Assessment of sustainable forest management criteria using indicators of the international programme ICP forests

This article gives an assessment of the international criteria for the sustainable management of forests on the basis of indicators according to the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests). The given program allows us to assess Criterion 2: Maintenance of Forest Health and Vitality and Criterion 4: Conservation and Improvement of the Biological Diversity of the Forest Ecosystem. The informative indicators of Criterion 2 are the degrees of the tree crown defoliation and the composition parameters of atmospheric deposition, soils, and dominant tree plant photosynthetic organs. The assessment of Criterion 4 is carried out on the basis of biodiversity items, including forest distribution by age and species composition, sets of forest community major groups (groups of forest types) and their quantitative representation in the area, number of tree and shrub species found in the forests, and species richness and species saturation. An attempt has been made to show whether the network of the permanent observation posts reflects in full measure the forest typological and age structure in Karelia and Karelian Isthmus.     

Tree diversity mitigates defoliation after a drought‐induced tipping point

Understanding the processes that underlie drought‐related tree vitality loss is essential for anticipating future forest dynamics, and for developing management plans aiming at increasing the resilience of forests to climate change. Forest vitality has been continuously monitored in Europe since the acid rain alert in the 1980s, and the intensive monitoring plots of ICP Forests offer the opportunity to investigate the effects of air pollution and climate change on forest condition. By making use of over 100 long‐term monitoring plots, where crown defoliation has been assessed extensively since 1990, we discovered a progressive shift from a negative to a positive effect of species richness on forest health. The observed tipping point in the balance of net interactions, from competition to facilitation, has never been reported from real ecosystems outside experimental conditions; and the strong temporal consistency of our observations with increasing drought stress emphasizes its climate change relevance. Furthermore, we show that higher species diversity has reduced the severity of defoliation in the long term. Our results confirm the greater resilience of diverse forests to future climate change‐induced stress. More generally, they add to an accumulating body of evidence on the large potential of tree species mixtures to face manifold disturbances in a changing world.     

Long-term sulphate and inorganic nitrogen mass balance budgets in European ICP Integrated Monitoring catchments (1990–2012)

Empirical evidence based on integrated environmental monitoring including physical, chemical and biological variables is essential for evaluating the ecosystem benefits of costly emission reduction policies. The international multidisciplinary ICP IM (International Cooperative Programme on Integrated Monitoring of Air Pollution Effects on Ecosystems) programme studies the integrated effects of air pollution and climate change on ecosystems in unmanaged and calibrated forested catchments. We calculated site-specific annual input-output budgets for sulphate (SO₄) and total inorganic nitrogen (TINNO₃-N + NH₄-N) for 17 European ICP IM sites in 1990–2012. Temporal trends for input (deposition) and output (runoff water) fluxes and the net retention/net release of SO₄ and TIN were also analysed. Large differences in the input and output fluxes of SO₄ and TIN reflect important gradients of air pollution effects in Europe, with the highest deposition and runoff water fluxes at IM sites located in southern Scandinavia and in parts of Central and Eastern Europe and the lowest fluxes at more remote sites in northern European regions. A significant decrease in the total (wet + dry) deposition of non-marine SO₄ and bulk deposition of TIN was found at 90% and 65% of the sites, respectively. Output fluxes of non-marine SO₄ in runoff decreased significantly at 65% of the sites, indicating positive effects of the international emission abatement actions in Europe during the last 20 years. Catchments retained SO₄ in the early and mid-1990s, but this shifted towards a net release in the late 1990s, which may be due to the mobilization of legacy S pools accumulated during times of high atmospheric SO₄ deposition. Despite decreased deposition, TIN output fluxes and retention rates showed a mixed response with both decreasing (9 sites) and increasing (8 sites) trend slopes, and trends were rarely significant. In general, TIN was strongly retained in the catchments not affected by natural disturbances. The long-term annual variation in net releases for SO₄ was explained by variations in runoff and SO₄ concentrations in deposition, while a variation in TIN concentrations in runoff was mostly associated with a variation of the TIN retention rate in catchments. The net release of SO₄ from forest soils may delay the recovery from acidification for surface waters and the continued enrichment of nitrogen in catchment soils poses a threat to terrestrial biodiversity and may ultimately lead to a higher TIN runoff through N-saturation. Continued monitoring and further evaluations of mass balance budgets are thus needed.     

