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Tree mineral nutrition is deteriorating in Europe

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.

Environmental drivers of ectomycorrhizal communities in Europe's temperate oak forests

Ectomycorrhizal fungi are major ecological players in temperate forests, but they are rarely used in measures of forest condition because large‐scale, high‐resolution, standardized and replicated belowground data are scarce. We carried out an analysis of ectomycorrhizas at 22 intensively monitored long‐term oak plots, across nine European countries, covering complex natural and anthropogenic environmental gradients. We found that at large scales, mycorrhizal richness and evenness declined with decreasing soil pH and root density, and with increasing atmospheric nitrogen deposition. Shifts in mycorrhizas with different functional traits were detected; mycorrhizas with structures specialized for long‐distance transport related differently to most environmental variables than those without. The dominant oak‐specialist Lactarius quietus, with limited soil exploration abilities, responds positively to increasing nitrogen inputs and decreasing pH. In contrast, Tricholoma, Cortinarius and Piloderma species, with medium‐distance soil exploration abilities, show a consistently negative response. We also determined nitrogen critical loads for moderate (9.5–13.5 kg N/ha/year) and drastic (17 kg N/ha/year) changes in belowground mycorrhizal root communities in temperate oak forests. Overall, we generated the first baseline data for ectomycorrhizal fungi in the oak forests sampled, identified nitrogen pollution as one of their major drivers at large scales and revealed fungi that individually and/or in combination with others can be used as belowground indicators of environmental characteristics.

Exceedance of critical loads and of critical limits impacts tree nutrition across Europe

Key messageExceedance of critical limits in soil solution samples was more frequent in intensively monitored forest plots across Europe with critical loads for acidity and eutrophication exceeded compared to other plots from the same network. Elevated inorganic nitrogen concentrations in soil solution tended to be related to less favourable nutritional status.ContextForests have been exposed to elevated atmospheric deposition of acidifying and eutrophying sulphur and nitrogen compounds for decades. Critical loads have been identified, below which damage due to acidification and eutrophication are not expected to occur.AimsWe explored the relationship between the exceedance of critical loads and inorganic nitrogen concentration, the base cation to aluminium ratio in soil solutions, as well as the nutritional status of trees.MethodsWe used recent data describing deposition, elemental concentrations in soil solution and foliage, as well as the level of damage to foliage recorded at forest plots of the ICP Forests intensive monitoring network across Europe.ResultsCritical loads for inorganic nitrogen deposition were exceeded on about a third to half of the forest plots. Elevated inorganic nitrogen concentrations in soil solution occurred more frequently among these plots. Indications of nutrient imbalances, such as low magnesium concentration in foliage or discolouration of needles and leaves, were seldom but appeared more frequently on plots where the critical limits for soil solution were exceeded.ConclusionThe findings support the hypothesis that elevated nitrogen and sulphur deposition can lead to imbalances in tree nutrition.

Influence of canopy budget model approaches on atmospheric deposition estimates to forests

Accurate quantification of total nitrogen and acidifying deposition is a major source of uncertainty in determining the exceedance of critical loads in forest ecosystems. Monitoring of atmospheric deposition is frequently based on throughfall measurements in combination with the canopy budget model to calculate ion-exchange fluxes between the forest canopy and incident rainfall water. Various approaches for each step in the canopy budget model have been reported and compared, but combinations of different approaches were not yet assessed. Therefore, the present study quantified the range of estimated dry deposition and total deposition resulting from all possible combinations of canopy budget model approaches for three typical case studies: (i) total nitrogen and potentially acidifying deposition onto a forest canopy, (ii) the ratio of these deposition variables between adjacent coniferous and deciduous stands and (iii) the parameters of a deposition time trend analysis. The time step, type of precipitation data and tracer ion used in the model had a significant effect on the findings in the three case studies. In addition, including or excluding canopy leaching of weak acids and canopy uptake of nitrogen during the leafless season largely affected the results, while including or excluding canopy uptake of nitrate generally showed no effect. In general, the use of wet-only precipitation data can be recommended, along with sodium as a tracer ion and the inclusion of weak acids. We conclude that further research should focus on the assumptions of inertness of the tracer ion and the equal deposition efficiency of base cations and the tracer ion and on the quantification of weak acids in rainfall and throughfall water. Since local or tree-species specific effects might influence the results obtained in this study, a similar analysis is recommended for other tree species and regions when using the canopy budget model.

