Interactive Effects of Tree Mixing and Drought on a Primary Forest Pest

Abstract

Climate change and biodiversity erosion are two major threats to the vitality of the world's forests. However, it is difficult to predict the extent to which tree diversity and environmental conditions interact to modify forest health, and in particular resistance and/or tolerance to insect pests. We used a tree diversity experiment with an irrigation treatment to investigate the effect of mixing and diluting host pines (Pinus pinaster) amongst broadleaved trees on pine stem borer (Dioryctria sylvestrella) infestations under contrasting drought conditions. We further tested whether the attack patterns of this primary pest resulted from direct effects of tree diversity and drought, or was indirectly mediated by their combined effects on pine vigor. The total number of stem borer attacks per plot significantly decreased with tree species richness, i.e., increased with the density of pines, being maximum in pure plots. There were more attacks in irrigated plots. Mixing pines with birches resulted in lower plot infestations but only in irrigated plots. The probability of individual tree being attacked was higher in irrigated plots and decreased with increasing pine density. More vigorous trees, i.e., with higher radial growth, were more likely to be attacked. We suggest that the pine stem borer is attracted by the volatile organic compounds released by the resin exuding from bark cracks. Bark fissuring increases with radial growth, which is enhanced by irrigation and reduced intraspecific competition in low-density pine plots. The presence of birch limits plot infestation, probably because it disrupts host-finding behavior through repellence by non-host volatiles. This phenomenon is mainly observed when pines are vigorous, i.e., when they are irrigated, and particularly attractive to the stem borer. Our results confirm that more severe droughts decrease the attacks by primary pests feeding on tree trunks, by reducing host tree vigor, and that focal tree species can obtain protection against specialized insect pests when mixed with non-host species. The functional characteristics of herbivorous insects are therefore to be taken into account in predicting the interactive effects of climate change and loss of diversity on forest health.

Associational resistance to both insect and pathogen damage in mixed forests is modulated by tree neighbour identity and drought

“…The impact of mixed forests on pathogens has been less extensively studied, but several papers report cases of associational resistance (Hantsch et al, 2014;Pautasso, Holdenrieder, & Stenlid, 2005). However, previous studies showed mixed forests do not always show associational resistance to pests and pathogens (Kambach, Kühn, Castagneyrol, & Bruelheide, 2016;Schuldt et al, 2010), suggesting that associational effects may be context dependent, for instance, on abiotic conditions (Jactel, Poeydebat, van Halder, & Castagneyrol, 2019).…”

Urban Forestry in Mogi Guaçu, São Paulo State, Brazil

“…This is because the number of trees not attacked by pests increases by the most effective dilution and by the physical and chemical masking done by non-host species (Table 1). The increase in all trophic levels makes the community more resistant to biotic and abiotic disorders (DASSOU & TIXIER, 2016;FIELD et al, 2020;GUO et al, 2019;GUYOT et al, 2015;JACTEL et al, 2019;KLAPWIJK et al, 2016).…”

Tree diversity effects on forest productivity: disentangling the effects of tree species addition vs. substitution

“…These findings are also of great ecological relevance because they demonstrate that it is possible to diversify pine monocultures with addition of birch at an early age and then benefit from ecosystem services such as pest protection (Damien et al, 2016;Jactel et al, 2019) and increased diversity of predatory insects (Jouveau et al, 2020) without compromising the wood production of the target species (here maritime pine). Long-term simulations of pine and birch growth on the study sites supported our results (Morin et al, 2020) showing that the ecosystem services associated with pine monoculture diversification can persist as the stand ages.…”