Study on the Dynamics of Stem Sap Flow in Minqin Wind and Sand Control Haloxylon ammodendron Forest, China

Abstract

In this study, we obtained real-time data on the stem fluid flow of Haloxylon ammodendron in the growing season in Minqin, China, based on thermal diffusion (TDP) monitoring technology, and analyzed the dynamic changes of stem fluid flow in it to provide important supporting evidence for understanding the water consumption of H. ammodendron during its growth. The results indicate that the fluid flow in the stem of the H. ammodendron increases with increasing growth age and also decreases as the H. ammodendron grows older and declines. The average daily sap flow rates of H. ammodendron stems were 0.956, 1.059, 1.460 and 0.570 cm³·(cm²·h)⁻¹ at 5, 10, 15 and 20 years, respectively, and the cumulative sap flow masses during the growing season from May to October were 610.173, 423.386, 1041.186 and 430.212 kg, respectively. H. ammodendron stem sap flow increases with the thickening of H. ammodendron’s ground diameter. The average daily sap flow rate of H. ammodendron stems at different stem levels ranged from 0.276 to 2.132 cm³·(cm²·h)⁻¹, and the cumulative sap flow mass during the growing season ranged from 121.656 to 1722.810 kg. The larger the diameter of the H. ammodendron at different forest ages, the earlier the sap flow initiation time and the higher the peak. The stem flow initiation time was 7:00–8:00, and the average daily maximum fluid flow rate was 2.493–5.536 cm³·(cm²·h)⁻¹, with the peak occurrence time advancing with age. The sap flow variation of H. ammodendron at different stand ages reflects the water-consuming process of H. ammodendron growth and its response to a drought environment, while the sap flow variation of H. ammodendron at different diameter classes shows that the individual growth differences of H. ammodendron also yield obvious competitive advantages. The results of the analysis can provide theoretical support for the estimation of ecological water use in desert H. ammodendron sand-fixing forests.

1. Introduction

Haloxylon ammodendron is the most widely distributed desert plant in the deserts of Central Asia and the largest tree species used in the dry zone for sand fixation and afforestation, and it is a typical desert sand fixation plant [1]. In China, Minqin has nearly 35,000 hm² of planted H. ammodendron forest, accounting for 51.5% of the total planted forest area, dominating the sand-fixing forests [2]. As early as the late 1950s, Minqin successfully introduced H. ammodendron for afforestation on sand dunes, and carried out large-scale promotion. The initial planting spacing of 1~2 m later increased to 2~3 m, but with the reduction in surface water resources and the decline of the water table, the water consumption of H. ammodendron growth increased, and as soil moisture has been unable to support the growth of initial H. ammodendron forests, there was bound to be a high rate of H. ammodendron death. The silvicultural density decreased to about 1200 plants hm⁻² after 2010, but it is still higher than the precipitation-carrying density of 570–600 plants hm⁻² [3]. The excessive density of H. ammodendron silviculture causes transpiration water depletion, which puts H. ammodendron in severe water stress, resulting in degradation or even dieback. The study of the water consumption process of H. ammodendron is a key link in maintaining the survival and growth of artificial sand-fixing forests in arid areas. At present, artificial H. ammodendron forests with different silvicultural years and different stand structures are commonly distributed in the Minqin desert area, providing good natural resources for the study of water consumption dynamics during H. ammodendron growth. Plants absorb water through the root system and rely on transpiration pull to make the fluid flow within the xylem of the plant body; therefore, the transpiration water consumption of plants can be characterized by trunk sap flow [4,5]. At present, a lot of research has been carried out on water depletion in desert vegetation [6,7,8,9], and there are more studies on H. ammodendron sap flow, involving the study of evapotranspiration and influencing factors in artificial H. ammodendron forests in different regions [10], the seasonal variation of H. ammodendron stem sap flow and its relationship with meteorological factors [11], and the response of H. ammodendron sap flow to soil moisture [12]. There are also reports on studies of sap flow, water consumption and environmental factors in native H. ammodendron trunks in desert areas [13,14], and H. ammodendron sap flow is mostly explored in studies of daily changes in H. ammodendron sap flow, seasonal changes, water consumption characteristics and its meteorological and soil moisture factors, while a more systematic analysis of sap flow dynamic changes during the growth of desert H. ammodendron is lacking. Water consumption by plant transpiration is an important component in the water cycle and energy balance of forest ecosystems, an important indicator reflecting the water status of plants, and a key factor affecting regional and even global climate [15]. Poyatos et al. [16] summarized the existing research results on the relationship between stem sap flow and transpiration, stating that under normal circumstances, plant stem sap flow could accurately reflect the transpiration efficiency and water use status of a single plant in each time period. In this study, the typical sand fixation plant H. ammodendron in the Minqin oasis desert transition zone in northwest China was taken as the research object. The stem sap flow of different age classes of H. ammodendron during the whole growth season was continuously measured using a TDP pin-type plant stemflow meter, and the dynamic changes of stem sap flow of H. ammodendron in different forest ages and diameter classes were analyzed, so as to grasp the water consumption law at play in the growth process of H. ammodendron. This provides a theoretical basis for the restoration and protection of degenerated H. ammodendron forest and the construction of new H. ammodendron shelter forests.