Modeling the effects of varying data quality on trend detection in environmental monitoring

Detection of changes in ecosystem characteristics is a principal tool for identifying and understanding the effects of anthropogenic activities on the condition and functioning of ecosystems. It is widely known that temporal trends can be blurred by the imprecision of the data. Research program managers are aware of the difficulties surrounding representative sampling and therefore enforce strict sampling protocols. Standardized sampling can be so effective that the initially much smaller uncertainty in the instrumental analysis becomes substantial. However, until now the effect of the quality of the instrumental analysis on the time required for trend detection has only rarely been quantified. In this paper, we present a novel technique and theoretical computations for the detection of trends in single and combined indices. The theory is clarified with examples from the International Co-operative Programme on Assessment and Monitoring of Air Pollution on Forests (ICP Forests). Moreover, the theoretical computations were made for normalized or scaled distributions and are therefore equally valid outside the field of environmental monitoring. The results show that, when sampling protocols largely reduce the variability of representative sampling, poor quality of the instrumental analysis blurs the data such that environmental monitoring or long-term ecological research programs can lose the ability to detect trends by causing up to three decades-long delay in detecting changes. We can thus conclude that high quality of the instrumental analysis is a prerequisite for a sensitive monitoring program.     

Changes in crown leaching composition induced by a sudden increase in atmospheric deposition. A case study in South-Western Bulgaria

Rain and throughfall chemistry has been monitored for five years (1987–1991) in three adjacent stands (Norway spruce, Silver fir and Scots pine) situated at an altitude of 1500 m above sea level in the Rhodopes mountains (South-Western Bulgaria). Throughfall collectors have been set up near the stem, below the opening between the crowns, and halfway between the stem and the border of the tree crown. Pollutant concentration in bulk precipitation strongly increased during the period of investigation. Because base cation deposition increased together with sulfate deposition, the increase in proton deposition remained moderate. The increase in throughfall concentration was parallel for different tree species. The time course of mineral concentration in throughfall was approximately similar to that of rainfall but the inter-event variability in concentration was reduced. Net throughfall fluxes of Ca, Mg and K increased during the study period whereas net throughfall fluxes of Cl, Na and S remained more stable. Although the wet deposition of protons increased, the proton input in net throughfall decreased or remained stable in relation to the increase in base cation concentration. This suggests a low dry deposition in the study region and conversely an increased net leaching of base cations. However, the seasonal variations in net throughfall as well as the time course of the difference between the concentrations of throughfall samples collected at different distances from the stems indicates that most of the increase in the net throughfall of Ca and Mg was due to dry deposition. The difficulties associated with the use of ions such as Na as deposition tracers for other base cations is emphasized.     

Assessments of impacts of nitrogen deposition on beech forests: results from the Pan-European Monitoring Programme

The article reviews effects of nitrogen (N) deposition on beech forest ecosystems in Europe. On the basis of beech plots of the Pan-European Monitoring Programme of ICP Forests and the EU, the deposition of N compounds as well as input-output budgets are listed and compared with studies in North America. The authors also discuss the critical threshold for N leaching. At present, N is leached in 10% of the plots evaluated. An in-depth evaluation of a beech plot in central Germany is presented. The high N leaching results in a considerable increase (four times higher N content in 2000 compared to 1965) in the export of nitrate from the beech forests from a nearby source. Finally, ecophysiological indicators (N content in beech leaves, fine root system, N content, root/shoot ratios) are discussed as a result of high N input.     

