Does atmospheric deposition of nitrogen threaten Swedish forests?

Abstract

The health and productivity of forests is fundamentally important to many societies, and the culture and economy of Sweden are intimately linked with Sweden's forests. Are the health and productivity of Sweden's forests at risk from too much nitrogen from acid deposition? We evaluated this question by posing a number of specific questions, and synthesized information from the extensive research in Sweden on N deposition, fertilization and forest growth. We addressed the questions: Have Swedish forest soils acidified in recent decades? Are Swedish forest saturated with N, or do they have an excess of N? Will excessive N lead to forest decline? Will liming or vitalisation fertilization improve forest nutrition and health? We examined the ideas behind these questions, the available evidence, and whether the evidence supported or refuted the ideas. Several studies have documented reductions in soil pH (measured in water) in Swedish forests over periods of several decades. This acidification has been accompanied by increases in ionic strength of soil solutions, reduced base saturation and increased soil organic matter. The importance of each of these acidifying processes has not been investigated directly, but evidence supports a substantial role for each mechanism in at least some cases. Current expectations about rates of mineral weathering are not consistent with the evidence; rates of weathering appear to be greater than expected, and to differ depending on tree species. Swedish forests are not saturated with N; only a few stands near the southwest coast (with the highest deposition rates) show leaching losses of N that rival N deposition rates. Across all regions of Sweden, inadequate supplies of N limit forest growth. Within each region, some stands fail to respond to N fertilization (or respond with a decrease in growth), which is common for other forest types in other countries. Stands that have received heavy fertilization with N may become responsive to further fertilization with phosphorus (P) or base cations, and fertilization with trace amounts of boron (B) may be important on some soils. No evidence supports any widespread responsiveness of the forests to fertilization with other elements unless N is also added. Vitalisation fertilization (with non-N nutrients) has not demonstrated substantial improvements in tree growth, although most experiments have focused on N-limited sites rather than N-excess sites. Liming studies from Sweden and around Scandinavia indicate that forest health typically suffers after liming, including growth losses of 5 to 10% lasting one or more decades. The forests of Sweden averaged about 30% greater growth per hectare in the 1990s than in the 1950s, and extensive forest inventories show no indication of abnormal forest declines within this overall picture of improving growth. Hypotheses of declining forest growth as a function of the ratio of base cations to aluminum in soil solution can be tested with N fertilization experiments. The cation ratios uniformly declined with N fertilization, but growth typically increased, refuting the idea that cation ratios can represent changes in forest productivity. We conclude that no evidence supports the hypothesis of past or current deposition of N has reduced the health or productivity of Sweden's forests; the opposite may have occurred. We also stress that several important questions cannot be addressed satisfactorily with current information. In particular, we recommend additional research on sites where N leaching rivals N deposition, elucidating mechanisms controlling N leaching and responses of trees to excessive levels of soil N.