2. Materials and Methods

2.1. Study Site

The study site is located near the Minqin Desert Control Comprehensive Experiment Station of Gansu Province at the southeastern edge of the Badain Jaran Desert (39°08′56″~39°09′02″ N, 103°36′54.4″~103°38′1.21″ E, altitude 1380 m). The region has a typical continental climate with a 7.3 °C annual average temperature, a 3289.1 °C effective temperature of greater than 10 °C, 110 mm of annual average precipitation mainly concentrated in the period from July to September, more than 2640 mm of potential evaporation, 168 d frost-free days, 3181 h of sunshine, 630 kJ cm⁻² of solar radiation, and year-round prevailing northwest wind with an annual average wind speed of 4.1 m s⁻¹. The soil is mostly sandy with poor nutrients and serious wind erosion. The landform is mostly semi-fixed sandy land with 3–10 m-high sand dunes. The vegetation of the study site was dominated by xerophyte shrubs, semi-shrubs and annual and perennial herbs, including shrubs H. ammodendron, Nitraria tangutorum and Artemisia arenaria, and herbs Phragmites australis, Kali collinum, Halogeton glom-eratus and Agriophyllum squarrosum, etc.

H. ammodendron is a typical regional artificial sand-fixing forest vegetation, due to different years of afforestation and the formation of different age structures of H. ammodendron stands, wherein the serious declining H. ammodendron stands are above 20 years of age, the distribution of stands is sparse, and the forest trees show a large area of death. We carried out supplementary planting and afforestation. The age structure of H. ammodendron forests is more complex. The elderly H. ammodendron more than 20 years old mainly rely on supplementary planting and afforestation, while the pure forest-age H. ammodendron is more common in mobile dune afforestation. The 5–20-year age groups of H. ammodendron were concentrated in the desert area, with basically the same climatic conditions, a neat structure of each stand, and the uniform growth of trees, and there were some differences in the density and growth of H. ammodendron stands affected by soil moisture in the stand, but the differences in soil moisture and stand density between stands of different ages were not significant (p < 0.05) (

Table 1. The information of plot and sample trees for H. ammodendron.

Stand Age	Number	Plant Height/cm	Ground Diameter/cm	Crown Breadth/cm	Stand Density/(Plant.hm−2)	Soil Moisture Content of 0–100 cm/%	Average Temperature from May to October/°C
20	N20-1	285.5	12.48	285 × 240	460 ± 247.09	2.032 ± 1.415	19.42 ± 6.28
	N20-2	300.5	12.97	227 × 266
	N20-3	272.5	6.46	197 × 164
	N20-4	211	7.16	186 × 185
15	N15-1	306.3	7.91	360 × 268	880 ± 162.86	2.896 ± 1.452	19.43 ± 6.40
	N15-2	400	7.6	250 × 238
	N15-3	356	9.04	398 × 350
	N15-4	370.4	6.2	236 × 198
10	N10-1	248.5	4.72	412 × 315	853 ± 172.57	2.003 ± 1.185	19.42 ± 6.28
	N10-2	253.5	4.51	322 × 217
	N10-3	293.5	5.42	365 × 328
	N10-4	275	6.39	366 × 278
5	N5-1	257	5.63	219.5 × 201.5	138 ± 20.09	2.839 ± 1.437	19.42 ± 6.29
	N5-2	199.5	5.78	212.7 × 135.4
	N5-3	216.7	5.28	211.5 × 113.7
	N5-4	113.4	4.43	182.6 × 141.8

).

2.2. Determination of the Stem Sap Flow and Water Consumption of H. ammodendron

Using the Minqin semi-fixed dune land artificial H. ammodendron as the research object, the age of H. ammodendron stands was determined by the age of silviculture and the H. ammodendron diameter class was divided by the frequency distribution of ground diameter through the preliminary survey of artificial H. ammodendron stands. The sample plots were selected from 5-, 10-, 15- and 20-year-old H. ammodendron forests; four H. ammodendron plants with similar growths were selected as the research target, and TDP-30 pin sensors were installed in each sample plant. The sensor probe length was 30 mm, the heating resistance was 50 Ω, and the heating voltage was 3.0 V. We installed the probes about 40 cm from the ground, referring to the TDP pin-type plant stem flow system instructions installed the sensor, sealed the probe jack with Plasticine, fixed the probe with plastic foam, and then wrapped and sealed it with tinfoil to reduce interference from sunlight and rain. The installed sensors were connected to the CR1000 data collector and then turned on, and a laptop computer was connected to the data collector to set the working parameters and enter the corresponding stem area values with a collection time interval of 10 min. The data were collected periodically and the sap flow rate was monitored synchronously during the growing season (May–October) in the sample plots of H. ammodendron. At the end of the H. ammodendron stem fluid flow monitoring, the basic growth indicators of the H. ammodendron sample plants were investigated and monitored (Table 1). After the installation of the equipment was completed, it was connected to the solar cell and the experiment started. The TDP consists of two probes (T-Type) with copper constantan thermocouples and a special heating wire. The two thermocouples are connected in the same way and the signal corresponds directly to the difference in temperature between the two probes of the sensor. The upper probe is heated using a constant current, while the lower probe is used as a reference. When the power is activated, the heat transfer from the tree sapwood increases with the increase in stem flow rate, and the heat diffusion from the sapwood increases with the increase in stem flow, resulting in a decrease in the heating source temperature. When the stem sap flow velocity is zero or very small, the temperature difference (dT) between the two sensors is maximum. With the increase in stem sap flow, the temperature difference between the two sensors decreases, and so we could obtain the stem sap flow data of the sample tree, collecting and recording them through the CR1000 data collector to synchronously monitor the sap flow rate of H. ammodendron in the growth season (May–October).

