Rich dynamics of a vegetation–water system with the hydrotropism effect

In recent years, with the abnormal global climate change, the problem of desertification has become more and more serious. The vegetation pattern is accompanied by desertification, and thus, the study of the vegetation pattern is helpful to better understand the causes of desertification. In this work, we reveal the influences of hydrotropism on the vegetation pattern based on a vegetation–water system in the form of reaction–diffusion equations. Parameter ranges for the steady-state mode obtained by analyzing the system show the dynamic behavior near the bifurcation point. Furthermore, we found that vegetation hydrotropism not only induces spatial pattern generation but also promotes the growth of vegetation itself in this area. Therefore, through the study of vegetation patterns, we can take corresponding preventive measures to effectively prevent land desertification and improve the stability of the ecosystem in the region.


Introduction
Vegetation is an important part of nature and plays a leading role in the ecosystem, known as the "Ecological Engineer" [1,2]. As a producer, vegetation can convert carbon dioxide into carbohydrates through photosynthesis and release oxygen and store energy. Moreover, vegetation coverage on the ground can not only reduce water and soil loss and protect slope land but also prevent wind and sand fixation and prevent desertification [3,4]. In nature, the growth of vegetation will be affected by climatic conditions, geographical environment, and human activities. Nowadays, with the rapid development of irrationality in the human society and global climate change, the vegetation ecosystem has been seriously damaged, and the problem of land desertification is becoming more and more serious [5][6][7].
In particular, in dry and semi-dry areas, because of its climatic characteristics, the problem of land desertification is particularly prominent. In the process of land desertification, vegetation distribution is uneven, but there are certain rules. We call this uneven and regular spatial distribution of vegetation as the vegetation pattern [8][9][10]. In addition, different vegetation pattern structures have different significances to the function of the ecosystem, such as the strip pattern can be used as a sign of semi-desert [11]. For the study of vegetation spatial patterns in these areas, many scholars established a series of a dynamic system. Based on ecologically realistic assumptions, Klausmeier established a model with vegetation and water in 1999 and gave suitable parameters, and two pattern types can be found by numerical simulations (regular patterns and irregular patterns). This model helps us understand how rainfall and grazing affect vegetation in semi-arid regions and demonstrates the importance of nonlinear mechanisms to the spatial structure of plant community [12]. In 2001, Von Hardenberg proposed a new vegetation-water system, which simulates the competition of vegetation roots for water resources [13]. In 2007, Gilad constructed a mathematical system for the study of the woody plant ecosystem in arid areas and captured various feedback mechanisms between biomass and water resources through this system [14]. Water diffuses freely in soil, and the original Klausmeier system does not consider the diffusion of water. Therefore, Vander Stelt took the diffusion of water into account on the basis of Klausmeier's system in 2012 [15]. In 2015, Zelnik simplified Gilad's model and combined with empirical data to study the dynamics of Namibia's Andromeda ecosystem. The research showed that the pattern trend changes gradually in the spatially expanded ecosystem [16]. Moreover, other scholars have also established relevant systems to study vegetation patterns [17][18][19][20][21][22][23][24][25].
It is known to all that water is the source of life. Vegetation needs water for photosynthesis and respiration to obtain nutrients needed for vegetation growth. Moreover, when transpiration takes away a lot of heat, water can maintain the normal temperature of vegetation to maintain life activities [26,27]. Vegetation absorbs water from the soil mainly through the root system to provide water necessary for life activities. In response to a moisture gradient, the roots of vegetation will show the characteristics of hydrotropism [28][29][30][31]. In dry and semi-dry areas, soil water distribution is uneven because of less and concentrated rainfall [32,33]. Therefore, the vegetation root system will absorb the water in other humid areas through the extension of the root system to meet its own growth and development in these areas. At present, it is not clear how the hydrotropism effect of vegetation roots affects the growth and distribution of vegetation. Therefore, to reveal the influence of hydrotropism of vegetation roots on vegetation growth and distribution, we build a spatial system with root hydrotropism effects based on the system of Klausmeier in this paper [34][35][36].
The following is the content arrangement of this article. In the first place, we propose a reaction-diffusion system with hydrotropism effects and analyze the existence and stability Schematic chart of hydrotropism of vegetation roots and the arrow direction indicates that the soil moisture is getting higher and higher. The roots of vegetation grow in areas to obtain more water resources. about the equilibrium point. In the next section, we derive the amplitude equation and its coefficients through the method of multi-scale analysis. In the fourth section, on the basis of the theoretical results obtained in the previous section, we conduct a numerical simulation to obtain the dynamic behavior and show the hydrotropism of vegetation's impact on the vegetation pattern. Finally, the last section gives conclusions and discussions on the effect of hydrotropism on vegetation growth and distribution.

