Hugues Titeux

Mobility of radiocesium in three distinct forest floors.

The degree of mixing of organic matter with minerals in organic and hemi-organic horizons of forest soils largely differs between humus types. As clay minerals might control the mobility of radiocesium in these forest floor horizons, plant contamination could greatly vary with the kind of humus. We measured the mobility of radiocesium in the upper O, OAh and Ah horizons of three acid forest soils with three distinct humus types: eumoder, dysmoder and fibrimor. We used two different approaches: a physico-chemical test quantifying the radiocesium interception potential (RIP) and a biological assay simulating an experimental rhizosphere. The results show that the (137)Cs horizon-to-plant transfer is directly governed by RIP, and thus by frayed edge sites born by weathered micaceous minerals. The inverse relationship between RIP and organic matter content indicates that in the three sites investigated the mixing of organic residues with Cs-fixing minerals is a key process in 137Cs mobility. These Cs-fixing clay minerals indeed decrease in the sequence eumoder > dysmoder > fibrimor because they are more diluted in forest floor with less bioturbation. Our results suggest that humus type might be an important parameter in classifying forest soils with respect to their ability to transfer radiocesium to the above standing vegetation.

Metal complexing properties of forest floor leachates might promote incipient podzolization in a Cambisol under deciduous forest

The characteristics of both the mineral substrate and the humus layer are of key importance in podzol formation. However, the properties of the forest floor leachates are seldom considered. We measured the complexation capacity by Cu2+ titration (CuCC), the optical density (E-4/E-6), as well as the concentrations of dissolved organic carbon (DOC) and of major and trace ions in the liquid extracts of forest floors of two Cambisols and two Luvisols on loess under deciduous forest. The vegetation consists of a mixture of beech, oak and maple. The humus type is a fibrimor in one Cambisol and a moder in the three other soils. Both in the solid phase and in the liquid extracts of the forest floors, the Si/(Al + Fe) and C/(Al + Fe) atomic ratios are much larger in the fibrimor than in the moders. In the liquid extracts, the complexation capacity (CuCC) and the density of metal binding sites (CuCC/DOC are larger in the fibrimor than in the moders. These large CuCC and CuCC/DOC in the fibrimor leachate are linked with relatively small stability constant of the humus-Cu complex and relatively large E-4/E-6 ratio, i.e. with reactive organic ligands with low molecular weight and weak aromatic character. These characteristics are associated with current incipient podzolization in the Cambisol with a fibrimor, and might be considered as an indicator of a substantial podzolizing potential of the forest floor

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.

Nutrient supply at the local tree level in mixed forests of sessile oak and beech

In mixed-species forests, tree species composition can affect nutrient return through litter fall. This in turn is expected to have an effect on soil available nutrients, which could influence the nutrient status at the local tree level. Using ion-exchange resins, we estimated resin available soil nutrients at two depths beneath target trees of sessile oak and beech in the Belgian Ardennes. First we tested whether resin available nutrients were related to tree nutrition, using foliar nutrient concentrations as a proxy. In a second step, we tested whether local litter fall, through total nutrient return or litter species composition, affected resin available nutrients. In a final stage, we tested the impacts of local stand composition, as an integrated proxy of above- and belowground processes, and compared them to those of litter composition. With the exception of P for oak, nutrient supply was only poorly related to foliar nutrient concentrations for both target species. The effects of litter fall on nutrient supply were driven by litter species composition and not by total nutrient inputs. Litter composition and local stand composition effects were in close agreement. Our results show that nutrient supply to target trees in mixed-species stands is affected by local neighbourhoods, yet to a limited extent. Direct translation of resin available nutrients into foliar concentrations is probably hampered by complex capture patterns.