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Article

Caragana microphylla (Shrub) Seedlings Exhibit Better Growth than Surrounding Herbs Under Drought Conditions

by
Zhengyu Wang
1,
Chengyi Tu
2,
Jingjing Fan
1,
Chuchen Wu
1,
Zhenglin Lv
1,
Ruining Liu
1 and
Ying Fan
1,*
1
College of Geography and Environment, Shandong Normal University, Jinan 250358, China
2
School of Economics and Management, Zhejiang Sci-Tech University, Hangzhou 310018, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(3), 1142; https://doi.org/10.3390/su17031142
Submission received: 12 November 2024 / Revised: 22 January 2025 / Accepted: 28 January 2025 / Published: 30 January 2025
Browse Figures
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Abstract

:
Shrub encroachment is a global ecological issue. The changes in growth dynamics between shrub seedlings and herbs are pivotal in determining shrub encroachment, yet their response to varying rainfall regimes remains unclear. We conducted a precipitation manipulation experiment (three precipitation (P) amount treatments: P−25% (225 mm), P (300 mm), P+25% (375 mm); three drought interval treatments: DI4, DI6, DI8) on a mixture of Caragana microphylla (shrub) seedlings and four herbs (Neotrinia splendens, Campeiostachys dahurica, Lolium multiflorum and Medicago sativa), analyzing their ecophysiological and growth responses. The results showed the following: (1) Under P−25%, herb growth was inhibited, while shrub seedlings thrived. Compared to P, C. microphylla significantly increased by 138% in aboveground biomass (AGB), while herb AGB decreased by 10%. (2) Under P+25%, herbs exhibited superior growth to shrub seedlings. Compared to P, four herbs significantly increased by 53% in AGB, while C. microphylla growth did not significantly respond. (3) Under DI8, shrub seedlings exhibited superior growth compared to herbs. Compared to DI4, C. microphylla significantly increased by 90% in AGB, while herb growth did not significantly respond. Our results indicate that drier conditions suppressed herb growth while promoting shrubs. However, increased precipitation amounts stimulated herb growth but not shrubs. These results could explain the process of shrub encroachment and provide a theoretical basis for predicting the pattern of shrub expansion under future rainfall regimes.

