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Climate change impacts, adaptive capacity, and vulnerability of European forest ecosystems

Soil microbial communities mediate the decomposition of soil organic matter (SOM). The amount of carbon (C) that is respired leaves the soil as CO2 (soil respiration) and causes one of the greatest fluxes in the global carbon cycle. How soil microbial communities will respond to global warming, however, is not well understood. To elucidate the effect of warming on the microbial community we analyzed soil from the soil warming experiment Achenkirch, Austria. Soil of a mature spruce forest was warmed by 4 °C during snow-free seasons since 2004. Repeated soil sampling from control and warmed plots took place from 2008 until 2010. We monitored microbial biomass C and nitrogen (N). Microbial community composition was assessed by phospholipid fatty acid analysis (PLFA) and by quantitative real time polymerase chain reaction (qPCR) of ribosomal RNA genes. Microbial metabolic activity was estimated by soil respiration to biomass ratios and RNA to DNA ratios. Soil warming did not affect microbial biomass, nor did warming affect the abundances of most microbial groups. Warming significantly enhanced microbial metabolic activity in terms of soil respiration per amount of microbial biomass C. Microbial stress biomarkers were elevated in warmed plots. In summary, the 4 °C increase in soil temperature during the snow-free season had no influence on microbial community composition and biomass but strongly increased microbial metabolic activity and hence reduced carbon use efficiency.

Experimental warming effects on the microbial community of a temperate mountain forest soil.

The response of forest ecosystems to increased atmospheric CO2 is constrained by nutrient availability. It is thus crucial to account for nutrient limitation when studying the forest response to climate change. The objectives of this study were to describe the nutritional status of the main European tree species, to identify growth-limiting nutrients and to assess changes in tree nutrition during the past two decades. We analysed the foliar nutrition data collected during 1992-2009 on the intensive forest monitoring plots of the ICP Forests programme. Of the 22 significant temporal trends that were observed in foliar nutrient concentrations, 20 were decreasing and two were increasing. Some of these trends were alarming, among which the foliar P concentration in F. sylvatica, Q. Petraea and P. sylvestris that significantly deteriorated during 1992-2009. In Q. Petraea and P. sylvestris, the decrease in foliar P concentration was more pronounced on plots with low foliar P status, meaning that trees with latent P deficiency could become deficient in the near future. Increased tree productivity, possibly resulting from high N deposition and from the global increase in atmospheric CO2, has led to higher nutrient demand by trees. As the soil nutrient supply was not always sufficient to meet the demands of faster growing trees, this could partly explain the deterioration of tree mineral nutrition. The results suggest that when evaluating forest carbon storage capacity and when planning to reduce CO2 emissions by increasing use of wood biomass for bioenergy, it is crucial that nutrient limitations for forest growth are considered.

Tree mineral nutrition is deteriorating in Europe.

Global warming has the potential to increase soil respiration (RS), one of the major fluxes in the global carbon (C) cycle. RS consists of an autotrophic (RA) and a heterotrophic (RH) component. We combined a soil warming experiment with a trenching experiment to assess how RS, RA, and RH are affected. The experiment was conducted in a mature forest dominated by Norway spruce. The site is located in the Austrian Alps on dolomitic bedrock. We warmed the soil of undisturbed and trenched plots by means of heating cables 4 °C above ambient during the snow-free seasons of 2005 and 2006. Soil warming increased the CO2 efflux from control plots (RS) by ∼45% during 2005 and ∼47% during 2006. The CO2 efflux from trenched plots (RH) increased by ∼39% during 2005 and ∼45% during 2006. Similar responses of RS and RH indicated that the autotrophic and heterotrophic components of RS responded equally to the temperature increase. Thirty-five to forty percent or 1 t C ha−1 yr−1 of the overall annual increase in RS (2.8 t C ha−1 yr−1) was autotrophic. The remaining, heterotrophic part of soil respiration (1.8 t C ha−1 yr−1), represented the warming-induced C loss from the soil. The autotrophic component showed a distinct seasonal pattern. Contribution of RA to RS was highest during summer. Seasonally derived Q10 values reflected this pattern and were correspondingly high (5.3–9.3). The autotrophic CO2 efflux increase due to the 4 °C warming implied a Q10 of 2.9. Hence, seasonally derived Q10 of RA did not solely reflect the seasonal soil temperature development.

Carbon losses due to soil warming: Do autotrophic and heterotrophic soil respiration respond equally?

