Soil Compaction Effects on Growth of Young Ponderosa Pine Following Litter Removal in California's Sierra Nevada

Increased use of heavy equipment and more frequent entry into forest stands has increased the potential for soil compaction and decreased productivity. We examined compaction and tree growth relationships on three California soils of contrasting textures (clayey, loamy, and sandy loam) on plots from which the organic soil horizon had been removed. Compacted and noncompacted treatments were compared. Changes in bulk density (D b ), soil strength, and total porosity, measured during the growing season, were greatest in the 15‐ to 30‐cm depth at all sites. Bulk density increases were greatest in the loamy soil (30%) and least in the sandy loam (23%). Total porosity decrease in the upper 45 cm averaged 20, 9, and 13% for the clay, loam, and sandy loam textures, respectively. In the 30‐ to 45‐cm soil depth, compacted soils reached critical water potentials (<−1.5 MPa) 50 d sooner in the loam and 67 d sooner in the clay. In the sandy loam, compaction extended the period of plant‐available water for 86 and 48 d in the 1‐ to 15‐ and 15‐ to 30‐cm soil depths. Midday stem water stress was greater for trees in compacted plots of loamy and clayey textures, but less on sandy loam. Soil compaction did not reduce tree growth universally in these 3‐ to 8‐yr‐old plantations. Effects were detrimental, insignificant, and beneficial for clayey, loamy, and sandy loam soils, respectively. Results show that compaction effects depend strongly on soil texture and soil water regime. Soil physical values, per se, are not always reliable criteria for evaluation.