Agricultural site productivity: principles derived from long-term experiments and their implications for intensively managed forests

Long-term agricultural field experiments provide relevant information for questions being asked about the sustainability of managed forests, particularly as management regimes become more intensive. In this paper, concepts and criteria related to sustainable site productivity are reviewed and findings from a range of long-term agricultural field experiments are evaluated. Based on this evaluation, the following site productivity principles are identified: (1) soil organic matter is the Link between most management systems and sustainable site productivity; (2) nutrient deficiencies can be corrected; (3) soil texture is a key variable affecting soil organic matter and site productivity; (4) return of crop residues enhances soil organic matter and site productivity; and (5) productive cropping systems have environmental benefits. While technological advances have proven to be highly successful for enhancing long-term crop yields, they also have the potential to mask underlying declines in site productivity. This hypothesis needs to be rigorously tested. Agricultural field experiments show that crop productivity can be sustained for long time periods when appropriate management approaches are applied. Management requirements to achieve sustainability differ, however, depending on site-specific edaphic and climatic characteristics and the needs of the crop. Site productivity principles derived from agricultural field studies are also highly relevant to sustaining site productivity of managed forests. Several characteristics of managed forests should be considered when extrapolating conclusions from agricultural experiments. These include harvest removals of biomass and nutrients, residue type and distribution, management and system characteristics, and soil types used. Many characteristics associated with even intensively managed forests are sought as goals of conservation cropping systems and should contribute toward sustaining long-term site productivity. (C) 2000 Elsevier Science B.V. All rights reserved.