2.3. Calculation of Stem Fluid Flow of H. ammodendron

We used the analysis software provided by Ecotek to process and calculate the original data. The liquid flow rate formula of the plug-in stem flow meter was the Granier empirical formula [17]:

$${\mathrm{V}}_{\mathrm{h}}=\mathsf{\alpha }{\mathrm{k}}^{\mathsf{\beta }}=3600\times 0.0119\times {\left(\frac{\mathsf{\Delta }\mathrm{Tmax}-\mathsf{\Delta }\mathrm{T}}{\mathsf{\Delta }\mathrm{T}}\right)}^{1.231}$$

(1)

In Equation (1), V_h is stem sap flow velocity [cm³·(cm²·h)⁻¹], Δ is the maximum temperature difference between two probes at Tmax zero flux (°C), ΔT is the temperature difference between two probes for specific flux (°C), K is the dimensionless unit, and α and β depend on the heat coefficient.

According to the stem sap flow velocity, the liquid flow mass of a single plant can be calculated, and the calculation formula is:

$$\mathrm{Q}={\mathrm{V}}_{\mathrm{h}} \times {\mathrm{A}}_{\mathrm{s}} \times \mathrm{T}/ 1000$$

(2)

In Formula (2), Q is the mass of stem sap flow (kg), As is the sapwood area (cm²), and T is the time (h).

After the monitoring of stem sap flow of H. ammodendron is completed, the basal diameter and bark thickness of H. ammodendron sample plants are measured, and the sapwood thickness of H. ammodendron sample plants is measured with a growth cone. The formula for calculating the sapwood area is:

$$\mathrm{As}=\mathsf{\pi }{\left(\mathrm{r}-{\mathrm{r}}_{\mathrm{b}} \right)}^2 -\mathsf{\pi }{\left(\mathrm{r} -{\mathrm{r}}_{\mathrm{b}} - {\mathrm{r}}_{\mathrm{s}}\right)}^2$$

(3)

In Equation (3), As is sapwood area (cm²), r is base diameter (cm), r_b is bark thickness (cm), and r_s is sapwood thickness (cm).

The liquid flow of the H. ammodendron stem was collected once every 10 min, and the liquid flow rate and liquid flow mass of the H. ammodendron stem every 10 min were calculated according to Equations (1) and (2), respectively. The average value of the stem flow rate in 1 day was taken as the daily flow rate, the cumulative stem flow mass in 1 day was taken as the daily flow mass, the average value of the measured daily flow mass in a certain time period was taken as the daily average flow mass, and the monthly measured daily average flow mass under different weather conditions was determined. The monthly liquid flow quality was obtained by combining the weighted accumulation of the number of days with different weather occurring in each month.

2.4. Determination of Soil Moisture

We made dig soil profiles beside the four age classes of the H. ammodendron sample trees, installed EC-5 soil moisture sensors at 5, 20, 50, 100 and 150 cm, connected the soil moisture sensors at different levels to EM50 series data collectors, and downloaded data with ECH2O Utility software. The monitoring time was from April 2020 to November 2020, and the data monitoring interval was set as 1 h.

2.5. Investigation of H. ammodendron Forest

An artificial measurement method was used to investigate the H. ammodendron forest. A tower ruler was used to measure the height and crown width of H. ammodendron, and a vernier caliper was used to measure the ground diameter of H. ammodendron. We counted the number of H. ammodendron forests in each age class, and measured the forest area to obtain the stand density index.

2.6. Measurement of Meteorological Factors

The meteorological monitoring tower built by Gansu Institute of Desert Control is located beside the test site; the meteorological tower can record the temperature (T), relative air humidity (RH), photosynthetic effective radiation (PAR), net radiation (Rn), atmospheric pressure (P), wind speed (Ws) and precipitation (Rain) every 10 min.

2.7. Statistical Analysis

When processing the data, we selected the daily variation data of the stem sap flow of 4 age classes of H. ammodendron with similar trends and small differences for several consecutive days in each month, combined with the measured temperature and soil moisture, taking the representative data of several days to reduce the error. SPSS data processing software was used for statistical analysis. We used Excel2019 to process the TDP acquisition data and meteorological data, and then plotted them.

