Interactive effects of tree species composition and water availability on growth and direct and indirect defences in Quercus ilex

Abstract

Plant diversity has often been reported to decrease insect herbivory in plants. Of the numerous mechanisms that have been proposed to explain this phenomenon, how plant diversity influences plant defences via effects on growth has received little attention. In addition, plant diversity effects may be contingent on abiotic conditions (e.g. resource and water availability). Here, we used a long‐term experiment to explore the interactive effects of tree species composition and water availability on growth, direct (i.e. phenolics) and indirect (i.e. volatile organic compounds – VOCs) defences and leaf herbivory in Quercus ilex. We quantified herbivory by chewing insects, phenolic compounds and VOCs in Q. ilex trees growing in stands differing in tree species composition (Q. ilex, Q. ilex + Betula Pendula, Q. ilex + Pinus pinaster and Q. ilex + B. pendula + P. pinaster) and water availability (irrigated versus control). Both direct and indirect defences were affected by tree species composition, but such changes were not mediated by changes in tree stem diameter. Quercus ilex trees growing in stands with P. pinaster had the lowest concentration of both direct and indirect defences. Importantly, the effects of tree species composition on VOCs were exacerbated on irrigated blocks. Despite variation in defences, tree species composition did not affect herbivory in Q. ilex. Accordingly, we did not find any association between defences and insect herbivory. Our results suggest that changes in the micro‐environment rather than growth‐defence associations may mediate tree diversity effects on defences. In addition, reduced defensive investment in more diverse stands could negatively impact tree resistance masking the beneficial effects of species diversity at reducing insect herbivory.

Microbial biomarkers of tree water status for next‐generation biomonitoring of forest ecosystems

“…In 2015, Q. robur pre-dawn water potential was lower in non-irrigated blocks than in irrigated blocks (−0.37 and −0.19 MPa respectively; Castagneyrol et al, 2017). The same pattern was observed for B. pendula in 2016 (−1.63 and −0.43 MPa for non-irrigated and irrigated blocks respectively; Castagneyrol et al, 2018) and for Q. ilex in 2019 (−0.49 and −0.18 MPa for non-irrigated and irrigated blocks respectively;Galmán et al, 2022).…”

“…This was true for all tree species except Q. robur for which the effect of irrigation treatment on summer water deficit was weaker. The experimental design thus allowed us to compare the phyllosphere microbiota of trees that have repeatedly experienced water deficit during summer (Castagneyrol et al, 2017(Castagneyrol et al, , 2018Galmán et al, 2022) to the microbiota of unstressed trees in irrigated plots.…”

Understanding the phytochemical diversity of plants: Quantification, variation and ecological function

“…Among studies testing for functional effects of phytochemical diversity, only a few have done it in an information context (Table S1), examining how variation in both individual and community level volatile diversity can deter herbivores (e.g. Doyle 2009, Zu et al 2020, 2022, Galmán et al 2022) or attract parasitoids of herbivores (Wan et al 2017). In the case where phytochemical compounds act as floral scent to attract pollinators, some studies have examined variation in richness between e.g.…”

Neighbouring tree effects on leaf herbivory: Insect specialisation matters more than host plant leaf traits

“…Although both the identity and diversity of the neighbouring trees and the leaf traits of the focal saplings affected the level of insect herbivory, the variability in the leaf traits of the saplings was neither regulated by the neighbouring trees (Table S6) nor provided the functional link between tree diversity and insect herbivory (Figure S1). These findings are not surprising, given that highly controlled experiments produced opposing results, with evidence supporting (Muiruri et al., 2019) or contradicting (Castagneyrol et al., 2018; Galmán et al., 2022) the leaf trait‐mediated tree diversity effects. Of course, we cannot rule out the potential that unmeasured traits like constitutive or induced defence (Kostenko et al., 2017; Poeydebat et al., 2021) or plant apparency (e.g.…”

“…However, because few recent studies investigating the role of plant leaf traits in mediating the effects of neighbouring tree diversity and identity on insect herbivory in highly controlled experiments yielded inconsistent results (e.g. Galmán et al., 2022; Muiruri et al., 2019), it is unclear whether leaf traits can mediate neighbouring tree effects under natural conditions.…”