Study on the Water Mechanism of Sparse Grassland Decline of Ulmus pumila L.

Abstract

Ulmus pumila L. occupies an important niche in arid ecosystems. This study aimed to investigate the sap flow characteristics of declining Ulmus pumila L. in arid regions and its relationship with environmental factors. During the 2023 growing season (June to October), continuous sap flow monitoring was conducted using thermal dissipation probes (TDPs) on Ulmus pumila L., along with measurements of soil moisture, air temperature, relative humidity, solar radiation, wind speed, and vapor pressure deficit (VPD). The results showed that when the sap flow rate of elm individuals reached 0.92 mL/cm²/h, the trees entered an extremely severe decline stage. Sap flow rates were significantly positively correlated with net solar radiation, relative humidity, VPD, and soil moisture, but negatively correlated with wind speed and real-time rainfall. VPD was identified as the key factor influencing sap flow across different decline stages, while solar radiation was critical in assessing the severity of decline. A weakened correlation between sap flow and solar radiation marked the onset of severe decline. Additionally, soil moisture exhibited a significant positive effect on sap flow rates overall. These findings not only advance our theoretical understanding of plant ecology in arid areas but also offer practical insights for managing Ulmus pumila L. decline, thus contributing to more sustainable resource management and environmental protection strategies.

1. Introduction

With global climate change and increasing human activities, the imbalance between water supply and demand for trees has intensified, particularly in vulnerable savanna ecosystems where water stress is acute [1,2,3]. Research indicates that drought stress heightens tree sensitivity to water needs [4,5,6]. Both drought and high-temperature stress further exacerbate imbalances in water transport processes, notably affecting taller, older trees and reducing the average age and size within forest communities [7,8]. This cascading stress leads to a decline in forest health and productivity, impairing carbon sequestration and accelerating forest transitions toward degradation [9,10]. Such degradation not only increases tree mortality rates, contributing to shifts in species composition and ecosystem functions but also weakens forests’ resilience against future environmental changes [11,12].

Tree decline is a complex biological phenomenon, extending beyond mere water transport issues. Recent studies indicate that water imbalance and carbon starvation are primary causes of this decline [9,13]. For instance, under drought and heat stress, reduced water transport efficiency has led to widespread tree mortality [14,15]. However, defining and identifying the stages of tree decline remains challenging. Accurate identification of tree decline stages is crucial for effective forest management. This limitation underscores the importance of accurate stage identification for effective forest management and conservation, highlighting the need to deepen our understanding of tree decline mechanisms—particularly changes in water transport dynamics over time. By bridging this gap, researchers can better inform efforts to sustain forest health and ecosystem resilience in the face of climate change. Existing research shows that decline is a progressive, irreversible process [16]. When trees enter this phase, their water transport mechanisms undergo substantial changes. Declining trees can initially sustain relatively high sap flow rates under drought, but this rate sharply decreases as drought intensifies [17]. As such, a drop in sap flow rate is often considered a key indicator of a tree entering the decline phase. Stem sap flow, a crucial pathway for water transport, not only reflects the water uptake from soil via roots but also affects canopy transpiration rates [18]. This sap flow impacts not only individual tree growth but also significantly influences the ecosystem’s water cycle and biodiversity [19,20]. Consequently, the sap flow rate is influenced by various environmental factors, including vapor pressure deficit and solar radiation. Compared to the traditional thermal pulse method, the thermal diffusion probe (TDP) method has become widely adopted due to its continuous data output and higher measurement accuracy [21]. Although stem sap flow measurement is an effective approach to assessing the water-use status of individual trees, research is still largely focused on sap flow–environmental factor relationships rather than the mechanisms of water transport dynamics in trees under progressive decline [22,23,24]. This gap in understanding is particularly relevant as semi-arid and arid regions face rising challenges related to water scarcity, where trees’ water-use strategies are essential to maintaining ecosystem function and biodiversity.