Roads as nitrogen deposition hot spots

Mobile sources are the single largest source of nitrogen emissions to the atmosphere in the US. It is likely that a portion of mobile-source emissions are deposited adjacent to roads and thus not measured by traditional monitoring networks, which were designed to measure long-term and regional trends in deposition well away from emission sources. To estimate the magnitude of near-source nitrogen deposition, we measured concentrations of both dissolved inorganic nitrogen (DIN) and total dissolved nitrogen (inorganic + organic) (TDN) in throughfall (i.e., the nitrogen that comes through the forest canopy) along transects perpendicular to two moderately trafficked roads on Cape Cod in Falmouth MA, coupled with measurements of both DIN and TDN in bulk precipitation made in adjacent open fields at the same transect distances. We used the TDN throughfall data to estimate total nitrogen deposition, including dry gaseous nitrogen deposition in addition to wet deposition and dry particle deposition. There was no difference in TDN in the bulk collectors along the transects at either site; however TDN in the throughfall collectors was always higher closest to the road and decreased with distance. These patterns were driven primarily by differences in the inorganic N and not the organic N. Annual throughfall deposition was 8.7 (±0.4) and 6.8 (±0.5) TDN kg N ha^?1 year^?1 at sites 10 and 150 m away from the road respectively. We also characterized throughfall away from a non-road edge (power line right-of-way) to test whether the increased deposition observed near road edges was due to deposition near emission sources or due to a physical, edge effect causing higher deposition. The increased deposition we observed near roads was due to increases in inorganic N especially NH₄ ⁺. This increased deposition was not the result of an edge effect; rather it is due to near source deposition of mobile source emissions. We scaled these results to the entire watershed and estimate that by not taking into account the effects of increased gaseous N deposition from mobile sources we are underestimating the amount of N deposition to the watershed by 13–25 %.     

Impacts of air pollution and climate change on forest ecosystems--emerging research needs

Outcomes from the 22nd meeting for Specialists in Air Pollution Effects on Forest Ecosystems "Forests under Anthropogenic Pressure--Effects of Air Pollution, Climate Change and Urban Development", September 10-16, 2006, Riverside, CA, are summarized. Tropospheric or ground-level ozone (O3) is still the phytotoxic air pollutant of major interest. Challenging issues are how to make O3 standards or critical levels more biologically based and at the same time practical for wide use; quantification of plant detoxification processes in flux modeling; inclusion of multiple environmental stresses in critical load determinations; new concept development for nitrogen saturation; interactions between air pollution, climate, and forest pests; effects of forest fire on air quality; the capacity of forests to sequester carbon under changing climatic conditions and coexposure to elevated levels of air pollutants; enhanced linkage between molecular biology, biochemistry, physiology, and morphological traits.     

广州市森林土壤水化学和元素收支平衡研究

为了探讨酸沉降对森林生态系统的影响,于1998年4月至1999年3月对广州市3个不同林分试验点的降雨,林内穿透雨和土壤渗透水进行了物理量测定和化学分析,据此计划了元素的收支平衡,研究表明：（1）SO4^2-和Ca^2 是土壤水中最主要的阴,阳离子,土壤水中的主要组分浓度的变化与树冠和凋落物淋溶的贡献以及土壤中一系列物理化学过程（如硝化作用,吸附作用,阳离子交换反应）有关;（2）土壤水中H^ 和Al^3 浓度的增高是土壤酸化的重要指标,可能对森林植被构成潜在危害;（3）S－沉降（＞110kg/hm^2.a)可能不是目前土壤酸化的主要原因,也没引起土壤中碱性离子的大量淋失,由于研究区土壤有较强的吸附So4^2-的能力和容量,So4^2-和与之关系密切的Ca,Mg,K等元素在土壤中处于积累阶段;（4）过量的N沉降（＞20kg/hm^2.a)和NH4^ 的硝化 作用是广州市森林土壤进一步酸化的主要原因,由于该地区NOx排放量逐年上升,因此N沉降对森林生态系统的负面影响会进一步加剧。     

Effect of fog on sea salt deposition on peat soil in boreal <Emphasis Type="Italic">Picea glehnii</Emphasis> forests in Ochiishi,eastern Hokkaido,Japan

