The Impact of Bamboo (Phyllostachys edulis) Expansion on the Water Use Patterns of Broadleaf Trees

Abstract

The expansion of bamboo (Phyllostachys edulis) affects the growth status of trees in colonized forests, but there has been insufficient research on changes in tree water physiology. In this study, we used stable δ²H, δ¹⁸O, and ¹³C isotope ratios to analyze the water sources and water use efficiency (WUE) of bamboo, deciduous broadleaf trees (Alniphyllum fortunei), and evergreen broadleaf trees (Machilus pauhoi and Castanopsis eyrei) in a bamboo-expended broadleaf forest (BEBF), a bamboo-absent broadleaf forest (BABF), and a bamboo forest (BF). We found that the expansion of bamboo had no significant effect on the water sources and WUE of deciduous broadleaf trees, but altered the water sources of evergreen broadleaf trees. During the growing season, evergreen broadleaf trees decrease their uptake fractions of surface soil water by 7.1% to 9.6% and increased their uptake fractions of middle soil water by 5.8%~9.4%. Conversely, during the non-growing season, they increased their uptake fractions of surface soil water by 11.9% and decreased their uptake fractions of deeper soil water by 5.6%~12.9%. Additionally, after expanding into broadleaf forests, bamboo increased its uptake proportion of surface and shallow soil water by 20.0% and 9.4% during the growing season. Its WUE also improved, increasing by 20.0 μmol/mol and 13.0 μmol/mol during the growing and non-growing seasons, respectively. These results indicate that as bamboo expands into broadleaf forests, it enhances its competitiveness for water resources by changing its water use strategy. Compared to deciduous broadleaf trees, evergreen broadleaf trees exhibit more flexible water use strategies under the conditions of bamboo expansion. Our research reveals, for the first time, how broadleaf trees adjust their water use strategies in response to bamboo expansion, and uncovers the mechanisms behind bamboo expansion into evergreen broadleaf forests from the perspective of water use strategies. This will aid future forest management under the conditions of bamboo expansion.

1. Introduction

Water is the transport medium for substances such as photosynthetic products and growth hormones in plants, and it directly participates in various physiological activities [1], making it especially important for plant growth and development. The pattern of water use by plants largely determines how ecosystems respond to changes in environmental water conditions. However, research on plant water use has mostly focused on arid and semi-arid regions, primarily concentrating on the relationship between water use strategies and climate factors [2,3,4]. In recent years, broadleaf forests have faced serious bamboo (Phyllostachys edulis) expansion issues [5,6]. Bamboo is a water-intensive plant with high water demands during its growth [7,8], which can restrict the water availability conditions for broadleaf trees and subsequently affect their growth and development. To survive, broadleaf trees must adjust their water use strategies to ensure that they obtain sufficient water amid long-term water competition. Some species with water use strategies that are insensitive to environmental changes may be eliminated due to water limitations. Therefore, studying the water use strategies of broadleaf trees is crucial for understanding how they respond to the expansion of bamboo.

Plant water use strategies are mainly reflected in two aspects. The first is the source of water, which is the manifestation of plant water use strategies, with available water primarily coming from soil water, atmospheric precipitation, and groundwater [9]. The proportion of a plant’s utilization of these potential water sources varies with changes in soil moisture content and precipitation [2,10,11]. The second is water use efficiency (WUE), which refers to the biomass accumulated per unit weight of water consumed (the ratio of photosynthetic rate to transpiration rate) [12,13]. WUE is an important indicator of a plant’s drought resistance and is closely related to environmental factors that affect photosynthesis and transpiration, such as light and water availability [14,15,16]. After bamboo expands into broadleaf forests, what impact does it have on soil water availability, and how do the water use strategies of broadleaf trees change?

Bamboo has a strong advantage in water competition compared to broadleaf trees due to several biological characteristics. Firstly, as a clonal plant, bamboo can share water among its clones through rhizomes [17]. Secondly, bamboo’s extensive underground root system and high biomass of fine roots, about six times that of broadleaf trees, form a root network in the soil [18]. As bamboo expands into broadleaf forests, its fine root distribution tends to be in the upper (surface and shallow) soil layer [18], thus intensifying competition for upper soil water with broadleaf trees. This may lead to a lack of available upper soil water for broadleaf trees. Several studies have found that when some species experience insufficient upper soil water, they undergo adaptive changes, shifting to use water from deeper soil layers or groundwater to reduce interspecies water competition [4,19,20,21]. Additionally, under inadequate water conditions, trees close their stomata to increase WUE for survival and growth [22,23,24,25]. Therefore, do broadleaf trees increase their use of deep soil water and improve WUE to adapt to the environment after bamboo expansion?

