Experimental Evidence

Since the 1940s, experiments with additions of nutrients on hundreds of plots in the field have shown a more or less strong N limitation on forest growth in Fennoscandian forests (Tamm, 1991; Binkley and Hogberg, 1997). In such trials, additions of NII4NO, increase forest production, but they frequently acidify the mineral soil (Fig. 5; Tamm, 1991; Binldey and Hogberg, 1997). This means that the BC/A1 (or Ca/Al) ratio is lowered, while forest production increases, and definitely implies that the N supply has proximal control of forest productivity.

Furthermore, trees have a low demand for Ca in relation to the supply. Extensive laboratory tests, as well as field evidence, suggest that the demand for N is more than an order of magnitude higher than the demand for Ca in the two major conifer species, Picea abies and Pinus sylvestris (Ingestad, 1979; Linder, 1995), while in boreal forests, foliar analysis frequently shows that the concentration of N is at most only twice the concentration of Ca (e.g., Ed-fast etai, 1990).

FIGURE 5 The basic correlation between the supply of N (and forest production) and soil base saturation (cf. Fig. 2), and the effects (arrows) of experimental treatments on forest growth.

Base saturation

FIGURE 5 The basic correlation between the supply of N (and forest production) and soil base saturation (cf. Fig. 2), and the effects (arrows) of experimental treatments on forest growth.

Any positive effects of Ca on tree growth, as suggested by the correlation between Ca supply and forest growth, cannot thus be direct, but can potentially occur in a longer perspective, provided increases in soil pH increase soil N turnover. However, in typical boreal forest soils with a C/N ratio > 30 in the mor-layer, the reverse, i.e., increased N immobilization, commonly occurs after liming (Persson and Wiren, 1996) and is most likely the reason why forest growth often declines over a period of several decades after liming (Fig. 5; Derome et al, 1986).

It is noteworthy that experimental additions of elements do not, in the shorter term, necessarily mimic a higher natural level of supply. Microorganism communities likely evolve in relation to specific site conditions, to which sudden changes in nutrient supply rate, or pH, are a perturbation. Possibly, as suggested by the liming trials, it may take decades before the microorganism community is in balance with a new chemical regime. This contrasts to natural conditions, e.g., at a site like Betsele, where the discharge area has, because of its position in the landscape, maintained a relatively high and stable pi I since the last deglaciation 9200 year ago, while the surrounding soils in recharge areas have been acidified gradually through podzolization (but with forest fires as major intermittent disturbances leading to transient increases in soil pH).

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