1. Introduction

Shrub-encroached grassland refers to the increase in the density, coverage and biomass of indigenous woody or shrubby plants in arid and semi-arid grassland ecosystems [1]. This phenomenon has been observed in various regions around the world, including North America, Asia, Africa, Australia, and even the Arctic. In the global context, shrub encroachment into grasslands occupies approximately 10% to 20% of the total area in arid and semi-arid regions, impacting approximately 500 million hectares of land [2,3]. In China, approximately 5.1 million hectares of grassland in Inner Mongolia have been encroached by Caragana microphylla Lam. [4,5]. The rate of shrub encroachment was the highest on the Qinghai-Tibet Plateau, where at least 39% of alpine meadows has been replaced by shrub-encroached grassland [6]. Shrub encroachment leads to changes in soil structure, such as enhanced water content and soil infiltrability [7,8]. It also changes both the structure and function of grassland ecosystems, including vegetation productivity, biological diversity, and carbon storage [9] and threatens the sustainable development of grasslands and animal husbandry. Consequently, shrub encroachment has become a significant ecological and environmental issue in arid and semi-arid regions globally.
Previous studies have identified several factors contributing to shrub encroachment, including climate change (e.g., global warming and alterations in rainfall patterns), rising atmospheric CO2 concentrations, overgrazing, and fire suppression [10]. Among them, rainfall plays a significant role. The change in rainfall regime affects the moisture conditions for plant survival, thus significantly influencing the characteristics of shrub patches (e.g., size, shape, coverage) in arid and semi-arid regions [11]. Climate models predict that global warming will lead to an increase in total global precipitation, more extreme rainfall events, and intensified regional drought conditions [12,13]. Research on the Inner Mongolia region indicates that over the years, there has been a decreasing trend in precipitation in the eastern part and an increasing trend in the western part [14]. Additionally, there has been an increase in both the intensity and frequency of droughts in the region [15].
The response of shrubs and herbs to changes in rainfall patterns will directly affect the development trend of shrub encroachment into grassland under future climate change [16,17]. Some studies suggest that an increase in precipitation acts as a driver of shrub encroachment within a certain precipitation threshold [18]. On the contrary, research also confirms that increased precipitation leads to heightened competition between shrubs and herbs, which ultimately suppress the growth of shrubs [4,19]. When precipitation decreases and plants face drought stress, deep-rooted shrubs can utilize deep soil moisture to sustain growth and gain a competitive advantage, thereby promoting shrub encroachment into grassland [17,20]. The contradictory results observed may be attributed to a lack of separate consideration of precipitation amount and drought interval (DI) as two distinct variables. When total precipitation remains constant, heavy rainfall events with longer intervals between precipitation events and an increased amount of precipitation per event result in comparatively reduced water evaporation, allowing moisture to penetrate deeper soil layers [21]. Deep-rooted shrubs can utilize this moisture. However, it also increases the risk of herbs that rely on shallow soil moisture being water-limited [17,22]. Some research results indicated that extending the interval between precipitation events significantly increases the annual net primary productivity (ANPP) of shrubs and decreases the ANPP of herbs [23,24]. Currently, the response of shrub encroachment to changes in rainfall and its underlying mechanisms remains controversial, possibly due to insufficient consideration of the complexity of rainfall changes. Multiple combinations of DI and the amount of individual rainfall could lead to either increased or decreased soil moisture. Therefore, it is necessary to treat precipitation amount and DI as two independent variables and conduct precisely controlled experiments on their combinations to determine how rainfall conditions affect shrub encroachment.
Moreover, the shrubs and herbs in shrub-encroached grassland exhibit a mosaic distribution. The differences in competitiveness between shrubs and herbs are fundamental factors in the shrub encroachment, with competition primarily occurring at the interface between shrub and herb patches, composed of shrub seedlings and herbs. At the edges of shrub patches, where shrub seedlings predominantly compete with surrounding herbs, the outcome of this competition determines the potential for further expansion of shrub patches. If shrub seedlings have an advantage over the surrounding herbs in acquiring various resources (mainly water), then herb patches will be gradually replaced by shrubs, and vice versa. Additionally, within scattered shrub seedlings dispersed among herb patches, their competitive interactions dictate whether new shrub patches can form within the herb areas. Therefore, understanding and managing the competitive dynamics between shrubs and herbs are pivotal for grasping the trajectory of shrub encroachment in grassland ecosystems.
However, there is currently insufficient clarity regarding the growth differences between shrub seedlings and herbs under varying precipitation amounts and DI. Therefore, this study focuses on C. microphylla, the largest invasive shrub of China, and four common herbaceous species (Neotrinia splendens (Trin.) M.Nobis, P.D.Gudkova & A.Nowak; Campeiostachys dahurica (Turcz. ex Griseb.) B.R.Baum, J.L.Yang & C.Yen; Lolium multiflorum Lam.; Medicago sativa L.) that coexist with C. microphylla in shrub-encroached grasslands in Inner Mongolia. By controlling the amount and interval of rainfall, we simulated future rainfall regimes to explore the dynamic changes in shrub seedlings and herbs growth under varying moisture conditions, and to predict the expansion trend of shrub patches. This provides theoretical support for managing and restoring grasslands, thereby offering insights for more effective resource conservation efforts, disaster mitigation, and advancement in ecological research.

2. Materials and Methods

2.1. Experimental Design

The experiment was conducted from September 2022 to February 2023 in the intelligent glass greenhouse located at the Shandong Normal University in Jinan, Shandong Province, China (36°32′20″ N, 116°49′51″ E). This greenhouse employs smart control systems and advanced glass materials (high-strength, high-transparency glass to ensure uniform illumination), enabling automated environmental monitoring and control, including adjustments in temperature, humidity, and light, to meet the diverse growth requirements of the plants. The greenhouse environment was set according to the natural conditions of shrub-encroached grasslands during summer in Inner Mongolia. The greenhouse provided all-day temperature and humidity control, maintaining the temperature at 20~24 °C/10~15 °C (daytime/night-time), and the humidity at 50~55%. Additionally, supplementary lighting was applied daily from 5:00 to 19:00 to mimic summer daylight, resulting in a photosynthetically photon flux density (PPFD) of approximately 900 μmol·m−2·s−1.
This study includes the following species: C. microphylla (Fabaceae, shrub), N. splendens (Poaceae, perennial herb), C. dahurica (Poaceae, perennial herb), L. multiflorum (Poaceae, annual or biennial herb), and M. sativa (Fabaceae, perennial herb). Plant seeds and the soil used for seedling cultivation were collected from the shrub-encroached grasslands in Inner Mongolia. The soil texture is sandy and sandy loam, with low organic matter content and the presence of calcic horizon. In September 2022, the seeds were sown in a seedling tray. When seedlings of all species reached 3~4 leaves, they were transplanted into pots (dimensions: length 15 cm; width 15 cm; height 12 cm) according to the planting scheme (Figure 1). Each pot involved C. microphylla (6 individuals), N. splendens (3 individuals), C. dahurica (3 individuals), L. multiflorum (3 individuals), and M. sativa (3 individuals). In November, once all seedlings of transplanted species in pots were growing normally without any mortality (about one week after transplantation), the watering started according to different precipitation amounts and DI. Before precipitation manipulation, shrub and four herb seedlings showed similar seedling heights among different pots (Figure 2). The pots were rotated weekly to eliminate variations in light and temperature, ensuring a uniform growth environment.
According to the meteorological data from the Inner Mongolia Autonomous Region, the average annual precipitation amount is 328.3 mm [14]. Therefore, the original rainfall regime was set as 300 mm per year (P), and roughly 2/3 of it occurred during the summer [25] with approximately one rainfall event every 4 days (DI4), equivalent to 8.9 mm of rainfall every 4 days. Moisture conditions were controlled via changes in the precipitation amount and DI. According to the trends in precipitation changes across different regions of Inner Mongolia [14], we set the precipitation amount as ±25% (P−25%, P+25%) of the P to simulate future water availability. Furthermore, extreme rainfall events and an extension of DI will probably occur in the northern grassland areas of China [26]. Thus, DI was set as 6 days (DI6) and 8 days (DI8). The experiment was conducted by incorporating three precipitation amount levels and three drought interval levels. There were 9 (3 × 3) treatments, including P−25% DI4, P−25% DI6, P−25% DI8, P DI4, P DI6, P DI8, P+25% DI4, P+25% DI6, and P+25% DI8. Each treatment group had three replicates, totaling 27 (9 × 3) pots. Based on the surface area of the pots, the volume of water for each precipitation treatment is detailed in Table 1. We watered the pots for three months according to the planned amount and duration. In February 2023, the heights of each plant in all experimental pots were recorded, while the leaf water potential (ΨL) and net photosynthetic rate (Pn) of C. microphylla and C. dahurica in each respective pot were measured. After all measurements were completed, the plants were harvested. We measured the aboveground biomass (AGB) and belowground biomass (BGB) of C. microphylla, and the total AGB of the four herb species in each pot.

