Leaf volatile emissions of Betula pendula during autumn coloration and leaf fall.

Deciduous trees remobilize the nitrogen in leaves during the process of autumn coloration, thus providing a high quality food source for aphids preparing to lay over-wintering eggs. It has been suggested that aphids may use volatile organic compounds (VOCs) to: (a) select leaves where nutrient remobilization has started and induced defenses are reduced; and (b) detect the time of leaf abscission. We analyzed VOCs emitted by the foliage of Betula pendula Roth. during autumn coloration and from leaf litter just after leaf fall. We tested the hypothesis that costly, photosynthesis-related terpenes and other herbivore-induced VOCs related to attraction of aphid parasitoids and predators are reduced during the coloration process. We also investigated if the VOC emission profile of abscising leaves is different from that of early stage yellowing leaves. Enemy-luring compounds (E)-ß-ocimene, linalool, and (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) were emitted only from the green foliage. Methyl salicylate (MeSa), known to recruit predatory bugs and attract migrant aphids, was emitted until the first stage of color change. Cis-3-hexenol, an indicator of cellular disintegration, became dominant in the emissions from abscising leaves and from fresh leaf litter. We discuss the ecological significance of the observed changes in birch leaf VOC profiles during the process of autumn senescence.

Birch (Betula spp.) leaves adsorb and re-release volatiles specific to neighbouring plants--a mechanism for associational herbivore resistance?

Plant-emitted semi-volatile compounds have low vaporization rates at 20-25 degrees C and may therefore persist on surfaces such as plant foliage. The passive adsorption of arthropod-repellent semi-volatiles to neighbouring foliage could convey associational resistance, whereby a plant's neighbours reduce damage caused by herbivores. We found that birch (Betula spp.) leaves adsorb and re-release the specific arthropod-repelling C(15) semi-volatiles ledene, ledol and palustrol produced by Rhododendron tomentosum when grown in mixed association in a field setup. In a natural habitat, a higher concentration of ledene was released from birches neighbouring R. tomentosum than from birches situated > 5 m from R. tomentosum. Emission of alpha-humulene, a sesquiterpene synthesized by both Betula pendula and R. tomentosum, was also increased in R. tomentosum-neighbouring B. pendula. In assessments for associational resistance, we found that the polyphagous green leaf weevils (Polydrusus flavipes) and autumnal moth (Epirrita autumnata) larvae both preferred B. pendula to R. tomentosum. P. flavipes also preferred birch leaves not exposed to R. tomentosum to leaves from mixed associations. In the field, a reduction in Euceraphis betulae aphid density occurred in mixed associations. Our results suggest that plant/tree species may be protected by semi-volatile compounds emitted by a more herbivore-resistant heterospecific neighbour.

From plants to birds: higher avian predation rates in trees responding to insect herbivory.

BACKGROUND: An understanding of the evolution of potential signals from plants to the predators of their herbivores may provide exciting examples of co-evolution among multiple trophic levels. Understanding the mechanism behind the attraction of predators to plants is crucial to conclusions about co-evolution. For example, insectivorous birds are attracted to herbivore-damaged trees without seeing the herbivores or the defoliated parts, but it is not known whether birds use cues from herbivore-damaged plants with a specific adaptation of plants for this purpose. METHODOLOGY: We examined whether signals from damaged trees attract avian predators in the wild and whether birds could use volatile organic compound (VOC) emissions or net photosynthesis of leaves as cues to detect herbivore-rich trees. We conducted a field experiment with mountain birches (Betula pubescens ssp. czerepanovii), their main herbivore (Epirrita autumnata) and insectivorous birds. Half of the trees had herbivore larvae defoliating trees hidden inside branch bags and half had empty bags as controls. We measured predation rate of birds towards artificial larvae on tree branches, and VOC emissions and net photosynthesis of leaves. PRINCIPAL FINDINGS AND SIGNIFICANCE: The predation rate was higher in the herbivore trees than in the control trees. This confirms that birds use cues from trees to locate insect-rich trees in the wild. The herbivore trees had decreased photosynthesis and elevated emissions of many VOCs, which suggests that birds could use either one, or both, as cues. There was, however, large variation in how the VOC emission correlated with predation rate. Emissions of (E)-DMNT [(E)-4,8-dimethyl-1,3,7-nonatriene], beta-ocimene and linalool were positively correlated with predation rate, while those of highly inducible green leaf volatiles were not. These three VOCs are also involved in the attraction of insect parasitoids and predatory mites to herbivore-damaged plants, which suggests that plants may not have specific adaptations to signal only to birds.

Effect of long-term forest fertilization on Scots pine xylem quality and wood borer performance.

We tested whether changes in long-term nutrient availability would affect the xylem quality and characteristics of Scots pine trees as a food source for the larvae of the xylophagous wood borer Hylotrupes bajulus L. (Cerambycidae). We looked for an effect of host plant growth and xylem structural traits on H. bajulus larval performance, and looked for delayed effects of long-term forest fertilization on xylem chemical quality. In general, larval performance was dependent on larval developmental stage. However, the growth of larvae also varied with host plant quality (increases in the concentration of nitrogen and carbon-based secondary compounds of xylem were correlated with a decrease in the larval growth rate). The greater annual growth of trees reduced tracheid length and correlated positively with second-instar H. bajulus growth rate. This is consistent with the hypothesis that intrinsic growth patterns of host plants influence the development of the xylophagous wood borer H. bajulus.