Low-intensity insect herbivory could have large effects on ecosystem productivity through reduced canopy photosynthesis

Abstract. Our current understanding of the effect of insect herbivory on ecosystem productivity is limited. Previous studies have typically quantified only the amount of leaf area loss or have been conducted during outbreak years when levels of herbivory are much higher than on average. These set-ups often do not take into account the physiological changes taking place in the remaining plant tissue after insect attack or do not represent typical, non-outbreak herbivore densities. Here, we estimate the effect of non-outbreak densities of insect herbivores on gross primary productivity in a temperate oak forest both through leaf area loss and through changes in leaf gas exchange. We first conduct a meta-analysis to assess evidence of herbivory-induced changes in photosynthesis in the literature. We then estimate how canopy primary productivity changes with decreasing and increasing levels of herbivory by using a canopy upscaling model and the average leaf-level effect based on the literature. The meta-analysis revealed a wide range of effects of herbivory on leaf photosynthesis, ranging from a reduction of 82 % to an increase of 49 %. On average, herbivory reduces the photosynthetic rate in the remaining leaf tissue by 16 % [6 %–27 %; 95 % CI]. The gross primary productivity of an oak stand under normal (5 % leaf area loss) levels of herbivory is estimated on average to be 13 % [5 %–21 %] lower than that of a non-herbivorized stand, once physiological changes in the intact plant tissue are considered. We propose that the effect of insect herbivory on primary productivity is non-linear and determined mainly by changes in leaf gas exchange and the pattern at which herbivory spreads through the canopy. We call for replicated studies in different systems to validate the relationship between insect herbivory and ecosystem productivity proposed here.