2.2. Measurement Indicators and Methods

2.2.1. Measurement of Leaf Water Potential

The ΨL of plants was measured using a PMS 1515 D pressure chamber (PMS Instrument Company, Corvallis, OR, USA). Measurements were taken between 14:00 and 16:00 on the day before watering (pre-watering ΨL) and on the first day after watering (post-watering ΨL) from well-developed C. microphylla seedlings and C. dahurica, focusing on fully expanded, mature leaves from the upper part of the plant.

2.2.2. Measurement of Net Photosynthetic Rate

The Pn (μmolCO2·m−2·s−1) of C. microphylla seedlings and C. dahurica were measured using an LI-6400 XT portable photosynthesis system (LI-COR Biosciences, Lincoln, NE, USA) with a leaf chamber fluorometer at 9:00~11:30 before watering. The flow rate was set at 500 μmol/s, and the photosynthetically active radiation (PAR) was set at 1500 μmol·m−2·s−1. After clamping the leaves in the leaf chamber, the value was allowed to stabilize before being recorded.

2.2.3. Determination of Biomass Parameters

The aboveground parts of C. microphylla and four herb species (N. splendens, C. dahurica, L. multiflorum and M. sativa) were harvested from the ground surface, while the belowground parts of C. microphylla were carefully extracted to preserve root integrity. Each plant sample was placed in a labeled envelope, and then de-enzymed at 110 °C in an oven for 15~20 min, followed by drying at 75 °C until reaching a constant weight. The AGB and BGB of each plant were individually weighed and recorded using a balance with a precision of 0.01 g. AGB is the dry matter weight of stems and leaves. BGB is the dry matter weight of the roots. Total biomass (TB) is the sum of AGB and BGB. The root–shoot ratio (R/S) is the ratio of BGB to AGB. Precipitation use efficiency (PUE) is the ratio of vegetation biomass to precipitation amount.

2.3. Data Processing and Analysis

All data processing and statistical analyses were performed using Excel 2019 and SPSS 27.0 (IBM Corporation, Armonk, NY, USA). Figures were generated using Origin 2021 (OriginLab Corporation, Northampton, MA, USA). The effects of precipitation amount, drought interval, and their interaction on plant morphological and physiological parameters were analyzed by two-way ANOVA. When a significant factor effect or interaction was found, one-way ANOVA followed by multiple comparisons (Duncan’s multiple range test, p < 0.05) was used to test for differences between different treatments.

