Response of water retention characteristics in litters based on the leaf type Open Access

Forests primarily intercept precipitation, regulate runoff, improve soil composition, and affect soil erosion processes through the canopy layer, the litter layer, and the soil layer (Hartanto et al. 2003; Xia et al. 2019; Zhang et al. 2021). The litter layer, as the second active layer of forest hydrological effects (Tu et al. 2022), plays a key role in achieving hydrological effects in mountainous areas. Acting like a sponge covering the ground surface (Chen et al. 2018), it absorbs some of the precipitation passing through the vegetation canopy, changes the distribution process of precipitation, and allows some of it to slowly infiltrate into the soil (Liu et al. 2017). Additionally, the presence of litter increases the roughness of the surface soil layer, which slows down runoff and flow velocity on the slope, effectively regulating water storage (Dunkerley et al. 2001).

While research on the water-holding characteristics of forest litter has been widespread in recent years (Zhou et al. 2018a, b; Xie & Su 2020), studies on the water-conservation capacity of litter leaves are rarely addressed. The shape of leaves in forest litter greatly affects the capacity and efficiency of water utilization. For instance, coniferous litter has a smaller surface area that is coated with hydrophobic substances, which makes it less effective at retaining water on the leaf surface (Cesarano et al. 2016). However, it can compact well and block the flow path through the litter (Li et al. 2020). On the other hand, broad-leaved litter has a larger surface area, allowing water to displace more and facilitating water retention on the leaf surface (Zhao et al. 2019). However, the research results regarding the water-holding characteristics of needle-leaf forests and broad-leaf forests are quite controversial (Zhou et al. 2018a). This discrepancy may be attributed to the differences in leaf shape characteristics among various species. Therefore, conducting comparative studies on the water-holding capacity of different leaf shapes of forest litter is of significant scientific importance for a thorough understanding and evaluation of the hydrological ecological functions of forest litter and the construction of soil and water-conservation forests (Cui & Pan 2023).

The soaking method is a traditional technique used to determine the water-holding capacity of litter layers (Shen et al. 2019). By conducting soaking tests, the water-holding characteristics of litter layers can help characterize their water-conservation function and partial rain interception ability in forest lands to a certain extent. The indoor soaking method is the most widely employed technique to measure the water-holding capacity of litter. Many scholars, such as Su & Liu (2022), Zhou et al. (2018a, b), and Bai et al. (2021), have conducted studies on the water-holding capacity of litter using the indoor soaking method. This method primarily involved drying the litter, placing it in nylon mesh bags or other permeable containers, immersing it in water for a specific period, measuring the change in mass, and analyzing the hydrological effects of the litter to determine its water-holding characteristics. Tang et al. (2024) revealed that a significant impact of different Pinus massoniana stands in subtropical mountainous areas on the water-holding capacity of the litter by using the indoor soaking method. Yang et al. (2023) discovered that forests with large trees and diverse tree species can improve the water-holding capacity of litter through indoor immersion tests. Gun et al. (2023) found, through indoor soaking experiments, that the water-holding capacity of the litter layer in four subtropical limestone secondary forests in China is logarithmically related to soaking time. It is important to note that the water retention characteristics of litter may vary under different types and environmental conditions.

This study selected typical artificial forest land and collected litter from three tree species: P. massoniana, C. camphora, and M. grandiflora, as research objects. The purpose of the study is to compare and analyze their water absorption and retention characteristics to improve understanding of the hydro-ecological functions of forests and provide a theoretical foundation for enhancing forest ecological benefits. Furthermore, these findings will assist in developing scientifically sound water and soil conservation strategies.

In this study, three common tree species, including coniferous species (P. massoniana), ovoid species (C. camphora), and oval species (M. grandiflora), were collected from forest plantations in Huaxi District, Guizhou Province (106°37′40.8″E; 26°27′28.7″N). The selected types of leaf litter present on the forest floor had not decomposed. For litter collection, we followed the methodology outlined by Helvey (1964). All the collected litter samples were packed in sealed plastic bags and promptly transported to the laboratory. Subsequently, the leaves were dried in an oven at 70 °C for 48 h in preparation for the rainfall simulation experiment. The objective of this process was to maintain consistent moisture levels in the leaf litter prior to the onset of precipitation, thereby facilitating improved comparisons of results (Zhao et al. 2019). The characteristics of leaf litters are shown in Table 1.

The undecomposed litter layer, weighing 120 g (unit area biomass of 0.50 kg m⁻²) and 250 g (unit area biomass of 1.00 kg m⁻²) for each treatment, was then placed in a nylon mesh bag with an approximate aperture of 1 mm. The bag was then immersed in a container filled with water for a water immersion test. After being submerged for 5 min, 15 min, 30 min, 1 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h and 24 h, the bags were removed and the litter was left to drain until no water droplets fall, with the time of drainage being recorded. The litter was subsequently weighed using an electronic balance. Each group of litter undergoes three repeated tests.