Stable isotope techniques are widely used to study plant water use strategies, due to their sensitivity, accuracy, and speed [3,26]. By analyzing the hydrogen (²H) and oxygen (¹⁸O)-stable isotope composition in plant xylem water and potential water sources, the proportion of water sources utilized by plants can be quantified [27]. Additionally, the analysis of plant leaf carbon-stable isotopes (δ¹³C) can estimate the long-term WUE of plants [13,28]. Based on these analyses, we utilize stable isotope techniques to examine uptake fractions of various potential water sources and the WUE of broadleaf trees and bamboo in a bamboo-absent broadleaf forest (BABF), bamboo-expended broadleaf forest (BEBF), and bamboo forest (BF) within the Qiyunshan National Nature Reserve, Jiangxi Province, China, where bamboo expansion is relatively severe. Our aim is to answer the following scientific questions: After bamboo expansion into broadleaf forests, what strategic changes do broadleaf trees adopt for water use? Do broadleaf trees increase their use of deep soil water? Do they improve their WUE? Validating these hypotheses can reveal the water use patterns of broadleaf trees, providing insights into the water consumption characteristics and trends of evergreen broadleaf forests under bamboo expansion. This can offer valuable insights for the restoration of broadleaf forests.

2. Materials and Methods

2.1. Study Area

The research was conducted in the Qiyunshan National Nature Reserve (QNNR), located in Jiangxi Province, China (113°54′–114°07′ E, 25°41′–25°54′ N, Figure 1), with an elevation of approximately 2060 m above sea level. The QNNR belongs to the humid subtropical monsoon climate zone, which is characterized by a warm climate and abundant rainfall [29,30]. The average annual precipitation in the QNNR ranges from 1522 to 1660 mm, and the average annual temperature is between 18.0 and 18.4 °C [31]. The dominant vegetation in the QNNR is evergreen broadleaf forest, but many areas still have bamboo forests. The canopy structure in this region primarily consists of two layers: the upper canopy, featuring fast-growing deciduous trees along with some mature evergreen trees (maximum height: 15–20 m), and the understory, which is predominantly populated by mature and young shade-tolerant species.

2.2. Experimental Design

Three experimental blocks—B1, B2, and B3—were established in June 2021 (Figure 1 and Figure 2). Each block contained three sample plots measuring 20 m × 20 m, representing bamboo-absent broadleaf forest (BABF), bamboo-expended broadleaf forest (BEBF), and bamboo forest (BEBF), totaling nine plots. The sample plots were spaced more than 10 m apart, and were all located on a northwest-facing clay loam slope with an incline of 16 to 20 degrees. Before the establishment of the nature reserve, the area experienced human-induced disturbances, leading to the formation of secondary broadleaf forests with similar stand densities across the plot. Following the creation of the nature reserve in 1997, the broadleaf forests began a process of natural recovery. During this recovery, the broadleaf forests neighboring the bamboo forests were gradually encroached upon by the expansion of bamboo, leading to the formation of bamboo–broadleaf mixed forests and bamboo stands.

The broadleaf area primarily consisted of Alniphyllum fortunei, Machilus pauhoi, Castanopsis eyrei, Castanopsis fargesii, and Daphniphyllum oldhamii in both the BEBF and BABF of the study area. Three tree species—A. fortunei, M. pauhoi, and C. eyrei—were selected as the focus of our research due to their presence in both forest types. Additionally, we also focused our study on bamboo in the BEBF and BF.