3. Results

3.1. Ecophysiological Responses

3.1.1. Leaf Water Potential

Two-way ANOVA (Table 2) revealed that the precipitation amount significantly affected the pre-watering ΨL of C. microphylla (p < 0.05) and DI significantly impacted the post-watering ΨL of C. microphylla (p < 0.05), while their interaction did not exert significant effects (p > 0.05). Both pre- and post-watering ΨL of C. microphylla showed an increasing trend with increasing precipitation amounts (Figure 3a,b). Under DI4, the pre-watering ΨL of C. microphylla reached a peak of −2.55 ± 0.93 MPa at P+25%, signifying a notable 46% increase compared to P−25% (p < 0.05). Under P−25%, the post-watering ΨL of C. microphylla at DI8 was significantly higher compared to DI4 and DI6, by 48% in each case (p < 0.05).
DI significantly affected the pre-watering and post-watering ΨL of C. dahurica (p < 0.05), while the precipitation amount and their interaction did not exert significant effects (p > 0.05) (Table 2). Before watering, the ΨL of C. dahurica (Figure 3c) decreased with the extension of DI under P−25%, P, and P+25%. Especially under P+25%, the ΨL of C. dahurica at DI8 exhibited a significant decrease of 124% compared to DI4 (p < 0.05). However, the influence of precipitation amount variation on the ΨL of C. dahurica before watering was minimal. After watering, the ΨL of C. dahurica decreased with the extension of DI under both P−25% and P+25% (Figure 3d). Particularly under P+25%, it was significantly reduced by 46% at DI8 compared to DI4 (p < 0.05).

3.1.2. Net Photosynthetic Rate

Two-way ANOVA (Table 2) indicated that DI did not significantly impact the Pn of C. microphylla (p > 0.05), while the precipitation amount and their interaction exhibited a significant effect (p < 0.05). As shown in Figure 4a, under DI4, the Pn of C. microphylla increased significantly by 17% at P−25% compared to P (p < 0.05), while at P+25%, there was a slight increase compared to P, though this difference was not statistically significant (p > 0.05). Furthermore, the Pn of C. microphylla increased with the extension of DI, and DI8 exhibited a notable 13% increase compared to DI4 under P (p < 0.05).
Two-way ANOVA (Table 2) suggested that precipitation amount significantly influenced the Pn of C. dahurica (p < 0.05), while DI and their interaction had no significant effect (p > 0.05). As shown in Figure 4b, both decreased and increased precipitation amounts led to a decrease in the Pn of C. dahurica. Specifically, under DI4, the Pn of C. dahurica decreased by 17% at both P−25% and P+25%, compared to P. Among the three DIs (DI4, DI6, DI8), the Pn of C. dahurica reached its peak under P, registering values of 12.37 ± 0.13, 12.46 ± 0.77 and 12.19 ± 1.97 μmolCO2·m−2·s−1, respectively.

3.2. Growth Responses

3.2.1. Plant Height

Precipitation amount had a significant impact on the plant height of N. splendens, C. dahurica and L. multiflorum (p < 0.05) but not on C. microphylla and M. sativa (p > 0.05). DI had a significant impact on the plant height of C. dahurica (p < 0.05) but not on C. microphylla, N. splendens, L. multiflorum and M. sativa (p > 0.05). The interaction between precipitation amount and DI had no significant effect on any of the four herb species (p > 0.05) (Table 2). Plant height serves as a primary indicator of water and nutrient utilization by plants. Under DI4, P−25% and P+25% both promoted the growth of C. microphylla plant height, showing increases of 41% and 11%, respectively, compared to P, but these differences were not statistically significant (p > 0.05). In addition, prolonged DI also promoted the growth of C. microphylla height (Figure 5a). The plant height of N. splendens, C. dahurica and M. sativa all increased at P−25% and decreased at P+25% (Figure 5b,c,e). Under P+25%, the plant height of C. dahurica was significantly decreased by 23% at DI8 compared to DI4 (p < 0.05) (Figure 5c). Moreover, under DI4, the plant height of L. multiflorum decreased by 18% and 15% at P−25% and P+25%, respectively, compared to P (Figure 5d).

3.2.2. Biomass

Two-way ANOVA (Table 2) suggested that DI had no significant impact on the AGB, BGB and TB of C. microphylla (p > 0.05), while precipitation amount and their interaction had a significant effect (p < 0.05). The trends in AGB, BGB and TB of C. microphylla exhibited similar variations, with precipitation amounts of P−25% and P+25% both stimulating the accumulation of C. microphylla biomass (Figure 6). Under DI4, the AGB, BGB and TB of C. microphylla increased by 138%, 153% and 147% at P−25%, respectively, compared to P, with significant differences (p < 0.05). At P+25%, the AGB, BGB, and TB increased by 29%, 73%, and 47%, respectively, but these differences were not statistically significant (p > 0.05). In addition, extending the DI promoted the biomass accumulation of C. microphylla. Under P, the AGB and TB of C. microphylla increased significantly by 90% and 131% at DI8, respectively, compared to DI4 (p < 0.05). However, at DI6, there was a slight non-significant decrease in AGB, BGB, and TB of C. microphylla compared to DI4 (p > 0.05). The AGB, BGB and TB of C. microphylla attained their minimum values under P DI6, which were 0.20 ± 0.12 g, 0.10 ± 0.08 g and 0.30 ± 0.20 g, respectively.
Similarly (Table 2), DI had no significant impact on the AGB of herbs (p > 0.05), while precipitation amount and their interaction had a significant effect (p < 0.05). The increase in precipitation amount facilitated the accumulation of AGB in herbs (Figure 7). Under DI4, the total AGB of the herbs significantly increased by 53% at P+25% compared to P (p < 0.05). Under P−25% and P, the total AGB of the herbs showed an increase with the extension of the DI, but these differences were not statistically significant (p > 0.05). However, under P+25%, the total AGB of the herbs decreased with the extension of the DI and DI8 demonstrated a significant 23% reduction compared to DI4 (p < 0.05).