We investigated seasonal changes in the chemical properties of precipitation (bulk deposition, throughfall and stem flow) in Picea glehnii forests and neighboring Sphagnum communities in three ombrotrophic mires in Ochiishi district, northern Japan, to clarify the contribution of fog to nutrient addition to mires. Na⁺ and Cl^– dominated the bulk deposition, followed by Mg²⁺, Ca²⁺ and SO₄^2–, implying an oceanic influence on mire chemistry. Differences in chemical properties among bulk deposition, throughfall and stem flow increased with proximity to the coastline. There was little difference in electrical conductivity (EC) among bulk deposition, throughfall and stem flow during the period of high fog frequency, which was approximately 17 fog days per month from June to August, but there were large differences in EC during the period of low fog frequency, which was approximately 5 fog days per month from September to November. In general, throughfall and stem flow were enriched with Na⁺, Mg²⁺, Ca²⁺, Cl^– and SO₄^2– at the P.glehnii canopy, and seasonal trends in ionic concentration showed almost the same trend as EC. This seasonal pattern of atmospheric deposition chemistry showed that sea salt deposition on mires depends on fog occurrence. Sea salt is washed out of the atmosphere by fog when fog covers the forest canopy and, hence, throughfall and stem flow did not lead to the enrichment of chemical constituents during passage through the canopy in these mires during the season of high fog occurrence.     

Towards comparable assessment of the soil nutrient status across scales—Review and development of nutrient metrics

Nutrient availability influences virtually every aspect of an ecosystem, and is a critical modifier of ecosystem responses to global change. Although this crucial role of nutrient availability in regulating ecosystem structure and functioning has been widely acknowledged, nutrients are still often neglected in observational and experimental synthesis studies due to difficulties in comparing the nutrient status across sites. In the current study, we explain different nutrient‐related concepts and discuss the potential of soil‐, plant‐ and remote sensing‐based metrics to compare the nutrient status across space. Based on our review and additional analyses on a dataset of European, managed temperate and boreal forests (ICP [International Co‐operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests] Forests dataset), we conclude that the use of plant‐ and remote sensing‐based metrics that rely on tissue stoichiometry is limited due to their strong dependence on species identity. The potential use of other plant‐based metrics such as Ellenberg indicator values and plant‐functional traits is also discussed. We conclude from our analyses and review that soil‐based metrics have the highest potential for successful intersite comparison of the nutrient status. As an example, we used and adjusted a soil‐based metric, previously developed for conifer forests across Sweden, against the same ICP Forests data. We suggest that this adjusted and further adaptable metric, which included the organic carbon concentration in the upper 20 cm of the soil (including the organic fermentation‐humus [FH] layer), the C:N ratio and of the FH layer, can be used as a complementary tool along with other indicators of nutrient availability, to compare the background nutrient status across temperate and boreal forests dominated by spruce, pine or beech. Future collection and provision of harmonized soil data from observational and experimental sites is crucial for further testing and adjusting the metric.     

Topography strongly affects atmospheric deposition and canopy exchange processes in different types of wet lowland rainforest,Southwest Costa Rica

Bulk precipitation and throughfall were collected in a wet lowland rainforest in SW Costa Rica on an event basis to allow modelling the contributions of dry deposition and canopy exchange to nutrient inputs and internal cycling of nutrients. Estimates based on bulk precipitation underestimated total atmospheric deposition to tropical rainforests by up to 10-fold ignoring the contributions of dry deposition. Canopy exchange contributed most of the aboveground inputs to the forest soil of Na⁺, about half for K⁺, 10% for P and Mg²⁺ and negligible for N, C and other elements. Tree species composition did not account for the differences found in net throughfall between forest sites, and vegetation structure (plant area index) had only a small effect on net throughfall. Forest regrowth affected net throughfall through reduced soil fertility and differences in leaf traits. Topography most significantly affected net throughfall via increased dry deposition at sites of higher elevation and via soil fertility and increased canopy exchange at down slope sites.     

The effect of stand age on throughfall chemistry in spruce stands in the Potok Dupniañski catchment in the Silesian Beskid Mountains, southern Poland

