Tree Diversity and Forest Resistance to Insect Pests: Patterns, Mechanisms, and Prospects

Abstract

Ecological research conducted over the past five decades has shown that increasing tree species richness at forest stands can improve tree resistance to insect pest damage. However, the commonality of this finding is still under debate. In this review, we provide a quantitative assessment (i.e., a meta-analysis) of tree diversity effects on insect herbivory and discuss plausible mechanisms underlying the observed patterns. We provide recommendations and working hypotheses that can serve to lay the groundwork for research to come. Based on more than 600 study cases, our quantitative review indicates that insect herbivory was, on average, lower in mixed forest stands than in pure stands, but these diversity effects were contingent on herbivore diet breadth and tree species composition. In particular, tree species diversity mainly reduced damage of specialist insect herbivores in mixed stands with phylogenetically distant tree species. Overall, our findings provide essential guidance for forest pest management. Expected final online publication date for the Annual Review of Entomology, Volume 66 is January 11, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Climate affects neighbour‐induced changes in leaf chemical defences and tree diversity–herbivory relationships

“…In fact, we found evidence that tree diversity effects on herbivory were dependent on climate. This result could partly explain the variable effects of tree diversity on herbivory previously reported in the literature (Brezzi et al, 2017;Castagneyrol et al, 2014;Jactel et al, 2020;Kambach et al, 2016;Ratcliffe et al, 2017;Schuldt et al, 2010Schuldt et al, , 2014Vehviläinen et al, 2007;Wein et al, 2016) including in studies focusing on birch trees (Castagneyrol et al, 2018;Haase et al, 2015;Muiruri et al, 2019;Setiawan et al, 2014). Specifically, we provided evidence for changes in associational effects along the mean annual temperature gradient: associational resistance of birch to insect herbivory occurred in cold conditions, whereas no associational effects could be detected in warm conditions.…”

The hidden role of multi-trophic interactions in driving diversity-productivity relationships

“…As long as producer species are not limited to access distinct resource compartments, multi-trophic interactions consistently enhance net diversity effects. Whether herbivores are predominantly specialists or generalists determines if such effects are strong or negligible, respectively (Thébault & Loreau 2003;Jactel et al 2021). In our simulations, generalism is constraint by predator-prey body-mass ratios known from aquatic and belowground ecosystems (Schneider et al 2016).…”

“…This similarly results from a reduction in monoculture productivity. Such top-down reductions due to herbivory can be compensated in more species-rich producer communities (e.g., Jactel et al 2021), where trophic interactions shape the composition and interactions among producer species (e.g., Naeem et al 1994;Thébault & Loreau 2003;Brose 2008;Zhao et al 2019). By addressing the interplay of resource-use complementarity and multi-trophic interactions, our study synthesizes bottom-up and top-down drivers of BEF relationships.…”

Hydraulic diversity stabilizes productivity in a large-scale subtropical tree biodiversity experiment

“…28,34 and species asynchrony is thus likely driven by idiosyncratic growth responses to climatic variability. Next, tree growth in mixtures is shaped by tree-tree interactions such as resource partitioning and biotic feedbacks 20,29,30,44 which may in turn be modulated by changes in climatic conditions [45][46][47] . Finally, intrinsic rhythms like mast seeding 26,27 may induce asynchronous growth dynamics.…”

“…Species react differently to climatic conditions e.g.2,28 and species asynchrony is thus likely driven by differential growth responses of species in mixture to climatic variability. Next, tree growth in mixtures is shaped by tree-tree interactions such as resource partitioning and biotic feedbacks 21,29,30,42 which may in turn be modulated by variation in climatic conditions [43][44][45] . Finally, intrinsic rhythms like mast seeding can influence species productivity and its inter-annual variability 26 thereby inducing species asynchrony.…”