Patterns of throughfall deposition, nitrate seepage, and soil acidification in contrasting forest edges

Subtitle:Patronen van doorvaldepositie, nitraatuitspoeling en bodemverzuring in contrasterende bosranden

Due to forest fragmentation, forest edges as ecotone boundaries between forest and open area such as heathland, pasture, or agricultural land are increasingly dominant features in landscapes around the world. In Flanders, the northern part of Belgium, forests are strongly fragmented: the relative amount of forest edge in the total forested area amounts to almost 60 % when considering an edge depth of only 50 m. Forest edges have a vast influence on the flux of nutrients or pollutants from the atmosphere towards the forest ecosystem. Due to these so-called edge effects, throughfall deposition of nitrogen (N) and potentially acidifying pollutants [sulfate (SO42-), nitrate (NO3-), and ammonium (NH4+)] is enhanced from the front of the edge to up to more than 100 m from the edge, by up to a fourfold compared with the forest interior. On top of that, Flanders suffers from N and SO42- deposition levels that are among the highest in Europe. In the interior of forests, high levels of N (NHx + NOy) and sulphur (S, SOx) deposition are associated with N saturation, increased levels of nitrate seepage, soil acidification, and eventually vitality decreases, floristic shifts, and declines in forest biodiversity. These facts argue for mitigating measures that reduce the input of N and potentially acidifying pollutants in forest ecosystems. In order to formulate recommendations for the design and management of forest edges for the mitigation of edge effects, we tried to get insight into (i) the impact of forest type and edge structure on the extent of edge effects on throughfall deposition, and (ii) the impact of enhanced input of N and potentially acidifying pollutants on two ecosystem responses at edges, i.e., nitrate seepage and soil acidification. For this reason, we sampled throughfall deposition, soil solution, and soil and determined stand structure characteristics along transects perpendicular to edges of contrasting forest stands, from the edge front up to a distance of 128 m from the edge. We focused on forest ecosystems on sandy soils, which are most vulnerable to nitrate seepage and soil acidification, and in regions with intensive livestock breeding and consequently high NH3 emissions. In addition, a wind tunnel study was performed in which we simulated edge effects in model forests with differing structure. Two field studies demonstrated that the throughfall deposition of N and potentially acidifying deposition in pine stands are subject to edge effects that penetrate the forest to a deeper extent and cause a higher level of deposition enhancement at the edge front than in deciduous oak and birch stands. The additional input of N and potentially acidifying pollutants in the oak and birch stands was 10 to 15 times lower than in the pine stands. The difference between a pine and an adjacent birch edge in additional input was higher for the potentially acidifying pollutants than for the so-called base cations. This indicates that the higher additional input of potentially acidifying pollutants in pine edges is not compensated by a higher input of ‘potentially neutralising’ base cations, so higher rates of soil acidification can be expected at pine edges in comparison with birch edges. By means of a wind tunnel study and two field studies, the impact of edge structure on edge patterns of (throughfall) deposition was explored. In the wind tunnel study, a gradual forest edge (i.e., a gradual transition of vegetation height in front of the forest edge) deflected the wind flow and decelerated wind speed in front of the forest. Consequently, it induced a decrease in the level of deposition enhancement at the edge front in comparison with a steep forest edge, particularly in sparse forest models. A field study, in which throughfall deposition patterns were measured in adjacent steep and gradual edges, confirmed the wind tunnel results on the lower level of deposition enhancement at gradual edges, but also pointed to a decrease in penetration depth of edge effects. With a steep transition, the additional N and S deposition induced by edge effects in the forest was, on average, three times higher than the additional N and S deposition in the forest and the gradual edge vegetation itself in the case of a gradual transition. In addition, the field study pointed to an important prerequisite in terms of shape and size of the gradual edge vegetation for the mitigation of edge effects on N and potentially acidifying deposition. In the wind tunnel study, edge effects penetrated further into a sparse model forest than in a dense forest, which was caused by the stronger deceleration of wind speed and turbulence in the dense forest. This effect of canopy density, expressed as leaf area index (LAI), was verified in a field study on Corsican and Scots pine stands with differing LAIs. LAI was found to be a key driver in the processes causing edge effects on deposition as it determined both the level of deposition enhancement at the edge front and the penetration depth of the edge effects. The overall increase in deposition in the entire edge zone of forests displayed an optimum at an intermediate LAI level, with small penetration depths in forests with high LAI and low levels of deposition enhancement in forests with low LAI. In both deciduous and coniferous forests, higher NO3- seepage fluxes occurred in forest edges to up to approximately 80 m from the edge. In the first 10 - 20 m from the forest edge, however, the NO3- seepage was no straightforward reflection of N input via throughfall deposition. The difference between N input and NO3- output via soil nutrient leaching of about 25 kg N per ha per year was partly attributed to the higher N uptake by the trees and the higher seepage fluxes of dissolved organic nitrogen, but mainly to the enhanced N retention in the soil. Higher pH values and higher levels of exchangeable base cations in the upper mineral soil at edges indicated that, in contrast with what was expected based on the higher input of potentially acidifying pollutants at edges, the rate of soil acidification was lower at edges than in interior zones. We presumed that edge gradients in microclimate, input of so-called base cations via throughfall deposition, and biomass production together with drift of agriculturally applied lime fertilizer were the main causes for the higher N retention in the soil and lower rates of soil acidification at edges. Based on the results this thesis, we provided edge layout and management suggestions for the purpose of mitigating edge effects on the deposition of N and potentially acidifying pollutants. We suggest, on a scale of increasing impact on the edge patterns of deposition but decreasing ease of implementation: (i) early and frequent thinning of forests, (ii) layout of gradual edge vegetation at steep edges, and (iii) conversion of coniferous pine plantations to deciduous forest types. Particularly at edges exposed to the prevailing wind directions (in Flanders, the south to westerly oriented edges), efforts should be made to reduce the extent of edge effects. It should be kept in mind, however, that mitigating measures, such as adjusted edge layout and management, do not provide the key solution for the environmental problem of air pollution. Instead, the achievement of vast emission reductions is the most fundamental prerequisite for full and sustainable protection and recovery of forest ecosystems and ecosystems in general.

ISBN:9789059892835

Publication year:2009

Accessibility:Closed