Mathematical model
In dry and semi-dry regions, soil water replenishment was mainly derived from rainfall due to their geographical environment. After the rainfall reaches the ground, one part of the rainfall penetrates into the soil and becomes groundwater or surface runoff, and the other part is lost to the atmosphere through the transpiration of vegetation and evaporation of the ground. These areas receive less rainfall because of their climatic conditions. Therefore, soil water distribution in these areas is uneven, forming a moisture gradient. In response to a moisture gradient, the roots of vegetation will show the characteristics of hydrotropism to get more water for basic life activities ( Figure 1).
The spatial motion of hydrotropism of vegetation roots is to absorb more water. In this sense, it is assumed that the diffusion rate of vegetation hydrotropism is proportional to the diffusion speed of water. Consequently, based on the Klausmeier system [12], we establish a system with water diffusion and vegetation hydrotropism effects as follows: where P is the vegetation biomass and W is the soil water volume; parameters A, M, L, D 1 , D 2 represent rainfall, plant mortality, evaporation rate, vegetation diffusion rate, and water diffusivity, respectively. The term ξ zW zX describes the surface runoff which is proportional to the slope of terrain, where ξ is a constant downhill runoff flow velocity, and the term RWP 2 indicates the absorption of soil moisture by the roots of vegetation, reflecting the mechanism of long-distance inhibition and short-distance promotion [37]. D is the hydrotropism rate of the vegetation.
For the convenience of mathematical analysis, we reduce the numbers of parameters by dimensionless transformation on the system (2.1) and obtain the following system: In this paper, we mainly study the flat ground without considering the slope's influence on the formation of the pattern structure. Therefore, our system is finally simplified as follows:

Order
Corresponding equation

Mathematical analysis of the model
3) without considering diffusion is as follows: dp dt Make the right end of Eq. 2.4 equal to 0, and calculate the equilibrium point. System (2.3) has three equilibrium points, including a semi trivial steady-state solution and two non-trivial steady-state solutions: (1) E 0 = (p 0 , w 0 ) = (0, a), which corresponds to no vegetation.
Because the equilibrium point E 1 is always unstable when diffusion is not considered, we will only study the equilibrium point E 2 given in the following section. First, linearize system (2.3) at E 2 to obtain zp zt a 11 p + a 12 w + d 1 Δp + dΔw, where a 11 m, a 12 We set a perturbation to the uniform stationary solution (p 2 , w 2 ) and expand it in Fourier space: Substituting Eq. 2.6 variables into Eq. 2.5, one can obtain its following characteristic equation: It is equivalent to the following equation:

Pattern form Expression Existence interval Stability
Homogeneous state Then, necessary conditions for system (2.3) to generate bifurcation behavior are as follows: According to the necessary conditions of Hopf bifurcation (2.9) and Turing bifurcation (2.10), we select a as the control variable, and the system branch diagram of system (2.3) can be drawn, as shown in Figure 2. At the same time, the dispersion relationship of system (2.3) is demonstrated in Figure 3.
Obviously, Figure 3 shows that within an appropriate parameter range, as the parameter d increases, the real part of the eigenvalue gradually increases and the Turing patterns appear.

Amplitude equations
Generally, the pattern structure is influenced by the most active mode of the system. The amplitude equation can describe the system's dynamic behavior around the most active mode [10]. Therefore, we can use the amplitude equation to research the system's dynamic behavior around the Turing bifurcation point. The pattern structure is represented by three pairs of resonance modes (] j , − ] j ), which are 120°angles. For this paper, FIGURE 4 For different pattern structures, the parameter values are shown in Table 3. (a 1 )-(c 1 ) is the water pattern, and (a 2 )-(c 2 ) is the vegetation pattern. Among them, (a 1 ) and (a 2 ) are spot patterns, (b 1 ) and (b 2 ) are mixed patterns, and (c 1 ) and (c 2 ) are strip patterns.
Frontiers in Physics frontiersin.org  Frontiers in Physics frontiersin.org 06
Frontiers in Physics frontiersin.org 07 through the method of multi-scale analysis, the amplitude equation of system (2.3) and its coefficients are derived. Near a = a T , the solution can be written in the following form:

