Search for top‐down and bottom‐up drivers of latitudinal trends in insect herbivory in oak trees in Europe

Abstract

Aim The strength of species interactions is traditionally expected to increase toward the Equator. However, recent studies have reported opposite or inconsistent latitudinal trends in the bottom‐up (plant quality) and top‐down (natural enemies) forces driving herbivory. In addition, these forces have rarely been studied together thus limiting previous attempts to understand the effect of large‐scale climatic gradients on herbivory. Location Europe. Time period 2018–2019. Major taxa studied Quercus robur. Methods We simultaneously tested for latitudinal variation in plant–herbivore–natural enemy interactions. We further investigated the underlying climatic factors associated with variation in herbivory, leaf chemistry and attack rates in Quercus robur across its complete latitudinal range in Europe. We quantified insect leaf damage and the incidence of specialist herbivores as well as leaf chemistry and bird attack rates on dummy caterpillars on 261 oak trees. Results Climatic factors rather than latitude per se were the best predictors of the large‐scale (geographical) variation in the incidence of gall‐inducers and leaf‐miners as well as in leaf nutritional content. However, leaf damage, plant chemical defences (leaf phenolics) and bird attack rates were not influenced by climatic factors or latitude. The incidence of leaf‐miners increased with increasing concentrations of hydrolysable tannins, whereas the incidence of gall‐inducers increased with increasing leaf soluble sugar concentration and decreased with increasing leaf C : N ratios and lignins. However, leaf traits and bird attack rates did not vary with leaf damage. Main conclusions These findings help to refine our understanding of the bottom‐up and top‐down mechanisms driving geographical variation in plant–herbivore interactions, and indicate the need for further examination of the drivers of herbivory on trees.

Herbivory on the pedunculate oak along urbanization range in Europe: effects of local forest cover and insect feeding guild

“…We therefore refrain from putting forth any particular mechanism that may underlay the patterns we observed. However, we speculate that denser tree canopies may have buffered micro-climatic variations (Coley and Barone, 1996;Dale and Frank, 2014;Yamasaki and Kikuzawa, 2003), which may have been particularly favourable to chewing herbivores that are external feeders (Savilaakso et al, 2009) and at the same time unfavorable to leaf-galling herbivores, as they benefit from high temperatures (Valdés-Correcher et al, 2021). Alternatively, top-down forces also vary with local canopy cover and may consequently influence insect herbivory.…”

“…Urbanization and local canopy cover were slightly negatively correlated (Pearson r = -0.38, P < 0.05), and were independent of latitude (Urbanization: Pearson r = 0.02, P > 0.05; Local canopy cover: Pearson r = 0.04, P > 0.05) and climate (Temperature and urbanization: Pearson r = -0.02, P > 0.05; Temperature and local canopy cover: Pearson r = -0.12, P < 0.05; Precipitation and urbanization: Pearson r = 0.03, P > 0.05; Precipitation and local canopy cover: Pearson r = 0.01, P > 0.05). Although latitude was negatively correlated with temperature (Pearson r = -0.76,P < 0.05) and precipitation (Pearson r = -0.70,P < 0.05) which could have caused collinearity issue, a previous study found that climatic variables were better predictors of variation in herbivory and therefore decided to only include climatic variables in the models (Valdés-Correcher et al, 2021).…”

“…The present study is part of an ongoing citizen science project that involved so far a total of 93 participants, including 41 scientists and 52 partner schools from 17 European countries (Castagneyrol et al, 2019;Valdés-Correcher et al, 2021) covering most of the native geographical range of the pedunculate oak (Figure 1A ). We geolocalized and surveyed a total of 298 mature oak trees in forest and urban areas during 2018 (n = 132), 2019 (n = 56), and 2020 (n = 113) (Figure 1 ).…”

“…Three response variables were used to characterize leaf herbivory (Valdés-Correcher et al, 2021): leaf damage (i.e., the percentage of leaf area removed by insect herbivores), leaf-miner incidence (i.e., the proportion of leaves with leaf-mines) and leaf-gall incidence (i.e., the proportion of leaves with galls). Herbivory was visually scored by assigning each to one of the following classes: 0, 0.1-5.0, 5.1-10.0, 10.1-15.0, 15.1-25.0, 25.1-50.0, 50.1-75.0 or >75%, where the percentage represented the proportion of leaf surface removed by chewing herbivores, or mined by leaf-miners.…”

“…To control for variability in herbivory that is influenced by local climatic conditions (Valdés-Correcher et al, 2021), we extracted spring temperature and precipitation (mean temperature and precipitation in April-June) data from the WorldClim database (Hijmans et al, 2005) on the basis of the oak coordinates. Spring temperature and precipitation correspond to the period when most of the partners collected the leaves and also the main period of activity of insect herbivores on oak.…”