3. Results

3.1. Characteristics of Stem Sap Flow of Haloxylon ammodendron

The statistical results of stem sap flow rate and sap flow rate of different age classes of H. ammodendron are shown in Figure 1 and

Table 2. Statistical of sap flow of H. ammodendron at different ages.

Sap Flow	Age/Year	Month						Total
		May	June	July	August	September	October
Average of diurnal sap flow/kg	20	0.927 ± 0.522 b	2.508 ± 0.984 b	3.861 ± 1.670 b	3.436 ± 1.049 b	2.940 ± 0.658 b	0.382 ± 0.309 b	2.342 c
	15	3.828 ± 1.432 a	5.188 ± 3.201 a	8.065 ± 3.460 a	7.156 ± 2.991 a	6.232 ± 2.260 a	3.486 ± 0.783 a	5.659 a
	10	1.025 ± 0.383 b	2.427 ± 1.086 b	4.424 ± 0.941 b	3.488 ± 2.175 b	2.156 ± 1.004 b	0.326 ± 0.275 b	2.301 c
	5	2.828 ± 1.585 a	4.303 ± 1.568 a	4.731 ± 0.940 b	4.469 ± 1.433 b	2.679 ± 1.568 b	0.908 ± 1.201 b	3.318 b
Monthly of sap flow/kg	20	28.725 ± 16.187 b	75.299 ± 29.529 b	119.701 ± 51.771 b	106.523 ± 32.512 b	88.187 ± 19.740 b	11.847 ± 9.566 b	430.212 c
	15	118.665 ± 44.406 a	155.641 ± 96.016 a	250.025 ± 107.267 a	221.822 ± 92.723 a	186.970 ± 67.787 a	108.063 ± 24.268 a	1041.186 a
	10	31.765 ± 11.865 b	72.810 ± 32.581 b	137.132 ± 29.169 b	106.892 ± 67.419 b	64.683 ± 30.120 b	10.104 ± 8.512 b	423.386 c
	5	87.368 ± 49.149 a	129.105 ± 47.041 a	146.657 ± 29.135 b	138.54 ± 44.437 b	80.369 ± 47.039 b	28.134 ± 37.237 b	610.173 b

Different lowercase letters within the same column indicate significant differences between different ages at the 0.05 level; similarly for the following tables.

. The results show that the sap flow of the H. ammodendron stem had obvious daily and seasonal variation patterns. The daily variation process of stem flow showed a single-peaked or multi-peaked curve; the stem flow velocity was high in the daytime and low in the evening; the peak occurred at noon, and the maximum daily average flow velocity was 2.203~5.673 cm³·(cm²·h)⁻¹, which then began to decline, and the lowest stem flow velocity was reached in the evening. The seasonal growth variation in stem flow was obvious, and the stem flow was small in May when the H. ammodendron sprouted and grew. The average daily flow rate ranged from 0.247 to 1.238 cm³·(cm²·h)⁻¹, and the cumulative monthly fluid flow mass ranged from 28.725 to 118.665 kg. The H. ammodendron began to grow in June, and the stem fluid flow gradually increased from then, with the fastest growth in July and August. The average daily fluid flow rate ranged from 0.847 to 2.087 cm³·(cm²·h)⁻¹, and the cumulative monthly fluid flow mass ranged from 106.523 to 250.025 kg. In September, H. ammodendron growth decreased, the stem fluid flow slowed down, and the fluid flow rate decreased. In October, H. ammodendron growth basically stopped, the stem fluid flow was very small, and the average daily fluid flow rate ranged from 0.100 to 0.799 cm³·(cm²·h)⁻¹; the cumulative monthly fluid flow mass ranged from 10.104 kg to 108.063 kg.

The stem sap flow rate of H. ammodendron varied greatly with age class. The 15-year-old H. ammodendron grew vigorously with maximum stem sap flow, with an average sap flow rate of 1.460 cm³·(cm²·h)⁻¹ and a maximum sap flow rate of 5.259 cm³·(cm²·h)⁻¹ during the May–October growing season, and an average daily sap mass of 5.659 kg and accumulated sap mass of 1041.186 kg during the May–October growing season. The growth of the 20-year-old H. ammodendron declined and the stem sap flow was significantly weakened, with an average sap flow rate of only 0.570 cm³·(cm²·h)⁻¹, a maximum flow rate of 2.203 cm³·(cm²·h)⁻¹, an average daily sap mass of 2.342 kg, and a cumulative sap mass of 430.212 kg during the growing season from May to October. The growth of the 10-year-old H. ammodendron was weak, and the stem sap flow was small; the average sap flow rate was 1.059 cm³·(cm²·h)⁻¹ and the maximum sap flow rate was 5.673 cm³·(cm²·h)⁻¹ during the growing season from May to October. The average daily sap mass was 2.301 kg and the cumulative sap mass was 423.386 kg. The 5-year-old H. ammodendron showed stronger growth and greater stem sap flow, with an average sap flow rate of 0.956 cm³·(cm²·h)⁻¹, a maximum sap flow rate of 2.783 cm³·(cm²·h)⁻¹, an average daily sap mass of 3.318 kg, and a cumulative sap mass of 610.173 kg during the growing season from May to October. Among the changes in the flow of H. ammodendron at different forest ages, the change in the stem flow of H. ammodendron at 15 years was significantly different from that of other forest ages (p < 0.05). The difference in stem flow between the 10-year-old and 20-year-old H. ammodendron plants was not significant (p < 0.05). The stem flow of 5-year-old H. ammodendron was significantly different from that of the 10-year and 15-year-old H. ammodendron plants from May to June, and the difference between July to October and from the 20-year-old H. ammodendron was significant (p < 0.05) (Table 2).

