Estimating Uncertainty in Ecosystem Budgets.

Ecosystem budgets require a large amount of information, and this information is commonly assembled at multiple spatial scales and even multiple locations.  Early ecosystem nutrient budgets reported values for pools and fluxes without any indication of uncertainty, which led to some unjustified conclusions, such as a missing source of N in northern hardwood forests.  We review current examples of efforts to propagate estimates of uncertainty through budgetary calculations.  We consider the contribution of spatial variation using a case study from the Bartlett Experimental Forest comparing stand-level budgets at three stages of ecosystem development. Aboveground biomass and nutrient content varied 2-fold among young (12-16 yr old) stands, was intermediate in stands aged 26-29 years old (1.4 fold), and least in old (114-121 yr old) stands (only 5% variation in biomass across stands).  Species contributions varied more than total biomass at all ages of stands, with major species varying up to 2-fold in their contributions to biomass even in the old stands.  Belowground biomass was more variable in coarse than fine root fractions, because of the spatial scale of sampling.  Total N and organic P pools in soils varied 1.8 fold across six sites; total exchangeable cations varied 1.5 (K) to 2.3 fold (Ca).  Replicating measurements is probably better than using mathematical propagation of uncertainty estimates, because of the importance of spatial variation in ecosystems.