Indicator value of trees for soil reaction in cool-humid forests

Acid rain, soil mineral weathering, and the interception of dust and sea salt still obscure the evaluation of tree effects on soil reaction in mature natural forests. The null hypothesis that tree species and size do not differentiate soil pH was tested in four ways: (i) comparison of an evergreen conifer ecosystem with a history of elevated exposure to acidifying emissions vs. a deciduous angiosperm ecosystem in a rural area; (ii) soil trenching to exclude mineral nutrient and water uptake by trees in the two ecosystems; (iii) comparisons of soil pH and the accumulation of decomposed organic material on the soil surface (Oh) beneath boreal angiosperms, boreal conifers, and temperate angiosperms at the landscape level; and, (iv) stand-level comparisons of large trees and small trees as a proxy for cumulative ecosystem effects. Results show for mature mixed forests on acid soils below pH 5 that tree species and size do not differentiate soil pH although varying tree species composition among forest stands differentiates the magnitude of acid rain impacts and indicates the disturbance history of forest vegetation at the landscape level. Low exchangeable base cation pools compared to annual turnover in foliage corroborates the notion of homoeostasis in mixed and structurally diverse natural forests. Natural conifer admixture in northern hardwoods moderates spring soil warming and thereby slows down element cycling. However, conifer admixture does not exacerbate biogenic soil acidification. Significantly reduced soil pH beneath ruderal angiosperms at the landscape level suggests a high vulnerability of cool-humid forests on acid soils to biogenic acidification caused by the destruction of the forest canopy.