Ulmus pumila L. is widely distributed across semi-arid and arid regions globally, thriving in the arid grasslands and desert margins of Central Asia and North America due to its robust adaptability. It is also a dominant species in the sparse grasslands of northern China [25], where it plays a crucial role in maintaining ecosystem stability in these fragile arid and semi-arid environments [26]. Despite its adaptability, *U. pumila* can still decline under extreme or prolonged drought conditions. Particularly in regions with annual precipitation below 400 mm, the survival of these trees is significantly challenged, with large areas of Ulmus pumila L. forests entering stages of decline [14,15].

This study aims to investigate the water transport characteristics of U. pumila across various stages of decline, specifically examining its responses within the soil–plant–atmosphere continuum under drought stress. By exploring how water transport dynamics shift as trees progress through stages of decline, we seek to enhance the understanding of U. pumila’s water-use strategies and address a critical gap in the field. This research contributes novel insights essential for developing informed water-management practices that support ecosystem resilience in drought-prone forest ecosystems.

2. Materials and Methods

2.1. Study Area

The research area was situated in dry grassland and desert regions in the north-central region of Yanchi County, Wuzhong City, Ningxia, China (Figure 1). The region experienced an average annual temperature of 7.1 °C, with maximum temperatures reaching 37.0 °C and minimum temperatures dropping to −29.5 °C. Annual sunshine duration averaged 2852.9 h, and the frost-free period spanned approximately 128 days. Average annual precipitation was 285 mm, with over 80% of rainfall concentrated between July and September. The region faced significant aridity, as annual evaporation was 9.6 times the annual rainfall. The average wind speed was 2.7 m/s, and there were 45.8 days of strong winds each year, primarily between November and April, with maximum wind speeds reaching 15–18 m/s. The area experienced 20.6 sandstorm days annually, peaking in spring. Soils in the study area were mainly recent alluvium, with analyses from 24 profile points indicating surface soil pH values between 8.0 and 8.5, an average total salt content of 0.1%, organic matter content of 0.57%, hydrolyzed nitrogen at 28.57 ppm, and available phosphorus at 8.45 ppm. Since the afforestation in 1975, Ulmus pumila L., Caragana korshinskii, and Populus L. have thrived in the experimental area, with understory vegetation including Pennisetum flaccidum, Agropyron cristatum (L.), Glycyrrhiza uralensis Fisch, and Stipa breviflora maintained at a coverage of 10% to 15%. However, the research area’s Ulmus pumila L. mortality rate has increased since the severe drought year of 2021, which is ascribed to water imbalance.

2.2. The Setting and Investigation of Sample Plots

The purpose of this study was to assess the decline of artificial Ulmus pumila L. in the Haba Lake Nature Reserve. In the urban south forest farm of the protected area, we conducted an extensive preliminary survey to identify Ulmus individuals across different decay stages within an artificial forest, approximately 35 years in age. Based on forest structure and to minimize environmental heterogeneity, we selected 22 Ulmus trees with consistent age and diameter class for intensive, long-term monitoring. Although the sample size is modest, relative to the population, this choice was necessary to maintain the precision of measurements and the feasibility of long-term monitoring within resource constraints: Level I (dieback rate < 25%), Level II (25% ≤ dieback rate < 50%), Level III (50% ≤ dieback rate < 75%), and Level IV (75% ≤ dieback rate < 100%) (Figure 2). Each level included at least four trees to ensure representativeness of the sample, and we confirmed that each level contained trees representative of the corresponding decay degree. This sampling strategy reflects a balance between capturing variability and managing the practical limitations of intensive monitoring, funding, and labor.