The statistical analysis was conducted using SPSS 20.0 software. One-way analysis of variance and a mean comparison based on the Duncan (p < 0.01) were employed to assess the variations among the treatments.

The relationship between the water absorption rate and the soaking time of three types of leaf litters under various unit biomass conditions is presented in Table 2. The dynamic process of the water absorption rate for each litter layer over time follows a power function relationship, with the equations showing a good-fitting effect. The determination coefficient R² for each fitted equation was greater than 0.99, indicating high accuracy in the models.

Figure 2 displays that the water-holding capacity of pine litter leaves stabilized after soaking for 4 h, while the water-holding capacity of magnolia and camphor litter leaves tended to stabilize after soaking for 6–8 h. This suggested that broadleaf litter required more time than coniferous litter to reach saturation, indicating a greater advantage in intercepting, holding, and storing water.

A regression analysis was conducted on the relationship between water holding in litter layers and soaking time, resulting in formulas that demonstrate the relationship between water holding in various types of litter layers and their soaking time, as displayed in Table 3. It was determined that the relationship between the cumulative water holding of each litter layer and the soaking time adheres to a logarithmic function.

As indicated in Table 4, there were significant differences in the characteristics of litter layers under natural conditions, such as natural moisture content, maximum water-holding capacity, and effective water retention capacity. The data in the table revealed that the natural moisture content of the litter layers of the three species was the highest in the camphor tree, followed by magnolia, and the lowest in pine. Additionally, both the W_m and W_av of the three litter layers were greater when the biomass per unit area was 1.00 kg m⁻² compared to 0.50 kg m⁻². However, the R_m and R_av were lower when the biomass per unit area was 1.00 kg m⁻² compared to 0.50 kg m⁻², although their differences were minimal. Additionally, C. camphora and M. grandiflora were compared to P. massoniana. Furthermore, the R_m and R_av of litter layers with different leaf shapes followed the order of M. grandiflora > C. camphora > P. massoniana. When the biomass per unit area was 0.50 kg m⁻², the W_m and R_m of the three litter layers showed a similar pattern, with the R_m of M. grandiflora reaching 171.21%, while P. massoniana was only 125.49%. However, when the biomass per unit area of the litter layer is 1.00 kg m⁻², the W_m and R_m of litter layers with different leaf shapes exhibit a different pattern from their effective interception characteristics, with C. camphora > M. grandiflora>P. massoniana.

Litter has a dense, interconnected cell structure that can effectively retain water and slow down water loss (Li et al. 2013, 2020). The research results of this experiment indicate that as soaking time increases, the water-holding capacity of three different leaf litter types initially increases rapidly, then gradually slows down, ultimately reaching a saturation point. Initially, the water absorption rate rapidly decreases within the first hour of soaking but still maintains a relatively high water absorption capacity. This is because the leaf litter was in a dry state before soaking, with a large difference in cell surface water potential, allowing the leaves to absorb water quickly. After 4 h, the water absorption rate of different layers of leaf litter with varying accumulation levels reaches 0. Between and 10 and 24 h later, the water retention capacity of the leaf litter layer reaches a stable saturation point. This finding is consistent with the research conducted by Xia et al. (2019) and Jiang et al. (2019).

Du et al. (2019) discussed the reasons for the opposite correlation between leaf litter holding capacity and rainfall intensity for two minor differences, which may be caused by the physical properties of forest leaf litter, such as leaf shape and leaf wax. When leaves are rich in non-hydrophilic substances such as oils, their hydrophobic properties are strong, leading to poor water absorption and storage capabilities, resulting in relatively weaker water-holding capacity (Li et al. 2021). When the surface area of fallen leaves increases, the contact area with water increases and their ability to absorb water also increases (Li et al. 2013). Our study indicates that the water absorption rate and water-holding capacity of P. massoniana leaf litter are lower than those of M. grandiflora and C. camphora with the same accumulation amount. The characteristics of P. massoniana leaves containing oils and having a smaller surface area than C. camphora and M. grandiflora indicate that leaf shape and leaf wax affect the water absorption capacity of leaf litter itself and thus affect its water retention capacity. In addition, under the same accumulation conditions, the W_av and R_av of leaf litter layers with different leaf types are ranked as follows: M. grandiflora > C. camphora > P. massoniana. This is related to the differences in leaf thickness and leaf area of the fallen leaves (Rajão et al. 2023). Zagyvai-Kiss et al. (2019) proposed that the water-holding capacity of forest leaf litter depends only on the dry weight of leaf litter per unit area and is not related to the type of leaves it is composed of (needleleaf or broadleaf), which only applies to the characteristics of the leaves themselves. Different types of forest leaf litter exhibit significant differences in leaf morphology. The leaf area negatively affects the density of the leaf litter layer, resulting in varying densities that create distinct pores. These pores play a crucial role in retaining rainfall in its natural state (Rajão et al. 2023). Naturally, the moisture content in leaf litter layers of the three tree species is the highest in C. camphora, followed by magnolia, and the lowest in pine trees. With increasing biomass per unit area, compared to the other two types of leaf litter, the variations in W_m and R_m of C. camphora are the greatest, which may be related to the R₀ of C. camphora.