3.2.3. Precipitation Use Efficiency

The precipitation amount, DI, and their interactions significantly affected the PUE of C. microphylla (p < 0.05) (Table 2). The PUE derived from both aboveground and belowground parts of C. microphylla demonstrated a similar trend (Figure 8). Specifically, under DI4, the PUE increased significantly by 215% for aboveground parts and 247% for belowground parts at P−25% compared to P (p < 0.05). Moreover, the PUE for belowground parts increased by 41% at P+25% compared to P, but the difference was not statistically significant (p > 0.05). Under both P−25% and P+25%, the PUE of C. microphylla decreased with the extension of DI. Particularly under P−25%, the derived PUE from both aboveground and belowground parts of C. microphylla exhibited significant decreases of 58% and 69%, respectively, at DI8 compared to DI4 (p < 0.05).
Similarly, the precipitation amount, DI, and their interactions significantly affected the PUE of herbs (p < 0.05) (Table 2). Both decreases and increases in precipitation amount resulted in a higher PUE calculated from the aboveground parts of the herbs (Figure 9). Under DI4, the P−25% and P+25% showed a 20% and 22% increase in PUE, respectively, compared to the P, with a significant difference (p < 0.05). Under the three precipitation amounts (P−25%, P, P+25%), the PUE of the herbs decreased with an extension of the DI. It reached the lowest point under DI8, with values of 25.06 ± 1.49, 23.32 ± 4.16, and 17.84 ± 2.87 gC·m−2·mm−1, representing a significant decrease of 44%, 38%, and 61%, respectively, compared to DI4 (p < 0.05).

3.2.4. Root–Shoot Ratio

Two-way ANOVA (Table 2) suggested that precipitation amount significantly impacted the R/S of C. microphylla (p < 0.05), while DI and their interaction had no significant effect (p > 0.05). As shown in Figure 10, under DI6, the R/S of C. microphylla significantly increased by 58% and 122% at P+25% compared to P−25% and P, respectively (p < 0.05). Under DI8, the R/S of C. microphylla increased with the increase in precipitation amount, showing a significant increase of 162% at P+25% compared to P−25% (p < 0.05). There was no significant difference in the R/S of C. microphylla under other precipitation treatments (p > 0.05).

4. Discussion

4.1. Reduced Rainfall Boosts Shrub Growth Whereas Increased Has Little Impact

Plant growth and development in arid and semi-arid grassland ecosystems are restricted by soil moisture levels [27]. The response of early growth and biomass allocation of plant seedlings to precipitation changes affects the subsequent growth and survival ability of seedlings. Seedling survival determines the dynamics, persistence, and expansion of plant populations [28]. This study found that the AGB of C. microphylla seedlings and herbs both increased in P+25%, though the extent of their responses varied (Figure 6a and Figure 7). As this experiment was conducted in pots with shallow depths, C. microphylla seedlings and the herbs shared the same soil layer, inevitably leading to competition for water resources. In this study, an increase in BGB and R/S of C. microphylla seedlings was observed under P+25% (Figure 6b and Figure 10), corroborating this notion. Optimal partitioning theory suggests that plants preferentially allocate resources to the organ encountering the most limiting resource [29,30]. When resources are scarce belowground, such as water and nutrients, plants will allocate more biomass to the roots, promoting root growth [31]. Therefore, it is suggested that with increasing precipitation amounts, stronger water competition abilities are exhibited by herbs compared to shrub seedlings, promoting their biomass accumulation and PUE enhancement (Figure 7 and Figure 9). Although an increase in pre-watering ΨL of C. microphylla seedlings was observed with increasing precipitation amount (Figure 3a), no significant changes were observed in their Pn, biomass, or PUE (Figure 4a, Figure 6 and Figure 8). Therefore, the results of this study indicate that P+25% was more beneficial to the growth of herbs, while the growth of C. microphylla seedlings was minimally affected. These findings are consistent with the results of a precipitation manipulation experiment conducted by Zhu et al. [19] in Inner Mongolia, which demonstrated that increased precipitation significantly promoted the growth of herbs, but had no significant impact on shrub growth.
Additionally, it was observed in this study that under P−25%, the pre-watering ΨL of C. microphylla seedlings decreased compared to P (Figure 3a). This may be attributed to reduced soil moisture, leading to water deficits within shrub seedlings. To meet physiological needs, shrub seedlings lower their ΨL to enhance water absorption from the soil [32]. Under P−25%, Pn of the C. microphylla seedlings significantly increased compared to P, promoting their growth, biomass accumulation and enhancing their PUE (Figure 4a and Figure 8). Consequently, with decreasing precipitation amount, the AGB, BGB, and TB of C. microphylla seedlings significantly increased (Figure 6). In contrast, under P−25%, the pre-watering ΨL of C. dahurica remained unchanged (Figure 3c). This may be attributed to the plant’s strategy of minimizing water loss by stomatal closure and reduced transpiration, which enables adaptation to arid conditions [33]. However, stomatal closure restricts carbon dioxide uptake, thus reducing photosynthetic activity [34]. Consequently, the Pn of C. dahurica significantly decreased under P−25%, adversely impacting its growth (Figure 4b). The total AGB of herbs decreased with a reduction in precipitation amount (Figure 7). This indicates that reduced precipitation amount (P−25%) inhibits the growth of herbs while promoting C. microphylla seedlings, which is consistent with the results of other precipitation manipulation studies [17,19].
This study suggests that drought may trigger shrub encroachment in grasslands, while the alleviation of drought conditions may not necessarily reverse this process. Previous research has shown that droughts gradually provide shrubs with a competitive advantage by altering the competition between plants and reallocating water resources, leading to shrub encroachment [22]. However, once shrubs gain a competitive advantage through drought and form shrublands, it becomes difficult for herbs to reclaim their ecological position [35].