2.3. Experimental Design

The experiment was conducted from June to October 2023, prior to the onset of the local rainy season, with the aim of assessing the decay status of Ulmus trees in the artificial forest of the Haba Lake Nature Reserve. Through field surveys, we selected a representative plot of approximately 400 square meters within the protected area, avoiding environmental factors, soil texture, and slope aspects that could influence the experimental results. In this plot, elm individuals exhibiting varying decay grades (mild, moderate, severe, and extremely severe) were randomly interspersed throughout the study area, rather than organized into separate clusters by decay grade. Notably, in stands with different levels of overall degradation, the proportion of elms within each decay grade varied significantly. Despite these variations, the study ensured that trees across all decay stages were well distributed across the plot, minimizing any potential biases from localized degradation patterns. We utilized the TY-TDP pin-type plant stem flow measurement system to assess all experimental trees, placing the probes at the same height on the same side of the trees (Figure 3). To evaluate soil moisture dynamics, we employed a simple random sampling (SRS) method within the plot containing the measured trees. Three sampling points were selected to ensure representative measurements of soil moisture changes before and after rainfall. Concurrently, we monitored meteorological data in the experimental area in real-time (hourly frequency) using data from the on-site meteorological station and the Global Energy Resource Prediction website (https://power.larc.nasa.gov/data-access-viewer/, accessed on 14 July 2024).

2.4. Determination of Soil Moisture Content

This study employed a random sampling method to grid the plot and collect soil samples at three randomly selected orientations. Soil samples were measured daily before and after rainfall. Soil samples were taken at intervals of 20 cm in the soil layer from 0 to 2 m using a soil auger. Each sample was placed in an aluminum box, and weighed, and this process was repeated three times for each depth. The samples were then dried in a constant temperature oven at 110 °C until they reached a constant weight to determine the absolute soil moisture content for each layer. The average values and standard deviations of the three measurements were calculated using the STDEV function in Excel software and plotted with error bars representing positive and negative deviations.

The soil-water storage is calculated based on the measured soil moisture content using the following formula:

$$w_c={\displaystyle \frac{w_0-w_1}{w_1}}\times 100\%$$

(1)

$$v_{w^{\prime }}=w_1\times v^{-1}$$

(2)

$$w_s=w_c\times v_{w^{\prime }}\times H$$

(3)

In the formula, $w_c$ represents the soil moisture content (%), $w_0$ is the weight of wet soil (g), and $w_1$ is the weight of soil after drying (g). $v_{w^{\prime }}$ is the soil bulk density (g·cm⁻³), and $v$ is the volume of the cutting ring (cm³). The soil-water storage ($w_s$) is calculated in millimeters (mm), with $H$ representing the soil depth (m).

2.5. Stem Sap Flow Measurement

To minimize the influence of morphological index variation, we selected Ulmus pumila L. with consistent diameter at breast height (DBH), height, and canopy coverage among the declining individuals. The experimental group comprised Ulmus pumila L. with similar DBH to reduce the impact of this variable on the data. We employed the TY-TDP needle-type plant sap flow measurement system, ensuring that all probes were positioned on the same side and at equal heights of the test trees. For specific groups with comparable levels of decline, additional probes were placed at the base diameter of the Ulmus pumila L. To mitigate interference from sunlight and other environmental factors, the probes were wrapped in aluminum foil and plastic. Following the manufacturer’s instructions, we installed the equipment, and data collection was conducted using the CR1000X system, with a collection interval of 10 min, followed by regular data downloads. The branch sap flow (g·cm⁻²·h⁻¹) is calculated using the Granier formula:

$$\mathrm{K}={\displaystyle \frac{{\mathrm{DT}}_{\max }-\mathrm{DT}}{\mathrm{DT}}}$$

(4)

$${\mathrm{J}}_{\mathrm{s}}=0.714\times {\mathrm{K}}^{1.231}\times 60$$

(5)

$$\mathrm{F}={\mathrm{J}}_s\times A_s$$

(6)

$$E=F\times \mathsf{{\rm T}}$$

(7)

