Input–output nutrient budgets as a diagnostic tool for sustainable forest management

Abstract

Maintaining, and if possible increasing, soil fertility is a major goal for sustainable forest management, because it determines to a large extent the site's capacity for wood production. As, generally, forest soils are poor, and the demand for wood is still increasing, the boundary of resilience of each ecosystem must be known. Several cases must be identified: the case of natural or native species forests, either planted or not, which are extensively managed and the case of intensively managed plantations. In this paper, input–output budgets are calculated according to temporal and spatial scales. The measurement of the main inputs and outputs is discussed. When all the fluxes entering and leaving the selected compartment are measured, the budget provides a realistic picture of the direction of change in soil fertility. In other cases, the validity of the budget has to be considered according to the proposed hypothesis. A balanced budget means that the element considered is not depleted from the soil. This does not mean that no changes occur in the ecosystem. A positive budget means that the element considered is accumulating in the system, which is not always favorable. A negative budget means the element considered is depleted in the system, but it has to be related to the available soil reserves and to the rate of flux into available pool to be a real index of risk for the ecosystem. Information from several case studies shows the interest of these budgets because (i) they are able to characterize depletion before the direct analysis of the system would be able to indicate it, and (ii) they give useful quantitative data for recommendations applicable to forest management. These budgets have the disadvantage of being labor-intensive and expensive. They are also site-specific and give little direct information for identifying the mechanisms of nutrient tranformations within the ecosystem. The relationships between forest production and nutrient availability are not sufficiently known to be able to quantify the variation in production when nutrient depletion is observed. On account of the difficulty in calculating such a budget, the most efficient strategy could be the following: (i) calculate budgets in a representative network of sites; (ii) relate the individual budgets to more easily available parameters; and (iii) propose more simple indicators for managers. Land classification and the potential impact of forest management is required for giving appropriate recommendations to managers.