Drivers of earthworm incidence and abundance across European forests

Earthworms have a significant influence on the structure, composition and functioning of forest ecosystems,
but in spite of their role as ecosystem engineers, little is known on the factors controlling their
distribution across European forests. Optimised sampling techniques, as well as more advanced statistical
tools and geographical information systems have facilitated studies at the landscape scale. But these,
and even larger-scale studies, are scarce due to data limitations, taxonomic inconsistencies and practical
issues in linking existing databases. In this continental-scale field-based study we used boosted
regression tree modelling to identify and evaluate the relative importance of environmental factors
explaining earthworm incidence (presence/absence) and abundance (density and biomass) in European
forests. To parameterise our models earthworms were sampled in six forest landscapes along a latitudinal
gradient from the boreal north to the Mediterranean south in spring or autumn of 2012, together
with several environmental variables. Earthworms were sampled using a combined method of mustard
extraction and hand sorting of litter and a soil monolith, after which they were weighed and identified to
functional group (epigeic, endogeic and anecic). We found that litter- and soil-related variables best
explained earthworm incidence and biomass in European forests, leaving only a minor role to climaterelated
variables. Among the litter related variables, understory vegetation played an important role in
explaining earthworm incidence and abundance. The relative importance of explanatory variables
differed between models for incidence, density and biomass and between earthworm functional groups.
Our results suggested that threshold values for soil C:N ratio, forest floor pH and understory plant
biomass and plant nutrient concentrations have to be attained before earthworms can occur. Beyond
these threshold values, variables like soil C:N ratio, tree litter C:P ratio and forest floor mass further explain earthworm biomass. Mechanisms behind these observations are discussed in the light of future earthworm distribution modelling at continental scale.