Tree species-soils relationships in old-growth forests of the Oregon Coast Range

Graduation date: 2007Tree species directly and indirectly affect soil nutrient cycles. I sought to characterize soils and foliage associated with four common canopy tree species (Douglas-fir, western hemlock, western redcedar, and bigleaf maple) in mixed-species old-growth forests of the Oregon Coast Range and to determine whether and how soils differ among the tree species. Sampling was replicated at eight forest sites to assess the generality of tree-species soils relationships across this region. Forest floor (Oe+Oa horizon) and mineral soils (0 to 10 cm and 10 to 20 cm depth) were analyzed for pools and concentrations of major elements (C, N, P, Ca, Mg, K, and H). Foliar nutrient concentrations, which reflect active nutrient cycling by the trees, were also\ud determined for C, N, P, Ca, Mg, and K. Results were analyzed with respect to two complementary conceptual models of tree species-soils relationships. In a depth-based model, trees were inferred to influence soil properties if surface mineral soils (0 to 10 cm) differed beneath the canopies of tree species, but deeper mineral soils (10 to 20 cm) did not. Using a context-dependence model, I assessed whether species-based differences in each soil nutrient diverged, converged, or were constant as nutrient status increased across sites. Douglas-fir soils were characterized by greater mass of forest floor relative to other species and, at high-C and -N sites, large pools of C and N in mineral soils. Western hemlock soils and foliage were generally poor in bases, particularly Ca. Western redcedar soils and foliage were high in Ca and were low in P, and soil P was especially low at high-P sites. Bigleaf maple soils and foliage were rich in P and base cations, and soils had high available nitrate relative to other species at nitrate-rich sites. Overall, the depth-based model was best supported by data for pools of the weatherable elements P and Ca, while the context-dependence model was best supported by data for the atmospherically derived, biologically fixed elements C and N. This apparent dichotomy in patterns for soil pools of rock-derived vs. atmospherically-derived nutrients merits further investigation. Forest management or natural successional processes that foster stand dominance by a single tree species are likely to reduce soil nutrient heterogeneity relative to that of current old-growth forests, and in some cases, may reduce soil fertility