4.2. Extended Drought Intervals Benefit Shrub Growth

The frequency and interval of rainfall play pivotal roles in shaping plant survival, growth, and species composition [36]. In arid and semi-arid regions, where evaporation rates are high, the temporal distribution of precipitation directly impacts soil water content, consequently influencing plant growth [37]. Currently, the majority of studies suggest that small rainfall events primarily affect the physiological activities of soil surface-level microorganisms or vegetation that respond rapidly to water availability [38,39], while plant growth and reproduction necessitate continuous and substantial rainfall events [40]. Studies have indicated that extended DI and increased precipitation per event benefit the growth of deep-rooted shrubs that can utilize water more flexibly [41]. Conversely, the growth of herbs, which rely on moisture in shallow soil layers, may be constrained due to limited water availability [22]. Although in this experiment both shrubs and herbs shared the same soil layer, this study still reached a similar conclusion to the one mentioned above, i.e., that extended DI is advantageous for the growth of C. microphylla seedlings. This finding is likely related to the competition for water resources between shrubs and herbs. With extended DI, the pre-watering ΨL of C. microphylla exhibited an increasing trend, accompanied by a significant rise in its Pn (Figure 3a and Figure 4a). This indicates that C. microphylla has obtained adequate water through competition in water-limited environments to sustain its photosynthesis and growth [42]. This competitive advantage enabled C. microphylla to effectively accumulate photosynthetic products under drought conditions, increasing both AGB and BGB (Figure 6). This finding is consistent with the outcomes of field experiments conducted by Yu et al. [24], where the interval between precipitation events was extended while maintaining a constant total annual precipitation amount, leading to an enhanced ANPP of shrubs. In contrast, extended DI resulted in a decline in pre-watering ΨL for C. dahurica (Figure 3 c). This may result from competition for water resources between shrubs and herbs, with herbs exhibiting a comparatively lower capacity for water acquisition, thereby inhibiting their growth.

4.3. Shrub Encroachment Trend Under Future Rainfall Regimes

In the context of climate change, global warming will alter the hydrological cycle both globally and regionally, leading to changes in rainfall patterns, such as variations in total precipitation amounts, imbalanced distribution of DI, and an increase in extreme events like droughts [12]. Previous studies have indicated that from 1970 to 2019, annual precipitation in Inner Mongolia exhibited a slight overall increasing trend. Specifically, precipitation significantly increased in the eastern and NW region, while a decreasing trend was observed in the SW region and certain areas of the eastern region [25]. Under P−25%, herb growth was inhibited, while the growth of C. microphylla seedlings was enhanced, which may further intensify shrub encroachment in the Inner Mongolian grasslands. Conversely, under P+25%, it is more favorable for the growth of herbs, with no significant impact on the growth of C. microphylla seedlings, potentially inhibiting shrub encroachment to a certain extent in the Inner Mongolian grasslands.
Furthermore, the global precipitation patterns will experience an increase in the frequency of extreme rainfall events and periodic droughts under the influence of climate change [13,43,44]. In China, a notable change has been observed in the frequency and intensity of extreme precipitation events, with an increase in the number of heavy rainfall events and a decrease in the number of moderate and light rainfall events [45]. In the arid and semi-arid regions of northern China, an extension of precipitation intervals has been observed, along with an increase in the frequency of heavy rainfall events [46,47]. With the extension of DI, the growth of C. microphylla seedlings was better and the biomass increased significantly, while there was no significant change in herb biomass. Therefore, under the projected scenario of prolonged DI in the future, shrub encroachment on the Inner Mongolia grassland may be further aggravated.