3.2. Variation of Sap Flow in the Stem of H. ammodendron at Different Diameter Classes

Statistical analysis of the daily variation of stem sap flow rate in different diameter classes of H. ammodendron during the growing season showed (Figure 2) that the sap flow rate of H. ammodendron increased with the increase in diameter. The ground diameter growth of the 20-year-old H. ammodendron was 6.5~13.0 cm, and the daily average liquid flow velocity of different ground diameters increased with the diameter class in the order of 0.276, 0.682 and 1.070 cm³·(cm²·h)⁻¹, while the maximum liquid flow velocity was 0.535, 2.224 and 2.767 cm³·(cm²·h)⁻¹, respectively. The ground diameter of the 15-year-old H. ammodendron ranged from 6.2 cm to 9.0 cm, the average daily flow velocities were 1.199, 1.843 and 2.132 cm³·(cm²·h)⁻¹, respectively, and the maximum flow velocities were 2.806, 4.711 and 5.465 cm³·(cm²·h)⁻¹. The ground diameter of the 10-year-old H. ammodendron ranged from 4.5 cm to 6.4 cm, the average daily flow velocities were 0.651, 1.889 and 2.020 cm³·(cm²·h)⁻¹, respectively, and the maximum flow velocities were 1.904, 4.406 and 5.536 cm³·(cm²·h)⁻¹. The ground diameter of the 5-year-old H. ammodendron ranged from 4.4 cm to 5.8 cm. The average daily flow velocities were 0.574, 0.931 and 1.152 cm³·(cm²·h)⁻¹, respectively, and the maximum flow velocities were 1.145, 2.244 and 2.493 cm³·(cm²·h)⁻¹. As regards the stem sap flow of H. ammodendron in different forest ages, the sap flow of H. ammodendron in larger diameter classes started earlier and had a higher peak value. The starting time of sap flow of H. ammodendron with a large ground diameter was 7:00–8:00, and the peak value of sap flow of the 20-year-old H. ammodendron appeared from 12:00–13:00. Peak sap flow occurred at 13:00–16:00 for the 15-year-old H. ammodendron, and peak sap flow was between 11:00 and 14:00 for the 10-year-old H. ammodendron. The peak value of the 5-year-old H. ammodendron sap flow appeared between 10:00 and 11:00, and the time of reaching the peak value of sap flow was earlier with a lower age.

The statistical results regarding the monthly stem sap flow quality of different stem grades of H. ammodendron (

Table 3. Monthly of sap flow of H. ammodendron at different diameters.

Age	Ground Diameter	Month						Total
		May	June	July	August	September	October
20	6.46 cm	12.132 ± 0.385 b	21.543 ± 0.648 b	45.983 ± 2.055 b	29.348 ± 0.894 b	5.038 ± 0.157 c	7.611 ± 0.265 a	121.656 ± 0.734 b
	7.16 cm	5.001 ± 0.165 b	117.900 ± 1.552 a	163.079 ± 2.266 a	212.704 ± 2.125 a	145.387 ± 1.647 b	7.827 ± 0.262 a	651.899 ± 1.336 a
	12.97 cm	78.707 ± 1.942 a	156.430 ± 2.504 a	76.322 ± 1.734 b	271.458 ± 2.399 a	200.907 ± 1.960 a	29.230 ± 1.203 a	813.053 ± 1.957 a
15	7.60 cm	43.493 ± 1.191 c	140.459 ± 2.809 a	174.657 ± 1.653 b	134.561 ± 1.836 b	71.500 ± 0.954 c	44.640 ± 0.309 c	609.310 ± 1.459 c
	7.91 cm	193.837 ± 1.961 b	142.437 ± 2.724 a	268.040 ± 3.158 a	194.693 ± 2.100 b	196.383 ± 1.275 b	76.681 ± 0.456 b	1072.526 ± 1.946 b
	9.04 cm	176.647 ± 6.013 a	216.451 ± 5.829 a	377.157 ± 7.729 a	371.068 ± 6.215 a	349.935 ± 5.033 a	231.533 ± 1.918 a	1722.810 ± 5.456 a
10	4.51 cm	51.321 ± 0.610 c	47.126 ± 1.927 a	160.099 ± 1.251 b	12.985 ± 0.317 b	10.211 ± 0.410 c	2.596 ± 0.085 b	284.346 ± 0.767 a
	4.72 cm	65.394 ± 1.417 b	93.049 ± 1.468 a	119.920 ± 0.864 b	152.638 ± 1.855 a	110.834 ± 0.790 a	1.091 ± 0.032 b	542.926 ± 1.071 a
	5.42 cm	83.079 ± 1.492 a	92.994 ± 1.242 a	123.870 ± 0.942 a	154.302 ± 5.401 a	79.411 ± 0.826 b	20.743 ± 0.833 a	554.399 ± 1.789 a
5	4.43 cm	43.376 ± 1.022 a	52.386 ± 2.204 c	115.520 ± 0.681 b	89.127 ± 0.962 b	52.1123 ± 0.082 a	19.876 ± 0.451 a	372.397 ± 1.023 b
	5.28 cm	81.375 ± 4.204 a	97.061 ± 2.137 b	126.386 ± 1.130 b	137.146 ± 2.281 a	108.481 ± 5.107 a	26.108 ± 1.319 a	576.556 ± 2.697 b
	5.63 cm	120.804 ± 1.646 a	190.297 ± 2.254 a	178.963 ± 1.436 a	170.451 ± 1.948 a	79.945 ± 0.985 a	34.934 ± 1.927 a	775.395 ± 1.699 a