The chemical composition of throughfall depends on the age of the Norway spruce (Picea abies Karst) stands and season of the year. The pH of throughfall decreased and the amount of hydrogen ion in throughfall deposited to the soil increased with increasing age of spruce stands, especially in the winter season. Concentrations of K+, H+, SO4(2-), Mn2+, and NH4(+) in throughfall were higher than bulk precipitation for the whole year and K+, H+, and Mn2+ concentrations were higher in throughfall in winter and the growing season. This indicates that these ions were washed out or washed from the surface of needles and/or the bark, and that NO3(-), NH4(+), Ca2+, Mg2+, Fe2+, and Zn2+ were absorbed in the canopy. The effect of high nitrogen deposition, above critical loads, and an increase in the amount of sulfur and in the sum of the strong acids (S-SO4(2-) and N-NO3(-)) that reached the soil with throughfall may have implications for the vitality of spruce stands, especially in older age classes. The application of Principal Component Analysis (PCA) has led to identification of five factors responsible for the data structure ("mineral dust", "acidic emissions", "heavy metals-dust particles", "ammonium [NH4(+)]", and "H+"). They explain more than 60% of the total variance system. The strong positive correlation between stand age class and ionic concentrations in throughfall occurs for all year and the winter period for ions within the following categories: "acidic emissions", SO4(2-) + NO3(-); "heavy metals-dust particles", Fe2+ + Mn2+ + Zn2+; "mineral dust", Na+ + K+ + Ca2+ + Mg2+; "NH4(+)"; and "H+". The strength of the relationship decreases in the growing period, probably due to processes occurring in the canopy (adsorption, leaching, etc.).     

Buffering capacity through cation leaching of Pinus radiata D. Don canopy

The change in the composition of atmospheric deposition as it passes through the forest canopy on two 10-year-old Pinus radiata D. Don forests (Manzanal and Posadero) was studied in the Basque Country, analysing the concentration of different constituents in bulk precipitation and throughfall. The precipitation at the study sites was bimodal with two maxima in the year; one in late spring and another one in late autumn. Posadero had a mean annual precipitation of 1223.6 mm with an interception loss of 27.5% by the forest canopy and Manzanal had a mean annual precipitation of 978.6 mm with the interception loss being 22.2% of it. Constituent concentrations followed a similar seasonal variation, increasing during the summer when precipitation decreases. At both study sites the chemical species analysed in bulk precipitation and throughfall were characterised by the fact that they came from three distinct sources: acidic pollution, marine and terrestrial origin. Concentration of constituents in bulk precipitation in Manzanal was higher than in Posadero, most probably due to the smaller amount of precipitation that falls in this study site. The precipitation at Manzanal had a significantly lower pH than at Posadero. The amounts of sulphate, nitrate-nitrogen, organic nitrogen and protons that fell in the bulk precipitation at Manzanal (the polluted site) were higher than those that fell at Posadero (less polluted site). The concentration of organic nitrogen in the bulk precipitation of the polluted site was significantly related to the hydrocarbon concentration measured in the atmosphere in the nearby town of Muskiz. Throughfall in Manzanal had higher amounts of sulphate, nitrate-nitrogen, calcium, magnesium, sodium, potassium and chloride than in Posadero. This fact suggests that both dry deposition and canopy leaching were an important source of throughfall constituents in Manzanal. The amount of manganese measured in Posadero throughfall was higher than that found in Manzanal throughfall. The pH in the throughfall did not show any significant difference between sites and was significantly higher than in bulk precipitation. Thus, canopies in the study sites seem to be able to neutralise very efficiently the acidic load of bulk deposition. Despite this buffering capacity of the canopies, the soil at Manzanal appeared to be more acidic than at Posadero, probably due to the liberation of protons in the rhizosphere when the neutralising pacity of the canopy is `recharged'. This soil acidification may be leading to a greater solubilization of aluminium in the polluted site which could suffer from cation nutrient deficiencies in the future.     

Mass transfer limitation of sulfate in methanogenic aggregates

The role of mass transfer limitation of sulfate as a factor governing the competition between sulfate reducing and methane producing bacteria in methanogenic aggregates was theoretically evaluated by the calculation of steady-state sulfate microprofiles using a reference set of parameters obtained from the literature. The shooting method was used as a numerical technique for solving the mathematical model. The effect of the parameters on mass transport limitation was tested by varying each reference value of the parameters with a factor of 3. Sulfate limitation within granules prevailed at moderate (0.1 kg m(-3)) and low sulfate concentrations in the bulk liquid, at high maximum sulfate utilization rates (3.73 x 10(-5) kg SO(4) (2-) kg(-1) VSS S(-1) or biomass concentrations (40 KG VSS m(-3)), and in large aggregates (radius of 7.5 10(-4) m). The effective diffusion coefficient of sulfate and the affinity constant were less determinative for the penetration depth of sulfate within a granule. (c) 1994 John Wiley & Sons, Inc.     

