•Soil losses from cropped hillslopes compromise food security over the long-term.•Causal chain (crop structure-throughfall-soil crust-runoff-soil loss) was explored.•Multiple regression analysis was ...applied to 24 variables measured in 30 micro-plots.•By crusting soils, throughfall kinetic energy enhances runoff and soil loss.•A switch from annual crops to perennial tree plantations can enhance soil erosion.
Display omitted
In Montane Southeast Asia, deforestation and unsuitable combinations of crops and agricultural practices degrade soils at an unprecedented rate. Typically, smallholder farmers gain income from “available” land by replacing fallow or secondary forest by perennial crops. We aimed to understand how these practices increase or reduce soil erosion. Ten land uses were monitored in Northern Laos during the 2015 monsoon, using local farmers' fields. Experiments included plots of the conventional system (food crops and fallow), and land uses corresponding to new market opportunities (e.g. commercial tree plantations). Land uses were characterized by measuring plant cover and plant mean height per vegetation layer. Recorded meteorological variables included rainfall intensity, throughfall amount, throughfall kinetic energy (TKE), and raindrop size. Runoff coefficient, soil loss, and the percentage areas of soil surface types (free aggregates and gravel; crusts; macro-faunal, vegetal and pedestal features; plant litter) were derived from observations and measurements in 1-m2 micro-plots. Relationships between these variables were explored with multiple regression analyses. Our results indicate that TKE induces soil crusting and soil loss. By reducing rainfall infiltration, crusted area enhances runoff, which removes and transports soil particles detached by splash over non-crusted areas. TKE is lower under land uses reducing the velocity of raindrops and/or preventing an increase in their size. Optimal vegetation structures combine minimum height of the lowest layer (to reduce drop velocity at ground level) and maximum coverage (to intercept the largest amount of rainfall), as exemplified by broom grass (Thysanolaena latifolia). In contrast, high canopies with large leaves will increase TKE by enlarging raindrops, as exemplified by teak trees (Tectona grandis), unless a protective understorey exists under the trees. Policies that ban the burning of multi-layered vegetation structure under tree plantations should be enforced. Shade-tolerant shrubs and grasses with potential economic return could be promoted as understorey.
•Throughfall kinetic energy (TKE) can be affected by under-canopy dead branches.•The present unit TKE was much lower than that reported, with fewer/no dead branches.•Empirical unit TKE model only ...considering upper-canopy structures overestimates.•6–15 sand-filled splash cups are appropriate for stand-scale TKE measurements.•Branch pruning may be unnecessary for soil erosion control.
Among soil erosion processes, the initial stage of splash soil erosion caused by throughfall kinetic energy (TKE) plays a crucial role in soil conservation within forest ecosystems. Throughfall (TF) through the upper canopy structure considerably influences TKE in coniferous plantations. Recent studies have revealed that the under-canopy structure laden with dead branches in unmanaged coniferous plantations reduces the TF; however, its specific impacts on TKE and appropriate sampling strategies remain unexplored. This study used 40 splash cups (4 for free kinetic energy and 36 for TKE) for one event (total: 600 splash cups for 15 events) in an unmanaged 36-year-old Japanese cypress plantation laden with dead branches. Additionally, unstratified and stratified Monte Carlo simulations were used to determine optimal sample sizes for stand-scale TKE estimation. Results demonstrated a strong correlation between TKE (J m−2) and TF (mm) (R2 = 0.98) across all events. The stand-scale unit TKE of 12.5 J m−2 mm−1 was much lower than those in seven previous studies with fewer or no dead branches (range: 16.4 to 28.2, median: 18.8 J m−2 mm−1). The previously developed empirical stand-scale unit TKE model considering under-canopy structure solely exhibited a 1.7-fold overestimation. Among the stand structures, a positive relationship of TKE with the distance between a sampling point and its nearest trunk (Dmin) was observed, which was positively correlated with the lowest dead branch height. This indicated that under-canopy dead branches weakened the TKE, likely by reducing TF and raindrop fall velocity. Stratified Monte-Carlo simulation considering the Dmin related spatial patterns of TKE provided a more efficient approach for capturing variability. To achieve high precision with potential errors of ≤5–10 %, a total sample size of 6–15 was appropriate. Our findings implied that the presence of under-canopy dead branches mitigates the soil erosion risk in unmanaged Japanese cypress plantations.
