Host Genetics and Environment Drive Divergent Responses of Two Resource Sharing Gall-Formers on Norway Spruce: A Common Garden Analysis

Consistent community genetic effects in the context of strong environmental and temporal variation in Eucalyptus

Provenance translocations of tree species are promoted in forestry, conservation, and restoration in response to global climate change. While this option is driven by adaptive considerations, less is known of the effects translocations can have on dependent communities. We investigated the relative importance and consistency of extended genetic effects in Eucalyptus using two species-E. globulus and E. pauciflora. In E. globulus, the dependent arthropod and pathogen canopy communities were quantified based on the abundance of 49 symptoms from 722 progeny from 13 geographic sub-races across 2 common gardens. For E. pauciflora, 6 symptoms were quantified over 2 years from 238 progeny from 16 provenances across 2 common gardens. Genetic effects significantly influenced communities in both species. However, site and year effects outweighed genetic effects with site explaining approximately 3 times the variation in community traits in E. globulus and site and year explaining approximately 6 times the variation in E. pauciflora. While the genetic effect interaction terms were significant in some community traits, broad trends in community traits associated with variation in home-site latitude for E. globulus and home-site altitude for E. pauciflora were evident. These broad-scale trends were consistent with patterns of adaptive differentiation within each species, suggesting there may be extended consequences of local adaptation. While small in comparison to site and year, the consistency of genetic effects highlights the importance of provenance choice in tree species, such as Eucalyptus, as adaptive divergence among provenances may have significant long-term effects on biotic communities.

The extended consequences of genetic conductivity: Mating distance affects community phenotypes in Norway spruce

Anthropogenic landscape-level alterations such as habitat fragmentation and long distance translocation of genetic material are currently altering the genetic connectivity and structure of forest tree populations globally. As the susceptibility of individual trees to dependent organisms is often genetically determined, it is possible that these genetic changes may extend beyond individuals to affect associated communities. To test this, we examined how variation in crossing distance among the progeny of 18 controlled crosses of Norway spruce (Picea abies) populations occurring across central Sweden affected chemical defense, and subsequently, a small community of galling Adelges aphids infecting planted trees at two common garden trails. Although crossing distance did not influence growth, vitality or reproduction in the studied population, it did influence the expression of one candidate defensive chemical compound, apigenin, which was found in higher concentrations within outcrossed trees. We also show that this variation in apigenin induced by crossing distance correlated with susceptibility to one member of the galling community but not the other. Furthermore, the effect of crossing distance on galling communities and the general susceptibility of Norway spruce to infection also varied with environment. Specifically, in the more benign environment, inbred trees suffered greater gall infection than outcrossed trees, which is contrary to general predictions that the effects of inbreeding should be more pronounced in harsher environments. These findings suggest that the effects of variation in crossing distance in forest trees can extend beyond the individual to influence whole communities.

Phytochemical variation in treetops: causes and consequences for tree-insect herbivore interactions

The interaction of plants and their herbivorous opponents has shaped the evolution of an intricate network of defences and counter-defences for millions of years. The result is an astounding diversity of phytochemicals and plant strategies to fight and survive. Trees are specifically challenged to resist the plethora of abiotic and biotic stresses due to their dimension and longevity. Here, we review the recent literature on the consequences of phytochemical variation in trees on insect-tree-herbivore interactions. We discuss the importance of genotypic and phenotypic variation in tree defence against insects and suggest some molecular mechanisms that might bring about phytochemical diversity in crowns of individual trees.

The Extended Community-Level Effects of Genetic Variation in Foliar Wax Chemistry in the Forest Tree Eucalyptus globulus

Genetic variation in foundation trees can influence dependent communities, but little is known about the mechanisms driving these extended genetic effects. We studied the potential chemical drivers of genetic variation in the dependent foliar community of the focal tree Eucalyptus globulus. We focus on the role of cuticular waxes and compare the effects to that of the terpenes, a well-studied group of secondary compounds known to be bioactive in eucalypts. The canopy community was quantified based on the abundance of thirty-nine distinctive arthropod and fungal symptoms on foliar samples collected from canopies of 246 progeny from 13 E. globulus sub-races grown in a common garden trial. Cuticular waxes and foliar terpenes were quantified using gas chromatography - mass spectrometry (GC-MC). A total of 4 of the 13 quantified waxes and 7 of the 16 quantified terpenes were significantly associated with the dependent foliar community. Variation in waxes explained 22.9% of the community variation among sub-races, which was equivalent to that explained by terpenes. In combination, waxes and terpenes explained 35% of the genetic variation among sub-races. Only a small proportion of wax and terpene compounds showing statistically significant differences among sub-races were implicated in community level effects. The few significant waxes have previously shown evidence of divergent selection in E. globulus, which signals that adaptive variation in phenotypic traits may have extended effects. While highlighting the role of the understudied cuticular waxes, this study demonstrates the complexity of factors likely to lead to community genetic effects in foundation trees.