Quantifying Carbon and Nutrient Input From Litterfall in European Forests Using Field Observations and Modeling

Litterfall is a major, yet poorly studied, process within forest ecosystems globally. It is important for carbon dynamics, edaphic communities, and maintaining site fertility. Reliable information on the carbon and nutrient input from litterfall, provided by litter traps, is relevant to a wide audience including policymakers and soil scientists. We used litterfall observations of 320 plots from the pan-European forest monitoring network of the “International Co-operative Programme on Assessment and Monitoring of AirPollution Effects on Forests” to quantify litterfallfluxes. Eight litterfall models were evaluated (four using climate information and four using biomass abundance). We scaled up our results to the total European forestarea and quantified the contribution of litterfall to the forest carbon cycle using net primary production aggregated by bioregions (north, central, and south) and by forest types (conifers and broadleaves). The 1,604 analyzed annual litterfall observations indicated an average carbon input of 224 g C · m2· year 1 (annual nutrient inputs 4.49 g N, 0.32 g P, and 1.05 g K · m2), representing a substantial percentage of net primary production from 36% in north Europe to 32% in central Europe. The annual turnover of carbon and nutrient in broadleaf canopies was larger than for conifers. The evaluated models provide large-scale litterfall predictions with a bias less than 10%. Each year litterfall in European forests transfers 351 Tg C, 8.2 Tg N,0.6 Tg P, and 1.9 Tg K to the forestfloor. The performance of litterfall models may be improved by including foliage biomass and proxies for forest management.

Environment and host as large-scale controls of ectomycorrhizal fungi

Explaining the large-scale diversity of soil organisms that drive biogeochemical processes—and their responses to environmental change—is critical. However, identifying consistent drivers of belowground diversity and abundance for some soil organisms at large spatial scales remains problematic. Here we investigate a major guild, the ectomycorrhizal fungi, across European forests at a spatial scale and resolution that is—to our knowledge—unprecedented, to explore key biotic and abiotic predictors of ectomycorrhizal diversity and to identify dominant responses and thresholds for change across complex environmental gradients. We show the effect of 38 host, environment, climate and geographical variables on ectomycorrhizal diversity, and define thresholds of community change for key variables. We quantify host specificity and reveal plasticity in functional traits involved in soil foraging across gradients. We conclude that environmental and host factors explain most of the variation in ectomycorrhizal diversity, that the environmental thresholds used as major ecosystem assessment tools need adjustment and that the importance of belowground specificity and plasticity has previously been underappreciated.Analyses of data from 137 forest plots across 20 European countries show that ectomycorrhizal fungal diversity is strongly influenced by environmental and host species factors and provide thresholds to inform ecosystem assessment tools

Author Correction: Environment and host as large-scale controls of ectomycorrhizal fungi

Change history: In the HTML version of this Article, author ‘Filipa Cox’ had no affiliation in the author list, although she was correctly associated with affiliation 3 in the PDF. In addition, the blue circles for ‘oak’ were missing from Extended Data Fig. 1. These errors have been corrected online.

VARIABILITY OF OZONE DEPOSITION VELOCITY OVER A MIXED SUBURBAN TEMPERATE FOREST

A 10-year long dataset of half-hourly ozone (O3) fluxes was used to study the variability in deposition velocity (υd) over a mixed temperate suburban forest. Average (median) υd amounted to 0.70 (0.46) cm s-1, with day- and night-time average (median) of 0.98 (0.73) cm s-1 and 0.46 (0.30) cm s-1, respectively. It was found that the precipitation form had a marked impact on υd and the deposition efficiency (υd/υdmax), with highest values measured when the canopy was dew-wetted or covered with snow. The analysis further evidenced that traffic volume led to increased deposition due to the presence of chemical reactions between O3 and nitric oxide (NO) above the canopy surface. During the working week, daytime values of υd, υd/υdmax and the O3 fluxes (F) were found to be significantly higher than the weekend values, especially during the winter half-year. In a next step, half hourly deposition data were aggregated into day- and night-time monthly values, for a correlative study with measured environmental variables. Monthly average night-time/daytime υd and υd/υdmax were positively correlated with the relative humidity at the canopy surface (RH(z0’)) and negatively correlated with the water levels below the ground surface. During the daytime, monthly υd and υd/υdmax were additionally increased during the working-week when traffic volume was high. There existed, however, substantially different weather conditions, in which unaccounted covariates with a totally different meteorological signature controlled the υd and F. It was speculated that, among other, biogenic volatile compounds (BVOCs) could have contributed to O3 quenching in some (spring) months with severe drought stress.

Responses of forest ecosystems in Europe to decreasing nitrogen deposition

Average nitrogen (N) deposition across Europe has declined since the 1990s. This resulted in decreased N inputs to forest ecosystems especially in Central and Western Europe where deposition levels are highest. While the impact of atmospheric N deposition on forests has been receiving much attention for decades, ecosystem responses to the decline in N inputs received less attention. Here, we review observational studies reporting on trends in a number of indicators: soil acidification and eutrophication, understory vegetation, tree nutrition (foliar element concentrations) as well as tree vitality and growth in response to decreasing N deposition across Europe. Ecosystem responses varied with limited decrease in soil solution nitrate concentrations and potentially also foliar N concentrations. There was no large-scale response in understory vegetation, tree growth, or vitality. Experimental studies support the observation of a more distinct reaction of soil solution and foliar element concentrations to changes in N supply compared to the three other parameters. According to the most likely scenarios, further decrease of N deposition will be limited. We hypothesize that this expected decline will not cause major responses of the parameters analysed in this study. Instead, future changes might be more strongly controlled by the development of N pools accumulated within forest soils, affected by climate change and forest management.