Tree Diversity, Initial Litter Quality, and Site Conditions Drive Early-Stage Fine-Root Decomposition in European Forests

Abstract

Decomposition of dead fine roots contributes significantly to nutrient cycling and soil organic matter stabilization. Most knowledge of tree fine-root decomposition stems from studies in monospecific stands or single-species litter, although most forests are mixed. Therefore, we assessed how tree species mixing affects fine-root litter mass loss and which role initial litter quality and environmental factors play. For this purpose, we determined fine-root decomposition of 13 common tree species in four European forest types ranging from boreal to Mediterranean climates. Litter incubations in 315 tree neighborhoods allowed for separating the effects of litter species from environmental influences and litter mixing (direct) from tree diversity (indirect). On average, mass loss of mixed-species litter was higher than those of single-species litter in monospecific neighborhoods. This was mainly attributable to indirect diversity effects, that is, alterations in microenvironmental conditions as a result of tree species mixing, rather than direct diversity effects, that is, litter mixing itself. Tree species mixing effects were relatively weak, and initial litter quality and environmental conditions were more important predictors of fine-root litter mass loss than tree diversity. We showed that tree species mixing can alter fine-root litter mass loss across large environmental gradients, but these effects are context-dependent and of moderate importance compared to environmental influences. Interactions between species identity and site conditions need to be considered to explain diversity effects on fine-root decomposition.

Fungal community composition and genetic potential regulate fine root decay in northern temperate forests

“…Environmental controls over fine root decay are often weaker and more variable than the decay of aboveground litter (Beidler & Pritchard, 2017;See et al, 2019;Wambsganss et al, 2021), and our results suggest these patterns may arise because fungal community composition and genetic variation overwhelm the direct effects of the environment on fine root decay. Unlike the abundance of fungal functional groups and genetic decay potential, which accounted for up to 24% of the variation in decay rates we observed (Figures 1 and 3), none of the environmental variables we measured directly predicted fine root decay (Figure 4a,b).…”

Increased belowground tree carbon allocation in a mature mixed forest in a dry versus a wet year

“…Another less known belowground flux is the root litter, estimated to constitute around 20% of NPP globally (McCormack et al., 2015). Furthermore, forest type and tree species were found to change the root decomposition rate, with the Mediterranean forest the most prominent, and Quercus fine roots are faster to decompose than Pinus (Wambsganss et al., 2022). In agreement, the belowground C sink was found to be an important component in the global C cycle as calculated in a semi‐arid forest 50 km south of this forest site (Qubaja, Grünzweig, et al., 2020; Qubaja, Tatarinov, et al., 2020).…”

Decoupling of nitrogen, phosphorus, and carbon release from fine and coarse roots during 7 years of decomposition

“…We found opposite relationships between root diameter and nutrient dynamics for nitrogen and phosphorus, with greater proportional P release from coarse roots (diameter ≥2 mm), and greater proportional N release from fine roots (diameter ≤2 mm, with higher initial N concentration). Roots with higher initial nutrient concentrations generally do not have higher decomposition rate and nutrient release rate except for N (Chen et al., 2002; Cusack et al., 2009; Freschet et al., 2021; Fujii & Takeda, 2010; Parton et al., 2007; Wambsganss et al., 2021). Furthermore, our data strongly suggest that while rates of P release during decomposition can be predicted as a function of mass loss, N release cannot (Figure S4), which challenged the assumption that these fluxes are linearly related to decomposition (Cusack et al., 2009; Parton et al., 2007).…”

“…Higher initial N and P concentrations consistently lead to faster decomposition rates during the early stages of fine root decomposition worldwide (See et al., 2019; Jiang et al., 2021), but effects on later stages of decomposition and nutrient release remain largely unexplored. In addition, there is increasing recognition for the role of other rock‐derived nutrients in regulating litter decomposition, including magnesium (Mg; Goebel et al., 2011; Wambsganss et al., 2021), calcium (Ca; See et al., 2019) and silicon (Si; Schaller & Struyf, 2013). A better understanding of how these less‐reported elements interact to affect long‐term rates of decomposition and nutrient release is therefore needed.…”