Proton budgets for a monitoring network of European forested catchments: impacts of nitrogen and sulphur deposition

Proton (H⁺ ion) budgets were calculated for 17 forested sites in Europe using open field (bulk) and throughfall deposition and runoff or soil leachate data. Proton budgets integrate information about the complex chemical and biological processes that govern the generation or consumption of acidity in the ecosystem into a single parameter. The sites belong to the multidisciplinary ICP integrated monitoring network, set up to assess the environmental impacts of transboundary air pollution. Mean annual H⁺ budgets were estimated to quantify the relative importance of different biogeochemical processes, with special emphasis on the N deposition. N deposition exceeded S deposition on an equivalent basis at the studied sites. Model based estimates for quantifying the impact of agreed international emission reduction measures showed that the relative importance of N deposition still is likely to increase in the future. Base cation weathering and ion exchange were the main processes for proton consumption. Sites on base poor soil material showed low base cation fluxes, export of acidity and high external/internal H⁺ source ratios. At these sites the dissociation of organic acids was commonly a significant internal H⁺ source. Depending on deposition inputs, the importance of N processes on the H⁺ budget varied between −4.5 mequiv. m⁻² a⁻¹ (small H⁺ consumption) and 46.2 mequiv. m⁻² a⁻¹ (H⁺ production). A relationship between the deposition inputs and the H⁺ production from N transformations was also observed, with higher H⁺ production at higher deposition levels. Net release of sulphate (and associated H⁺ production) was observed at many sites, being consistent with observations from other recent European budget studies.     

The contribution of nitrogen deposition to the eutrophication signal in understorey plant communities of European forests

We evaluated effects of atmospheric deposition of nitrogen on the composition of forest understorey vegetation both in space and time, using repeated data from the European wide monitoring program ICP‐Forests, which focuses on normally managed forest. Our aim was to assess whether both spatial and temporal effects of deposition can be detected by a multiple regression approach using data from managed forests over a relatively short time interval, in which changes in the tree layer are limited. To characterize the vegetation, we used indicators derived from cover percentages per species using multivariate statistics and indicators derived from the presence/absence, that is, species numbers and Ellenberg's indicator values. As explanatory variables, we used climate, altitude, tree species, stand age, and soil chemistry, besides deposition of nitrate, ammonia and sulfate. We analyzed the effects of abiotic conditions at a single point in time by canonical correspondence analysis and multiple regression. The relation between the change in vegetation and abiotic conditions was analyzed using redundancy analysis and multiple regression, for a subset of the plots that had both abiotic data and enough species to compute a mean Ellenberg N value per plot using a minimum of three species. Results showed that the spatial variation in the vegetation is mainly due to “traditional” factors such as soil type and climate, but a statistically significant part of the variation could be ascribed to atmospheric deposition of nitrate. The change in the vegetation over the past c. 10 years was also significantly correlated to nitrate deposition. Although the effect of deposition on the individual species could not be clearly defined, the effect on the vegetation as a whole was a shift toward nitrophytic species as witnessed by an increase in mean Ellenberg's indicator value.     

On the tracks of Nitrogen deposition effects on temperate forests at their southern European range – an observational study from Italy

