Effects of simulated N deposition on understorey vegetation of a boreal coniferous forest

Abstract

Summary 1.We report the short-term effects of simulated nitrogen (N) deposition on the understorey of a boreal forest in northern Sweden. Doses of 15 N double-labelled NH 4 NO 3 (0·5, 12·5, 25·0 and 50·0 kg N ha -1 ) were applied to 1 m 2 plots in the summer of 1995 and plants were harvested the following autumn. 2. No significant treatment effects were found in either above-or below-ground biomass which was distributed as follows: the ericaceous shrub Vaccinium myrtillus contributed 76%, the grass Deschampsia flexuosa 4%, and the bryophytes Dicranum majus and Pleurozium schreberi 20%. 3. The recovery of applied N in these species was 24, 27, 27 and 32% of the 0·5, 12·5, 25·0 and 50·0 kg N ha -1 applications, respectively, and thus the recovery increased with the N dose. 4. In the 0·5 kg N ha -1 treated plots, the highest concentrations of fertilizer-derived 15 N were found in the bryophytes, while in plots given the three higher N applications, leaves of D. flexuosa had the highest concentrations. 5. N application resulted in elevated concentrations of free amino acid N, which indicate increased storage of N in plant tissues. Furthermore, the N application resulted in increased damage to V. myrtillus by natural enemies. The severity of disease caused by two foliar parasitic fungi showed a clear numerical response to N treatment, as did the amount of herbivory as a result of Lepidoptera larvae. Key-words: Free amino acids, parasitic fungi and herbivore damage, nitrogen uptakeFunctional Ecology (1998) 12, 691-699 691 Shaver & Kedrowski 1986;Lähdesmäki et al. 1990;Baxter, Emes & Lee 1992;Näsholm et al. 1994;Ohlson, Nordin & Näsholm 1995). Thus, it can be predicted that changes in the N supply will lead to changed interactions between plants, parasitic fungi and herbivorous insects (de Nooij, Biere & Linders 1992;Hatcher 1995). Predictions of how elevated N supply will affect species composition and community structure in a specific ecosystem therefore must include considerations of plant-herbivore and plant-fungus interactions.As a contribution to the discussion concerning critical loads for N in boreal coniferous forests, we have established a field experiment in northern Sweden, where the background deposition of N is low. Thus, both short-and long-term responses to elevated N availability can be studied. Although species replacement has been reported in several studies, little is known about the processes which precede changes in species composition of boreal understorey vegetation. We here describe the initial effects of simulated N deposition, in the range that ambient N deposition occurs in southern Sweden (Lövblad et al. 1992), on plant N uptake, plant biochemistry, and both plant-herbivore and plant-parasitic fungus interactions. Materials and methods STUDY SITEThe experiment was conducted within the Svartberget Research Forest (64°14 'N, 19°46 'E) in northern Sweden, 60 km north-west of Umeå. The area is located in the middle boreal zone (Ahti, Hämet-Ahti & Jalas 1968). The background deposition of N in t...

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