Forest diversity effects on insect herbivores: do leaf traits matter?

Abstract

Summary Insect herbivore damage and abundance are often reduced in diverse plant stands. However, few studies have explored whether this phenomenon is a result of plant diversity effects on host plant traits. We explored indirect effects of tree species diversity on herbivory via changes in leaf traits in a long‐term forest diversity experiment in Finland. We measured 16 leaf traits and leaf damage by four insect guilds (chewers, gall formers, leaf miners and rollers) on silver birch ( Betula pendula ) trees growing in one‐, two‐, three‐ and five‐species mixtures. A decline in the frequency of birch in mixed stands resulted in reduced leaf area. This, in turn, mediated the reduction in chewing damage in mixed stands. In contrast, associational resistance of birch to leaf miners was not trait‐mediated but driven directly by concurrent declines in birch frequency as tree species richness increased. Our results show that leaf trait variation across the diversity gradient might promote associational resistance, but these patterns are driven by an increase in the relative abundance of heterospecifics rather than by tree species richness per se . Therefore, accounting for concurrent changes in stand structure and key foliar traits is important for the interpretation of plant diversity effects and predictions of associational patterns.

Positive outcomes between herbivore diversity and tree survival: Responses to management intensity in a Patagonian forest

“…Intermediate harvesting intensities promoted the largest diversity, evenness and richness of arthropod herbivore guilds on planted native tree saplings, supporting the intermediate disturbance hypothesis (Schowalter, 2012;Liebhold et al, 2018;Muiruri et al, 2019). Specifically, intermediate harvesting intensities could generate optimal temperature, humidity and light conditions for most of the forestdwelling arthropod herbivores (Gossner et al, 2014;Soler et al, 2016) as well as host greater plant quality and density (Coulin et al, 2019).…”

“…We found no studies analyzing the relation between herbivore guild diversity and herbivore rates, but there are some examples of positive relations between herbivore diversity and abundance (Neves et al, 2010(Neves et al, , 2014Kuchenbecker & Fagundes, 2018). Guild diversity could enhance herbivore abundance (Lewinsohn et al, 2005) potentially explaining greater leaf consumption rates (Siemann et al, 1996;Novotny et al, 2010Novotny et al, , 2012Pinheiro et al, 2016;Muiruri et al, 2019), because of niche complementarity (Gable et al, 2012;Valencia-Cuevas and Tovar-Sánchez, 2015). Finally, diversity indices and herbivory rates were higher during the second year, as well as the regional temperature (SMN, 2019), consistent with the idea that temperature is suboptimal for arthropods in temperate environments such as Patagonia (Cornelissen, 2011;Garibaldi et al, 2011).…”

“…On the other hand, herbivory could increase with management intensity in the planted individuals due to a higher rate of encounter of these plants (apparency hypothesis, Castagneyrol et al, 2013) or because of the typical higher average temperature of harvested plots (higher temperature in temperate environments improves the performance of arthropods, optimal temperature hypothesis, Cornellisen, 2011). Alternatively, intermediate management intensity would favour herbivory either because planted individuals survive and grow better (plant vigour hypothesis, Cornelissen et al, 2008) or because there is greater plant diversity (intermediate disturbance hypothesis, Schowalter, 2012;Liebhold et al, 2018;Muiruri et al, 2019). In contrast, it is also predicted that herbivory would be more intense at either extreme of management intensity because planted individuals are highly affected by shade (low management intensity) or high radiation levels (high management intensity), and herbivores would prefer stressed plants (plant stress hypothesis, White, 2009).…”

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

“…Several reviews have shown that tree diversity can promote plant productivity (Zhang, Chen, & Reich, 2012;Liang et al, 2016;Jactel et al, 2018), which in turn could result in reduced production of leaf chemical defences because of a trade-off between growth and defences (Coley, Bryant, & Chapin, 1985;Herms & Mattson, 1992;Endara & Coley, 2011). Consistently, increased tree diversity was found to be associated with lower concentrations of leaf chemical defences, including polyphenols, tannins, glycosids and alkaloids (Castagneyrol et al, 2018;Muiruri et al, 2019;Rosado-Sánchez, Parra-Tabla, Betancur-Ancona, Moreira, & Abdala-Roberts, 2018;Walter et al, 2012; but see Moreira, Abdala-Roberts, Parra-Tabla, & Mooney, 2014).…”

How to better estimate leaf area index and leaf angle distribution from digital hemispherical photography? Switching to a binary nonlinear regression paradigm

“…Canopies are loci of numerous biological, physical, chemical and ecological processes, most of which are affected by canopy structures (Muiruri et al, ; Zhao & Jackson, ). Two key canopy structural characteristics are leaf area index (LAI) and leaf angle distribution (LAD) that quantify the amount and orientation of foliage, respectively.…”