Nitrogen Retention During Forest Succession: Testing a Revised Conceptual Model

Abstract:

An enduring concept in ecosystem ecology, proposed by Vitousek and Reiners in a 1975 paper, states that retention of a limiting nutrient such as nitrogen (N) in an ecosystem is controlled by the rate of biomass accumulation. This conceptual model would predict that over the course of ecosystem development after forest disturbance, N retention should be high in young, rapidly growing forests, and N retention should decline later in succession as the rate of biomass accumulation slows. Long-term data from the Hubbard Brook Experimental Forest in NH show that the reference watershed did not follow this pattern, in that a mature forest with near-zero net biomass accumulation had very high N retention and little N leaching. We proposed an alternative hypothesis that accounts for the exchange of N between the vegetation and the mineral soil. In our revised conceptual model, the soil serves as a source of N to the trees early in succession, and a sink for N later in succession as the N re-accumulates in the soil. We tested this hypothesis in a chronosequence of forest stands in the White Mountains of NH. Patterns of vegetation N stocks, root growth and nitrate retention in the mineral soil generally supported the hypothesis, but other plant and soil variables showed only weak patterns or no patterns at all. One clear pattern to emerge was the low capacity for N retention in both the soil and the vegetation in old-growth forests.