Parallel increases in insect herbivory and defenses with increasing elevation for both saplings and adult trees of oak (Quercus) species.

Herbivory is predicted to increase toward warmer and more stable climates found at lower elevations, and this increase should select for higher plant defenses. Still, a number of recent studies have reported either no evidence of such gradients or reverse patterns. One source of inconsistency may be that plant ontogenetic variation is usually not accounted for and may influence levels of plant defenses and herbivory. We tested for elevational gradients in insect leaf herbivory and leaf traits putatively associated with herbivore resistance across eight oak (Quercus, Fagaceae) species and compared these patterns for saplings and adult trees. To this end, we surveyed insect leaf herbivory and leaf traits (phenolic compounds, toughness and nutrients) in naturally occurring populations of each oak species at low-, mid- or high-elevation sites throughout the Iberian Peninsula. Leaf herbivory and chemical defenses (lignins) were unexpectedly higher at mid- and high-elevation sites than at low-elevation sites. In addition, leaf chemical defenses (lignins and condensed tannins) were higher for saplings than adult trees, whereas herbivory did not significantly differ between ontogenetic stages. Overall, elevational variation in herbivory and plant chemical defenses were consistent across ontogenetic stages (i.e., elevational gradients were not contingent upon tree ontogeny), and herbivory and leaf traits were not associated across elevations. These findings suggest disassociated patterns of elevational variation in herbivory and leaf traits, which, in turn, are independent of plant ontogenetic stage.

Galmán A ，Abdala-Roberts L ，Covelo F ，Rasmann S ，Moreira X ... -  《-》

Inducibility of chemical defences in young oak trees is stronger in species with high elevational ranges.

Elevational gradients have been highly useful for understanding the underlying forces driving variation in plant traits and plant-insect herbivore interactions. A widely held view from these studies has been that greater herbivory under warmer and less variable climatic conditions found at low elevations has resulted in stronger herbivore selection on plant defences. However, this prediction has been called into question by conflicting empirical evidence, which could be explained by a number of causes such as an incomplete assessment of defensive strategies (ignoring other axes of defence such as defence inducibility) or unaccounted variation in abiotic factors along elevational clines. We conducted a greenhouse experiment testing for inter-specific variation in constitutive leaf chemical defences (phenolic compounds) and their inducibility in response to feeding by gypsy moth larvae (Lymantria dispar L., Lepidoptera) using saplings of 18 oak (Quercus, Fagaceae) species. These species vary in their elevational distribution and together span >2400 m in elevation, therefore allowing us to test for among-species elevational clines in defences based on the elevational range of each species. In addition, we further tested for elevational gradients in the correlated expression of constitutive defences and their inducibility and for associations between defences and climatic factors potentially underlying elevational gradients in defences. Our results showed that oak species with high elevational ranges exhibited a greater inducibility of phenolic compounds (hydrolysable tannins), but this gradient was not accounted for by climatic predictors. In contrast, constitutive defences and the correlated expression of constitutive phenolics and their inducibility did not exhibit elevational clines. Overall, this study builds towards a more robust and integrative understanding of how multivariate plant defensive phenotypes vary along ecological gradients and their underlying abiotic drivers.

Galmán A ，Petry WK ，Abdala-Roberts L ，Butrón A ，de la Fuente M ，Francisco M ，Kergunteuil A ，Rasmann S ，Moreira X ... -  《-》

Biotic and abiotic factors associated with altitudinal variation in plant traits and herbivory in a dominant oak species.

It is generally thought that herbivore pressure is higher at lower elevations where climate is warmer and less seasonal, and that this has led to higher levels of plant defense investment at low elevations. However, the generality of this expectation has been called into question by recent studies. We tested for altitudinal gradients in insect leaf damage, plant defenses (phenolic compounds), and nutritional traits (phosphorus and nitrogen) in leaves of the long-lived tree Quercus robur, and further investigated the abiotic factors associated with such gradients. We sampled 20 populations of Q. robur distributed along an altitudinal gradient spanning 35-869 m above sea level, which covered most of the altitudinal range of this species and varied substantially in abiotic conditions, plant traits, and herbivory. Univariate regressions showed that leaf herbivory, phenolics, and phosphorus increased toward higher elevations, whereas leaf nitrogen did not vary with altitude. Multiple regression analyses indicated that temperature was the single most important factor associated with herbivory and appears to be strongly associated with altitudinal variation in damage. Leaf phenolics were also correlated with herbivory, but in a manner that suggests these chemical defenses do not underlie altitudinal variation in damage. In addition, we found that variation in leaf traits (phenolics and nutrients) was in turn associated with both climatic and soil variables. Overall, these findings suggest that altitudinal gradients in herbivory and defenses in Q. robur are uncoupled and that elevational variation in herbivory and plant traits responds mainly to abiotic factors.

Abdala-Roberts L ，Rasmann S ，Berny-Mier Y Terán JC ，Covelo F ，Glauser G ，Moreira X ... -  《-》

Greater phylogenetic distance from native oaks predicts escape from insect leaf herbivores by non-native oak saplings.

Moreira X ，Vázquez-González C ，Encinas-Valero M ，Covelo F ，Castagneyrol B ，Abdala-Roberts L ... -  《-》

Interactive effects of plant neighbourhood and ontogeny on insect herbivory and plant defensive traits.

Moreira X ，Glauser G ，Abdala-Roberts L 《Scientific Reports》