Abstract
BackgroundIn contrast with the negligible contribution of the forest understorey to the total aboveground phytobiomass of a forest, its share in annual litter production and nutrient cycling may be more important. Whether and how this functional role of the understorey differs across forest types and depends upon overstorey characteristics remains to be investigated.MethodsWe sampled 209 plots of the FunDivEUROPE Exploratory Platform, a network of study plots covering local gradients of tree diversity spread over six contrasting forest types in Europe. To estimate the relative contribution of the understorey to carbon and nutrient cycling, we sampled non-lignified aboveground understorey biomass and overstorey leaf litterfall in all plots. Understorey samples were analysed for C, N and P concentrations, overstorey leaf litterfall for C and N concentrations. We additionally quantified a set of overstorey attributes, including species richness, proportion of evergreen species, light availability (representing crown density) and litter quality, and investigated whether they drive the understorey’s contribution to carbon and nutrient cycling.Results and conclusionsOverstorey litter production and nutrient stocks in litterfall clearly exceeded the contribution of the understorey for all forest types, and the share of the understorey was higher in forests at the extremes of the climatic gradient. In most of the investigated forest types, it was mainly light availability that determined the contribution of the understorey to yearly carbon and nutrient cycling. Overstorey species richness did not affect the contribution of the understorey to carbon and nutrient cycling in any of the investigated forest types.
Highlights
In contrast with the negligible contribution of the forest understorey to the total aboveground phytobiomass of a forest, its share in annual litter production and nutrient cycling may be more important
We focus on the mass and nutrient content of plant material that dies off every year, being the fallen leaves for the overstorey and the non-lignified aboveground biomass for the understorey, assuming that these compartments are the main contributors to annual carbon and nutrient cycling in temperate forest
We explored the relative contribution of the understorey to plot-level nutrient cycling, by comparing P and N concentrations and stocks in overstorey leaf litterfall with those measured in the non-lignified fraction of understorey biomass
Summary
In contrast with the negligible contribution of the forest understorey to the total aboveground phytobiomass of a forest, its share in annual litter production and nutrient cycling may be more important Whether and how this functional role of the understorey differs across forest types and depends upon overstorey characteristics remains to be investigated. Studies on forest ecosystem functioning generally focus on trees and their functional characteristics, while ignoring the understorey, the herbs, grasses, ferns, dwarf shrubs, mosses and seedlings growing at the forest floor. Despite their small stature, understorey plants can play an important functional role, especially in open forests where overstorey productivity is low (Landuyt et al 2019a). Knowledge on how nutrient and carbon cycling (i.e. the annual input of carbon and nutrients into the soil from dying plant material) by the understorey varies across overstorey types will be crucial to quantify potential trade-offs or synergies between overstorey and understorey functioning, which need to be accounted for when optimizing forest ecosystem functioning (Landuyt et al 2019a)
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