The functional role of understorey herbs for litter decomposition in temperate forests

Litter decomposition is a vital process in forest carbon (C) and nutrient cycling, and it is known to be regulated by initial litter quality and climate variables. In forests, components like understorey vegetation are indispensable to ecosystem stability and ecological function. For example, the understorey layer may play an equally important role in regulating the soil microclimate as the overstorey tree canopy. Additionally, the understorey layer may also enhance forest floor litter quality, by providing a diverse and nutrient-rich litter to the forest litter layer, thus, directly and indirectly regulating litter turnover processes. However, traditionally, understorey plants (the ‘understorey’ in the following contest is a shorter synonym for ‘the forest understorey herb layer’) have been considered as a constraining factor in silviculture, as the understorey vegetation competes with tree seedlings for resources, i.e., light, nutrients, and water. In recent decades, there has been a growing recognition that the understorey layer plays an important role in driving ecosystem processes and functioning. However, we still know relatively little about the effects of the understorey layer on the decomposition of tree litter, which forms the biggest natural source for C and nutrients in the topsoil of most forests. This thesis aimed to unravel the relative importance of the functional role of understorey herbs for forest litter decomposition, by testing its effects on the soil microclimate, its further impact on litter decomposition, and by testing its mixture (direct) effects on the decomposition of tree litter in temperate forests. We conducted an understorey removal field experiment and a mesocosm experiment with understorey plant species to investigate the regulatory effects of the understorey layer on litter decomposition, using green tea and rooibos teabags as standardized litter. We measured effects of understorey removal and percentage cover on the soil microclimate (temperature and water content) and litter decomposition. We found that the removal of the understorey significantly increased mean soil temperatures in the topsoil by 0.21 ± 0.03°C in the beginning of the incubation. The understorey layer could significantly buffer hot and cold thermal extremes at crest or trough of temperature changes, respectively, as temperature increases with fluctuations. Similarly, the soil moisture content was on average 6.32 ± 0.81 % higher in understorey removal than in the control plots. Additionally, during our short-term incubation, there was no significant difference in mass loss and moisture content of both green tea and rooibos tea between of the understorey treatments. Nonetheless, the percentage cover of the understorey vegetation, under natural conditions or under the environmental enrichment conditions, was negatively correlated with mass loss of both green tea and rooibos tea. To test for possible mixture effects of understorey herbaceous litter on the decomposition of overstorey tree species litter, we combined the (nitrogen and phosphorus rich) litter from two herbaceous species (Anemona nemorosa and Aegopodium podagraria) with each of five tree species (Fagus sylvatica L., Quercus robur L., Acer pseudoplatanus L., Fraxinus excelsior L., and Tilia platyphyllos Scop.). These tree species represent a gradient in litter quality. Two herbaceous litter addition experiments were performed, one under laboratory conditions and the other in-situ in an ancient temperate mixed deciduous forest in northern Belgium. We found that the addition of A. podagraria litter on top of tree litter led to a higher litter moisture content, abundance of soil fauna, and enriched litter nutrient content, resulting in a constant synergistic (positive) effect on fresh tree litters in laboratory conditions and on the litter decomposed for half a year in the field. The addition of A. nemorosa litter had, however, antagonistic (negative) effects on recalcitrant tree litter, despite it also significantly increased the abundances of soil fauna. Our findings highlight that, despite the fact that the overstorey canopy is commonly identified as the major driver for variation in the forest microclimate, the understorey herbaceous layer can also play an important role, by regulating soil microclimate when it is alive, or by providing nutrient-rich litter when it is decomposing. However, contrary to our expectations, the warmer and wetter soil environment caused by understorey removal did not stimulate litter decomposition, instead, the percentage cover of the understorey layer showed a consistent negative correlation with litter decomposition. In addition, the litter inputs from the understorey plant community can have a substantial influence on the decomposition of tree litter, i.e. mixture effects that operate through nutrient leaching and changing the community of soil biota. As an overlooked litter inflow to the forest litter layer, the presence of herb litter on top of tree litters improves microenvironmental conditions for decomposition of tree litters and increases soil fauna abundance in the mixture systems. Nonetheless, the most likely explanation for mixture effects of herb litter on tree litter should be attributed to the transfer of nutrients from herb litter towards the tree litter. Together, these experimental results imply that the understorey plant community contributes to the regulation of the soil biotic and abiotic environment, with possibly important consequences for the nutrient and carbon dynamics in forest ecosystems