Different lowercase letters within the same column indicate significant differences between different ages at the 0.05 level.

) show that the mass of stem sap flow increased with the increase in diameter, and the difference in sap flow quality between months is obvious. The cumulative stem sap flow mass of the 20-year-old H. ammodendron during the growing season ranged from 121.656 to 813.053 kg, with significant differences (p < 0.05) between different diameter classes in September, and non-significant differences (p < 0.05) between different diameter classes in October. The cumulative stem sap flow mass of the 15-year H. ammodendron ranged from 609.310 to 1722.810 kg, with significant differences (p < 0.05) between different diameter classes in May, September and October, and non-significant differences (p < 0.05) between different diameter classes in June. The cumulative stem sap flow mass of the 10-year-old H. ammodendron ranged from 284.346 to 554.399 kg, with significant differences (p < 0.05) between different diameter classes in May and September and non-significant differences (p < 0.05) between different diameter classes in June.

The cumulative stem sap flow mass of H. ammodendron for the 5-year-old plant ranged from 372.397 to 775.395 kg, with significant differences (p < 0.05) between different diameter classes in June and non-significant differences (p < 0.05) between different diameter classes in May, September and October.

The stem sap flow of the 20-year-old H. ammodendron reached the maximum in July and August, with different diameter classes reaching 45.983, 212.704 and 271.458 kg, respectively. The stem sap flow of the 15-year-old H. ammodendron reached the maximum in July, with different diameter classes reaching 174.657, 268.040 and 371.068 kg, respectively.

The stem sap flow of the 10-year-old H. ammodendron reached its maximum in July and August, and the different diameter classes reached 160.099, 152.638 and 154.302 kg, respectively. The stem sap flow of the 5-year-old H. ammodendron reached its maximum in July and August, and the different diameter classes reached 115.520, 137.146 and 178.963 kg, respectively.

3.3. Correlation Analysis between Stem Sap Flow and Meteorological Factors of H. ammodendron in Different Forest Age

From 1 May to 30 October 2020, the stem flow rate of four age classes of H. ammodendron in each month showed a significant correlation with meteorological factors (

Table 4. The correlation analysis between stem sap flow of H. ammodendron in different age classes and with meteorological factors.

Age Class	Months	T/°C	RH/%	Rn/(Wm−2)	Ws/(ms−1)
20 years	5	0.685 **	−0.565 **	0.385	0.557 **
	6	0.274 *	−0.147	0.241	0.168
	7	0.479 **	−0.363	0.427 *	0.382 **
	8	0.574 **	−0.550 **	0.442 *	0.185
	9	0.569 **	−0.513 **	0.207	0.503 **
	10	0.434 **	−0.274	0.187	0.214
15 years	5	0.743 **	−0.602 **	0.758 **	0.516 **
	6	0.128	0.189	0.112	0.125
	7	0.586 **	−0.691 **	0.597 **	0.605 **
	8	0.544 **	−0.448 **	0.480 **	0.385 **
	9	0.659 **	−0.547 **	0.630 **	0.551 **
	10	0.423 *	−0.16	0.553 **	0.068
10 years	5	0.835 **	−0.726 **	0.366 *	0.785 **
	6	0.165	−0.208	0.038	0.105
	7	0.613 **	−0.717 **	0.646 **	0.693 **
	8	0.826 **	−0.791 **	0.544 **	0.722 **
	9	0.725 **	−0.645 **	0.229	0.619 **
	10	0.363 *	−0.221	0.392 *	0.195
5 years	5	0824 **	−0.731 **	0.687	0.712 **
	6	0.352	−0.288	0.186	0.331
	7	0.827 **	−0.732 **	0.789 **	0.709 **
	8	0.822 **	−0.770 **	0.642 **	0.785 **
	9	0.769 **	−0.613 **	0.207	0.503 **
	10	0.462 *	−0.274	0.243	0.271

