Hättenschwiler Stephan, Körner Christian
Botanisches Institut der Universität Basel, Schönbeinstrasse 6, CH-4056 Basel, Switzerland Fax: +41-61-267 35 04; e-mail:
Oecologia. 1997 Dec;113(1):104-114. doi: 10.1007/s004420050358.
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.
迫切需要开展关于大气二氧化碳浓度升高和氮沉降对森林树木影响的生态系统水平实验。在此，我们展示了在自然养分贫瘠的山地森林土壤（0.7米高，350千克重）上九个云杉（欧洲云杉）模型生态系统的数据。每个系统由六棵7岁（收获时）的树组成，每棵树代表不同的基因型，还有一个草本下层植被层（三种物种）。这些模型生态系统在模拟的瑞士山地气候年周期中，暴露于三种不同的二氧化碳浓度（280、420、560微升/升）和三种不同的湿氮沉降速率（0、30、90千克/公顷·年）下，为期3年。生态系统总生物量不受二氧化碳浓度影响，但随氮沉降增加而增加。然而，随着二氧化碳浓度升高，根系生物量分配增加，导致二氧化碳浓度升高条件下生长的树木叶片质量比（LMRs）和叶面积比（LARs）显著降低。与二氧化碳浓度升高相反，氮沉降增加了地上植物部分的生物量分配，因此随着氮沉降增加，LMR和LAR更高。我们未观察到二氧化碳×氮对生长、生物量生产或分配的交互作用，也未观察到基因型×处理的交互作用。云杉树冠的最终叶面积指数（LAI）在420微升/升时比在280微升/升时小19%，在560微升/升时小27%，但未因氮沉降增加而显著改变。二氧化碳浓度升高时较低的LAI主要源于树枝较短（每棵树针叶较少），部分源于针叶凋落物增加。与氮沉降无关，云杉群落地上总氮含量随二氧化碳浓度升高而下降（与280微升/升相比，420微升/升时下降18%，560微升/升时下降31%）。氮沉降对地上总氮含量有相反影响（与0千克氮/公顷·年相比，30千克时增加18%，90千克时增加52%）。我们的结果表明，在天然森林土壤的竞争条件下，大气二氧化碳浓度升高可能不会导致生态系统生物量生产增加，但氮沉降可能会。二氧化碳浓度升高时LAI的降低表明冠层水平光合碳吸收的异速生长下调。单位地面面积树木氮质量因二氧化碳浓度升高而大幅下降，这可能表明二氧化碳导致土壤氮有效性降低。