Mountain terrestrial and aquatic ecosystems are responsive to external drivers of change, especially climate change and atmospheric deposition of nitrogen (N). We explored the consequences of a temperature-warming trend on stream nitrate in an alpine and subalpine watershed in the Colorado Front Range that has long been the recipient of elevated atmospheric N deposition. Mean annual stream nitrate concentrations since 2000 are higher by 50% than an earlier monitoring period of 1991–1999. Mean annual N export increased by 28% from 2.03 kg N ha � 1 yr � 1 before 2000 to 2.84 kg N ha � 1 yr � 1 in Loch Vale watershed since 2000. The substantial increase in N export comes as a surprise, since mean wet atmospheric N deposition from 1991 to 2006 (3.06 kg N ha � 1 yr � 1 ) did not increase. There has been a period of below average precipitation from 2000 to 2006 and a steady increase in summer and fall temperatures of 0.121 Cy r � 1 in both seasons since 1991. Nitrate concentrations, as well as the weathering products calcium and sulfate, were higher for the period 2000–2006 in rock glacier meltwater at the top of the watershed above the influence of alpine and subalpine vegetation and soils. We conclude the observed recent N increases in Loch Vale are the result of warmer summer and fall mean temperatures that are melting ice in glaciers and rock glaciers. This, in turn, has exposed sediments from which N produced by nitrification can be flushed. We suggest a water quality threshold may have been crossed around 2000. The phenomenon observed in Loch Vale may be indicative of N release from ice features such as rock glaciers worldwide as mountain glaciers retreat.

Climate‐induced changes in high elevation stream nitrate dynamics

Evaluation of air pollution-related risks for Austrian mountain forests.

Since the early eighties air pollution by SO2 and Pb and, to a lesser degree by NOx and NH3, have been significantly reduced in Europe. This was done in part for the protection of forest ecosystems. The reductions are reflected in the pollutant impacts and inputs and have been verified through the bioindicator Norway spruce. In contrast, ozone concentrations increased at most of the evaluated measuring locations and trends were calculated based on the results of about 100 stations in Austria and Germany. Despite reduced emissions, large parts of the forest ecosystems are still affected by air-pollution impacts. Negative effects can be assessed using different legal standards and Critical Levels and Critical Loads, respectively: The legal standard for the evaluation of SO2 impact, as used in Austria, is well suited. The provisional European Critical Level for ozone, the AOT40 must, in order to be a meaningful criterion for field applications, be further refined. Continuing the Critical Load concept, the spatial risk of acid depositions in areas of high geomorphic variability were evaluated using the new parameter of 'critical soil depth'. With the help of the newly defined 'lead accumulation index', the accumulation of Pb from air pollution could be evaluated. Air-pollution inputs and the spatial acidification risk is directly related to altitude. Up to altitudes of 1000 m and 1100 m, SO2 impact and acid depositions, respectively were indicated and elevated ozone doses and Pb inputs were observed in the sensitive sub-alpine areas. These results underline the necessity for increased protection of mountain forest ecosystems, among others through a further reduction of emissions or forest-related strategies.

Evaluation of pollution-related stress factors for forest ecosystems in central Europe.

In the North Tyrolean Limestone Alps a site was investigated over a four-year period (1998–2001) in order to assess the nitrogen saturation status, the nitrogen budget (quantification of the net uptake of nitrogen by the canopy and of the nitrogen mineralization, nitrogen uptake from roots and N2O emission rates, proof of the origin of nitrate in the soil water with stable isotope analyses), and the effects of the actual nitrogen input on ground water quality. The main goals were to quantify the nitrogen input rate, the nitrogen pools in above-ground and below-ground compartments, nitrogen turnover processes in the soil as well as the output into the groundwater and into the atmosphere. The findings are based on continuous and discontinuous field measurements as well as on model results. While nitrogen input exceeded the Critical Loads of the WHO (1995), nitrogen deficiency and nutrient imbalances were verified by needle analyses. The atmospheric input of inorganic nitrogen was higher than the nitrogen output in 50 cm soil depth. A tracer experiment with15N helped to prove that not more than half of the applied nitrate could be discharged. This allows the conclusion that nitrogen is stored in the system and that the site cannot yet be said to be saturated with nitrogen. The same result was also obtained by modelling. In addition, it was proved that the nitrogen discharge did not stem from deposition but from processes within the system.

Investigations of nitrogen fluxes and pools on a limestone site in the Alps.

Goal, Scope and Background
Ozone is the most important air pollutant in Europe for forest ecosystems and the increase in the last decades is significant. The ozone impact on forests can be calculated and mapped based on the provisional European Critical Level (AOT40 = accumulated exposure over a threshold of 40 ppb, 10,000 ppb.h for 6 months of one growing season calculated for 24h day-1).

For Norway spruce, the Austrian main tree species, the ozone risk was assessed in a basis approach and because the calculations do not reflect the health status of forests in Austria, the AOT40 concept was developed.

Friedl Herman

Papers

Evaluation of air pollution-related risks for Austrian mountain forests.

Evaluation of pollution-related stress factors for forest ecosystems in central Europe.

Investigations of nitrogen fluxes and pools on a limestone site in the Alps.

Modelling critical levels of ozone for the forested area of Austria. Modifications of the AOT40 concept.

Monitoring the impact of sulphur with the Austrian Bioindicator Grid.