** Indicates extremely significant correlation (p < 0.01); * significant correlation (p < 0.05).

). The correlation between stem flow rate and T was the highest in May, August, September and October, and the correlation between stem flow rate and RH was the highest in June and July. The degrees of influence can be ranked as T > RH > Ws > Rn. The correlation between stem flow rate and Rn of the 15-year-old H. ammodendron was the highest in May and October, the correlation between stem flow rate and RH was the highest in July, and the correlation between stem flow rate and T was the highest in September. The degrees of influence of meteorological factors can be ranked as Rn > T > RH > Ws. The stem flow rates of 15-, 10- and 5-year-old H. ammodendron had no correlation with meteorological factors in June. The correlation between stem flow rate and T in 10-year-old H. ammodendron was the highest in May, August and September, the correlation between stem flow rate and RH was the highest in July, and that between stem flow rate and Rn was the highest in October. The influences of meteorological factors on stem flow rate of H. ammodendron can be ranked as T > RH > Ws > Rn. The stem flow rate of 5-year-old H. ammodendron had the highest correlation with T; the influence degrees of meteorological factors on the stem flow rate of H. ammodendron can be ranked as T > Ws > RH > Rn. The smaller the age class of H. ammodendron, the stronger the response of stem flow rate to meteorological factors.

4. Discussion

The sap flow in the stem of H. ammodendron depends mainly on changes in environmental factors, and meteorological factors play a decisive role in instantaneous changes in plant sap flow. Previous studies have shown that meteorological factors such as temperature, total solar radiation, and relative air humidity have significant effects on plant sap flow [17,18,19,20,21,22,23]. Soil moisture determines the overall level of plant sap flow to some extent; for example, Gurbantunggut Desert is affected by winter snow collection and the H. ammodendron sap flow quality is the highest in April. As H. ammodendron transpiration intensifies and soil moisture decreases, H. ammodendron transpiration becomes limited by moisture conditions, making H. ammodendron sap flow continuously stable at a lower level [24,25]. The pattern of sap flow in the stem of H. ammodendron monitored in the sample plots at different ages in this study was more or less the same, and the sap flow showed obvious diurnal variation. The diurnal variation was large, and the daytime sap flow was significantly higher than that at night. At the same time, the seasonal variation in sap flow was significant, with sap flow starting in May, increasing to a peak in July–August, and then gradually weakening until it stopped. The diurnal and seasonal variations in sap flow in the stem of the H. ammodendron are consistent with the sap flow processes of most desert plants such as Populus euphratica, Caragana korshinskii, Hippophae rhamnoides, and Lycium chinense [22,23,24,25,26,27,28,29,30].

In May, H. ammodendron branches begin to sprout; as the observation site’s temperature changes between 5 and 25 °C, windy weather promotes the transpiration rate of H. ammodendron, and these more suitable environmental conditions make H. ammodendron branches develop rapidly. The water consumption of H. ammodendron in July–August is much higher than that of the other three forest ages. The reason is that the temperature in July and August reaches the highest throughout the observation period, and the rainfall is smaller. The higher temperature makes the transpiration of H. ammodendron strong, and the water consumption increases. The larger canopy of H. ammodendron at 15 years makes the transpiration water consumption higher than that of the other two age classes.

The highest rainfall occurred in September, while the average water consumption of the four age classes of H. ammodendron was low, because the temperature decreased in September and the content of abscisic acid in H. ammodendron reached the highest [29]. The increase in abscisic acid encouraged H. ammodendron to enter a dormant state [30,31], resulting in the low water consumption of H. ammodendron. The correlation between the stem sap flow of four age classes of H. ammodendron in June and various meteorological factors is not significant. The reason is that the temperature in June is high, the rainfall is very little, the soil moisture content is low, and drought weakens the photosynthesis and transpiration of H. ammodendron. This is consistent with the conclusion of Pengfei Sun [13], who found that the correlation between stem sap flow and meteorological factors is not significant under soil water stress.

The daily variation in H. ammodendron fluid flow showed different unimodal or multi-peaked curves, which were related to rainfall and temperature. Before rainfall, the air humidity increases, the temperature decreases, and the photosynthetic rate decreases. During rainfall, the crown of H. ammodendron can directly absorb some water that will participate in the stem flow, resulting in a multi-peak pattern of stem flow changes. At higher temperatures, the self-protective shut-down of the stomata of H. ammodendron, in order to prevent water loss, halts photosynthesis, which significantly reduces the flow of stem sap [12,16].