2.3. Sample Collection and Data Calculation

2.3.1. Water Use Sources

We collected isotope samples from the branches of broadleaf trees and bamboo in the BABF, BEBF, and BF during the growing season (mid-August) and the non-growing season (mid-November) of 2021, with a total of 48 samples. Simultaneously, we also collected soil isotope samples from the 0–10 cm, 10–20 cm, 20–40 cm, 40–60 cm, and 60–80 cm soil layers using a soil auger. Each soil sample was a composite of samples taken from the east, west, south, and north directions of the sampling plants, totaling 240 soil samples. Groundwater samples were collected immediately from wells with three replicates when collecting plant and soil samples. Rainwater samples were collected using homemade rainwater collectors (consisting of a plastic funnel with a diameter of 20 cm at the top and an iron bucket with a diameter of about 15 cm at the bottom) during precipitation events from August to November 2021.

All isotope samples were promptly placed in 50 mL centrifuge tubes, sealed with Parafilm, and then taken back to the laboratory, where they were stored frozen at −20 °C for measuring the natural molar abundance ratios of water isotopes. First, we used a cryogenic vacuum distillation system (Li-2000) to extract water from the plant xylem and soil isotope samples. Next, we used a 5 mL syringe and a 0.22 μm filter to filter the xylem water, precipitation, soil water, and groundwater into 1.5 mL sample vials. Finally, we measured the hydrogen and oxygen isotope ratios of these samples using an elemental analyzer–isotope ratio mass spectrometer (Thermo Fisher Scientific IR-MS, Langenselbold, Germany) with testing accuracies of 0.5‰ and 0.1‰, respectively. Formula (1), used for calculating the isotopic ratios of hydrogen and oxygen in the samples, is as follows:

$$\delta X‰={\displaystyle \frac{R_{sample}-R_{standard}}{R_{standard}}}\times 1000$$

(1)

where δX represents δ²H or δ¹⁸O, and R_sample and R_standard represent the molar abundance ratios of the heavy to light isotopes (²H/¹H or ¹⁸O/¹⁶O) in the sample and the standard, respectively (V-SMOW, Standard Mean Ocean Water).

We assessed the relative contribution of different water sources to xylem water using the IsoSource mixing model [32]. We inputted the isotope values of xylem water, precipitation, soil water (soil profile was subdivided into five potential water sources) and groundwater into the IsoSource model, and then set two parameters: (1) source increment, established at 1%, which assigned the proportion of plant utilization among each water source in 1% increments to test possible proportion combinations; and (2) mass balance tolerance, set at 0.01, meaning that the difference between the weighted sum of the isotope values from each water source and the isotope value of the xylem water should not exceed 0.01.

We divided the soil profile into five potential soil water sources based on variations in root distribution, soil water content (SWC), and the isotope composition of soil water across different soil layers. These sources were 0–10 cm (surface soil water), 10–20 cm (shallow soil water), 20–40 cm (middle soil water), 40–60 cm (deep soil water), and 60–80 cm (deeper soil water).

Additionally, when collecting soil for isotope analysis, a portion of each soil sample was placed in aluminum boxes and taken back to the laboratory. The fresh weight of each sample was measured immediately upon arrival, and the samples were then placed in a drying oven at 105 °C until reaching a constant weight. The dry weight was recorded, and the soil water content (SWC) was calculated.

2.3.2. Water Use Efficiency

We collected leaf samples from broadleaf trees and bamboo during the growing season (mid-August) and the non-growing season (mid-November) of 2021 to analyze the WUE. First, the leaves were placed in a drying oven at 105 °C for 30 min to kill the greens, and then dried at 65 °C to a constant weight. The dried leaves were then ground into powder using a ball mill. Finally, the δ¹³C values of the leaves were measured using an elemental analyzer–isotope ratio mass spectrometer (Thermo Fisher Scientific IR-MS), with an accuracy of 0.2‰. Formula (2), used for calculating the isotopic ratios of carbon in the samples, is as follows:

$$\delta {}^{13}C‰={\displaystyle \frac{R_{sample}-R_{standard}}{R_{standard}}}\times 1000$$

(2)

where R_sample and R_standard represent the molar abundance ratios of the heavy to light isotopes (¹³C/¹²C) in the sample and the standard, respectively.