In the formula, $\mathrm{K}$ is a dimensionless variable, where $\mathrm{DT}$ represents the temperature difference between the two probes when the sap flow in the trunk’s xylem is zero, and ${\mathrm{DT}}_{\max }$ is the instantaneous temperature difference between the probes. The branch sap flow rate ${\mathrm{J}}_{\mathrm{s}}$ is expressed in kgH₂O·h⁻¹, and $A_s$ is the area of the sapwood (cm²). The total sap flow per plant $\mathrm{F}$ (kgH₂O·plant⁻¹) can be calculated based on the sap flow rate and the time interval, where the data storage interval is “10 min”. The plant’s transpiration water consumption is represented by $E$ (kgH₂O·plant⁻¹). $\mathsf{{\rm T}}$ is derived from the daytime branch sap flow duration (h), and the total transpiration is estimated by considering the number of branches and their growth conditions to calculate the transpiration water consumption of the Ulmus pumila L. trees.

2.6. Meteorological Data Acquisitio

The study site is located within the Haba Lake Chengnan Management Station, which is equipped with a small meteorological station recording hourly environmental factors including precipitation (P), temperature (T), relative humidity (RH), wind speed (WS), and vapor pressure deficit (VPD). Data collection is synchronized with the xylem flow measurement system. Additionally, photosynthetically active radiation (PAR) data, which refers to the solar spectrum between 400 and 700 nanometers and is essential for plant photosynthesis, were obtained from the Global Energy Resource Prediction website (https://power.larc.nasa.gov/data-access-viewer/ (accessed on 14 July 2024)).

2.7. Data Processing

Data obtained from field surveys and indoor experiments were initially organized and processed in Excel 2010 software. Utilizing the built-in statistical functions and filtering tools of Excel, we categorized the data and performed simple descriptive statistical analyses. Subsequently, single-factor Analysis of Variance (ANOVA) was conducted using SPSS 26.0 software. Cluster analysis was performed through the Pinecone Cloud-based web analysis platform, while correlation and difference tests were analyzed and plotted using Origin 2020 software. Additionally, structural equation modeling (SEM) was conducted using R-4.2.2 language to fit the relationships between various physiological and meteorological indicators with stem flow. Finally, the analyzed results were compiled, and flowcharts and model diagrams were drawn using CorelDRAW X8 (64-Bit) and PowerPoint software.

3. Results

3.1. Soil Moisture Characteristics of Degraded Ulmus pumila L. Forest Land

3.1.1. Monthly Scale Water Characteristics of Degraded Ulmus pumila L. Forest Land

As depicted in Figure 4a, we stratified the soil moisture in the forest land to investigate how this critical factor for plant growth is influenced by seasonal variations. This study compares soil moisture in different months within the degraded artificial forest land of the Haba Lake Nature Reserve. The soil layers are divided into three main strata: the micro-utilization layer (0–30 cm), which is directly affected by rainfall; the utilization layer (30–160 cm), serving as the water-retaining and water-infiltrating layer effectively utilized by plants; and the regulation layer (below 160 cm), which possesses some water regulation functions.

As shown in Figure 4b, we observed that the moisture in the utilization layer gradually moved upward with the change of months, indicating an upward shift of the higher moisture layer. Concurrently, the total soil-water storage exhibited a slow increasing trend. Combining the results from the structural equation modeling in Figure 4a, we can conclude that the variation in soil-water storage is primarily influenced by the changes in the moisture of the regulation layer. The high moisture points in the regulation layer also rose, and the moisture changes in the utilization layer were consistent with those in the regulation layer, although all measured moisture contents were below 6%. Furthermore, the soil-water storage was highest in October and lowest in August.

3.1.2. Change Rule of Soil Moisture After Rainfall in Degraded Forest Land

Soil moisture responds sensitively to rainfall events. As illustrated in Figure 5, the water content in the micro-utilization layer (0–30 cm depth) increased rapidly within 1–21 h post-rainfall, while the adjustment layer (below 160 cm) showed a delayed increase around 28.5 h and then returned to initial moisture levels. Between 48 and 54 h, the water content peaked at 100 cm and 200 cm depths, indicating substantial infiltration. After 67 h, the moisture levels across the 2 m depth began to decrease.

