The Impact of Tree Diversity on Different Aspects of Insect Herbivory along a Global Temperature Gradient - A Meta-Analysis

Abstract

Forests with higher tree diversity are often assumed to be more resistant to insect herbivores but whether this effect depends on climatic conditions is so far poorly understood. In particular, a forest’s resistance to herbivory may depend on mean annual temperature (MAT) as a key driver of plant and insect phenology. We carried out a global meta-analysis on regression coefficients between tree diversity and four aspects of insect herbivory, namely herbivore damage, abundance, incidence rate and species richness. To test for a potential shift of tree diversity effects along a global gradient of MAT we applied mixed-effects models and estimated grand mean effect sizes and the influence of MAT, experimental vs. observational studies and herbivores diet breadth. There was no overall effect of tree diversity on the pooled effect sizes of insect herbivore damage, abundance and incidence rate. However, when analysed separately, we found positive grand mean effect sizes for herbivore abundance and species richness. For herbivore damage and incidence rate we found a significant but opposing shift along a gradient of MAT indicating that with increasing MAT diversity effects on herbivore damage tend towards associational resistance whereas diversity effects on incidence rates tend towards associational susceptibility. Our results contradict previous meta-analyses reporting overall associational resistance to insect herbivores in mixed forests. Instead, we report that tree diversity effects on insect herbivores can follow a biogeographic pattern calling for further in-depth studies in this field.

Tree diversity drives associational resistance to herbivory at both forest edge and interior

“…While many reasons have been proposed to explain discrepancies in the literature, including insect herbivores' host specificity (Castagneyrol, Jactel, Vacher, et al, 2014;Jactel & Brockerhoff, 2007) or climatic conditions (Kambach, Kühn, Castagneyrol, & Bruelheide, 2016), the methodology of herbivory assessment may be another potential explanation. While many reasons have been proposed to explain discrepancies in the literature, including insect herbivores' host specificity (Castagneyrol, Jactel, Vacher, et al, 2014;Jactel & Brockerhoff, 2007) or climatic conditions (Kambach, Kühn, Castagneyrol, & Bruelheide, 2016), the methodology of herbivory assessment may be another potential explanation.…”

The relative importance of plant intraspecific diversity in structuring arthropod communities: A meta‐analysis

“…predators and parasitoids). Also, while earlier meta‐analyses on plant diversity effects often did not distinguish between herbivore responses in terms of abundance and damage (Castagneyrol, Jactel, Vacher, Brockerhoff, & Koricheva, ; Jactel & Brockerhoff, ), a recent meta‐analysis of tree species diversity effects on herbivores by Kambach, Kühn, Castagneyrol, and Bruelheide () has revealed that herbivore abundance, species richness and damage may be differently affected by plant diversity and it is important to distinguish between these response variables.…”

Individual tree traits shape insect and disease damage on oak in a climate‐matching tree diversity experiment

“…We used a tree diversity experiment replicated at two sites in the UK, in which local and climate‐matched oak provenances are planted as monocultures or in mixtures of provenances and species. 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, ; Kambach, Kühn, Castagneyrol, & Bruelheide, ; Walter et al, ). 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.…”

“…However, non-native provenances may also experience stressful growing conditions under current local climates, leading to reduced tree vigor. Plant stress generally has a negative effect on the performance of primary foliar insect herbivores and biotrophic pathogens, while plant vigor can have the opposite effect (Huberty & Denno, 2004;Jactel et al, 2012). Despite the potential importance of introduced provenances as a future-proofing strategy against climate change, few experimental studies exist comparing the growth characteristics and pest resistance of local versus climate-matched provenances under current climatic conditions (Barsoum, 2015;Broadmeadow et al, 2005).…”