Mechanisms of Soil Erosion in Subtropical Forests of China - Effects of Biodiversity, Species identity, Tree architecture and Spatial variability on Erosivity

Soil erosion is a major threat to ecosystems and agricultural land worldwide. To overcome severe soil loss, aff orestation is used as a common tool. However, the mechanisms of soil erosion in forests are understood rarely up to now. There is still a knowledge gap to what extent biodiversity and tree species identity aff ect soil erosion in early successional forest stands, which tree architectural and leaf traits account for these eff ects and which of these traits are important for the spatial variability of soil erosion. Therefore, this thesis investigated the influence of tree species richness (as a measure of biodiversity) and tree species identity on rainfall erosivity (measured as throughfall kinetic energy; TKE). Furthermore, this thesis concentrated on the spatial variability of TKE. Importance and influence of five tree architectural and nine leaf traits on these TKE properties were evaluated. In addition, the influence of leaf litter diversity and soil meso- and macrofauna on initial soil erosion was investigated. The experiments were carried out in a young subtropical forest of southern China in the framework of the BEF-China (Biodiversity and Ecosystem Functioning) project. Tree species richness eff ects on TKE were found only at the local neighborhood scale while plot-level e ffects of tree species richness on TKE were not found. This eff ect was attributed to the young age of the forest plantation. Crown cover, canopy layering or tree heights have not yet fully developed and thus only e ffects at a local neighborhood scale can be seen. Neighborhood e ffects on TKE were due to larger crown areas and taller tree heights in more diverse neighborhoods thus increasing TKE. TKE was highly species-speci c. TKE below Choerospondias axillaris and Sapindus saponaria were higher and TKE below Schima superba was lower than the mean TKE of all other eight species. Species-speci c eff ects of TKE occurred due to diff erences in tree architecture and leaf traits. By far, leaf habit, leaf area and tree height were most important in inducing species-speci c TKE diff erences by changing rain drop velocity and drop size. Furthermore, TKE was spatially variable. Below the fi rst branch of a tree individual TKE was lowest due to low rain drop velocities and small drop sizes. In contrast, TKE was highest in the middle of four tree individuals due to a low interception by a low LAI resulting in higher throughfall amounts. In addition, this thesis provides a ranking of abiotic and biotic factors according to their importance for predicting TKE. Leaf area, leaf area index, throughfall and tree height were the most important variables. These findings emphasize the interplay between abiotic factors as well as tree architectural and leaf traits for a successful TKE prediction. Considering soil erosion management, the erosive potential of TKE in the experimental forest plantation can be mitigated by smaller leaf areas than 70 cm2, lower tree heights than 290 cm, lower crown base heights than 60 cm, smaller leaf area index than 1, more than 47 branches per tree individual and by using single tree species neighborhoods. Initial soil erosion (measured as sediment discharge) was not influenced by leaf litter diversity, but positively aff ected by the presence of soil meso- and macrofauna. This faunal eff ect arises mainly from arthropods slackening and processing the soil surface and only marginally from fauna taking part in the decomposition of leaves leading to less coverage. Nevertheless, leaf litter coverage highly negatively influenced the occurrence of initial erosion. Summarizing, biodiversity eff ects on soil erosion were neither present by investigating tree species richness at plot-level nor by investigating leaf litter diversity. However, a positive e ffect on TKE at the local neighborhood indicates that this can change with a full-grown and dense tree canopy with further succession of the forest. It can be concluded that in young successional forest stands tree architecture, leaf traits, species identity and abiotic factors are more important in influencing the erosive potential of rain than biodiversity

Towards a methodical framework for comprehensively assessing forest multifunctionality

Funded by Deutsche Forschungsgemeinschaft. Grant Number: DFG FOR 891/1-3 National Natural Science Foundation of China. Grant Numbers: 30710103907, 30930005, 31170457, 31210103910 Swiss National Science Foundation (SNSF) Sino-German Centre for Research Promotion in Beijing. Grant Number: GZ 986Peer reviewedPublisher PD

Toward a methodical framework for comprehensively assessing forest multifunctionality