Overall, the water content in each soil layer stabilized progressively over time following rainfall, with fluctuations occurring predominantly during the daytime and stabilizing overnight. The soil texture may have influenced the moisture retention in the utilized layer, as the 30–60 cm depth remained relatively constant. Notably, the 60–100 cm layer and the utilized layer exhibited higher moisture levels at night post-rainfall (see Figure 5).

3.2. Variation Characteristics of Sap Flow Velocity of Decaying Ulmus pumila L.

3.2.1. Variation Characteristics of Daytime Sap Flow Velocity of Decaying Ulmus pumila L.

We examined the diurnal variation in sap flow of Ulmus pumila L. at four distinct decay stages during the growing season. The results show that sap flow velocity on rainy days was significantly lower than on the days preceding rainfall, with a particularly pronounced difference when rainfall exceeded 5 mm. This change was most evident for trees in the mild and extremely severe decay stages, which exhibited a weaker response to rainfall compared to other stages (Figure 6).

The average sap flow rates in the mild, moderate, severe, and extremely severe decay stages were 3.14, 4.48, 5.86, and 1.12 mL/cm²/h, respectively. Except in the extremely severe decay stage, sap flow velocity variability increased with decay severity, with differences across stages ranging between 1.3 and 1.7 mL/cm²/h. In the extremely severe stage, sap flow rate was lowest, with limited variability, averaging only one-third of the rate observed in mildly decayed trees.

As shown in Figure 7, five days post-rainfall, the daily sap flow rate gradually declined until reaching a stable baseline. This trend was well described by a natural logarithmic power function fit (p < 0.05).

3.2.2. Variation Characteristics of Nighttime Sap Flow Velocity of Decaying Ulmus pumila L. Trees

During the growing season, the sap flow velocity of Ulmus pumila L. at four different decay stages increased following rainfall, mirroring the diurnal pattern of stem flow velocity. However, the specific responses of mildly and severely decayed trees to rainfall remain ambiguous (Figure 8).

The average nighttime sap flow velocities for mildly, moderately, severely, and extremely severely decayed Ulmus pumila L. were 1.09, 0.95, 0.72, and 0.62 mL/cm²/h, respectively, indicating a decreasing trend with increased decay. The average difference between stages ranged from 0.10 to 0.13 mL/cm²/h. In the extremely severe decay stage, sap flow velocity was the lowest, with minimal fluctuation, approximately half of that in mildly decayed trees (Figure 8).

As shown in Figure 9, nighttime sap flow returned to average levels after rainfall, following a linear regression pattern (p < 0.05). Among decay stages, sap flow velocity in Ulmus pumila L. at the extremely severe decay stage was least responsive to rainfall. For other stages, the influence of rainfall on nighttime sap flow velocity generally increased with decay severity. Overall, trees at higher decay stages exhibited a more gradual decline in sap flow velocity.

3.2.3. Diurnal Variation Characteristics of Sap Flow Rate of Decaying Ulmus pumila L. Trees

During the daytime, the sap flow velocity of Ulmus pumila L. at the four different decay stages decreased gradually with reduced solar radiation. Except for the extremely severe decay stage, sap flow velocity over the entire day increased with decay severity. The peak sap flow duration in mildly and moderately decayed trees spanned 10–18 h and 9–18 h, respectively. At night, sap flow velocity generally decreased with decay severity, mirroring the daytime pattern (see Figure 10a).

The average daily sap flow rates for the four decay stages were 2.29, 2.99, 3.68, and 0.92 mL/cm²/h, respectively. Notably, the average daily flow velocity in trees at the extremely severe decay stage was less than half of that in mildly decayed trees, with a significant difference (p < 0.01) (see Figure 10b).

