In Situ Rainwater Harvesting System Slows Forest Decline through Increasing Soil Water Content, Fine-Root Traits, and Plant Hydraulic Conductivity
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1. Introduction
Land degradation is one of the most pressing ecological issues in the world and a constraint to ecosystem services of terrestrial ecosystems [1]. In order to restore degraded land, planting trees is encouraged worldwide as forests are vital in averting desertification, and enhancing biodiversity and ecosystem services [2]. However, forest trees consume more water than other vegetation species, such as grasslands [3], and tend to deplete soil moisture in both shallow and deep layers post-tree planting [4]. This, in turn, will hamper normal plant growth, affecting fine-root traits and plant hydraulic conductivity characteristics, and may result in plant mortality and forest degradation [5]. Therefore, the balance between soil water supply and plant water uptake is fundamental for the healthy development and sustainability of forest ecosystems [6], especially in water-limiting regions.
Situated in the upper and middle reaches of the Yellow River basin, the semi-arid Loess Plateau of China spans an area of 64 × 104 km2, representing one of the most severely eroded regions throughout the world. The severe water loss and soil erosion in this region have increased the fragility of the ecosystem [7]. To mitigate these challenges and improve ecosystem services, larger-scale afforestation campaigns have been implemented by the Chinese government in the past few decades, including the Grain for Green Project, which has increased forest cover by 4.9% from 2000 to 2008 [8]. Robinia pseudoacacia L., recognized for its strong nitrogen-fixing ability and fast growth, is extensively cultivated, comprising 90% of afforestation trees within this area [9]. However, due to the imbalance between soil water supply and plant water consumption, the formation of dry soil layers has been documented across the majority of regions in the Loess Plateau [4,10]. This has led to the widespread early degradation of Robinia pseudoacacia trees, characterized by evenly dried shoots on the top of tree crowns—affected trees are commonly referred to as “dwarf-aged trees”—typically initiating around the age of 30 years [11]. Thus, the repaid development of larger scale Robinia pseudoacacia trees is sustainable only if there is sufficient soil moisture content. Inadequate management of soil water resources may lead to a severe soil water deficit, negatively impacting tree growth and survival rates, exacerbating land degradation [6,12].
Plants primarily depend on their fine roots and vascular structures to acquire and transport water and nutrient resources within forest ecosystems [13]. Therefore, understanding the characteristics of fine-root distribution and vascular features is essential for unraveling plant water-use patterns and plant survival strategies. Previous studies have indicated that fine-root distribution and water transport characteristics through xylem conduits are influenced by various factors, with drought stress being a significant one [14,15,16,17]. During drought periods, soil moisture is reduced severely, which may negatively affect fine-root distribution and xylem vascular structures. This potentially leads to plant hydraulic transfer failure and negatively impacts plant growth and photosynthesis, eventually causing plant mortality if drought persists [18]. Even though in the past few decades many studies have been conducted to explore the response of fine-root distribution or plant hydraulic conductivity to drought stress for different tree species, there remains a knowledge gap regarding how drought concurrently affects plant fine-root distribution, vascular structure, and the corresponding hydraulic conductivity. Knowledge of this will be helpful in deciphering plant water use patterns and survival strategies, and also pivotal in addressing critical issues in forests, especially in regions with limited water resources.
In recent years, a novel rainwater-saving method, referred to as the in situ rainwater collection and infiltration system (IRICIS), has been developed and implemented in apple and Chinese jujube orchards across the Loess Plateau region [19]. The primary objective of this rainwater-saving system is to improve the infiltration of rainfall into root zones, thereby increasing soil moisture content to meet the water demands of trees during dry periods. The theoretical framework and advantages of this system, compared to traditional management practices, have been extensively documented in previous studies [20]. However, research is limited regarding the influence of IRICISs on soil moisture content, fine-root distribution traits, xylem vascular properties, and associated plant hydraulic conductivity characteristics for common afforested trees, such as Robinia pseudoacacia trees. Additionally, the impact of IRICISs on the physiological and water transport characteristics of declining Robinia pseudoacacia trees remains unexplored. Knowledge of this holds significant importance in developing rational plantation management strategies and in fostering the healthy and sustainable development of these forests. Leveraging this background, we measured soil water content, fine-root distribution, xylem anatomical properties, and corresponding plant hydraulic conductivity of six declining Robinia pseudoacacia forest stands (35 years old) in the semi-arid Loess Plateau for two successive years, in 2015 (a wet year with above-average precipitation) and 2016 (a drought year with below-average precipitation). We hypothesized that the construction of the in situ rainwater collection and infiltration system (IRICIS) would positively influence plant available moisture content, fine-root distribution, xylem vascular characteristics, and plant hydraulic conductivity in declining Robinia pseudoacacia forests during drought conditions. We also expected that the IRICIS system, designed to enhance rainfall infiltration and soil moisture retention, would mitigate the adverse effects of drought on the physiological and water transport characteristics of the declining Robinia pseudoacacia trees, ultimately mitigating the processes of tree degradation.
