The Classics-Long-term trends in forest ecosystem processes as affected by air pollution, drought... more The Classics-Long-term trends in forest ecosystem processes as affected by air pollution, drought or other extreme weather events ..
Nitrogen (N) depositions in Europe are decreasing, but this could not explain faster than expecte... more Nitrogen (N) depositions in Europe are decreasing, but this could not explain faster than expected improvement of N saturation indicators in temperate forests. Alongside there were local signs of initial recovery from acidification during the past three decades and enhanced leaching of dissolved organic carbon and nitrogen (DOC, DON). These two global change processes both affect total dissolved nitrogen (TDN) levels and often occur simultaneously, hence complicating mechanistic explanations for changing European forest N status. We aimed to test the hypothesis that forest N status in northwest Europe has started to improve. If this hypothesis is confirmed, we wanted to investigate to what extent such improvement is due to reduced N deposition. We evaluated the evolution of multiple N saturation indicators in five ICP Forests Level II plots in northern Belgium, using long-term soil solution and foliage datasets. The DON:TDN ratio (molar) in soil solution increased overall in the O horizon (mean 0.279-0.463, slope 0.023-0.037 yr −1) and in the mineral soil (mean 0.134-0.78, slope 0.007-0.051 yr −1) between 2005 and 2014. The DOC:NO 3 − ratio (molar) in soil solution increased in three plots in the O horizon (mean 6.84-22.15, slope 0.58-1.92 yr −1) and in four plots in the mineral soil (mean 2.07-25.32, slope −0.06-5.76 yr −1) between 2002 and 2014. The ratio of N and phosphorus (P) concentrations in foliage (mg g −1) and the ratio of base cations (Bc = Ca + K + Mg) and N concentrations in foliage (molar) remained unaltered between 1999 and 2013. Changes in the soil solution chemical composition thus confirmed an improvement in forest N status, despite sustained high NO 3 − concentrations, but biotic recovery appeared to be lagging behind. This demonstrates that insight in forest recovery from N saturation requires a multiple indicator approach, and further monitoring of tree nutritional status alongside soil processes is needed to monitor the evolution of European forest N status. 2depositions strongly decreased, using a selection of indicators based on
Sinds 1950 hebben Vlaamse bossen te lijden onder luchtvervuiling met zwavel en stikstof door land... more Sinds 1950 hebben Vlaamse bossen te lijden onder luchtvervuiling met zwavel en stikstof door landbouw, verkeer en industrie. Dit leidde tot ernstige bodemverzuring en een verminderde chemische bodemwaterkwaliteit. De voorbije 25 jaar is door (inter)nationale beleidsinspanningen vooral de uitstoot van zwavel gedaald. In vijf proefvlakken onderzoekt het INBO de impact van luchtvervuiling op Vlaamse bossen op lange termijn.
Gradients in soil moisture, texture, nutrient state and spacing patterns enable us to divide stan... more Gradients in soil moisture, texture, nutrient state and spacing patterns enable us to divide stands into separate substands, which have their own tree species composition and structures. Nevertheless, such a separation is just the beginning of a set of conclusions and observations. It is based upon differences in flora and chemical characteristics. As a matter of fact, every separate sub stand has its own microclimate in which a well defined complex of environmental variables regulates regeneration, flora presence and leaf decomposition. The results of such an intensive research should lead to the improvement of the stand tending.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
A 10-year long dataset of half-hourly ozone (O 3) fluxes was used to study the variability in dep... more A 10-year long dataset of half-hourly ozone (O 3) fluxes was used to study the variability in deposition velocity (υ d) over a mixed temperate suburban forest. Average (median) υ d 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 υ d and the deposition efficiency (υ d/ υ dmax), 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 O 3 and nitric oxide (NO) above the canopy surface. During the working week, daytime values of υ d, υ d/ υ dmax and the O 3 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 υ d and υ d/ υ dmax were positively correlated with the relative humidity at the canopy surface (RH(z 0 ′)) and negatively correlated with the water levels below the ground surface. During the daytime, monthly υ d and υ d/ υ dmax 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 υ d and F. It was speculated that, among other, biogenic volatile compounds (BVOCs) could have contributed to O 3 quenching in some (spring) months with severe drought stress.
Carbon taken up by the forest canopy is allocated to tree organs for biomass production and respi... more Carbon taken up by the forest canopy is allocated to tree organs for biomass production and respiration. Because tree organs have different life span and decomposition rate, the tree C allocation determines the residence time of C in the ecosystem and its C cycling rate. The study of the carbon-use efficiency, or ratio between net primary production (NPP) and gross primary production (GPP), represents a convenient way to analyse the C allocation at the stand level. Previous studies mostly focused on comparison of the annual NPP-GPP ratio among forests of different functional types, biomes and age. In this study, we extend the current knowledge by assessing (i) the annual NPP-GPP ratio and its interannual variability (for five years) for five tree organs (leaves, fruits, branches, stem and coarse roots), and (ii) the seasonal dynamic of NPP-GPP ratio of leaves and stems, for two stands dominated by European beech and Scots pine. The average NPP-GPP ratio for the beech stand (38%) was similar to previous estimates for temperate deciduous forests, whereas the NPP-GPP ratio for the pine stand (17%) is the lowest recorded till now in the literature. The proportion of GPP allocated to leaf NPP was similar for both species, whereas beech allocated a remarkable larger proportion of GPP to wood NPP than pine (29% vs. 6%, respectively). The interannual variability of the NPP-GPP ratio for wood was substantially larger than the interannual variability of the NPP-GPP ratio for leaves, fruits and overall stand and it is likely to be controlled by previous year air temperature (both species), previous year drought intensity (beech) and thinning (pine). Seasonal pattern of NPP-GPP ratio greatly differed between beech and pine, with beech presenting the largest ratio in early season, and pine a more uniform ratio along the season. For beech, NPP-GPP ratio of leaves and stems peaked during the same period in the early season, whereas they peaked in opposite periods of the growing season for pine. Seasonal differences in C allocation are likely due to functional differences between deciduous and evergreen species and temporal variability of the sink strength.
The effects of pollen on dissolved inorganic nitrogen (DIN) compounds in throughfall water are no... more The effects of pollen on dissolved inorganic nitrogen (DIN) compounds in throughfall water are not completely understood. We conducted a 7-day leaching experiment with pollen from silver birch (including a sterilized control), European beech, sessile oak, Scots pine, Corsican black pine and Norway spruce using an immersion medium containing nitrate (11.295 mg N l-1). Within 2 hours, pollen released substantial amounts of potassium (K +), phosphate (PO3-) and organic compounds. Solute concentrations of ammonium (NH4 +) were built up over time. In treatments with pollen from birch, oak and beech, nitrate (NO3-) concentrations started to decrease after 24-48 hours, while simultaneously nitrite (NO2-) emerged, but part of the inorganic nitrogen could no longer be detected in solution. For birch, sterilisation of the pollen made no difference, indicating that microorganisms on the pollen played no substantial role in the observed N transformations. Conditions in the samples were oxic (1.82-6.12 mg O2 l-1), rendering microbial denitrification unlikely. Our findings revealed that pollen from broadleaved deciduous trees can transform throughfall NO3into NO2and likely also nitric oxide (NO), probably through the nitrate reductase pathway. The synthesis of NH4 + might be part of a natural defence mechanism protecting reproductive organs against pathogens during pollination.
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