The main factors affecting the changes in stem sap flow of H. ammodendron differed due to different monitoring time and space scales. According to previous studies, the instantaneous change of H. ammodendron sap flow rate is mainly affected by meteorological factors, while soil temperature and humidity have little impact on the change in trunk sap flow rate at the daily scale, but there is a significant correlation between the time scale of seasons and the daily average sap flow rate [31,32]. In this study, we set up the simultaneous monitoring of H. ammodendron sap flow in different age sections, focusing on H. ammodendron growth processes and inter-individual variability at larger spatial and temporal scales, and minimizing the influence of meteorological environmental factors. The study found that the growth of the 20-year-old H. ammodendron declined, and the daily average liquid flow mass was only 2.342 kg. The accumulated liquid flow mass in the growth season of May–October was 430.212 kg, which is consistent with the water consumption of aged H. ammodendron in the same region (495~1232 kg in June–November) as measured by Zhang Xiaoyan et al. [10]. In contrast, the average daily liquid flow rate of the 10-year-old H. ammodendron was 1.059 cm³·(cm²·h)⁻¹, and the maximum liquid flow rate was 5.673 cm³·(cm²·h)⁻¹, which is similar to the maximum liquid flow rate (0.75~0.85 dm·h⁻¹) of the original H. ammodendron with a 33 mm stem class studied by Li Hao et al. [14]. The variation in H. ammodendron fluid flow at different diameter levels reflects the fact that there is also a clear competitive advantage yielded by growth differences. Large-diameter H. ammodendron plants have an obvious competitive advantage, growing with high water consumption that lasts longer, while small-diameter H. ammodendron plants have a weak competitive ability, consuming less water, growing more weakly and facing elimination.

In arid desert habitats, soil moisture is an important factor affecting the sap flow of H. ammodendron, and the dominant factors affecting the sap flow process of H. ammodendron stem vary greatly in different forest sites with different soil moisture conditions. Pengfei Sun et al. [13] studied the water consumption characteristics of sap flow in the trunk of native H. ammodendron in the Gurbantunggut Desert, and concluded that sap flow in the root zone of H. ammodendron was significantly correlated with soil moisture when the soil volumetric moisture content was between 8.7 and 12.1%. Sap flow was more influenced by meteorological factors when the soil moisture content was >12.1%, and sap flow was more influenced by plant physiological characteristics when the soil moisture content was <8.7%. In this study, the average volumetric moisture content of soil in the root zone (50–150 cm) of H. ammodendron at 20, 15, 10 and 5 years of age was 1.627%, 2.859%, 1.243% and 2.142%, respectively, and the change in soil level moisture at each age was basically the same and was consistent with the change in the flow rate of H. ammodendron stem sap, but the magnitude of level change was greater (Figure 3). The average soil moisture in June and August in the 150 cm-deep layer of 15-year-old H. ammodendron stands was 4.820%, which is significantly higher than the soil moisture in its upper layer, and the H. ammodendron was able to make full use of soil moisture to grow vigorously. The soil moisture in the shallow 50 cm layer of the 5-year-old H. ammodendron was slightly better than that in the lower soil layer, but the difference in soil level moisture was not significant, and the soil moisture from July to September was 2.555%, 1.763% and 2.108% as the soil layer deepened. H. ammodendron can take full advantage of better soil moisture conditions and grow rapidly. The shallow 50 cm soil of 10-year-old H. ammodendron was affected by the wind and sand environment, and the average soil moisture from June to August was only 0.628%, which is significantly lower than that in the lower soil. The soil level moisture conditions were relatively poor, but H. ammodendron can adapt to a stressful moisture environment and grow normally. The soil level moisture of the 20-year-old H. ammodendron was relatively stable; the 150 cm-deep layer’s moisture was 1.843%, which is significantly better than the upper layers. H. ammodendron can use limited soil moisture to survive, but its growth is obviously affected. It can be seen that H. ammodendron is highly tolerant to drought stress and can adapt to different soil water environment; it also exhibits different physiological drought resistance characteristics related to growth, which lead to significant changes in the stem fluid flow in different age stands [32].

5. Conclusions

The results of this study show that the density of 5-year-old H. ammodendron stands was small and that the water content in the forest land was good, which could promote the rapid growth of H. ammodendron; the stem sap flow was also significantly enhanced. The daily average sap flow rate was 0.956 cm³·(cm²·h)⁻¹, and the cumulative liquid flow mass in the growing season was 610.173 kg. The stem sap flow of the 10-year-old H. ammodendron was relatively weak, with a daily average sap flow rate of 1.059 cm³·(cm²·h)⁻¹, and a cumulative mass of 423.386 kg in the growing season, which was mainly restricted by the low soil moisture and high stand density of the H. ammodendron forest. Generally, 15-year-old H. ammodendron can make good use of deep soil water and grow vigorously. The stem sap flow was large. The daily average sap flow rate was 1.460 cm³·(cm²·h)⁻¹. The accumulated liquid flow mass in the growing season was 1041.186 kg. The 20-year-old H. ammodendron plant showed a large growth volume and the excessive consumption of soil water in the forest land, leading to decline and death. The stand density decreased, the stem sap flow significantly weakened, the daily average sap flow rate was 0.570 cm³·(cm²·h)⁻¹, and the cumulative liquid flow quantity in the growing season was 430.212 kg. The water consumption of H. ammodendron plantations in Minqin of China is the largest at the age of 15. Although 20-year-old H. ammodendron can use groundwater, its water consumption is low and the mortality rate in plantations is high. This may be because the water consumption of H. ammodendron forests causes the groundwater level in the forest area drop, and the groundwater absorbed by H. ammodendron is not sufficient to maintain its physiological activity. Therefore, it is necessary to reasonably plan the initial density of the H. ammodendron plantation so as to slow down its future consumption of soil moisture, and to thus maintain the stability of the H. ammodendron plantation and activate greater ecological benefits.