Historical Perspective

That N can limit production in temperate forests was first revealed through experiments conducted by Mitchell and Chandler in the U.S. (1939) and Hesselman and Romell (Fig. 1) in Sweden (reviewed by Tamm, 1991). Meanwhile, many forest soil scientists tended to focus on the role of other elements, notably the so-called base cations, possibly because benefits of liming had been recognized in agriculture. The first generation of liming trials in forests were, thus, initiated to see if the treatment could increase forest growth by increasing the biological turnover of organic matter, rather than to counteract soil acidification (Hiittl and Zottl, 1993). Implications of the fact that N, unlike Ca, Mg, K, etc., is not supplied by weathering of minerals (but see Holloway et al, 1998) went largely unnoticed for some time.

Viro (1951; 1955) conducted an extensive survey of links between soil chemistry and forest productivity in Finland. He observed a correlation between exchangeable Ca and forest productivity. Most interestingly, Dahl et al. (1961), in a reexamination of his material, commented that the correlation was not firm evidence of a direct limitation of forest growth by Ca supply. Rather, they thought there must be an influence of the supply of Ca on the turnover of N in the soil. Subsequently, Dahl et al. (1967), in a regional survey in Norway, demonstrated a strong correlation between %N and base saturation of the mor layer (Fig. 2). They also showed that plant community composition and forest productivity changed continuously along the regression of %N versus base saturation. A similar pattern was found in a large survey comprising 921 forests in southern Finland (Lahti and Vaisanen, 1987), where exchangeable Ca (and pH) in the soil was correlated with %N, and where there was also a correlation between these variables and plant community composition and forest productivity (Fig. 2).

Early in the 20th century, Cajander (1909; 1926) described relations between forest vegetation type (the composition of the understorey or "field-layer" plant community) and forest productivity. Forest productivity and the distribution of forest types are global biogeochemical cycles iN the climate system

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FIGURE 1 Lars-Gunnar Romell in action trying to pull nutrients through the soil-plant system. This drawing may be used to illustrate the fact that N supply is linked to the supply of other elements. It was made by the Norwegian Dyring in 1948 on wall paper in the Flaka hut, 60 km from Umea in northern Sweden, where more systematic research on forest biogeochemistry started in the 1920s.

FIGURE 1 Lars-Gunnar Romell in action trying to pull nutrients through the soil-plant system. This drawing may be used to illustrate the fact that N supply is linked to the supply of other elements. It was made by the Norwegian Dyring in 1948 on wall paper in the Flaka hut, 60 km from Umea in northern Sweden, where more systematic research on forest biogeochemistry started in the 1920s.

closely intimately linked with landscape topography, i.e., more productive forests with tall herbs in the understorey are regularly found in toe slope areas (e.g., Hagglund and Lundmark, 1977). In fact, one can reasonably well predict forest productivity based on topographic maps alone (Holmgren, 1994).

After the alarming reports of forest decline in Central Europe in the late 1970s, the focus of discussions on forest nutrition was on

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Base saturation [%]

FIGURE 2 Correlations between %N and base saturation of the mor-layer: (a) O, in a regional survey in Hedmark county, Norway (Dahl et ill., 1967); (b)O, in a larger regional survey of southern Finland (Lahti and Väisänen, 1987); and (c) □, along the 90 m long gradient at Betsele, northern Sweden (Giesler et til., 1998). The regression line is from Dahl et al. (1967).

the negative effects of acidity, especially of Al3+, on roots (e.g., Htitterman and Ulrich, 1984). A dominating idea was that N in soil was already, or would soon be, in excess of plant demand because of high levels of N deposition (e.g., Nihlgard, 1985; Aber et al., 1989), and this is no doubt true for some areas of Central Europe, in particular. A contemporary model of acid deposition effects on tree growth (Sverdrup et al., 1992) treated N as a nonlimiting nutrient and the base cations Ca, Mg, and K as limiting. Hence, liming of forests was advocated as a means to counteract the suspected negative effects of soil acidification on tree growth even in Sweden (Sverdrup et al., 1994) despite the comparatively low load of acid deposition there. However, there was substantial experimental evidence to suggest that (a) forests in Sweden (as elsewhere in Fennoscanclia) were still, in general, strongly N-limited, and (b) within reasonable limits, further acidification should not pose an immediate threat to forest productivity in Sweden (Binkley and Hogberg, 1997). During much of the discussions about soil acidity effects on tree growth, some basic interactions between other elements and N supply were overlooked. A lower forest productivity on more acidic soils was to some extent interpreted as a growth decline caused by soil acidification, and not as a natural condition related to a low N supply and not necessarily involving any effects of acid rain (see the example from Betsele below).

It should be pointed out here that there is a major difference between the deeper forest soils that predominate in Fennoscandia and the shallow soils that are regionally important in Norway, in particular. The latter have limited buffer capacity and are particularly sensitive to acid rain.

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