Production in the forests of the world, with the exception of lowland tropical forests (Vitousek and Sanford, 1986), is generally restricted by lack of nitrogen, particularly in northern temperate and boreal regions (Vitousek and Howarth, 1991), or lack of phosphorus, in the tropics (Lloyd et al., n.d.). Nitrogen fertilization is an efficient means for enhancing agricultural production and is very effective in enhancing the productivity of forest plantations in Mediterranean, temperate, and boreal climatic regions (e.g., McMurtrie and Landsberg, 1992; Linder et al., 1996). Addition of nitrogen and phosphorus promotes the activity of photosynthesis per leaf, and nitrogen in particular stimulates increases in the number and growth of leaves, increasing the area of leaves in vegetation canopies. Many experiments worldwide have demonstrated that the growth of temperate forests is very responsive to the application of fertilizers, particularly of nitrogen (Linder and Rook, 1984; Tamm, 1991). For example, recent long-term experiments have shown a four-fold increase in the growth of Norway spruce in response to annual, complete fertilizer applications (75 kg N ha-1 yr-1) at 64�N over the past 12 years and a doubling of growth at 57�N over the past 10 years (Linder, 1995; Bergh et al., 1999). Maritime commercial spruce forest is also responsive to applications of nitrogen (Taylor and Tabbush, 1990; Wang et al., 1991). Comparable information from tropical forests is not readily available. The additional growth of vegetation may also lead to an increase in the amount of organic matter in the soil-or at least minimize the decrease brought about by tillage, harvesting, and other agricultural and forestry management practices.
Inventory data from sample plots indicate that the growth of trees has been increasing across Europe (Spiecker et al., 1996), and these trends have been observed elsewhere. Wet and dry deposition of nutrients from the atmosphere may be contributing to this enhancement of forest growth. In general, annual total (wet and dry) deposition of nitrogen (oxidized and reduced) to forests in rural areas is in the range of 5 to 40 kg ha-1 yr-1; the smaller amounts are in more remote forests, particularly at high latitudes and in the tropics (e.g., Forti and Moreira-Nordemann, 1991; Eklund et al., 1997; Freydier et al., 1998). Larger amounts are deposited on forests close to cities and industrial centers from which there are substantial nitrogen emissions, as well as in the near vicinity of intensive agricultural pig and poultry enterprises; this deposition may lead to problems such as acidification and loss of biodiversity. It is likely that such nitrogen inputs are supporting additional growth of young forests of particular relevance to the Kyoto forests and hence carbon sequestration (see Section 1.4; Cannell et al., 1999; Valentini et al., 2000), although recently this effect has been disputed (Nadelhoffer et al., 1999). A key uncertainty is to what extent and for how long the current annual rate of nitrogen deposition can sustain the growth rate and NEP of forests on Kyoto lands.
Other reports in this collection