Abstract
Vegetative parts of bilberry (Vaccinium myrtillus) are important forage for many boreal forest mammal, bird and insect species. Plant palatability to insects is affected by concentration of nutrients and defense compounds in plants. We expected that palatability of bilberry leaves to insect herbivores is influenced by light availability and soil productivity (both affecting nitrogen concentration and constitutive carbon-based defense compound concentration) and herbivory by mammals (affecting nitrogen concentration and induced carbon-based defense compound concentration). We studied bilberry leaf herbivory under different light availability, soil productivity and mammalian herbivory pressure in small sampling units (1m x 1m) in boreal forest in Norway. We used generalized linear mixed models and generalized additive mixed models to model insect herbivory on bilberry leaves as a function of shade, soil productivity and mammalian herbivory. Observed insect herbivory on bilberry leaves increased with increasing shade levels. Predicted insect herbivory increased with increasing previous mammalian herbivory at high shade levels and this response was magnified at higher soil productivity levels. At low to intermediate shade levels, this response was only present under high soil productivity levels. Our results indicate that light availability is more important for variation in bilberry leaf palatability than soil nutrient conditions.
Highlights
Previous mammalian herbivory was present in 394 quadrats (87 %)
In many quadrats we observed little herbivory: in 151 quadrats (33 %) previous mammalian herbivory was 1 % or less; insect herbivory was in almost half of all quadrats (n = 221, 49 %) 1 % or less
The soil variables phosphorus (P) and nitrogen (N): some statistics and the PCA Correlation between ammonium lactate extractable phosphorus (P) and total nitrogen (N) was low (ρ = 0.181; as plotting P against N indicated that some values of N seemed to be small outliers, Spearman rank correlation coefficient was used)
Summary
- Some statistics for the variables insect herbivory and previous mammalian herbivory and the variable shade - The soil variables phosphorus (P) and nitrogen (N): some statistics and the PCA - Insect herbivory versus mammalian herbivory on bilberry (figure) - Ecological range for soil productivity of bilberry - Temperature & precipitation in the study area within the study period Some statistics for the variables insect herbivory and previous mammalian herbivory and the variable shade Mean insect herbivory was 2.0 % (range = 0 - 32 %, SE = 0.15 %). In 438 quadrats (96 %) insect herbivory was present.
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