The plants’ WUE was analyzed using foliar δ¹³C_p data [13,33]. First, we calculated changes in C isotopic discrimination (∆δ¹³C) from foliar δ¹³C_p data and air δ¹³C_a using Formula (3). The δ¹³C_a was determined based on Formula (7), where t represents the sampling year (2021 in this study) [34]. Next, the ∆δ¹³C value was used to calculate the ratio of internal leaf CO₂ concentration (C_i) to ambient air CO₂ concentration (C_a) through Formula (4), where a represents the diffusion through the stomata (4.4‰), and b represents the carboxylation fraction by ribulose-1,5-bisphosphate carboxylase/oxygenase (27‰) [13]. The value for C_a was obtained from Formula (6) [34]. Finally, we calculated the plant WUE using the C_i/C_a ratio and applied Formula (5), where 1.6 is the ratio of stomatal conductance to water vapor and CO₂.

$$∆{}^{13}C=\left({\displaystyle \frac{\delta {}^{13}C_a-\delta {}^{13}C_p}{1+\delta {}^{13}C_p/1000}}\right)$$

(3)

$$∆{}^{13}C=a+{\displaystyle \frac{(b-a)C_i}{C_a}}$$

(4)

$$WUE={\displaystyle \frac{\left(C_a-C_i\right)}{1.6}}$$

(5)

$$C_a=277.78+1.350e^{\left[0.01572\left(t-1740\right)\right]}$$

(6)

$$\delta {}^{13}C_a=-6.429-0.006e^{\left[0.0217\left(t-1740\right)\right]}$$

(7)

2.4. Statistical Analysis

We used an independent-sample t-test to analyze the differences in the uptake fractions of various potential water sources and WUE of broadleaf trees in BABF and BEBF, and bamboo in BF and BEBF. This analysis aimed to investigate the impact of bamboo invasion into broadleaf forests on the water use strategies of broadleaf trees as well as the adjustments in bamboo’s own water use strategies. All data were statistically analyzed using Excel 2019 and SPSS 22.0, with graphs generated using Origin 2022 software.

3. Results

3.1. Soil Moisture Characteristics

The expansion of bamboo increased the soil water content of broadleaf forests (Figure 3). From BABF to BEBF, the greatest increases in soil water content occurred in the 0–10 cm and 10–20 cm shallow soil layers, with increases of 26.5% (p < 0.01) and 22.3% (p < 0.01) during the growing season, and 24.5% (p < 0.01) and 21.8% (p < 0.01) during the non-growing season. From BEBF to BF, there was no significant change in water content in the 0–10 cm soil layer during either season. However, increases were noted in the 10–20 cm, 20–40 cm, 40–60 cm, and 60–80 cm soil layers, with changes of 5.0% (p > 0.05), 15.6% (p < 0.01), 11.9% (p < 0.01), and 5.9% (p > 0.05) during the growing season, and 7.3% (p < 0.05), 14.2% (p < 0.01), 18.9% (p < 0.01), and 9.8% (p < 0.05) during the non-growing season.

3.2. Isotopic Signatures of the Potential Water Sources

A local meteoric water line (LMWL) was fitted for δ²H and δ¹⁸O values of precipitation, and slope of the fitted line were lower than that of the global meteoric water line (GMWL), indicating stronger evaporation during the local precipitation process (Figure 4). The soil water isotope values in BABF, BEBF, and BF are mostly located to the right of the local meteoric water line during both the growing and non-growing seasons, suggesting that the soil water in these areas mainly originates from precipitation and has undergone evaporation-related enrichment.

The expansion of bamboo reduces the intensity of soil evaporation in broadleaf forests during both the growing and non-growing seasons (Figure 4). The slopes of the soil water lines in BEBF (SWL_BE) and BF (SWL_BF) are higher than those in BABF (SWL_BA), with slopes of 6.96 and 5.71 compared to 4.91 during the growing season, and 5.19 and 4.96 compared to 4.57 during the non-growing season. Additionally, bamboo expansion affects δ¹⁸O, but its impact varies by season (Figure 4). During the growing season, bamboo expansion leads to a depletion of δ¹⁸O in the soil, with isotope values for δ¹⁸O in BEBF and BF being 1.1‰ (p < 0.01) and 0.6‰ (p > 0.05) lower than in BABF, respectively. In the non-growing season, bamboo expansion results in an enrichment of δ¹⁸O in the soil of broadleaf forests, with the δ¹⁸O values in BEBF and BF being 1.1‰ (p > 0.05) and 1.8‰ (p < 0.01) higher than in BABF, respectively.

