A NEW DECIDUOUS HARDWOOD FOREST BIOMASS MACRONUTRIENT UPTAKE STOICHIOMETRY DETERMINED AT THE SMALL WATERSHED-SCALE

Forest biomass is capable of having a significant influence on the chemistry of pore, ground, and surface waters. Macronutrient uptake by an aggrading forest can reduce solute concentrations in waters. Macronutrients released from a degrading forest can result in solute concentrations higher than would be produced by chemical weathering alone. Only during steady-state-biomass conditions, when the growth of new vegetation is exactly balanced by the death and decay of old vegetation, can the influence of biomass on water chemistry be ignored. However, steady state conditions rarely exist. Quantifying the role of biomass, therefore, is critical if solute mass transfers between ecosystem reservoirs are to be accurately determined.

A new biomass stoichiometry is introduced that is based on stream-water chemistry of small watersheds located in the Mid-Atlantic USA and covered by deciduous hardwood forests. The first approach utilized two adjacent watersheds that only significantly differ in their percentage of forest cover. These watersheds are those of the Brubaker Run and the House Rock Run in southeastern Pennsylvania, and are collectively termed the Octoraro watersheds. Because these watersheds only differed by percent forest cover, the difference in their stream-water K⁺, Mg²⁺, and Ca²⁺concentrations reflects the biochemical behavior of the biomass. The second approach involved the Bear Branch watershed of north-central Maryland, which is developed on unreactive quartzite bedrock, and thus the biomass is the dominant influence on its stream water chemistry. Sodium, which is neither biologically cycled nor appreciably derived from cation-exchange sites, was included in all calculations to verify the methods. To yield meaningful results, stream-water-concentration data must be acquired over representative fractions of growing and dormant seasons.

The calculated biomass macronutrient uptake stoichiometry for the Octoraro watersheds is K_1.0Mg_1.0Ca_1.4. This stoichiometry compares very favorably with that of K_1.0Mg_1.1Ca_0.97 determined for the Bear Branch watershed. The comparability of these two stoichiometries may indicate that they have application to deciduous forest-covered watersheds outside of the Mid-Atlantic region of the USA.