• Energy Research

Abstract The uptake of carbon dioxide (CO2) from the atmosphere through photosynthesis is accompanied by an inevitable loss of water vapor through the stomata of leaves. The rate of leaf-level CO2 assimilation per unit stomatal conductance, i.e. intrinsic water-use efficiency (WUEi), is thus a key characteristic of terrestrial ecosystem functioning that is central to the global hydroclimate system. Empirical evidence and theory suggest a positive response of forest WUE to increased CO2 levels globally. Although evidence exists for a positive effect of ecosystem nitrogen (N) inputs on WUEi, it is not clear how trends in atmospheric N deposition have affected WUEi in the past. Here we combine twentieth-century climate and nitrogen deposition with stable isotope signature in tree rings and document a WUEi trend reversal at two sites in Switzerland, that matches the timing of a trend reversal in atmospheric N deposition. Using generalized additive models (GAMs), we fitted observed WUEi time series to multiple environmental covariates. This suggested N deposition to have a significant effect on long-term WUEi at the site that was exposed to higher N deposition levels. The ratio of the increase in WUEi in response to increase in CO2 (dWUEi/dCO2) declined by 96% after 1980 (from 0.53 to 0.02) in the beech forest and declined by 72% in the spruce forest (from 0.46 to 0.13) concurrent with a sharp decline in N deposition. Using the GAM model for two scenarios, we show that had N deposition levels not declined after 1980s, WUEi would have increased more strongly in response to increasing CO2. Although the increase in N deposition was limited to the 1950–1980 decades and the signals have declined with improvements in air quality across Europe, the role of atmospheric pollution must be reconsidered in interpretation of tree ring studies and for building environmental proxies that are pivotal to understanding future sink capacity of terrestrial ecosystems in response to climate change.

Abstract The uptake of carbon dioxide (CO2) from the atmosphere through photosynthesis is accompanied by an inevitable loss of water vapor through the stomata of leaves. The rate of leaf-level CO2 assimilation per unit stomatal conductance, i.e. intrinsic water-use efficiency (WUEi), is thus a key characteristic of terrestrial ecosystem functioning that is central to the global hydroclimate system. Empirical evidence and theory suggest a positive response of forest WUE to increased CO2 levels globally. Although evidence exists for a positive effect of ecosystem nitrogen (N) inputs on WUEi, it is not clear how trends in atmospheric N deposition have affected WUEi in the past. Here we combine twentieth-century climate and nitrogen deposition with stable isotope signature in tree rings and document a WUEi trend reversal at two sites in Switzerland, that matches the timing of a trend reversal in atmospheric N deposition. Using generalized additive models (GAMs), we fitted observed WUEi time series to multiple environmental covariates. This suggested N deposition to have a significant effect on long-term WUEi at the site that was exposed to higher N deposition levels. The ratio of the increase in WUEi in response to increase in CO2 (dWUEi/dCO2) declined by 96% after 1980 (from 0.53 to 0.02) in the beech forest and declined by 72% in the spruce forest (from 0.46 to 0.13) concurrent with a sharp decline in N deposition. Using the GAM model for two scenarios, we show that had N deposition levels not declined after 1980s, WUEi would have increased more strongly in response to increasing CO2. Although the increase in N deposition was limited to the 1950–1980 decades and the signals have declined with improvements in air quality across Europe, the role of atmospheric pollution must be reconsidered in interpretation of tree ring studies and for building environmental proxies that are pivotal to understanding future sink capacity of terrestrial ecosystems in response to climate change.

Changing environmental conditions may substantially interact with site quality and forest stand characteristics, and impact forest growth and carbon sequestration. Understanding the impact of the various drivers of forest growth is therefore critical to predict how forest ecosystems can respond to climate change. We conducted a continental-scale analysis of recent (1995–2010) forest volume increment data (ΔVol, m3 ha−1 yr−1), obtained from ca. 100,000 coniferous and broadleaved trees in 442 even-aged, single-species stands across 23 European countries. We used multivariate statistical approaches, such as mixed effects models and structural equation modelling to investigate how European forest growth respond to changes in 11 predictors, including stand characteristics, climate conditions, air and site quality, as well as their interactions. We found that, despite the large environmental gradients encompassed by the forests examined, stand density and age were key drivers of forest growth. We further detected a positive, in some cases non-linear effect of N deposition, most pronounced for beech forests, with a tipping point at ca. 30 kg N ha−1 yr−1. With the exception of a consistent temperature signal on Norway spruce, climate-related predictors and ground-level ozone showed much less generalized relationships with ΔVol. Our results show that, together with the driving forces exerted by stand density and age, N deposition is at least as important as climate to modulate forest growth at continental scale in Europe, with a potential negative effect at sites with high N deposition.

Changing environmental conditions may substantially interact with site quality and forest stand characteristics, and impact forest growth and carbon sequestration. Understanding the impact of the various drivers of forest growth is therefore critical to predict how forest ecosystems can respond to climate change. We conducted a continental-scale analysis of recent (1995–2010) forest volume increment data (ΔVol, m3 ha−1 yr−1), obtained from ca. 100,000 coniferous and broadleaved trees in 442 even-aged, single-species stands across 23 European countries. We used multivariate statistical approaches, such as mixed effects models and structural equation modelling to investigate how European forest growth respond to changes in 11 predictors, including stand characteristics, climate conditions, air and site quality, as well as their interactions. We found that, despite the large environmental gradients encompassed by the forests examined, stand density and age were key drivers of forest growth. We further detected a positive, in some cases non-linear effect of N deposition, most pronounced for beech forests, with a tipping point at ca. 30 kg N ha−1 yr−1. With the exception of a consistent temperature signal on Norway spruce, climate-related predictors and ground-level ozone showed much less generalized relationships with ΔVol. Our results show that, together with the driving forces exerted by stand density and age, N deposition is at least as important as climate to modulate forest growth at continental scale in Europe, with a potential negative effect at sites with high N deposition.

Resource allocation to different plant tissues is likely to be affected by high investment into fruit production during mast years. However, there is a large knowledge gap concerning species-specific differences in resource dynamics. We investigated the influence of mast years on stem growth, leaf production, and leaf carbon (C), nitrogen (N), and phosphorus (P) concentrations and contents in Fagus sylvatica, Quercus petraea, and Q. robur at continental and climate region scales using long-term data from the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) and similar datasets. We discussed the results in the light of opposing resource dynamics hypotheses: (i) resource accumulation before mast years and exhaustion after mast years (resource storage hypothesis), (ii) shifting resources from vegetative to generative compartments (resource switching hypothesis), and (iii) investing resources concurrently in both vegetative and generative compartments (resource matching hypothesis). Linear mixed-effects modelling (LMM) showed that both stem growth and leaf production were negatively influenced by weather conditions which simultaneously lead to high fruit production. Thus, the impact of generative on vegetative growth is intermixed with effects of environmental factors. Superposed epoch analyses and LMM showed that for mast behaviour in F. sylvatica, there are indicators supporting the resource storage and the resource switching hypotheses. Before mast years, resources were accumulated, while during mast years resources switched from vegetative to generative tissues with reduced stem and leaf growth. For the Quercus species, stem growth was reduced after mast years, which supports the resource storage hypothesis. LMM showed that leaf C concentrations did not change with increasing fruit production in neither species. Leaf N and P concentrations increased in F. sylvatica, but not in Quercus species. Leaf N and P contents decreased with increasing fruit production in all species, as did leaf C content in F. sylvatica. Overall, our findings suggest different resource dynamics strategies in F. sylvatica and Quercus species, which might lead to differences in their adaptive capacity to a changing climate.