Intensity, frequency and rate of insect herbivory for an alpine Rhododendron shrub: elevational patterns and leaf-age effects

Abstract

Uncertainty still exists on the directions and intensity of changes in leaf herbivory under scenarios of global warming. We, therefore, conducted an investigation on insect herbivory along an elevational gradient to explore how leaf herbivory may respond to future climate warming using a space-for-time substitution approach. We hypothesize that the leaf herbivory for alpine woody species should decline with elevation. We also guess the leaf herbivory may not differ between different leaf-age groups, for the old leaves are less attractive to insect due to their lower nutrients. To approve these assertions, we measured different aspects of leaf herbivory, i.e., the intensity (leaf area consumed per damaged leaf), frequency (percentage of leaves damaged), and rate (percentage of consumed leaf area over the total number of leaves), across different leaf-age groups for Rhododendron aganniphum var. schizopeplum along an elevational gradient (4280–4640 m) in the Sergymla Mountains, southeast Tibet. Related leaf traits of leaf mass per area (LMA) and nitrogen (N_mass), as well as total phenolics for 1-year-old leaves, were also investigated. In contrast with our expectation, the rate of herbivory did not vary with elevation, while the frequency and intensity reflected increasing and declining patterns, respectively. LMA and total phenolics tended to increase with elevation, while N_mass significantly declined. Further analysis indicated that N_mass and total phenolics mainly explained the variation of frequency of herbivory, while N_mass reflected a significant effect on the variation of intensity. No differences in herbivory were found between the leaf-age groups. Our results suggest that the lower intensity of leaf herbivory at higher elevations is mainly associated with the declined nutritional levels, while the higher frequency might be related to the higher costly anti-herbivore defenses like phenolics and the lower nutritional levels. Although the older leaves are exposed to herbivore attacks for a longer time, they possessed the same herbivory levels as current-year leaves partly due to their lower nitrogen concentrations. Both supporting the nutrient limitation hypothesis, i.e., plants with lower leaf nutrient levels possess less leaf herbivory. In all, the herbivory on the alpine Rhododendron is small in magnitude, but given the higher herbivory (for intensity at least) under persistent warming conditions and its potential impacts on mediating plant–insect interactions, insect herbivory should be included in predictions of climate change impacts on alpine ecosystems.