•Throughfall kinetic energy (TKE) under two banana canopy scales was considered.•Banana leaf was classified into five kinds of shapes based on canopy drip points.•The various leaf shapes had ...significantly different effects on splash erosion.•Canopy drip points have higher TKE than open field and canopy non-drip points.•The high TKE observed at drip points of banana canopy could aggravate soil erosion.
Banana (Musa nana Lour.) plants have a distinctive canopy pattern with their extremely long and wide leaves. However, the effects of such distinct leaf shapes on rainfall redistribution and splash erosion are still poorly understood. Here, we investigated the basic characteristics of throughfall (TF) distribution and throughfall kinetic energy (TKE) and clarified the effect of specific leaf shapes on TKE under the individual banana plant (IBP) and the whole banana plantation (WBP) canopies. We found that the TF under the IBP and WBP canopies was 80.2% and 84.7% of the incident rainfall, respectively. The spatial variability of TF was high, which was attributed to the prominent funnelling and shading effects of the banana canopy. The KE at canopy drip points was significantly higher than that at canopy non-drip points and in open fields under both the IBP and the WBP canopies, implying that the negative effect of banana canopy drip points was far greater than the protective effect of non-drip points on splash erosion. In addition, the leaves at canopy drip points were classified as valley (Va), overlap (Ov), leaf tip (Ti), breakage (Br) and complex shapes (Cs). Significant differences in TKE among these leaf shapes were found, and the significantly lower TKE in Ti and the greater TKE in Cs than those in Va, Ov and Br were observed. As a result, these leaf shapes potentially affected TF splash erosion under the banana canopy. Further ecohydrological studies are required to improve heavy TF splash erosion in banana plantation and to create environmental-friendly plantation.
Raindrop impact on bare soils is the initial phase of rainfall-induced soil erosion which is altered under any type of vegetation due to the interactions of rainfall with the canopies. This study ...examines the drop size distribution (DSD) and kinetic energy (KE) of raindrops above and below the birch tree (Betula pendula Roth.) canopy in a research plot in the city of Ljubljana, Slovenia using a one-year observation of 63 rainfall events and the effect of meteorological variables under moderate continental climate. Simultaneous measurement of the microstructures of open rainfall and throughfall was carried out using an optical disdrometer. The result of our analysis revealed that throughfall DSD showed two distinct major peaks (bimodal) occurring primarily on smaller drop sizes while open rainfall has only one. The cumulative drop number, median drop-volume diameter (D50), and drop fall velocity of throughfall were 16.4%, 26.6%, and 5.0% lower than those of open rainfall, respectively. Also, the relative volume percentage of raindrops > 1.5 mm is 1.5 times higher than those observed in throughfall drops which indicates that the presence of the canopy caused the fractionation of larger drops into smaller droplets. These reductions significantly differ depending on the phenoseasons of the canopy with the leafed state being higher than the leafless state. Similarly, the Kruskal-Wallis H test result revealed that birch tree elicits a statistically significant change in the kinetic energy of open rainfall, thus weakening the mean rainfall KE by 33.7%. On the other hand, KE is positively affected by the phenological condition of the canopy with higher attenuation being observed during its leafed state. Also, the correlation analysis demonstrated that vapor pressure deficit, air temperature, and relative humidity have stronger associations with throughfall kinetic energy among meteorological variables considered. These findings underscore the necessity of an optimized selection of tree species for afforestation programs.
Display omitted
•Below-canopy soil erosion is influenced by throughfall kinetic energy (TKE).•Throughfall has bimodal distribution with smaller median drop-volume diameter.•Birch tree canopy significantly reduces the mean kinetic energy of open rainfall.•TKE is strongly affected by canopy phenoseasons with higher reduction in leafed state.•Vapor pressure deficit has a stronger association with TKE among meteorological variables.
