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2016

Tamm Review: Revisiting the influence of nitrogen deposition on Swedish forests

Binkley, Daniel; Högberg, Peter

Abstract

Are the health and productivity of Sweden's forests at risk from too much nitrogen (N) from acid deposition? Twenty years ago we assessed the evidence available for several aspects of this question (Binkley and Hogberg, 1997). We found little evidence for risks other than potential shifts in ground flora, but concerns continued to arise across Europe and elsewhere. We took advantage of two decades of accumulated evidence to re-evaluate whether Swedish forests are threatened by N deposition. During this time, N deposition declined by about 25% across the southern half of Sweden, and sulfur deposition declined by more than half. The growth rates of forests across the country continued the long-term trend of increasing by about 1-1.5% annually; average growth rates are now about 20-25% greater than in the mid-1990s. Forest soils often acidify by about 0.5-1.5 pH units during a rotation, but some evidence indicated that acidification may have occurred beyond this age-related pattern. Any average change in soil pH across the country appeared to relate more strongly to increases in carbon concentrations rather than cation leaching and declining base saturation. No evidence of N saturation (with outputs matching inputs) has been reported in Sweden, and nitrogen-limitation remains widespread. Fertilization with elements other than N generally does not increase growth unless N is also added, especially on mineral soils. Long-term fertilization experiments demonstrated that growth responses depended heavily on the dose rate of N application, not just cumulative totals. Repeated low rates of addition (20-50 kg N ha(-1) yr(-1)) provided greater growth increases per kg N added than higher rates, illustrating that the possible impacts of N deposition could not be reliably gauged by shorter-term experiments with unrealistically high dose rates. The composition of ground flora appeared to be sensitive to N additions, in studies across geographic gradients of N deposition and with high rates of N fertilization. Any long-term effects of N deposition on ground flora may be difficult to separate from long-term changes in stand structure and growth. Aluminum toxicity concerns do not appear to be supported by evidence, and liming generally does not increase forest growth. We discuss broader implications that arise from this assessment, including approaches to evaluating support for assumptions, the varying quality of types of evidence, and in some cases the irreducibility of uncertainty about elucidating cause-and-effect responses in complex forest systems.

Published in

Forest Ecology and Management
2016, Volume: 368, pages: 222-239
Publisher: Elsevier