Biodiversity-ecosystem functioning (BEF) research has extended its scope from communities that are short-lived or reshape their structure annually to structurally complex forest ecosystems. The establishment of tree diversity experiments poses specific methodological challenges for assessing the multiple functions provided by forest ecosystems. In particular, methodological inconsistencies and nonstandardized protocols impede the analysis of multifunctionality within, and comparability across the increasing number of tree diversity experiments. By providing an overview on key methods currently applied in one of the largest forest biodiversity experiments, we show how methods differing in scale and simplicity can be combined to retrieve consistent data allowing novel insights into forest ecosystem functioning. Furthermore, we discuss and develop recommendations for the integration and transferability of diverse methodical approaches to present and future forest biodiversity experiments. We identified four principles that should guide basic decisions concerning method selection for tree diversity experiments and forest BEF research: (1) method selection should be directed toward maximizing data density to increase the number of measured variables in each plot. (2) Methods should cover all relevant scales of the experiment to consider scale dependencies of biodiversity effects. (3) The same variable should be evaluated with the same method across space and time for adequate larger-scale and longer-time data analysis and to reduce errors due to changing measurement protocols. (4) Standardized, practical and rapid methods for assessing biodiversity and ecosystem functions should be promoted to increase comparability among forest BEF experiments. We demonstrate that currently available methods provide us with a sophisticated toolbox to improve a synergistic understanding of forest multifunctionality. However, these methods require further adjustment to the specific requirements of structurally complex and long-lived forest ecosystems. By applying methods connecting relevant scales, trophic levels, and above? and belowground ecosystem compartments, knowledge gain from large tree diversity experiments can be optimized

How do newly-amended biochar particles affect erodibility and soil water movement? : a small-scale experimental approach

Biochar amendment changes chemical and physical properties of soils and influences soil biota. It is, thus, assumed that it can also affect soil erosion and erosion-related processes. In this study, we investigated how biochar particles instantly change erodibility by rain splash and the initial movement of soil water in a small-scale experiment. Hydrothermal carbonization (HTC)-char and Pyrochar were admixed to two soil substrates. Soil erodibility was determined with Tübingen splash cups under simulated rainfall, soil hydraulic conductivity was calculated from texture and bulk soil density, and soil water retention was measured using the negative and the excess pressure methods. Results showed that the addition of biochar significantly reduced initial soil erosion in coarse sand and silt loam immediately after biochar application. Furthermore, biochar particles were not preferentially removed from the substrate surface, but increasing biochar particle sizes partly showed decreasing erodibility of substrates. Moreover, biochar amendment led to improved hydraulic conductivity and soil water retention, regarding soil erosion control. In conclusion, this study provided evidence that biochar amendments reduce soil degradation by water erosion. Furthermore, this effect is detectable in a very early stage, and without long-term incorporation of biochar into soils

Rule-based analysis of throughfall kinetic energy to evaluate biotic and abiotic factor thresholds to mitigate erosive power

Below vegetation, throughfall kinetic energy (TKE) is an important factor to express the potential of rainfall to detach soil particles and thus for predicting soil erosion rates. TKE is affected by many biotic (e.g. tree height, leaf area index) and abiotic (e.g. throughfall amount) factors because of changes in rain drop size and velocity. However, studies modelling TKE with a high number of those factors are lacking. This study presents a new approach to model TKE. We used 20 biotic and abiotic factors to evaluate thresholds of those factors that can mitigate TKE and thus decrease soil erosion. Using these thresholds, an optimal set of biotic and abiotic factors was identified to minimize TKE. The model approach combined recursive feature elimination, random forest (RF) variable importance and classification and regression trees (CARTs). TKE was determined using 1405 splash cup measurements during five rainfall events in a subtropical Chinese tree plantation with five-year-old trees in 2013. Our results showed that leaf area, tree height, leaf area index and crown area are the most prominent vegetation traits to model TKE. To reduce TKE, the optimal set of biotic and abiotic factors was a leaf area lower than 6700mm2, a tree height lower than 290 cm combined with a crown base height lower than 60 cm, a leaf area index smaller than 1, more than 47 branches per tree and using single tree species neighbourhoods. Rainfall characteristics, such as amount and duration, further classified high or low TKE. These findings are important for the establishment of forest plantations that aim to minimize soil erosion in young succession stages using TKE modelling