The use of soil organic matter as a criterion of the relative sustainability of forest management alternatives: a modelling approach using FORECAST

The purpose of this study was to evaluate the usefulness and sensitivity of parameters that describe various soil organic matter characteristics for the evaluation of harvesting impacts on ecosystem function, using an ecosystem-level, hybrid simulation model (FORECAST, a model developed from its predecessor FORCYTE-11). Four separate sets of forest management options were applied to both a simulated unmanaged Douglas-fir forest (with a high accumulation of forest floor, coarse woody debris, and the nutrients contained therein) and to a simulated Douglas-fir forest growing on a site depleted in organic matter and nutrients. These four options consisted of the combination of two rotation lengths (40 vs. 80 years), and two levels of biomass utilization (full-tree vs. conventional tree length harvest). Based on the model simulations, productivity on the previously unmanaged site dropped significantly (to 59% of the value for the original, unmanaged forest) by the end of the second 40 year rotation when subjected to an intensive management option (40 year rotation, full-tree harvesting). However, a rapid drop in the active soil pool of decomposing organic matter occurred and was not rebuilt. As a result of this reduction in the size of the active soil pool, nitrogen deficiencies (N-demand greater than N-uptake) occurred in the subsequent rotations. In contrast, the initially nutritionally degraded site responded positively to an intermediate level of management (80 year rotations, stem-only harvesting), with a steady increment in the active soil organic matter pool size over the 240 year simulation period. This response indicates that sites degraded by past activities generally have the ability to recover (aggrade) if put under a less demanding management regime. Change in the mass and dynamics of active soil organic matter provides a more rapid and sensitive parameter than tree growth when attempting to assess the sustainability of management alternatives. Examples are presented which illustrate the importance of the starting state condition, as well as the management system being applied, when attempting to assess the sustainability of management alternatives.