Research Highlights: Leaf chemistry is a key driver of litter decomposition; however, studies directly comparing metabolites that are important for tree growth and defence across different woody species are scarce. Background and Objectives: Choosing 14 temperate woody species differing in their growth rates, nutrient demand, shade tolerance, and drought sensitivity, we hypothesized that the species would group according to their metabolite profiles based on their ecological background. Materials and Methods: We analysed total N and C, soluble amino acid, protein, and phenolic levels in green leaves and leaf litter of these species, each in two consecutive years. Results: Metabolite levels varied significantly across species and between the sampling years which differed in temperature and precipitation (i.e., colder/drier vs warmer/ wetter). Conclusions: The 14 woody species could not be grouped according to their green leaf or leaf litter metabolite profiles. In litter leaves, most of the variation was explained by total phenolics and total nitrogen levels, and in green leaves by total phenolics and total soluble amino acid levels. Local climate variation between the two consecutive years for green leaves or leaf litter led to significant differences in metabolite levels, although some of them were species-specific.

Forest soils are major sinks of terrestrial carbon, but this function may be threatened by mass outbreak events of forest pests. Here, we measured soil CO2-C and N2O-N fluxes from a Scots pine (Pinus sylvestris L.) forest that was heavily infested by the nun moth (Lymantria monacha L.) and an adjacent noninfested (control) forest site during one year. In the infested forest, net emissions of CO2-C were higher during main defoliation, summer and autumn, while indications of increased N2O-N emissions were found at one sampling date. On basis of this, a microcosm incubation experiment with different organic matter treatments was conducted. Soil treatments with needle litter, insect feces plus needle litter, and insect feces showed 3.7-, 10.6-, and 13.5-fold higher CO2-C emissions while N2O-N of the insect feces plus needle litter, and insect feces treatment was 8.9-, and 10.4-fold higher compared with soil treatments without added organic matter (control). Hence, the defoliation in combination with high inputs of organic matter during insect outbreaks distinctly accelerate decomposition processes in pine forest soils, which in turn alters forests nutrient cycling and the functioning of forests as carbon sinks.

Forest insects are among the most important factors of disturbance in European forests. The increase in and duration of drought stress events due to climate change not only increase the vulnerability of rural and urban forests but also predispose weakened stands to insect pest calamities. In this context, many German and European forest research institutes and environmental institutions report an increase in the densities and calamity developments of the oak processionary moth (Thaumetopoea processionea L.) not only in oak and mixed-oak forests but also in smaller areas where oak trees regularly occur, e.g., parklands, urban areas, copses, avenues, recreational forests, etc. It is expected that the oak processionary moth (OPM) will benefit from the overall weakened vitality of both individual oaks and oak stands in the future and that mass outbreaks will occur at an increased frequency. This paper reports on the effects that the OPM can have on tree performance for rural forest and urban oak trees by applying the chlorophyll fluorescence non-destructive diagnostic method for the identification and quantification of damage to oak leaves. The aim of the study was to investigate the effects of OPM frass activity on tree leaf health using chlorophyll fluorescence measurements, comparing infested host oaks with non-infested oaks in urban and forest environs. The study assessed: 1. the quantum efficiency of photosystem II (PS II), which counts as an indicator for leaf conditions, 2. the performance index, which indicates the efficiency of the photosynthetic light reaction, 3. the energy loss of the photosynthetic reaction, which is an indicator for cell damage, and 4. the degree of open reaction centers in PS II, which indicates how well light energy can be absorbed for photosynthesis. Infested urban and rural oaks showed a significantly reduced quantum yield of PS II by up to 10% compared to non-infested oak leaves. The leaf performance was significantly reduced by up to 35% for infested urban oaks and by up to 60% for infested forest oaks, respectively. The energy losses were two times higher for infested urban and forest oaks. However, OPM infestation led to a higher reduction in the photosynthetic performance in the leaves of forest oaks compared to that of urban oaks. In order to avoid permanent damage, suitable countermeasures must be taken quickly, as, immediately after pest infestation, the performance decreases significantly. A lower performance means a significant loss in biomass production as well as in tree vitality.