3.3. Variation Characteristics of Evapotranspiration of Decaying Ulmus pumila L. Trees

3.3.1. Single-Day Variation Characteristics of Water Consumption of Decaying Ulmus pumila L. Trees

The daily variation in cumulative flow of Ulmus trees at the four decay stages is not significantly different. Except for the extremely severely decayed type, the overall order of cumulative flow is mildly decayed < moderately decayed < severely decayed. The general rule for the cumulative flow of Ulmus trees at various decay stages is that the flow on rainy days is significantly lower than on sunny days. Trees with lower decay stages have higher cumulative flow on rainy days and lower on sunny days. The extremely severely decayed type only shows a cumulative flow slightly closer to other types during heavy rain (Figure 11a).

The daily water consumption of Ulmus trees at different decay stages are 4093.04 mL for mild decay, 4114.98 mL for moderate decay, 4116.75 mL for severe decay, and 566.22 mL for extremely severe decay. During the entire measurement period, the total water consumption for trees at each decay stage are 487.072 L for mild decay, 489.682 L for moderate decay, 489.893 L for severe decay, and 67.380 L for extremely severe decay. From the data distribution, it is observed that the daily water consumption of severely decayed trees is more dispersed, with greater variability (Figure 11b).

3.3.2. Variation Characteristics of Daily Scale Water Consumption of Decaying Ulmus pumila L.

As the degree of decline progresses, both the nighttime and daytime water consumption of Ulmus pumila L. decrease significantly. Trees with mild decay exhibit the highest water consumption on both rainy and sunny days, while those in the extremely severe decay stage show the lowest. Across all decline stages, nighttime water consumption is consistently lower than daytime consumption, indicating that Ulmus pumila L. transpiration is more prominent during the day, especially in mild and moderate decay stages where daytime water consumption is notably higher. Overall water consumption on rainy days is relatively low, particularly for trees in severe and extremely severe decay stages, suggesting that increased humidity reduces water demand. On sunny days, total daily water consumption is generally higher than on rainy days, following a consistent trend across decay stages: trees with mild decline have the highest total daily water consumption, while those with extremely severe decline have the lowest (see Figure 12a).

The proportion of nighttime water consumption to daily water consumption for Ulmus trees at different decay stages on rainy days are 56% (mild decay), 36% (moderate decay), 26% (severe decay), and 63% (extremely severe decay) and on sunny days are 62% (mild decay), 87% (moderate decay), 79% (severe decay), and 47% (extremely severe decay). For mildly decayed, moderately decayed, and severely decayed Ulmus trees, the proportion of nighttime water consumption to daily water consumption on sunny days generally shows an increasing trend, with the increase in proportion being greater as the decay severity of the Ulmus trees increases. However, the proportion of nighttime water consumption to daily water consumption for extremely severely decayed Ulmus trees shows a significant decrease on sunny days compared to rainy days (see Figure 12b).

3.4. Effects of Meteorological Factors on Sap Flow Rate of Trees

We employed structural equation modeling (SEM) to analyze the relationship between meteorological factors and the stem flow rate of Ulmus trees. The analysis revealed that the vapor pressure deficit (VPD) is the primary meteorological factor influencing the stem flow rate of Ulmus trees. Additionally, different stages of decay significantly affect the flow rate. When precipitation (P) and temperature (T) increase, the stem flow rate decreases. Precipitation (P), as real-time data, has a particularly close relationship with the stem flow rate. Considering the lag effect of rainfall, combined with the data from Figure 7, we can conclude that after the influence of rainfall, as the soil-water storage increases, the stem flow rate returns to the mean value after five days.

Under the same meteorological conditions, the response patterns of Ulmus trees with different decay stages to meteorological factors vary significantly, as shown in Figure 13. For mildly decayed Ulmus trees, the primary factor affecting their stem flow rate is the vapor pressure deficit (VPD). However, as the decay progresses from mild to severe, the influence of meteorological factors on the flow rate gradually diminishes, and the effect of many meteorological conditions shifts from positive to negative. When it comes to extremely severely decayed Ulmus individuals, the influence of meteorological factors slightly rebounds, but the positive increase in temperature and solar radiation leads to a decrease in the stem flow rate (Figure 14).

