title: Biomass allocation and canopy development in spruce model ecosystems under elevated CO2and increased N deposition.
abstract: Ecosystem-level experiments on the effects of atmospheric CO2enrichment and N deposition on forest trees are urgently needed. Here we present data for nine model ecosystems of spruce (Picea abies) on natural nutrient-poor montane forest soil (0.7 m2of ground and 350 kg weight). Each system was composed of six 7-year-old (at harvest) trees each representing a different genotype, and a herbaceous understory layer (three species). The model ecosystems were exposed to three different CO2concentrations (280, 420, 560 μl l-1) and three different rates of wet N deposition (0, 30, 90 kg ha-1year-1) in a simulated annual course of Swiss montane climate for 3 years. The total ecosystem biomass was not affected by CO2concentration, but increased with increasing N deposition. However, biomass allocation to roots increased with increasing CO2leading to significantly lower leaf mass ratios (LMRs) and leaf area ratios (LARs) in trees grown at elevated CO2. In contrast to CO2enrichment, N deposition increased biomass allocation to the aboveground plant parts, and thus LMR and LAR were higher with increasing N deposition. We observed no CO2×  N interactions on growth, biomass production, or allocation, and there were also no genotype × treatment interactions. The final leaf area index (LAI) of the spruce canopies was 19% smaller at 420 and 27% smaller at 560 than that measured at 280 μl CO2l-1, but was not significantly altered by increasing N deposition. Lower LAIs at elevated CO2largely resulted from shorter branches (less needles per individual tree) and partially from increased needle litterfall. Independently of N deposition, total aboveground N content in the spruce communities declined with increasing CO2(-18% at 420 and -31% at 560 compared to 280 μl CO2l-1). N deposition had the opposite effect on total above ground N content (+18% at 30 and +52% at 90 compared to 0 kg N ha-1year-1). Our results suggest that under competitive conditions on natural forest soil, atmospheric CO2enrichment may not lead to higher ecosystem biomass production, but N deposition is likely to do so. The reduction in LAI under elevated CO2suggests allometric down-regulation of photosynthetic carbon uptake at the canopy level. The strong decline in the tree nitrogen mass per unit ground area in response to elevated CO2may indicate CO2-induced reductions of soil N availability.
authors.affiliation: Botanisches Institut der Universität Basel, Schönbeinstrasse 6, CH-4056 Basel, Switzerland Fax: +41-61-267 35 04; e-mail: haetten@ubaclu.unibas.ch, , , , , , CH.
authors.forename: Stephan
authors.initials: S
authors.lastname: Hättenschwiler
authors.affiliation: Botanisches Institut der Universität Basel, Schönbeinstrasse 6, CH-4056 Basel, Switzerland Fax: +41-61-267 35 04; e-mail: haetten@ubaclu.unibas.ch, , , , , , CH.
authors.forename: Christian
authors.initials: C
authors.lastname: Körner
keywords: Key words Leaf area index; Litterfall; Nitrogen content; Picea abies; Productivity