Biomass allocation and canopy development in spruce model ecosystems under elevated CO and increased N deposition.

Ecosystem-level experiments on the effects of atmospheric CO enrichment 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 m of 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 CO concentrations (280, 420, 560 μl l) and three different rates of wet N deposition (0, 30, 90 kg ha year) in a simulated annual course of Swiss montane climate for 3 years. The total ecosystem biomass was not affected by CO concentration, but increased with increasing N deposition. However, biomass allocation to roots increased with increasing CO leading to significantly lower leaf mass ratios (LMRs) and leaf area ratios (LARs) in trees grown at elevated CO. In contrast to CO enrichment, N deposition increased biomass allocation to the aboveground plant parts, and thus LMR and LAR were higher with increasing N deposition. We observed no CO ×  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 CO l, but was not significantly altered by increasing N deposition. Lower LAIs at elevated CO largely 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 CO (-18% at 420 and -31% at 560 compared to 280 μl CO l). 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 year). Our results suggest that under competitive conditions on natural forest soil, atmospheric CO enrichment may not lead to higher ecosystem biomass production, but N deposition is likely to do so. The reduction in LAI under elevated CO suggests 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 CO may indicate CO-induced reductions of soil N availability.