4. Discussion

4.1. Daily Variations of Sap Flow Rate in Different Stages of Declining Ulmus Trees

The daily sap flow rate in plants varies depending on their physiological and ecological traits and their responsiveness to environmental factors. On typical sunny days, plants often display either unimodal or bimodal sap flow patterns [27]. For example, under sufficient soil moisture, Pinus tabulaeformis [28] and Betula platyphylla [29] show a unimodal pattern, while Juglans regia and Hippophae rhamnoides exhibit bimodal patterns [30]. This variation is closely tied to soil moisture availability [30]; for instance, Haloxylon ammodendron follows a unimodal pattern with sufficient moisture but shifts to a bimodal pattern under drought stress [31]. In this study, Ulmus trees, under continuous rainfall during the growing season, did not show a “midday pause” on sunny days. Instead, sap flow rates demonstrated a consistent unimodal “J”-shaped pattern across different growth stages, with elevated daytime rates and lower nighttime rates, similar to patterns observed in Malus, Haloxylon, and Vitis under sufficient moisture. Despite the water-stressed environment, Ulmus* maintained a unimodal sap flow pattern, with peak timing, duration, and rates varying across months, potentially due to changes in tree structure, trunk diameter, and decay-stage-specific environmental responses.

Nighttime sap flow supports essential water balance regulation, facilitating upward transport of materials from roots to canopy [32]. *Ulmus* trees in this study maintained nighttime sap flow from sunset to midnight, consistent with observations in other species [33,34,35]. Driven primarily by root pressure, nighttime flow replenishes internal water stores, especially in crowns and branches, ensuring adequate supply for the next day’s transpiration [32,36]. Nighttime sap flow can constitute up to 30% of total daily transpiration [36,37]. On rainy nights, nighttime sap flow proportions were as follows: severe > mild > moderate > extreme decay, with only severely decayed trees falling below 30%. Conversely, on sunny nights, the order shifted to the following: severe < mild < moderate < extreme decay, potentially due to net solar radiation, suggesting that severely decayed trees rely heavily on nighttime sap flow for moisture replenishment. Moreover, nighttime sap flow activity was higher at the beginning (June) and end (September and October) of the growing season, indicating that midday sap flow in the peak growing stage supports nutrient accumulation and storage.

4.2. Relationship Between Sap Flow Rate in Declining Ulmus Trees and Environmental Factors

Differences in sap flow rates among Ulmus trees at varying degrees of decline are attributed to tree deterioration and are influenced by a range of environmental factors [27,38]. Previous studies have found positive correlations between sap flow rates and solar radiation, air temperature, and vapor pressure deficit, with negative correlations to relative humidity [27,38,39,40]. However, this study shows that correlations between sap flow rates and these environmental factors change with the degree of decline. The dominant environmental factors affecting sap flow rates are significantly related to tree species and environmental conditions [40,41]. Our findings indicate that for degraded Ulmus trees, the sap flow rate is significantly correlated with vapor pressure deficit, and path analysis reveals that air temperature and saturated vapor pressure difference are the dominant factors. This contrasts with findings for Haloxylon [42], Ziziphus [43], and Castanea [41], further highlighting the complexity of environmental factors influencing transpiration in different tree species.

Meteorological factors impact sap flow in a timely manner, with tree water consumption reflecting changes in these factors as an open system [44]. By establishing sap flow models (regression equations of the sap flow rate versus environmental factors) for Ulmus trees at different decline stages, we can conveniently predict water consumption across various stages of decline, meeting the need for precise water management in Ulmus trees [45]. This study developed separate regression models for sap flow rates against environmental factors for Ulmus trees at different decline levels (Figure 13), providing a basis for estimating sap flow rates and subsequent water consumption during different growth periods and the entire growing season. However, due to limitations, this study only examined sap flow rates in Ulmus trees of similar age and trunk diameter, which may lead to deviations in applying this model to other Ulmus varieties and different diameter classes. Therefore, future studies on Ulmus water consumption characteristics should incorporate a comprehensive approach considering different varieties, trunk diameters, and soil moisture conditions, alongside physiological and morphological indices, to provide a more robust theoretical basis for water management in Ulmus trees.

