Abstract
Diversifying planted forests by increasing genetic and species diversity is often promoted as a method to improve forest resilience to climate change and reduce pest and pathogen damage. In this study, we used a young tree diversity experiment replicated at two sites in the UK to study the impacts of tree diversity and tree provenance (geographic origin) on the oak (Quercus robur) insect herbivore community and a specialist biotrophic pathogen, oak powdery mildew. Local UK, French, and Italian provenances were planted in monocultures, provenance mixtures, and species mixes, allowing us to test whether: (a) local and nonlocal provenances differ in their insect herbivore and pathogen communities, and (b) admixing trees leads to associational effects on insect herbivore and pathogen damage. Tree diversity had variable impacts on foliar organisms across sites and years, suggesting that diversity effects can be highly dependent on environmental context. Provenance identity impacted upon both herbivores and powdery mildew, but we did not find consistent support for the local adaptation hypothesis for any group of organisms studied. Independent of provenance, we found tree vigor traits (shoot length, tree height) and tree apparency (the height of focal trees relative to their surroundings) were consistent positive predictors of powdery mildew and insect herbivory. Synthesis. Our results have implications for understanding the complex interplay between tree identity and diversity in determining pest damage, and show that tree traits, partially influenced by tree genotype, can be important drivers of tree pest and pathogen loads.
Highlights
Much ecological research in recent decades has shown that plant spe‐ cies richness has substantial effects on different aspects of ecosystem functioning (Isbell et al, 2017)
Tree species diversity may modify plant apparency, which can be de‐ fined the height of focal trees relative to their surroundings, and has been shown to be an important predictor of insect herbivore loads on individual trees (Castagneyrol, Giffard, Péré, & Jactel, 2013; Régolini et al, 2014)
By studying replicate trials between years (2 years of data from one site) and between trials, we looked for consistent impacts of provenance identity and tree diversity across space and time
Summary
Much ecological research in recent decades has shown that plant spe‐ cies richness has substantial effects on different aspects of ecosystem functioning (Isbell et al, 2017). While provenance identity and tree diversity can both shape associated pest communities (Dickson & Whitham, 1996; Wimp, Martinsen, Floate, Bangert, & Whitham, 2005), the simultaneous impacts of diversifying both the genetic and species composition of forest stands have rarely been tested. We tested the impacts of varying both tree species and genetic diversity on oak (Q. robur) foliar insect herbivores, oak powdery mildew and the interaction between the two groups. There is increasing evidence that tree diversity effects on pest organisms are context dependent and can be modulated by abiotic conditions such as temperature and water availability impacting upon plant growth and functional traits (Castagneyrol, Moreira, & Jactel, 2018; Kambach, Kühn, Castagneyrol, & Bruelheide, 2016; Walter et al, 2012). 4. Differences in provenance susceptibility to insect herbivores and powdery mildew could lead to associational effects in mixed prov‐ enance plots, compared to provenance monocultures. Our results emphasize the importance of considering tree identity as well as diversity setting in the design of mixed forests, and illustrate how environmental context can affect plant growth, with impacts on forest pests and pathogens
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