3.3. Quantification of Water Use Sources for Bamboo and Broadleaf Trees

The expansion of bamboo increased its uptake fractions of surface and shallow soil water (S1 and S2) during the growing season (Figure 5a). In BF, the proportion of soil water from S1 and S2 was 8.0% and 16.9%, respectively, whereas in BEBF, it was 28.0% and 26.3% during the growing season. However, the magnitude of this change was relatively small during the non-growing season (Figure 5e).

Additionally, the effect of bamboo expansion on the water source of broadleaf trees varies depending on the species and season. For M. pauhoi and C. eyrei, bamboo expansion reduced their uptake fractions of surface soil water (S1) while increasing their uptake fractions of middle soil water (S3) during the growing season (Figure 5c,d). Conversely, in the non-growing season, bamboo expansion increased their uptake fractions of surface soil water (S1) and reduced their uptake fractions deep soil water (S4 and S5; Figure 5g,h). For A. fortune, the changes in the uptake fractions of various potential water sources were relatively small. During the growing season, A. fortune had the highest uptake fractions of surface soil water in BABF and BEBF (31.5% and 29.5%, respectively; Figure 5b), while in the non-growing season, it mainly utilized precipitation in BABF and BEBF (75.1% and 70.8%, respectively; Figure 5f).

3.4. Water Use Efficiency for Bamboo and Broadleaf Trees

After bamboo encroachment into the broadleaf forest, the water use efficiency (WUE) significantly increased (Figure 6). The WUE of bamboo rose from 40.2 μmol/mol and 46.7 μmol/mol in BF to 60.2 μmol/mol and 59.6 μmol/mol in BEBF during the growing and non-growing seasons, respectively, with increases of 48.8% and 27.7% (all p < 0.01). However, bamboo expansion had no significant impact on the WUE of the broadleaf trees in either season (Figure 6). In addition, there were significant differences in WUE among species. C. eyrei had significantly higher WUE than A. fortunei and M. pauhoi in BABF and BEBF (since A. fortunei is a deciduous species, its WUE during the non-growing season was not analyzed due to the absence of leaves during that period).

4. Discussion

4.1. Bamboo Expansion Increased Its Uptake Fractions of Surface and Shallow Soil Water and Improved Its WUE

Our research showed that after bamboo expansion into broadleaf forests, its uptake fractions of surface and shallow soil water (0–20 cm) increase during the growing season (Figure 5a). Variations in uptake fractions of potential water sources by plants are typically associated with root vertical distribution and soil water availability [10,21,35,36]. During bamboo expansion into broadleaf forests, its fine roots tend to distribute in the upper soils [18], which benefits bamboo by enhancing its ability to take up water from surface and shallow soil. This strategy of increasing active root systems in surface and shallow soil enhances bamboo’s competitiveness for resources like water, as absorbing water from surface and shallow soil layers reduces energy consumption and aids in nutrient uptake [36,37,38].

Unlike the growing season, in the non-growing season, the bamboo in both the bamboo forest (BF) and bamboo-expended broadleaf forest (BEBF) showed similar water use patterns, with the highest proportion of water uptake coming from surface and shallow soil layers (0–20 cm; Figure 5e). This could be related to the availability of soil water. Indeed, the water content in the middle (20–40 cm) and deep (40–80 cm) soil layers was significantly lower than in the surface and shallow layer in both the BF and BEBF (Figure 3b). Moreover, during the non-growing season, bamboo’s photosynthesis and transpiration were lower than during the growing season, resulting in a reduced demand for water. Thus, during this period, the surface and shallow soil water available for plants to use is relatively sufficient. Therefore, bamboo tends to used surface and shallow soil water, because the energy consumption for accessing these layers is relatively small compared to middle and deep soil layers [38]. Additionally, the proportion of clay particles in middle and deep soil is often higher than that in surface and shallow soil, which can reduce the water uptake capacity of bamboo roots [39].