2. Materials and Methods
2.1. Site Area
The study was conducted from 2015 to 2016 in the Yeheshan catchment, Fufeng County (34°33′ N, 107°54′ E, 1090 m a.s.l, Figure 1) in Shaanxi Province on the southern Chinese Loess Plateau. The average annual precipitation of the area is 580 mm with significant seasonal variations. Approximately 80% of the precipitation falls between May and October. The annual mean air temperature is 12.7 °C. The soil is silt loam according to the USDA (United States Department of Agriculture) classification, with average values of sand, 5.8%, silt, 73.4%, and clay, 20.9%.
Vegetation restoration has been extensively implemented since the late of 1980s, when Robinia pseudoacacia was widely planted. The forest coverage of the Yeheshan catchment is approximately 90%, and the Robinia pseudoacacia spans an area of ~87 km2. The main understory vegetation species in this area include Stipa bungeana and A. codonocephala.
2.2. Experiment Design
In this study, six declining Robinia pseudoacacia (35 years) experimental plots, each 20 × 20 m2 in size and with a density of 1200 trees/ha, were chosen. The mean height and diameter at breast height of these trees were 9.0 m and 15.5 cm, respectively. The ratio of leaf area index to sapwood area for these trees was 0.05 m2 cm−2. The mean crown length (m), the mean crown height (m), leaf area index (m2 m−2), and cumulative sapwood area (m2/ha) values of these selected trees were 4.2 m, 5.8 m, 1.96 m2 m−2, and 3.3 m2/ha, respectively. These sites were strategically constructed on sun-facing middle slopes (5–10°), where rain-fed agriculture was formerly conducted.
Three experimental sites were implemented with IRCISs at treated sites, while another three experimental sites were designated as control sites. In the treated sites, an IRCIS system was strategically installed upslope of individual trees. The IRCISs consisted of a semi-circular ridge, 1.0 m in radius and 0.2 m in height (Figure 2). During ridge construction, soil was excavated and repositioned to establish the main stem at the summit of the ridge, ensuring a mostly flat soil surface in the semicircle area. Inside the semicircular ridge, a soil pit measuring 0.8 m × 0.8 m × 0.8 m deep was excavated, and the downslope wall was positioned 1.0 m from the bole. A rainwater storage pit, covered with synthetic fiber fabric and containing grasses, leaves, and brushwood debris, was established. The pit, lined with polythene sheet, had a central hole measuring 3 cm in diameter, chaneling the rainwater accumulated from the fish-scale ridge into the pit. Rainwater collected in the pit could then infiltrate into the surrounding soil. This design effectively reduces runoff, leading to an increase in soil moisture content. In this study, the IRCISs were installed at the end of 2014 and the measurements were carried out in 2015–2016, and when constructing the rainfall harvesting system, the understory herbaceous plants around the IRCISs were cleared to eliminate the potential influence of understory vegetation on the experimental results.
Rainfall was measured utilizing a Geonor T-200B weighing precipitation gauge (Geonor, Eiksmarka, Norway). Based on rainfall data from 1958–2016, a curve depicting the distribution of rainfall frequencies was formulated. Every year was categorized as wet, normal, or dry, based on the percentage of total precipitation (<25% for dry, >75% for wet, and the remaining for normal years). The experimental sites treated in 2015 were designated as RC2015, and those treated in 2016 were labeled as RC2016, while the comparison experimental sites in 2015 were labeled as WT2015, and those in 2016 were designated as DT2016.
2.3. Soil Water Measurement
At each experimental site, three neutron probes (5.0 m) were randomly positioned between the tree and rainwater harvesting system in the treated plots and randomly in the control treatment. The volumetric soil moisture content was measured using a neutron probe. To minimize the influence of rainfall on soil water movement, a rain-free period of five successive days was designated as the measurement period. Sampling was conducted 12 times/year throughout the sampling period. Slow neutron counts were counted at 0.15 m intervals down to a depth of 0.8 m and then at 0.2 m intervals down to a soil depth of 5.0 m. The volumetric soil moisture content (θv) at each depth was determined by applying the calibration curve to the slow neutron count rate (CR):
Plant available moisture storage (PAMS) denoted the maximum quantity of soil water accessible for plant use, and was determined by subtracting the soil moisture content (SWC) from the plant permanent wilting point (PWP):
where ΔZ denotes the increment in soil depth. Based on measurement, the average value of PWP in this study was 0.072 cm3 cm−3.
2.4. Fine-Root Measurement
Fine-root distribution was examined employing the soil auger technique. Within every experimental site, soil samples (n = 10) were gathered using a cylindrical metal auger (0.09 m diameter and 0.1 m long). Soil cores were randomly selected between the tree and IRCIS pit in the treated plots and randomly in the control treatment areas. Moreover, soil cores were gathered at 0.2 m increments, extending to a depth of 5.0 m beyond which fine roots were not detected.