The water-holding capacity is one of the most important variables in interception modeling (Zagyvai-Kiss et al. 2019). The spatial distribution pattern of litter will also affect its water-holding characteristics. Previous studies have shown that different densities and distribution forms of litter in forests will affect surface water retention and infiltration, thereby affecting the distribution and cycling of soil moisture (Zhu et al. 2022; Rajão et al. 2023). We compared our results with early studies and found that litter interception mainly depends on the litter mass per unit area – the greater the mass, the larger the accumulation volume, the greater the water-holding capacity, the higher the water absorption rate, and the better the water retention performance (Zhu et al. 2021; Jourgholami et al. 2022; Cui & Pan 2023). In this study, the water-holding capacity of litter leaves with a unit area biomass of 1.00 kg m⁻² was higher than those with a unit area biomass of 0.50 kg m⁻² at different soaking times, indicating that the accumulation of litter leaves is a key factor affecting its water-holding capacity. When the unit area biomass is 0.50 kg m⁻², the water-holding process curves of C. camphora leaves and M. grandiflora leaves are more similar, showing a greater difference with the P. massoniana leaf litter layer. However, when the unit area biomass is 1.00 kg m⁻², the water-holding curve of C. camphora is higher than that of M. grandiflora, with different patterns in W_m and R_m among different leaf litter layers, namely C. camphora > M. grandiflora > P. massoniana. This may be influenced by the natural moisture content of the litter. C. camphora R₀ (27.7%) > M. grandiflora R₀ (11.46%), the higher the biomass of C. camphora, the greater the influence of R₀, and the greater the water-holding capacity. In addition, under the same accumulation amount, C. camphora leaf litter layer forms more pores than M. grandiflora, making it easier to retain moisture.

This study indicates that the water-holding capacity of each litter layer shows a rapid increase trend in the first 15 min, followed by a slower increase. Under rainfall conditions, precipitation reaching the litter layer is first intercepted and absorbed by the litter layer before slowly infiltrating into the soil or forming overland flow along the litter surface, delaying the time for precipitation to reach the soil layer, known as the lag time. This suggests that in actual rainfall, the first 15 min are an important stage for litter absorption, with decreased absorption in the later stages, directing more toward runoff distribution. After 10 h, the water-holding capacity of the litter layer reaches a stable saturation point. In simulated rainfall, litter is usually not completely wet and remains unsaturated; therefore, filling the voids in the litter layer may require more time (Du et al. 2019).

For different climate zones or geological or soil conditions, the relative water-holding characteristic values of coniferous forests, broad-leaved forests, and mixed forests may not be consistent (Zhou et al. 2018b). Therefore, our research results apply to mountainous hilly areas. Under different natural conditions, there may still be some differences in the water-holding capacity of litter. This study has certain limitations as it only explored the water retention characteristics of litter under soaking test conditions, lacking observation and validation under actual natural conditions. Future research could further explore the effects of different environmental factors, decomposition levels of litter, and vegetation types on water retention characteristics. Additionally, combining modeling and simulation methods could provide a more comprehensive study of the function of litter in soil hydrological cycles and vegetation ecosystems.

In summary, the water retention process of litter layers during soaking tests occurs in three stages: rapid water absorption, a gradual increase in water retention, and eventual saturation stability. The water absorption rates of litter layers vary dynamically with soaking time in a power function relationship, and the real-time water-holding capacity of litter layers changes logarithmically with soaking time. Broadleaf litter takes longer to reach saturation compared to conifer litter, indicating that broadleaf litter has greater advantages in water interception, retention, and storage.

The study was supported by the Guizhou Provincial Basic Research Program (Natural Science) (QKHJC-ZK[2023]YB065), the Guizhou Provincial Department of Education Higher Education Science Research Project-Youth Project (QJJ[2022]112), and the Technology Project of Power Construction Corporation of China (DJ-ZDXM-2023-07).

The data that support the findings of this study are available on request from the corresponding author, upon reasonable request.

The authors declare there is no conflict.