WUE reflects the adjustment in water use strategy of bamboo when it expands into other forest types from another perspective. We found that after bamboo expansion into broadleaf forests, its WUE increases during both the growing and non-growing seasons (Figure 6), which may be related to changes in soil water content. We observed that the soil water content was significantly higher in the BF compared to BEBF (Figure 3). In other words, the available soil water decreases after bamboo expands into broadleaf forests. This can induce a decrease in the stomatal conductance of bamboo, thereby leading to an increase in WUE [22,25,40]. This shows that bamboo is highly sensitive to changes in water availability and can quickly increase its WUE to adapt to water-scarce environments. Relevant studies have shown that higher resource capture capability and utilization efficiency are potential mechanisms for plants to successfully invade other forest types [41,42,43]. Therefore, we consider that during the process of bamboo expansion into broadleaf forests, the gradual shift of water uptake depth from deeper to shallow and surface layers during the growing season and the improvement in WUE might be the water use mechanisms for its successful expansion into broadleaf forests.

4.2. Impacts of Bamboo Expansion on the Water Source of Broadleaf Trees Vary by Species

The research findings indicate that the expansion of bamboo reduced the utilization rate of 0–10 cm surface soil water and increased the utilization rate of 20–40 cm middle-layer soil water by evergreen broadleaf trees (M. pauhoi and C. eyrei; Figure 5c,d) during the growing season. We speculate that this phenomenon is linked to the competitive advantage in water resources and root distribution of bamboo. Being a clonal plant, bamboo shares resources such as nutrients and water among ramets through rhizomes [8,17,44]. Additionally, its extensive underground root system, with fine root biomass roughly six times that of broadleaf trees, forms a dense root network in the soil [18]. These biological traits give bamboo a competitive advantage in acquiring water resources compared to trees. Moreover, after expanding into broadleaf forests, most of bamboo’s roots are primarily distributed in the upper soil layer [18], thus increasing the competitive pressure on broadleaf trees for surface soil water. Therefore, to avoid direct competition with bamboo, evergreen broadleaf trees quickly adjust their water uptake depth, shifting water sources from surface to middle-layer soil, indicating a great ecological plasticity.

However, the water use pattern of A. fortunei is less sensitive to bamboo expansion. It exhibits the highest proportion of water absorption from the 0–10 cm surface soil layer in both BABF and BEBF (Figure 5b). This is likely related to the nutrient availability in the surface soil and the biological characteristics of the species. Numerous studies have shown that plants tend to use surface and shallow soil water [36,45], and some tree species even rely on surface and shallow soil water when it is limited [46] due to higher nutrient availability in this soil. A. fortunei is a fast-growing, shade-intolerant, deciduous, broadleaf tree that requires more nutrients (such as nitrogen and phosphorus) to sustain photosynthesis compared to shade-tolerant, evergreen, broadleaf trees [47]. Based on this characteristic, A. fortunei likely stimulates the distribution of more roots in the surface soil to absorb nutrients and water from that layer. Additionally, the expansion of bamboo increases the water competition pressure on broadleaf trees while also enhancing soil moisture content, with a more significant increase in surface soil moisture (Figure 3a). This increase in soil water can partially compensate for the impact of bamboo competition on the availability of surface soil water for broadleaf trees. Therefore, after bamboo expansion, A. fortunei, which has a high nutrient demand, does not reduce its uptake rate of surface soil water.

Interestingly, during the non-growing season, the expansion of bamboo led to an increase in the uptake rate of 0–10 cm surface soil water and a decrease in the uptake rate of 40–80 cm deeper soil water by evergreen broadleaf trees (M. pauhoi and C. eyrei; Figure 5g,h). This water use pattern is largely attributed to changes in soil water availability following the expansion of bamboo. Our study revealed that the soil water content in BEBF was significantly higher than that in BABF (Figure 3b). The well-developed root system of bamboo may help reduce soil erosion in forests [48], thereby promoting water infiltration into the soil and increasing soil water content. Furthermore, during the non-growing season, bamboo experiences reduced photosynthesis and transpiration [49,50], decreasing its water consumption and thereby lowering the competition pressure on the broadleaf trees. When surface soil water is sufficient and available, plants are more inclined to use water from the surface soil because it contains higher nutrient levels and requires less energy to extract [38,45]. Consequently, during the non-growing season, the water uptake depth of evergreen broadleaf trees gradually shifts from deeper to surface layers. In contrast, deciduous broadleaf trees (such as A. fortunei) take up almost no water from the soil during the non-growing season (Figure 5b,f). This is because they mainly rely on transpiration pull to absorb water through their roots, and deciduous species have weaker transpiration during the non-growing season when they lack leaves. Thus, the uptake rate of soil water by deciduous broadleaf trees is very low during this period.

