Herbivory differentially alters litter dynamics of two functionally contrasted grasses

Abstract

Summary Herbivores can have contrasted impacts on litter quality and litter decomposition, through an alteration of leaf chemistry and leaf senescence. Depending on the context, herbivores can induce defensive secondary compounds and thus slow down litter decomposition or accelerate decomposition by short‐cutting nutrient resorption. Almost nothing is known for grasses, which contain smaller amounts of secondary compounds than forbs and trees. Because grasses span a gradient from exploitative species having a low C : N ratio and induced defences, to conservative species having a high C : N ratio and constitutive defences, we hypothesize that the litter dynamics of functionally contrasted grasses may be differentially altered by herbivores. In a mesocosm experiment, we assessed the litter decomposition rate of two subalpine grasses, the more exploitative Dactylis glomerata and the conservative Festuca paniculata, in the presence of two grasshopper species, Chorthippus scalaris and Euthystira brachyptera. We hypothesized that decomposition patterns depending on grass species and herbivory could be explained by the C : N ratio and the total phenolic content of fresh, senescent and decomposed leaves. Herbivory by grasshoppers induced the accumulation of phenolics in the fresh leaves of D. glomerata, but most of these compounds were lost during senescence. The decomposition rate of D. glomerata senescent leaves did not depend on herbivory, phenolics and N content or C : N ratio. In contrast, herbivory did not induce any phenolic accumulation in the grazed leaves of F. paniculata, but during senescence, phenolics disappeared in greater proportions in grazed leaves than in ungrazed leaves, probably due to the physical alteration of grazed leaves. Herbivory slowed down the decomposition rate of F. paniculata, which was correlated to the phenolic concentration of senescent leaves, but not to the C : N ratio or N content. Herbivory by grasshoppers differentially altered the litter decomposition rate of the two functionally contrasted grasses, having no effect on D. glomerata and slowing down F. paniculata. Thus, the combination of chemical and physical modifications of leaves by grazing and their interaction with grass traits may have either accelerating or decelerating effects on litter decomposition, with potentially complex outcomes at the ecosystem level.

Species traits as drivers of food web structure

“…The growing interest for trait‐based approaches to soil fauna will likely improve trait documentation, and thereby the precision of future analyses. As and have recently pointed out, the next challenge in biodiversity and ecosystem functioning studies will be to relate network ecology to ecosystem processes. Answers to these questions have the potential to upend existing theories by demonstrating the changing effect and importance of functional diversity across trophic levels, as suggested by and .…”

“…This method deals easily with non‐linear relationships and is more flexible than GLMs. GLMs, however, represent the trait‐matching centrality formalism developed by , which is grounded in food web theory and is essentially the common denominator to most trait‐matching methods ().…”

“…Several studies also revealed that trait identity may play an even more important role than diversity (, and ). Despite these advances, the applicability of a trait‐based approach to animal taxa and across trophic levels remains to be validated (, ).…”

Plant secondary metabolites: a key driver of litter decomposition and soil nutrient cycling

“…) and the influence of herbivory (Ibanez et al . ; Jackrel & Wootton ). Knowledge on the distribution of secondary metabolites within plants is still scant, and probably most of the PSM are found in higher concentrations in young rather than mature leaves (Barton & Koricheva ).…”

“…Standardization of methods and a systematic, accurate description of environmental variables are also necessary to carry forward research more efficiently. Difficulties with PSM studies arise from the fact that tissue PSM content can show temporal variations due to ontogenic and seasonal changes (Koricheva et al 1998;Barton & Koricheva 2010), abiotic stresses such as nutrient limitation (De Long et al 2016) and the influence of herbivory (Ibanez et al 2013;Jackrel & Wootton 2015). Knowledge on the distribution of secondary metabolites within plants is still scant, and probably most of the PSM are found in higher concentrations in young rather than mature leaves (Barton & Koricheva 2010).…”