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Plant-to-plant volatile-mediated communication and subsequent induced resistance to insect herbivores is common. Less clear is the adaptive significance of these interactions; what selective mechanisms favour plant communication and what conditions allow individuals to benefit by both emitting and responding to cues? We explored the predictions of two non-exclusive hypotheses to explain why plants might emit cues, the kin selection hypothesis (KSH) and the mutual benefit hypothesis (MBH). We examined 15 populations of sagebrush that experience a range of naturally occurring herbivory along a 300 km latitudinal transect. As predicted by the KSH, we found several uncommon chemotypes with some chemotypes occurring only within a single population. Consistent with the MBH, chemotypic diversity was negatively correlated with herbivore pressure; sites with higher levels of herbivory were associated with a few common cues broadly recognized by most individuals. These cues varied among different populations. Our results are similar to those reported for anti-predator signalling in vertebrates.

Purpose of Review Approximately 40 years ago, key papers indicating that volatile chemicals released by damaged plants elicited defense-related changes in their neighbors, brought prominence to the idea of plant communication. These studies were conducted with several tree species and the phenomenon observed was dubbed “talking trees.” Today there is a wealth of evidence supporting the idea that plants can send and receive information both above and belowground. However, while early reports of plant-plant communication concerned trees, the literature is now heavily biased towards herbaceous plants. The purpose of this review is to highlight recent research on tree-tree communication with an emphasis on synthesizing knowledge on the ecological relevance of the process. Recent Findings Aboveground, information is often provided in the form of biogenic volatile organic compounds (VOCs), which are released by both undamaged and damaged plants. The blends of VOCs released by plants provide information on their physiological condition. Belowground, information is conveyed through mycorrhizal networks and via VOCs and chemical exudates released into the rhizosphere. Recent findings have indicated a sophistication to tree communication with more effective VOC-mediated interactions between trees of the same versus a different genotype, kin-group, or chemotype. Moreover, common mycorrhizal networks have been shown to convey stress-related signals in intra- and interspecific associations. Together these two forms of communication represent “wireless” and “wired” channels with significance to facilitating plant resistance to herbivores. Summary In this review, we examine tree-tree communication with a focus on research in natural forest ecosystems. We particularly address the effects of tree-tree communication on interactions with herbivorous insects. Aboveground and belowground interactions are both reviewed and suggested implications for forest management and future research are presented.

Aims Biogenic volatile organic compound (BVOC) research has been mostly focused on foliar emissions. In this experiment, the main focus was on rhizosphere BVOC emissions of Scots pine seedlings under changing growth conditions. Methods Soil-growing Scots pines were exposed to increased air (0.5°C) and soil (4.0°C) temperature and N addition (30 kg N ha− 1 yr− 1) for three growing seasons in a field experiment. In addition to these factors, seedlings were exposed to bark herbivory by large pine weevils in two last seasons. Gas-chromatography and mass-spectrometry was used for analyzing the BVOC samples collected from pine rhizosphere. Results Almost 98 % of BVOCs were non-oxygenated monoterpenes (nMTs), 1 % oxygenated monoterpenes (oMTs), 0.5 % sesquiterpenes (SQTs), and 1 % other BVOCs. In both years, there was an interaction of warming, N addition and bark herbivory on rhizosphere BVOC emissions. In 2015, warming and N addition in single exposures decreased oMT emissions, while herbivory in single exposure increased oMT emissions. In 2016, the three-way interaction showed that the effects of warming, N addition and herbivory on BVOCs were mainly detected in single exposures. In 2016, warming decreased nMT, oMT, SQT and other BVOC emissions; N addition decreased oMT and SQT emissions; and herbivory decreased SQT and other BVOC emissions. Conclusions Warming and N addition in single exposures decreased the rhizosphere BVOC emissions. The effect of bark herbivory on BVOC emissions varied between the years from increase to decrease. It seems that under altered growth conditions Scots pine seedlings may reduce carbon investment to rhizosphere BVOCs.