Rubber is usually grown as a monoculture but there have been recent attempts to encourage rubber-based agroforestry systems to reduce adverse environmental impacts, including the reduction of soil ...erosion in Xishuangbanna, SW China. To estimate the influence of different types of rubber-based agroforestry systems on soil erosion processes, we measured the throughfall kinetic energy (TKE) under different vegetation types by using 640 sand-filled Tübingen splash cups. This study was conducted in Xishuangbanna Tropical Botanical Gardens under natural rainfall conditions. Our results indicated that in both rubber-based agroforestry systems and rubber monocultures, a significant linear positive correlation exists between TKE and rainfall amount. Rainfall amount is a critical factor that contributes to soil detachment in rubber plantations in this region. TKE under rubber plantation conditions was found to be notably higher than under open field conditions (ranging from 1.84 to 2.32 times greater). However, there was no significant difference under multiple canopies compared to monoculture. TKE values under the different rubber-based agroforestry systems were closely related to the canopy structure, and TKE and leaf area index were significantly negatively correlated. The spatial variability of TKE was higher in rubber-based agroforestry systems than in rubber monocultures. In addition, TKE was usually concentrated in 3–4m bands that did not have the protection of a sub-canopy. The fact that the erosion by TKE under rubber-based agroforestry was still high highlights the importance of selecting intercrops when constructing rubber-based agroforestry systems and of improving planting patterns.
•Rainfall was a critical factor affecting soil erosion under rubber plantations.•The effect of rubber-based agroforestry systems in reducing TKE was limited.•TKE appeared extreme in areas without the protection of the sub-canopy.•It is important to improve intercropping planting patterns.
This study evaluated the spatial distribution of the throughfall kinetic energy (TKE) on a small scale in a rubber plantation. The experiments used Tübingen splash cups with natural rainfall. The ...results indicate that the leaf area index did not significantly affect the TKE during the foliated season. There was no significant correlation between the TKE and the distance from the trunk. However, the lateral translocation of the throughfall in the canopy significantly affected the spatial distribution of the TKE, and high TKE points appeared in the middle and at the edge of the canopy. The results also show that the spatial distribution of the volume-specific TKE values was similar in different rainfall and rainfall intensity groups. The variogram of the spatial variability demonstrates that the TKE exhibited a strong spatial autocorrelation. We confirm that the rainfall redistribution is important for the spatial distribution of the TKE in a rubber plantation.
► Tree saplings (2 years, 1.2m) reduce rainfall kinetic energy by 59%. ► Planting density has a significant effect on throughfall kinetic energy. ► Throughfall KE is different among tree species. ► ...Plant architectural traits are substantial for interpreting differences in throughfall KE. ► Canopy closure and storage are most important when considering KE under saplings.
In order to estimate the influence of plant architectural traits on the erosivity of throughfall we studied throughfall kinetic energy (KE) under tree saplings in a plantation-like experiment in the humid subtropics. Our analyses of rainfall and throughfall KE are based on measurements using calibrated splash cups. Two experiments were carried out, one focusing on density effects and the other testing for species-specific effects and effects of species mixtures. The major architectural traits were measured to characterize sapling morphology. Mixed effects models were used for statistical analysis. In both models, rainfall KE was identified as the most important effect on throughfall KE. Overall, rainfall KE per area was reduced by 59% below the canopy of the studied saplings. We found a significant effect of sapling density on throughfall KE. This is primarily due to the relation between free throughfall and released throughfall. As free throughfall possesses a far higher KE than released throughfall originating from saplings, lower sapling density results in higher total throughfall KE. We also showed that the influence of density on throughfall KE decreases with increasing sapling height due to lateral canopy growth of the saplings.
Throughfall KE was significantly different among species. We attribute this to species-specific differences in crown architectural traits. These traits have opposite influence on throughfall KE and interact with each other. Depending on its magnitude, one crown trait can possibly superimpose contrary effects of others.
PDF