We studied forest monitoring data collected at permanent plots in Italy over the period 2000–2009 to identify the possible impact of nitrogen (N) deposition on soil chemistry, tree nutrition and growth. Average N throughfall (N‐NO₃+N‐NH₄) ranged between 4 and 29 kg ha^?1 yr^?1, with Critical Loads (CLs) for nutrient N exceeded at several sites. Evidence is consistent in pointing out effects of N deposition on soil and tree nutrition: topsoil exchangeable base cations (BCE) and pH decreased with increasing N deposition, and foliar nutrient N ratios (especially N : P and N : K) increased. Comparison between bulk openfield and throughfall data suggested possible canopy uptake of N_, levelling out for bulk deposition >4–6 kg ha^?1 yr^?1. Partial Least Square (PLS) regression revealed that ‐ although stand and meteorological variables explained the largest portion of variance in relative basal area increment (BAI_rel 2000–2009) ‐ N‐related predictors (topsoil BCE, C : N, pH; foliar N‐ratios; N deposition) nearly always improved the BAI_rel model in terms of variance explained (from 78.2 to 93.5%) and error (from 2.98 to 1.50%). N deposition was the strongest predictor even when stand, management and atmosphere‐related variables (meteorology and tropospheric ozone) were accounted for. The maximal annual response of BAI_rel was estimated at 0.074–0.085% for every additional kgN. This corresponds to an annual maximal relative increase of 0.13–0.14% of carbon sequestered in the above‐ground woody biomass for every additional kgN, i.e. a median value of 159 kgC per kgN ha^?1 yr^?1 (range: 50–504 kgC per kgN, depending on the site). Positive growth response occurred also at sites where signals of possible, perhaps recent N saturation were detected. This may suggest a time lag for detrimental N effects, but also that, under continuous high N input, the reported positive growth response may be not sustainable in the long‐term.     

Input-output budget of nitrogen and the effect of experimentally changed deposition in the forest ecosystems in central Japan

To evaluate the current nitrogen (N) status in Japanese forests, field measurements of rainfall, throughfall, litter layer percolation, and soil solution percolation were conducted in a red pine stand (Kannondai) and a deciduous stand (Yasato) located in central Japan. N input via throughfall was 31 and 14 kg ha(-1) year(-1) and output below rooting zone was 9.6 and 5.5 kg ha1 year(-1) in Kannondai and in Yasato, respectively. Two thirds of input N were retained in plant-soil systems. Manipulation of N input was carried out. Ionic constituents were removed from throughfall with ion exchange resin at removal sites and ammonium nitrate containing twice the N of the throughfall was applied at N addition sites periodically. SO4(2-) output below 20-cm soil layer changed depending on the input, while NO3- output was regulated mainly by the internal cycle and effect of manipulation was undetected. These Japanese stands were generally considered to have a larger capacity to assimilate N than NITREX sites in Europe. However, N output fluxes had large spatial variability and some sites in Kannondai showed high N leaching below rooting zone almost balanced with the input via throughfall.     

Insect herbivory, organic matter deposition and effects on belowground organic matter fluxes in a central European oak forest

Apart from the forest floor, the canopy of forested ecosystems functions as the second most important source for dissolved and particulate fractions of organic and inorganic C and N compounds. However, under mass outbreak situations of insect herbivores this flux path of organic matter is considerably intensified clearly exceeding C and N fluxes from the forest floor. In this paper we report on herbivore-altered C and N fluxes from the canopy to the forest floor and effects on forest floor nutrient fluxes during severe defoliating herbivory of the winter moth (Operophtera brumata) and the mottled umber moth (Eranis defoliaria) in an oak forest in Germany. Over the course of 6.5 months we followed the C and N fluxes with bulk deposition, throughfall solution, insect frass deposits (green-fall together with insect faeces) and with forest floor solution in an 117-yr-old oak (Quercus petraea) forest. Compared to the control, herbivore defoliation significantly enhanced throughfall inputs of total and dissolved organic carbon and nitrogen by a factor of 3 and 2.5 (for TOC and DOC), and by 1.4 and 1.3 times (for TNb and DNb), respectively. Frass plus green-fall C and N fluxes peaked in May with 592 kg C?ha^?1 and 33.5 kg N?ha^?1 representing 79.6% (for C) and 78.3% (for N) of the total C and N input over 2.5 months. The quantitative and qualitative C and N input via faeces and litter deposition significantly differ between the insect affected and non-affected site. However, the C and N fluxes with throughfall did not significantly correlate with forest floor leachates. In this context, forest floor fluxes of TOC, DOC and NO₃-N were significantly lower at the infested site compared to the control, whereas fluxes of NH₄-N together with DON were significantly higher. The study demonstrates the importance of linking the population and associated frass dynamics of herbivorous insects with the cycling of nutrients and organic matter in forest ecosystems, highlighting the remarkable alterations in the timing, amounts and nature of organic matter dynamics on the ecosystem level. Consequently, the ecology of phytophagous insects allows partly to explain temporal-spatial alterations in nutrient cycling and thus ecosystem functioning.