Phenolic metabolites are frequently implicated in chemical defense mechanisms against pathogens in woody plants. However, tree breeding programmes for resistance to pathogens and practical tree-protection applications based on these compounds seem to be scarce. To identify gaps in our current knowledge of this subject, we explored some of the recent literature on the involvement of phenolic metabolites in the resistance of northern forest trees (Pinus, Picea, Betula, Populus, and Salix spp.) to pathogens. Although it is evident that the phenolic metabolism of trees is often activated by pathogen attacks, few studies have convincingly established that this induction is due to a specific defense response that is capable of stopping the invading pathogen. The role of constitutive phenolics in the resistance of trees to pathogens has also remained unclear. In future studies, the importance of phenolics in oxidative stress, cell homeostasis and tolerance, and the spatial and temporal localization of phenolics in relation to invading pathogens should be more carefully acknowledged. Possibilities for future studies using advanced methods (e.g., metabolic profiling, confocal laser scanning microscopy, and use of modified tree genotypes) are discussed.Résumé : Les métabolites phénoliques sont souvent impliqués dans les mécanismes de défense chimiques contre les agents pathogènes chez les plantes ligneuses. Cependant, il semble y avoir peu de programmes d'amélioration des arbres pour la résistance aux agents pathogènes ou de mesures pratiques de protection des arbres qui sont basés sur ces composés. Dans le but d'identifier les lacunes dans nos connaissances actuelles sur ce sujet, nous avons exploré une partie de la littérature récente traitant du rôle des métabolites phénoliques dans la résistance des espèces forestières nordiques (Pinus, Picea, Betula, Populus et Salix spp.) aux agents pathogènes. Bien qu'il soit évident que le métabolisme des composés phénoliques chez les arbres est souvent déclenché par les attaques des agents pathogènes, il a rarement été établi de façon convaincante que cette induction est due à une réaction de défense spécifique capable d'enrayer l'invasion de l'agent pathogène. De plus, on n'est toujours pas certain du rôle que jouent les composés phénoliques constitutifs dans la résistance des arbres face aux agents pathogènes. Dans les études ultérieures, on devrait accorder plus d'attention à l'importance des composés phénoliques en lien avec le stress oxydatif, l'homéostasie et la tolérance des cellules ainsi qu'à la localisation spatiale et temporelle des composés phénoliques en relation avec l'invasion des agents pathogènes. La discussion porte sur la possibilité que les études futures utilisent des méthodes de pointes telles que le profilage métabolique, le microscope confocal à balayage laser et aient recours à des arbres dont le génotype a été modifié.[Traduit par la Rédaction]

Secondary attraction to aggregation pheromones plays a central role in the host colonization behavior of the European spruce bark beetle Ips typographus. However, it is largely unknown how the beetles pioneering an attack locate suitable host trees, and eventually accept or reject them. To find possible biomarkers for host choice by I. typographus, we analyzed the chemistry of 58 Norway spruce (Picea abies) trees that were subsequently either (1) successfully attacked and killed, (2) unsuccessfully attacked, or (3) left unattacked. The trees were sampled before the main beetle flight in a natural Norway spruce-dominated forest. No pheromones were used to attract beetles to the experimental trees. To test the trees' defense potential, each tree was treated in a local area with the defense hormone methyl jasmonate (MeJ), and treated and untreated bark were analyzed for 66 different compounds, including terpenes, phenolics and alkaloids. The chemistry of MeJ-treated bark correlated strongly with the success of I. typographus attack, revealing major chemical differences between killed trees and unsuccessfully attacked trees. Surviving trees produced significantly higher amounts of most of the 39 analyzed mono-, sesqui-, and diterpenes and of 4 of 20 phenolics. Alkaloids showed no clear pattern. Differences in untreated bark were less pronounced, where only 1,8-cineole and (-)-limonene were significantly higher in unsuccessfully attacked trees. Our results show that the potential of individual P. abies trees for inducing defense compounds upon I. typographus attack may partly determine tree resistance to this bark beetle by inhibiting its mass attack.

