 1999. Patterns of nitrogen availability within a forested watershed exhibiting symptoms of nitrogen saturation. Forest Ecology and Management 119: 247-257.