A two-stage process was employed to separate root samples from the soil. Firstly, soil samples were meticulously washed over a 5 mm sieve. Vegetation roots and other plant remnants were visually identified and removed. Secondly, Robinia pseudoacacia roots were categorized based on their size classes (diameter > 2 mm and diameter ≤ 2 mm) using a microscope with 10–40× magnification. Root samples underwent digital scanning using an Epson Perfection v700 photo scanner (Seiko Epson Corporation, Suwa, Nagano-ken, Japan) with a resolution of 600 dpi. The length of roots was determined utilizing WinRhizo Software version Pro 3.5 (Régent Instrument Inc., Quebec, QC, Canada, www.regent.qc.ca). The length density of fine roots (−3), was estimated as
where RL represents the length of fine root; RVs is the volume of soil samples. Additionally, the cumulative length density of fine roots (CFRLD) for each experimental site was calculated.
2.5. Xylem Anatomical Measurements
At each site, six trees were randomly selected, and sapwood samples were gathered from the diameter at breast height (DBH) of stems. Two cores for both current year sapwood (south facing and north facing; only current year sapwood is able to conduct water), each 5 mm in diameter and spaced 30 mm apart, were extracted using a Suunto increment borer (SUUNTO, Vantaa, Finland). Each core was sectioned transversely (20 μm thick), and 12 samples were obtained using a sliding microtome (Leica RM2235; Leica Microsystems Nussloch GmbH, Nussloch, Germany). The sections were then treated with a 1% safranin solution to improve the contrast between wood and conduit void space. After staining, section samples were placed in glycerol and prepared for microscopic analysis. Depending on the size and abundance of conduits, 4–6 images of each section were taken through a CCD digital camera (Guangzhou Mingmei Technology Co., Ltd., Guangzhou, China) linked to an Olympus CX 31 microscope (×40 magnification, Olympus Corp., Tokyo, Japan). Subsequently, conduit abundance and conduit diameter were obtained with the ImageJ software (version 1.8.0, US National Institutes of Health, Stapleton, NY, USA).
2.6. Hydraulic Conductivity Characteristics
The hydraulic conductivity KTH (Kg m MPa−1 s−1) of plant xylem conduit was calculated following Hagen–Poiseuille’s law as:
where η represents water viscosity (1.002 × 10−9 MPa s) and Dh represents the hydraulically weighted mean conduit diameter, estimated as follows:
where D represents the equivalent conduit diameter, and N is the number of conduits.
During this study period, branches aged one year old, positioned in the mid-upper canopy on the sun-exposed side of the plant, and roots from shallow soil layers (0–1.0 m), were utilized to evaluate the percentage loss of hydraulic conductivity (PLC). The values of PLC for both branches and roots were examined using the method suggested by Sperry et al. [21]. During the period of fine-root sampling, branch and root samples (~0.3–0.4 m in length, ~1.0 cm in diameter) were gathered. To mitigate embolism caused by excision, all branch and root samples were excised underwater. Subsequently, the collected samples were carefully enclosed in polyethylene film and transported to the experimental platform for further analysis. The values of PLC at each site were determined by averaging measurements from three segments (each 4-cm long) across three biological replicates. More detailed information about the measurement processes of PLC can be found in Ma et al. [13].
2.7. Statistical Analysis
One-way ANOVA was performed to test the difference between treatments for PAMS, FRLD, Dh, KTH, and PLC. All statistical analyses were assessed using SPSS software (version 25.0; SPSS Inc., Chicago, IL, USA) and p < 0.05 was considered statistically significant.
5. Conclusions
Due to the higher water demand of Robinia pseudoacacia trees, most Robinia pseudoacacia forest stands had significantly lower plant-available moisture content on the Loess Plateau, resulting in higher soil moisture deficits, especially in drought years. This, in turn, has hampered the healthy growth of Robinia pseudoacacia, leading to a decline and, in severe cases, mortality of trees. To minimize these negative effects, enhance the efficient use of soil water, and improve the resistance and resilience of forests to drought stress, diverse afforestation strategies, including engineering practices and rainfall harvesting measures, have been documented for sustainable artificial forest development. Our study demonstrates that the application of a rainfall harvesting system—the IRCIS—can effectively increase plant available moisture content of declining Robinia pseudoacacia forest stands. In particular, IRCIS treatment significantly increased xylem conduit diameter and plant hydraulic conductivity while substantially reducing the percentage loss of hydraulic conductivity in both roots and branches. Furthermore, IRCIS treatment significantly reduced the root biomass and distribution depth of Robinia pseudoacacia during different rainfall years. This implies that IRCISs are beneficial for plant growth and survival.
The response to rainwater harvesting may vary in different soil types due to variations in water retention, drainage, and other soil characteristics. The study’s findings may be specific to the soil conditions on the Loess Plateau. However, the results of this study can serve as a valuable reference for research on other soil types. The findings of this study are significant for devising strategic methodologies for the planning and management of rainwater resources. The adoption of these strategies, especially the incorporation of rainwater harvesting systems (IRCISs), offers a viable solution to counteract forest degradation induced by drought stress.
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