The critical load concept is used to establish the deposition levels which ecosystems can tolerate without significant harmful effects. Here we summarize work within the Swedish research program Abatement Strategies for Transboundary Air Pollution (ASTA) assessing the critical load of N for boreal forests. Results from both field experiments in an area with low background N deposition in northern Sweden, and from a large-scale monitoring study, show that important vegetational changes start to take place when adding low N doses and that recovery of the vegetation after ceasing N input is a very slow process. The data presented indicate that changes in key ecosystem components occur even at a lower rate of N input than the present recommended empirical critical load for boreal forest understorey vegetation of 10-15 kg N ha(-1) yr(-1). Based on the data presented, we suggest that the critical load should be lowered to 6 kg N ha(-1) yr(-1).

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Standardized tools are needed to identify and prioritize the most harmful non-native species (NNS). A plethora of assessment protocols have been developed to evaluate the current and potential impacts of non-native species, but consistency among them has received limited attention. To estimate the consistency across impact assessment protocols, 89 specialists in biological invasions used 11 protocols to screen 57 NNS (2614 assessments). We tested if the consistency in the impact scoring across assessors, quantified as the coefficient of variation (CV), was dependent on the characteristics of the protocol, the taxonomic group and the expertise of the assessor. Mean CV across assessors was 40%, with a maximum of 223%. CV was lower for protocols with a low number of score levels, which demanded high levels of expertise, and when the assessors had greater expertise on the assessed species. The similarity among protocols with respect to the final scores was higher when the protocols considered the same impact types. We conclude that all protocols led to considerable inconsistency among assessors. In order to improve consistency, we highlight the importance of selecting assessors with high expertise, providing clear guidelines and adequate training but also deriving final decisions collaboratively by consensus.

Efforts to introduce pathogen resistance into landscape tree species by breeding may have unintended consequences for fungal diversity. To address this issue, we compared the frequency and diversity of endophytic fungi and defensive phenolic metabolites in elm (Ulmus spp.) trees with genotypes known to differ in resistance to Dutch elm disease. Our results indicate that resistant U. minor and U. pumila genotypes exhibit a lower frequency and diversity of fungal endophytes in the xylem than susceptible U. minor genotypes. However, resistant and susceptible genotypes showed a similar frequency and diversity of endophytes in the leaves and bark. The resistant and susceptible genotypes could be discriminated on the basis of the phenolic profile of the xylem, but not on basis of phenolics in the leaves or bark. As the Dutch elm disease pathogen develops within xylem tissues, the defensive chemistry of resistant elm genotypes thus appears to be one of the factors that may limit colonization by both the pathogen and endophytes. We discuss a potential trade-off between the benefits of breeding resistance into tree species, versus concomitant losses of fungal endophytes and the ecosystem services they provide.

Conventions and policies for biodiversity conservation and climate change mitigation state the need for increased protection, restoration and climate change adaptation of forests. Much degraded land may be targeted for large-scale forest restoration, yet challenges include costs, a shortage of regeneration material and the need for restored forests to serve as a resource for communities. To ensure ecosystem function for the future, forest restoration programs must: (1) learn from the past; (2) integrate ecological knowledge; (3) advance regeneration techniques and systems; (4) overcome biotic and abiotic disturbances and (5) adapt for future forest landscapes. Historical forest conditions, while site-specific, may help to identify the processes that leave long-term legacies in current forests and to understand tree migration biology/population dynamics and their relationship with climate change. Ecological theory around plant-plant interactions has shown the importance of negative (competition) and positive (facilitation) interactions for restoration, which will become more relevant with increasing drought due to climate change. Selective animal browsing influences plant-plant interactions and challenges restoration efforts to establish species-rich forests; an integrated approach is needed to simultaneously manage ungulate populations, landscape carrying capacity and browse-tolerant regeneration. A deeper understanding of limiting factors that affect plant establishment will facilitate nursery and site preparation systems to overcome inherent restoration challenges. Severe anthropogenic disturbances connected to global change have created unprecedented pressure on forests, necessitating novel ecological engineering, genetic conservation of tree species and landscape-level approaches that focus on creating functional ecosystems in a cost-effective manner.