5. Conclusions

Soil moisture plays a crucial role in plant growth. Changes in rainfall patterns affect the moisture content of the soil, consequently affecting plant growth. This study aimed to examine the growth of C. microphylla seedlings and four herbs under varying rainfall patterns by controlling precipitation amount and DI. The results indicate that, under a constant DI, P+25% was more favorable for the growth of herbs. In contrast, P−25% promoted the growth of C. microphylla seedlings while inhibiting the growth of herbs. Moreover, under conditions where the amount of precipitation remained constant but the DI was extended, the growth of C. microphylla seedlings was further favored. In general, under drier conditions, the growth advantage of shrub seedlings was significantly higher than that of surrounding herbs. However, under the P+25% treatment, the growth of herbs was favored over that of shrub seedlings, while shrub seedlings did not exhibit growth disadvantages. Therefore, it is predicted that future rainfall events with increasing DI and larger single rainfall amounts in the Inner Mongolia region will be beneficial to shrub growth and promote continuous shrub encroachment into grasslands.

Author Contributions

Conceptualization, Z.W. and Y.F.; Formal analysis, Z.W., J.F., C.W. and Z.L.; Funding acquisition, Y.F.; Investigation, Z.W., J.F., C.W., Z.L. and R.L.; Methodology, Z.W. and Y.F.; Project administration, Y.F.; Resources, Y.F.; Supervision, Y.F.; Visualization, Z.W.; Writing—original draft, Z.W.; Writing—review and editing, Z.W., C.T., J.F. and Y.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research received funding support from the National Natural Science Foundation of China (grant number 42001021) and the China Postdoctoral Science Foundation (grant number 2019 M662427).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest. This article does not contain any studies with human participants or animals performed by any of the authors. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Cultivation arrangements in each pot.
Figure 1. Cultivation arrangements in each pot.
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Figure 2. Greenhouse exterior (a), interior (b), and distribution of replicate treatment diagram (c).
Figure 2. Greenhouse exterior (a), interior (b), and distribution of replicate treatment diagram (c).
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Figure 3. Effects of different rainfall patterns on leaf water potential (ΨL) of Caragana microphylla (a) pre-watering and (b) post-watering; ΨL of Campeiostachys dahurica (c) pre-watering and (d) post-watering. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
Figure 3. Effects of different rainfall patterns on leaf water potential (ΨL) of Caragana microphylla (a) pre-watering and (b) post-watering; ΨL of Campeiostachys dahurica (c) pre-watering and (d) post-watering. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
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Figure 4. Effects of different rainfall patterns on net photosynthetic rate (Pn) of (a) C. microphylla and (b) C. dahurica. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
Figure 4. Effects of different rainfall patterns on net photosynthetic rate (Pn) of (a) C. microphylla and (b) C. dahurica. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
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Figure 5. Effect of different rainfall patterns on plant height of C. microphylla and four herb species. (a) C. microphylla, (b) Neotrinia splendens, (c) C. dahurica, (d) Lolium multiflorum, (e) Medicago sativa. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
Figure 5. Effect of different rainfall patterns on plant height of C. microphylla and four herb species. (a) C. microphylla, (b) Neotrinia splendens, (c) C. dahurica, (d) Lolium multiflorum, (e) Medicago sativa. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
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Figure 6. Effects of different rainfall patterns on (a) aboveground biomass (AGB), (b) belowground biomass (BGB), (c) total biomass (TB) of C. microphylla. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
Figure 6. Effects of different rainfall patterns on (a) aboveground biomass (AGB), (b) belowground biomass (BGB), (c) total biomass (TB) of C. microphylla. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
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Figure 7. Effect of different rainfall patterns on total AGB of four herbs. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
Figure 7. Effect of different rainfall patterns on total AGB of four herbs. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
