Nitrogen Supply and Forest Productivity in a Landscape Perspective Hypotheses

There may be several explanations for the increase in forest productivity (and the correlation between base cations and N) down slope major slopes in Fennoscandian forest landscapes (Tamm, 1991; Giesler et al., 1998; Table 1). One category of these explanations (Fig. 3) refers to transport of N toward toe slope areas. As N is lost by leaching from large recharge areas, there should be a concentrated flux of N in the smaller groundwater discharge areas. Rohde (1987) reported that groundwater discharge areas on average comprise approximately 10% of the landscape in Swedish forests, which would lead to a ninefold increase per unit area of the N flux in the groundwater discharge areas as compared to that in recharge areas. Transport of N should increase as a result of disturbances, e.g., after a forest fire, the flux of NO,- should increase (Reiners, 1981). It may well be that the generally wetter ecosystems in toe slope positions acted as comparatively intact biological nutrient traps, while forests upslope were more damaged by fire (as fires usually climb upslope), and hence became more leaky. Also, according to this kind of explanation, the correlation between N and base cations occurs because they are solutes in the same water flow.

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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).

TABLE 1 Theoretical N Balance over a Period of 100 Years in a Discharge Area and Its Associated Recharge Area in Boreal Forest in Northern Sweden*

Recharge area

Discharge area

Deposition1' N: fixation1' Fire"

Leaching1' Lateral inflow

Balance

"It is assumed that the recharge area covers 90% of the total area. All figures are in kg N ha-1 100 year-1.

'Has increased from a low preindustrial (and pre-intensive-agricul-tural) background to 2 kgN ha-1 year-1 (Lovblad et al., 1995).

''Estimates based on Nohrstedt (1985).

"Assumes a fire return interval of 100 years (Zackrisson, 1977), and that fires consume mainly the upper part of the mor-layer and some woody debris on the ground.

''Leaching data from Degermark (1985; 1987; 1988; 1989). The high value given in brackets for the discharge area was calculated to bring its N-sequestration rate down to the level of that in the recharge area.

However, stream water and groundwater originating in recharge areas in unpolluted parts of Fennoscandia have never been reported to contain appreciable amounts of N (e.g., > 2 kg N ha-1 year" '), except in connection with disturbance, e.g., forest N fertilization and clearfelling (Tamm et al, 1974; Wiklander, 1981). After clearfelling, forests downslope of the felled area may show increased growth, which, given the strong N limitation, may be due to a flux of N from the clearfelling to the forest downslope (Lundell and Albrektsson, 1997). In contrast to the situation with N, the concentrations of base cations, notably of Ca, is higher than the N concentration in groundwater, in particular when the water has passed a long way through the soil and the bedrock. Data from a typical recharge area on acidic bedrock in northern Sweden show that the concentration of Ca is an order of magnit ude higher than that of total N in groundwater (Degermark, 1985; 1987; 1988; 1989). Back-of-the-envelope calculations suggest that losses of N from such recharge areas commonly are ~ 1 kg N ha 1 year-1, potentially contributing to a flux of ~9 kg N ha-1 year-1 in discharge areas, given that the latter compose 10% of the area (see above and Table 1).

Another category of explanations (Fig. 3) refers to in situ processes in the groundwater discharge areas. This category of explanations of the higher N supply in discharge areas includes (Tamm, 1991; Chapin et al., 1988; Giesler et al., 1998):

FIGURE 3 Possible interpretations of mechanisms underlying the increase in N supply that regularly occurs down slopes in Fennoscandian boreal forests. In the upper graph, the importance of the flux of N (and base cations) from recharge to discharge areas is stressed. In the lower graph, it is emphasized that fluxes of N down slopes are small, and that there must be processes promoting the high in situ N turnover in discharge areas, and that these processes are probably linked to the high pH (which is maintained by the high flux of base cations) in such areas.

conditions more conducive for N2-fixation, at present or earlier in site history, higher rates of autotrophic nitrification because of higher soil pH, higher rates of N mineralization and higher in situ flux of solutes to roots and mycorrhizae because of wetter conditions, and

FIGURE 3 Possible interpretations of mechanisms underlying the increase in N supply that regularly occurs down slopes in Fennoscandian boreal forests. In the upper graph, the importance of the flux of N (and base cations) from recharge to discharge areas is stressed. In the lower graph, it is emphasized that fluxes of N down slopes are small, and that there must be processes promoting the high in situ N turnover in discharge areas, and that these processes are probably linked to the high pH (which is maintained by the high flux of base cations) in such areas.

4. smaller losses of N during fires because of wetter conditions (and lower position in the landscape).

There is evidence from a study of 20 sites in central Sweden that N2 fixation by free-living microorganisms is positively correlated with pFI and extractable Ca (Nohrstedt, 1985), which commonly are higher in discharge as compared to recharge areas (but see the example from Betsele below). As regards 2 and 3, it is often held that N turnover (and especially autotrophic nitrification) in acid soils should be stimulated by an increase in pH (e.g., Alexander, 1977; Kreutzer, 1995). Water limitation is a complex issue, as it may affect activity of soil organisms or flux of solutes to roots (Chapin et al., 1988), as well as it might imply a direct water limitation on photosynthesis. Hence, it is difficult, even experimentally, to determine if water supply or its effects on N supply limit plant growth.

It is likely that processes of both categories described above (Fig. 3) are highly relevant. It appears complicated, but of great theoretical interest, to determine the relative importance of inflow of N versus increased in situ N turnover as components of the higher N availability in groundwater discharge areas.

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