Plant physical and chemical traits associated with herbivory in situ and under a warming treatment

Abstract

Plants protect themselves against herbivore attacks with physical traits and toxic secondary metabolites. Levels of plant defences and herbivore performance might shift under climate warming, particularly in alpine habitats, where herbivore pressure is currently low. Plant responses to warming should be driven by species‐specific shifts in physical and chemical defence traits. We investigated the association between plant leaf physical and chemical traits and herbivory under current and warmer climates in three grasslands along a subalpine to alpine gradient. Specifically, we measured the rate of in situ natural herbivory, and performed bioassays to measure overall plant species‐level resistance using the extreme generalist non‐native caterpillar Spodoptera littoralis. We simulated warmer conditions by using open‐top chambers and assessed the effect of warming on leaf physical and chemical traits, and how trait changes affect caterpillar performance. Natural herbivory and caterpillar performance were associated with plant physical traits, including specific leaf area, and with ordination axes representing dimensions of the plant chemical profile. We found that the warming treatment independently decreased the number of distinct chemical compounds per species, and marginally increased specific leaf area. Changes in leaf functional traits were not systematically associated with changes in caterpillar performance. Synthesis. Plant physical traits and chemical profiles are both related to natural herbivory and plant resistance against Spodoptera littoralis. While leaf physical and chemical traits of high elevation plants were modified by the warming treatment, these changes did not result in predictable effects on plant resistance against herbivores.

Plant palatability and trait responses to experimental warming

“…However, there was substantial variation in thermal responses among individual pairs. Descombes et al 9 found that warming modified physical and chemical phenotypes of all studied plant species. However, the trait changes did not result in a consistent effect on plant resistance against herbivores.…”

“…Higher temperatures are predicted to increase population densities of most insect species and cause alterations in their body size, life cycle duration and in the extent of their host plant exploitation 7 strengthening the intensity of the plant-herbivore interactions (e.g., 7,8 ). However, other studies showed that warming can reduce plant-herbivore interactions in some systems, and the responses thus seem to be species specific 5,6,9,10 . A possible explanation for this specificity is the differential effects of temperature on plant traits that determine plant palatability.…”

“…A possible explanation for this specificity is the differential effects of temperature on plant traits that determine plant palatability. This issue is, however, still largely unexplored (but see Descombes et al 9 ).…”

“…The differences in plant traits among these environments may thus also be caused by these other factors. To what extent it is truly the temperature that is responsible for the observed trait differentiation is thus often unknown (but see 9,16,17 ). In addition, field studies alone do not allow to distinguish whether the trait differentiation is due to phenotypic plasticity or genetic differentiation among the plants 18 .…”

“…Previous studies have often found contrasting results when comparing the effects of environment on plant-herbivore interactions among different plant species, indicating that plant species and environmental conditions interact to determine plant palatability 6,9,20,21 . For example, Lemoine et al 6 showed that overall open 1 Institute of Botany, The Czech Academy of Sciences, Zámek 1, 252 43 Průhonice, Czech Republic.…”