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Figure 8. Effects of different rainfall patterns on precipitation use efficiency (PUE) of C. microphylla. (a) PUE derived from the aboveground part and (b) PUE derived from the belowground part. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
Figure 8. Effects of different rainfall patterns on precipitation use efficiency (PUE) of C. microphylla. (a) PUE derived from the aboveground part and (b) PUE derived from the belowground part. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
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Figure 9. Effects of different rainfall patterns on PUE (drive from aboveground part) of herbs. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
Figure 9. Effects of different rainfall patterns on PUE (drive from aboveground part) of herbs. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
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Figure 10. Effects of different rainfall patterns on root–shoot ratio (R/S) of C. microphylla. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
Figure 10. Effects of different rainfall patterns on root–shoot ratio (R/S) of C. microphylla. P−25%, P, and P+25% represent a 25% decrease in precipitation amount, natural precipitation amount, and a 25% increase in precipitation amount, respectively. DI4, DI6 and DI8 indicate drought interval (DI) of 4 days, 6 days, and 8 days, respectively. Different uppercase letters show significant differences between the treatments of different DIs under the same precipitation amount (p < 0.05); different lowercase letters indicate significant differences between the treatments of different precipitation amounts under the same DI (p < 0.05). Error bar represents the standard error of the mean.
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Table 1. The watering volume of each group and corresponding precipitation amount.
Table 1. The watering volume of each group and corresponding precipitation amount.
Precipitation AmountP−25%
(225 mm)		P
(300 mm)		P+25%
(375 mm)
Drought Interval
DI4151 mL200 mL250 mL
DI6225 mL299 mL376 mL
DI8299 mL401 mL500 mL
Table 2. Results of two-way ANOVA for the effects of precipitation amounts (P) and drought intervals (DI) on plants ecophysiological and growth variables.
Table 2. Results of two-way ANOVA for the effects of precipitation amounts (P) and drought intervals (DI) on plants ecophysiological and growth variables.
VariableSpeciesPDIP × DI
FSig.FSig.FSig.
Pre-watering ΨL (MPa)C. microphylla4.0410.036 *2.4900.1111.4230.267
C. dahurica0.5880.56610.928<0.001 ***0.7500.571
Post-watering ΨL (Mpa)C. microphylla1.2270.3174.4220.027 *2.9080.051
C. dahurica1.3100.2943.9800.037 *1.9500.146
Pn (μmolCO2·m−2·s−1)C. microphylla14.912<0.001 ***0.1610.8534.5050.017*
C. dahurica11.7150.001 **0.0490.9520.5070.732
Aboveground PUE (gC·m−2·mm−1)C. microphylla42.183<0.001 ***13.204<0.001 ***6.9830.002 **
four herbs4.5310.025 *106.285<0.001 ***4.4820.011 *
Belowground PUE (gC·m−2·mm−1)C. microphylla28.955<0.001 ***18.897<0.001 ***9.138<0.001 ***
AGB (g)C. microphylla11.233<0.001 ***2.0420.1604.6100.011 *
four herbs25.181<0.001 ***0.0060.9945.4180.005 **
BGB (g)C. microphylla7.8490.004 **0.5290.5983.8940.019 *
TB (g)C. microphylla7.4640.005 **1.8720.1845.5720.005 **
R/SC. microphylla9.5330.002 **0.2880.7531.4160.271
Plant height (cm)C. microphylla3.5350.0510.2580.7751.3620.286
N. splendens9.8080.002 **1.6860.2160.2900.880
C. dahurica16.533<0.001 ***4.9610.019 *2.3290.095
L. multiflorum13.815<0.001 ***2.3270.1262.8770.053
M. sativa2.2610.1330.4960.6170.5250.718
Note: *, **, *** significance at 0.05, 0.01 and 0.001, respectively. ΨL is leaf water potential, Pn is net photosynthetic rate, AGB is aboveground biomass, BGB is belowground biomass, TB is total biomass, PUE is precipitation use efficiency; R/S is root–shoot ratio.
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Wang, Z.; Tu, C.; Fan, J.; Wu, C.; Lv, Z.; Liu, R.; Fan, Y. Caragana microphylla (Shrub) Seedlings Exhibit Better Growth than Surrounding Herbs Under Drought Conditions. Sustainability 2025, 17, 1142. https://doi.org/10.3390/su17031142

AMA Style

Wang Z, Tu C, Fan J, Wu C, Lv Z, Liu R, Fan Y. Caragana microphylla (Shrub) Seedlings Exhibit Better Growth than Surrounding Herbs Under Drought Conditions. Sustainability. 2025; 17(3):1142. https://doi.org/10.3390/su17031142

Chicago/Turabian Style

Wang, Zhengyu, Chengyi Tu, Jingjing Fan, Chuchen Wu, Zhenglin Lv, Ruining Liu, and Ying Fan. 2025. "Caragana microphylla (Shrub) Seedlings Exhibit Better Growth than Surrounding Herbs Under Drought Conditions" Sustainability 17, no. 3: 1142. https://doi.org/10.3390/su17031142

APA Style

Wang, Z., Tu, C., Fan, J., Wu, C., Lv, Z., Liu, R., & Fan, Y. (2025). Caragana microphylla (Shrub) Seedlings Exhibit Better Growth than Surrounding Herbs Under Drought Conditions. Sustainability, 17(3), 1142. https://doi.org/10.3390/su17031142

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