4.3. Bamboo Expansion Has No Significant Effect on the WUE of Broadleaf Trees

WUE is the ratio of photosynthesis rate to transpiration rate [12,13], which is affected by environmental factors such as light, temperature, and soil moisture [14,15,16]. For example, Aranda et al. (2007) found that the WUE of trees increases with higher light intensity, but decreases with greater soil water availability [51]. Our findings demonstrate that the expansion of bamboo had no significant effect on the WUE of the broadleaf trees (Figure 6). This is likely due to a combination of factors resulting from bamboo expansion. Firstly, light penetration may increase as some trees die during bamboo expansion [52], which is beneficial for surviving trees in improving WUE. Conversely, soil water content increased after the expansion of bamboo (Figure 3), which is not conducive to improving the WUE of surviving trees. Additionally, after the expansion of bamboo, soil temperature increases, causing a delay in the phenological activity of the cambium in broadleaf trees and extending their growth period [53]. This implies that the trees must sustain physiological activities and growth over a longer time, thereby increasing their water demand. Since photosynthetic efficiency declines in autumn, the trees may be unable to compensate for water consumption through increased carbon assimilation, leading to a reduction in WUE. Therefore, we observed no significant change in the WUE of broadleaf trees after the expansion of bamboo.

Additionally, we found that the WUE of the late-successional species, C. eyrei, was significantly higher than that of the early-successional species, A. fortunei, and the mid-successional species, M. pauhoi (Figure 6), consistent with the findings of Wang [54]. The differences in WUE among species may be related to specific leaf area (SLA). Our results indicate that the SLA of C. eyrei was significantly lower than that of M. pauhoi and A. fortunei (Table S1). A smaller SLA means that plants allocate more dry matter to constructing protective structures and mesophyll cells (palisade and spongy tissues) [55,56], which increases the surface area of mesophyll cells and enhances CO₂ assimilation [57]. Simultaneously, it increases the resistance in water transport paths, reducing the transpiration rate. Therefore, with a smaller SLA, plants lose less water when fixing the same amount of carbon, thus increasing WUE. This suggests that species with higher WUE may have a competitive advantage in community succession, making them more likely to become late-successional species.

This study highlights the adaptive strategies of broadleaf trees to bamboo expansion and provides relevant information on the impact of bamboo expansion on the water use patterns of broadleaf trees. The results show that broadleaf trees exhibit some adaptability to the bamboo expansion, suggesting that they can likely coexist stably with bamboo for a period of time. This finding is consistent with our previous study on the radial growth patterns of broadleaf trees [53]. However, the coexistence of broadleaf trees and bamboo may change as bamboo continues to expand. When bamboo reaches a certain threshold in number, the competition for shallow soil water between bamboo and broadleaf trees may intensify, potentially preventing broadleaf trees from absorbing enough water from the shallow soil to support their growth and survival. Broadleaf trees with water use patterns that are insensitive to bamboo expansion may be eliminated due to water limitations. The dynamics of this shift between broadleaf trees and bamboo, along with its potential mechanisms, need further validation through research.

5. Conclusions

Our research indicates that as bamboo expands into broadleaf forests, it gradually shifts its water uptake depth to shallower layers and increases its WUE, thereby enhancing its competitive advantage for water resources. The impact of bamboo expansion on the water use patterns of different types of broadleaf trees varies. Evergreen broadleaf trees exhibit flexible water use patterns: during the growing season, they shift their water uptake depth from the surface layer to the middle layer to reduce competition with bamboo for surface soil water; in the non-growing season, they adjust their uptake from deep to surface layers due to the increased soil water availability following bamboo expansion. In contrast, deciduous broadleaf trees adopt a more conservative water use strategy, maintaining the highest uptake fractions of surface soil water during the growing season and primarily relying on precipitation during the non-growing season, both before and after the expansion of bamboo. This indicates that evergreen broadleaf trees have a stronger adaptive capacity compared to deciduous broadleaf trees under the environmental conditions created by bamboo expansion.