4.3. Changes in Ulmus Tree Response to Environmental Factors During Decline

Our study reveals that Ulmus trees exhibit a significantly weaker response to photosynthetically active radiation (PAR) in the later stages of decline. This diminished response is primarily due to the progressive loss of photosynthetic capacity and transpiration efficiency, as evidenced by withering branches, reduced leaf area, and declining chlorophyll content, reflecting broader physiological deterioration [46,47,48].

In contrast, the tree’s sensitivity to vapor pressure deficit (VPD) follows a distinct pattern. Initially, as the tree declines, its response to VPD weakens. However, as the tree approaches mortality, VPD sensitivity increases unexpectedly. This resurgence in VPD sensitivity near the threshold of death is intriguing. We hypothesize that as Ulmus pumila succumbs to decline, its root water absorption capacity diminishes, and xylem conduits may become occluded, weakening the tree’s ability to respond to environmental stressors [49,50,51]. At the mortality threshold, the tree’s increased sensitivity to VPD may be due to heightened water demands for new shoot or root growth as the tree attempts to maintain essential functions [26,52,53]. Our findings highlight the complex nature of tree decline, which is not a uniform process but rather involves distinct physiological stages. While the tree’s response to environmental factors like VPD typically weakens during the early and mid-stages of decline, it may increase again as the tree seeks to support new growth in its final stages. Furthermore, VPD is an essential environmental element influencing the water flux of tree decay [54,55], and its influence is increased for elms when the tree dies. This heightened sensitivity is likely linked to the increased water demand for new shoot growth, which is crucial for sustaining physiological functions during the final stages of decline.

These observations underscore the importance of considering VPD in water management strategies for trees in decline, particularly in arid regions where water availability is limited. Understanding the dynamic response of Ulmus to environmental factors, especially VPD, can inform adaptive water management practices that address the unique needs of trees undergoing physiological decline.

4.4. Limitations of This Study

This study demonstrates that the Ulmus pumila L. sap flow rate and water-use efficiency decline significantly as the trees enter an irreversible stage of decline. Under drought and high-temperature conditions, the sap flow rate exhibits a negative correlation with soil moisture and relative humidity, highlighting the substantial impact of climate change on tree physiology [56,57,58]. As the decline progresses, trees’ water transport mechanisms undergo notable alterations. While the sap flow rate remains relatively high initially, it decreases sharply in severe decline stages, affecting not only individual trees but also potentially disrupting the ecosystem’s water cycle [59,60]. The sap flow rate is influenced by a range of environmental factors, including water vapor pressure deficit and solar radiation, underscoring the complex interplay between trees’ water-use capacity and environmental conditions [61,62].

Furthermore, implementing comprehensive management measures—such as improved soil moisture management and restorative planting—can enhance trees’ water-use efficiency and significantly increase the survival rates of declining species. Future research should prioritize studying resilient tree species and their responses to climate shifts, providing a scientific foundation for addressing ecological challenges brought by global change.

5. Conclusions

The daily variation of the sap flow rate during the growing season for Ulmus trees at different stages of decline exhibits an unimodal “J” shaped curve. Transpiration water consumption is relatively low during the late (September and October) and early (June) stages of the growing season, while it peaks in July and August, which are critical water demand periods for Ulmus trees. We established four regression models correlating sap flow rates with six environmental factors for Ulmus trees at varying degrees of decline, enabling the estimation of individual tree water consumption for different months and throughout the entire growing season. Preliminary estimates indicate that water consumption leading to the death of declining Ulmus trees should exceed 67.380 L, with VPD change after rainfall identified as the primary factor influencing sap flow regulation. Notably, one Ulmus tree exhibited no sap flow in mid-August, indicating complete mortality.