Variations in allometrical relationship between stand nitrogen storage and biomass as stand development

Exploring the relationship between tissue nitrogen content (N) and its biomass (M) is crucial for understanding ecosystem carbon balance and N cycling because N is one of the limiting nutrients for the primary production in most terrestrial ecosystems. Although much has been learnt about N utilization and distribution within plant body, how N and M are coupled at the community level, especially for ecosystems after disturbance, remains inadequately understood. Prior work indicates that N and M often co-vary in a manner that can be expressed as a power law, mathematically taking the form N = beta M super(ga), where beta is an allometric constant and alpha is the scaling exponent. Although it has been demonstrated that N scales as approximately the 3/4 power of M across a wide diversity of plant communities, considerable uncertainty exists as to whether this 3/4 scaling relationship is valid for all forested communities or whether it changes as a function of forest successional stage. To fill this gap, data on N and M were collected from different woody species of three different forest community-types (i.e. cove hardwoods, mesic, mixed-oak community, and the dry, mixed-oak community) over 20-years of succession following commercial clear-cutting in a watershed (WS7) located in the Coweeta Hydrologic Laboratory. Total stand nitrogen content scaled nearly isometrically with respect to total stand biomass (i. e. N proportional to M super( approximately 1.0)) for 16 out of the 18 successional stages of the three community types. The mean scaling exponents of N vs. M for the cove hardwood community, the mesic, mixed-oak community, and the dry, mixed-oak community were 0.96, 0.99 and 0.97, respectively. Our observations make it reasonable to suppose that the isometric scaling relationship of N vs. M should scale up from the level of individual trees to the level of an entire forest stand at each particular successional stage. Despite this homogeneity in N vs. M scaling exponents across the different successional stages and the three community-types, all three forest communities had allometric constants that decreased systemically with increasing stand age. The highest Y-intercept was observed immediately after clear-cutting, whereas the lowest Y-intercept was observed approximately 20 years after cutting. Specifically, N scales as the 0.85-power of M across entire data set as a consequence of the systemic decrease in the numerical value of the allometric constant