Soil Source, Seed Source, and Organic-Matter Content Effects on Douglas-Fir Seedling Growth

Douglas-fir [Pseudotsuga menziesii var. glauca (Beissn.) Franco] seedling success on harvested areas in northern Idaho is often limited on harsh sites because of moisture and nutrient stress. This study was conducted to determine the importance of soil source, seed source, and organic matter on seedling growth and nutrition. Soil and seed were taken from both high- and low-elevation sites at the Priest River Experimental Forest, Priest River, ID. In a greenhouse, seed was sown in both home-source soil (taken from local seed-collection zone) and nonsource soil (taken from opposite seed-collection zone) mixes. Soil mixes included 100% organic, 50% organic/50% mineral, or 100% mineral soil. Seedlings were measured for height, bud length, root collar diameter, dry root and shoot weight, total N, total P, K, and number of ectomycorrhizal roots tips g⁻¹ of dry root. Both soil and seed sources had similar nutrient and growth characteristics for each of the three soil mixes. One hundred percent organic soils, after only one growing season, produced seedlings that were significantly taller and had greater bud length, root collar diameter, and biomass than those growing in either mineral soil or the organic/mineral mix. Total N and P uptake was greater in the organic soil than in the mineral soil or the mix, but K uptake was greater in pure mineral soil. Low-elevation seed-source trees were always taller than their high-elevation counterparts, an apparent result of adaptation to longer growing seasons. The best ectomy-corrhizal colonization took place in the mineral soil mix. Ectomy-corrhizae were most prolific on seedlings grown in nonsource soil when compared with those grown in home-source soil. Regeneration success in the Inland Northwest will be substantially improved by maintaining the integrity of organic horizons on harvested areas and by adhering closely to seed-transfer-zone guidelines.