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Belowground C and N storage is important in maintaining forest productivity and to C[O.sub.2] sequestration. How these pools respond to management is poorly understood. We investigated effects of repeated applications of complete fertilizer and competing vegetation control with herbicides on C and N storage in forest-floor, fine-root, and mineral-soil C and N pools to 1-m depth at three Pinus ponderosa P. Lawson & C. Lawson var. ponderosa plantations across a site quality gradient in northern California. Belowground C pools without treatment were 66, 153, and 199 Mg C [ha.sup.-1] for the low-, intermediate-, and high-quality sites, respectively, and N pools were 5.1, 6.7, and 6.5 Mg N [ha.sup.-1], respectively. Treatments increased tree-bole volume at 20 yr as much as 400%, while changes in C and N pools belowground were less dramatic. Herbicide treatment increased forest-floor C pools 35% at the poorer quality site. Fertilization increased forest-floor C and N storage 46 to 106% at all sites. Fertilization decreased fine-root C pools at 0 to 0.3 m at the most productive site 43% and increased this N pool 43% at the least productive site, but did not influence fine-root pools to 1 m. Fertilization increased mineral-soil C pools on lower quality sites, resulting in 12 to 57% more belowground C storage. At the intermediate site, fertilization increased total belowground N storage 12%. Results of this study suggest that the major sequestration mechanism up to this point in stand development is through gains in tree biomass rather than storage in fine roots and soil belowground. Abbreviations: SOM, soil organic matter.

Human population growth places increasing demands on the world's forests. Sustaining these forests will depend to a considerable extent on sustaining soil structure and function. Traditional scientific concerns about sustaining timber production focused largely on soil fertility, but recent interest has focused attention on the plant and animal community and on 'biodiversity'. Neither of these approaches is satisfactory on its own. They should be replaced with an ecosystem analysis approach that includes the biotic and abiotic components. The most fundamental issue in sustainable forest management is the long-term productive capacity of individual sites, which is determined to a considerable extent by the availability of soil resources. Understanding the sustainability of site productivity requires knowledge of the determinants of leaf area and its photosynthetic efficiency, and of the allocation of net production. Keys that determine production ecology in boreal and cool temperate forests include low soil temperature, slow rate of litter decomposition and the resultant character of the forest floor, and, in some areas, impeded soil drainage; these lead to limitations on nutrient availability. Natural disturbance influences these factors and thereby plays a vital role in maintaining productivity and several measures of biological diversity. Without stand-replacing disturbances, closed forest may not be the climax vegetation in many western and northern Canadian landscapes. While forests and soils should be protected from 'damage', sufficient disturbance should be sustained to respect the disturbance-related ecology of these forests. Ecological site classification and use of ecosystem-level management simulation models are important for the design of appropriate disturbance regimes.

Many forest management treatments are directly aimed at maintaining or enhancing forest productivity. There may also be secondary effects that detract from this goal. We discuss three case studies in Washington state in which several mechanisms may have led to adverse secondary impacts. In the first study, pulp and paper (PII) sludges were mixed into soil and growth of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco], noble fir (Abies procera Rehder) and western white pine (Pinus monticola Douglas ex D. Don) was monitored. There was a significant negative correlation of height and diameter growth and C/N ratio for Douglas-fir and western white pine (0.05 level). In a second study, effects of 50 yr of red alder (Alnus rubra Bong.) and Douglas-fir growth on soil chemistry and stand productivity were compared. When the 50-yr-old stands were cut and red alder was established by planting into the soil of the former Douglas-fir and red alder forests, lowered available P in the soil of the previous red alder stand was observed. In a third study, high rates of low C/N ratio organic matter (300 Mg [ha.sup.-1]) were added in municipal biosolids (N amount about 8000 kg [ha.sup.-1]) to Douglas-fir and grand fir [Abies grandis (Dougl.) Forbes] plantations. Excess organic N in the biosolids apparently mineralized, nitrified, and contributed to soil acidification and accelerated cation leaching. Severe Mg deficiency (0.25 g [kg.sup.-1] in biosolids-treated vs. 0.93 g [kg.sup.-1] in untreated area) might be the cause of observed foliar chlorosis and poor growth rates.