Organic Matter Retention after Harvesting Coast Douglas-fir: Altered Microclimate, Vegetation Structure, and Soil Nutrient Pools Interact to Regulate Forest Productivity.

Historically, logging debris left after harvesting coast Douglas-fir (Pseudotsuga menziesii var. menziesii) was viewed as an impediment to tree planting and a potential fire risk, and was thus gathered and removed. More recently, logging debris has been seen as a potential resource for producing biofuels. However, over a decade of research across a gradient in site productivity in western Washington and Oregon has shown that organic matter retention can benefit soil quality and Douglas-fir productivity. The debris shields the soil surface, reducing ground-level solar radiation, soil temperature, and evaporative losses. Reductions in light, temperature, and germination sites from debris decreased abundance of competitive forbs and grasses and increased abundance of less-competitive vine species. At the same time, accretions of mineral soil carbon, nitrogen, potassium, and magnesium were greater with debris retention, particularly during the first five years after forest harvesting when differences in plant community structure were most acute. These changes in microclimate, plant community structure, and soil chemistry associated with organic matter retention generally have been beneficial to forest productivity, although magnitude of individual responses has varied among sites and intensities of competing vegetation control. For example, on a coarse-textured soil, reductions in Scotch broom (Cytisus scoparius) abundance from debris retention resulted in higher survival and 46% more volume of planted Douglas-fir by age 10 years. In contrast, retention of logging debris on finer-textured soils was associated with increases in growth but not survival of planted Douglas-fir. Combining competing vegetation control with organic matter retention generally resulted in additive, and sometimes synergistic, increases in tree growth. Thus, our studies have identified potential benefits to sustained forest productivity from organic matter retention across a gradient of sites in western Washington and Oregon, depending on soil characteristics and intensity of competing vegetation control.