No abstract

Research on the combined effects of climate change and nitrogen deposition on reproductive traits, and especially on the production of viable seeds, is still scarce despite their importance for population persistence and expansion. Hence, in this study we set out to investigate the direct and indirect effects of the above-mentioned global change drivers on seed viability in the coniferous shrub Juniperus communis L. In many parts of its European range, juniper is increasingly threatened, partly because of a lack of sexual reproduction. We hypothesised that this regeneration failure is partly due to poor seed viability. Using data from 39 populations throughout Europe, we were able to demonstrate that a strong, triangular-shaped relationship exists between the percentage of viable seeds produced and the percentage of juniper seedlings occurring in a population, which indicates that the species is indeed partly seed limited. Furthermore, based on an extended dataset of 42 populations, we found that seed viability was negatively affected by temperature, measured as mean annual growing degree-days, and nitrogen deposition (but not by drought). Suggestions are made about the processes behind the observed patterns, but more research is required. Nevertheless, our results do raise serious concerns for the conservation of juniper in light of the predicted rise in temperature and global nitrogen emissions. Furthermore, it is likely that similar patterns can also be observed for other species. Unravelling all these interacting effects of GC requires either well-designed, but inevitably complex, experiments or large-scale observational studies covering broad environmental gradients (cf. Sagarin et al. 2006). In this study, we adopt the latter approach to investigate the direct and indirect effects of GC drivers on seed viability in the coniferous shrub Juniperus communis L. A good understanding of the factors explaining the variation in seed viability in juniper is of particular interest, as poor seed viability may be linked with strong population declines that are actually occurring in large parts of its range, including northwest European lowlands and Mediterranean mountain regions. In Flanders (i.e. the northern part of Belgium), the number and sizes of juniper populations have declined dramatically in the past two decades, and at present only three populations remain that contain more than 100 individuals (Adriaenssens S., Baeten L., Crabbe S., Verheyen K., Ghent University, Ghent, unpublished results). Also, in surrounding regions, including Wallonia (southern Belgium; The main reasons for the juniper decline probably include habitat destruction, habitat degradation and very limited recruitment in remaining populations (e.g. Verheyen et al. 2005). Due to its protected status, the destruction of juniper habitat is at present less common, and for the same reason, actions are being undertaken to prevent further habitat degradation in extant populations. Degradation of juniper habitat is mostly related to progress...

Living trees are the main source of biogenic volatile organic compounds (BVOCs) in forest ecosystems, but substantial emissions originate from leaf and wood litter, the rhizosphere and from microorganisms. This review focuses on temperate and boreal forest ecosystems and the roles of BVOCs in ecosystem function, from the leaf to the forest canopy and from the forest soil to the atmosphere level. Moreover, emphasis is given to the question of how BVOCs will help forests adapt to environmental stress, particularly biotic stress related to climate change. Trees use their vascular system and emissions of BVOCs in internal communication, but emitted BVOCs have extended the communication to tree population and whole community levels and beyond. Future forestry practices should consider the importance of BVOCs in attraction and repulsion of attacking bark beetles, but also take an advantage of herbivore-induced BVOCs to improve the efficiency of natural enemies of herbivores. BVOCs are extensively involved in ecosystem services provided by forests including the positive effects on human health. BVOCs have a key role in ozone formation but also in ozone quenching. Oxidation products form secondary organic aerosols that disperse sunlight deeper into the forest canopy, and affect cloud formation and ultimately the climate. We also discuss the technical side of reliable BVOC sampling of forest trees for future interdisciplinary studies that should bridge the gaps between the forest sciences, health sciences, chemical ecology, conservation biology, tree physiology and atmospheric science.

No abstract

This study assessed the foliar uptake of 15 N-labelled nitrogen (N) originating from wet deposition along with leaf surface conditions, measured by wettability and water storage capacity. Foliar 15 N uptake was measured on saplings of silver birch, European beech, pedunculate oak and Scots pine and the effect of nitrogen form (NH 4 + or NO 3 − ), NH 4 + to NO 3 − ratio and leaf phenology on this N uptake was assessed. Next to this, leaf wettability and water storage capacity were determined for each tree species and phenological stage, and the relationship with 15 NH 4 + and 15 NO 3 − uptake was examined. Uptake rates were on average five times higher (p<0.05) for NH 4 + than for NO 3 − and four times higher for deciduous species than for Scots pine. Developing leaves showed lower uptake than fully developed and senescent leaves, but this effect was tree species dependent. The applied NH 4 + to NO 3 − ratio did only affect the amount of N uptake by senescent leaves. The negative correlation between measured leaf Water Air Soil Pollut (2011) 219:43-57 contact angles and foliar N uptake demonstrates that the observed effects of tree species and phenological stage are related to differences in leaf wettability and not to water storage capacity.