Abstract
Climate change may foster pest epidemics in forests, and thereby the fluxes of elements that are indicators of ecosystem functioning. We examined compounds of carbon (C) and nitrogen (N) in insect faeces, leaf litter, throughfall and analysed the soils of deciduous oak forests (Quercus petraea L.) that were heavily infested by the leaf herbivores winter moth (Operophtera brumata L.) and mottled umber (Erannis defoliaria L.). In infested forests, total net canopy-to-soil fluxes of C and N deriving from insect faeces, leaf litter and throughfall were 30- and 18-fold higher compared with uninfested oak forests, with 4333 kg C ha−1 and 319 kg N ha−1, respectively, during a pest outbreak over 3 years. In infested forests, C and N levels in soil solutions were enhanced and C/N ratios in humus layers were reduced indicating an extended canopy-to-soil element pathway compared with the non-infested forests. In a microcosm incubation experiment, soil treatments with insect faeces showed 16-fold higher fluxes of carbon dioxide and 10-fold higher fluxes of dissolved organic carbon compared with soil treatments without added insect faeces (control). Thus, the deposition of high rates of nitrogen and rapidly decomposable carbon compounds in the course of forest pest epidemics appears to stimulate soil microbial activity (i.e. heterotrophic respiration), and therefore, may represent an important mechanism by which climate change can initiate a carbon cycle feedback.
Highlights
Insect pests in forests have formerly been recognized as discrete biotic disturbance occurring at relatively small spatial scales and short time intervals
We examined compounds of carbon (C) and nitrogen (N) in insect faeces, leaf litter, throughfall and analysed the soils of deciduous oak forests (Quercus petraea L.) that were heavily infested by the leaf herbivores winter moth (Operophtera brumata L.) and mottled umber (Erannis defoliaria L.)
total organic carbon (TOC) fluxes did not differ between forests (p = 0.885, n = 30), while total nitrogen (TN)
Summary
Insect pests in forests have formerly been recognized as discrete biotic disturbance occurring at relatively small spatial scales and short time intervals. Widespread defoliation is associated with considerable fluxes of organic matter 2 and nutrients via insect faeces, litter fragments and throughfall (precipitation) Such element fluxes in forests are rarely considered in biogeochemical analyses (or included only in a condensed form in large-scale operations) [4,5,6], but are expected to influence ecosystem functioning by modifying microbial growth and organic matter decomposition, and thereby their feedback to plants and other ecosystem components. [9]) predicts that herbivores have a decelerating effect on nutrient cycling, with herbivores selectively feeding on nutrient-rich plants, and thereby increasing the abundance of nutrient-poor (and chemically or physically defended) plants This subsequently results in decreased decomposition and nutrient turnover and a reduced productivity from plants. The lack of knowledge on the biogeochemical consequences of pest outbreak populations attacking millions of hectares of forested land annually contributes further to this unpredictability [2,4,5,12]
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