Multi-scale analysis
Making c p w , N N 1 N 2 , system (3.1) can be written as follows: Expand a, c, L, N as follows: L T , b, h 2 , h 3 are as follows: Decompose the time scale (T = t, T 1 = εt, T 2 = ε 2 t), then the derivative of the amplitude A with respect to time is Substituting (3.4) into formula (3.3), we get the equation of different order of ε (see Table 1).
For the order of ε, where L T is the model's linear operator at the critical point, and p 1 w 1 is a linear combination of the eigenvectors corresponding to zero eigenvalues of the linear operator L T . The solution of equation is .
For the order of ε 2 , According to the solvability condition of Fredholm, to guarantee the existence of equation's non-trivial solution, the right term of Eq. 3.8 must be the same as the L + T zero eigenvalue conjugate. The zero characteristic value of L + T is 1 Use F j p and F j w to represent the coefficients corresponding to e i]jr in F p and F w .
. According to the conjugation condition, Then, It can be found that the second-order term's coefficient value is more than 0 from all the aforementioned equations, which will cause the amplitude of W j to diverge, and higher-order terms need to be introduced to saturate it. Therefore, the solution of Equation 3.8 is written in the following form: (3.12) Similarly, use H j p and H j w to represent the coefficients corresponding to e i]jr in H p and H w .

Pattern phase transition induced by soil water diffusion
In this section, we will study the influence of soil water diffusion on vegetation in dry and semi-dry places. In these areas, the lack of water resources causes the diffusion of soil water mainly caused by concentration differences, that is, absorption water feedback. For different vegetation, the ability to absorb water is different, which will lead to the concentration difference of soil water. It is reflected in the parameter d 2 in our model; the greater the concentration difference of soil water, the greater is the parameter d 2 . In order to better reveal the effects of different water diffusion intensities on the vegetation pattern, we do not consider the hydrotropism of vegetation and fix the parameters d = 0 and d 1 = 1; then, we can change the parameter d 2 to reflect different water diffusion intensities. Figure 5 shows the change in the corresponding vegetation pattern with the change in the parameter d 2 . With the increase in d 2 , the vegetation pattern structures change in the following sequence, gap pattern, mixed pattern, strip pattern, and spot pattern. Moreover, we also find that when d 2 ≤ d 1 , no vegetation pattern is generated; when d 2 > d 1 , with the increase in the parameter d 2 , the gap between the pattern becomes larger and larger. In fact, the difference in the soil water concentration reflects the difference in the water absorption capacity among vegetation. The greater the diffusion intensity of the water beside the vegetation, the stronger is the absorption capacity of the vegetation for water, which will promote its own growth more. Conversely, as the intensity of diffusion of water next to the vegetation increases, more water will flow to its location, which will suppress the growth of the nearby vegetation. It shows that the vegetation pattern is formed through the long-range competition and short-range promotion mechanism.

Influence of hydrotropism on the vegetation pattern
Due to the climatic conditions and geographical environment in dry and semi-dry regions, the soil water resource distribution is uneven, and a water gradient is formed. Because of the presence of the moisture gradient, roots exhibit hydrotropism characteristics. For different vegetation, the intensity of the hydrotropism of the vegetation root is different. Some vegetation roots are sensitive to soil moisture, but some vegetation roots are relatively weak. Therefore, in this section, we study the influence of different vegetation growth abilities of water on the vegetation in the area. In our spatial model, we fixed other diffusion parameters, that is, d 1 = 1, d 2 = 1, and observed the influence of hydrotropism on the vegetation pattern structures by changing the parameter d. Figure 6 shows the pattern structures of vegetation under different root hydrotropism intensities. Through theoretical analysis and numerical simulation, we know that when we do not consider the hydrotropism of vegetation roots in our model, let d 1 = d 2 , in which no vegetation pattern will be generated. Therefore, the hydrotropism of vegetation roots can induce the generation of vegetation patterns.
In Figure 6, we show the influence of the parameter d on the vegetation pattern structures. When the parameter d is small, the gap pattern appears ( Figure 6A). When the parameter d increases slightly, the gap pattern begins to disappear, the strip pattern appears, and the vegetation pattern structure becomes mixed gap and strip pattern structures ( Figure 6B). When the parameter d continues to increase, the gap pattern completely disappears, showing the strip pattern structure ( Figure 6C). Subsequently, with the increase in the d parameter, the spot pattern gradually appears (Figures 6D,F). To sum up, gradually enhance the hydrotropism effect of roots, then the vegetation pattern's structure changes in the following sequence: the gap pattern, mixed pattern, strip pattern, and spot pattern. In Figure 7, we show the realistic vegetation distribution corresponding to numerical simulation pattern structures.
Moreover, in Figure 8, we show the relationship between the root hydrotropism effect and average vegetation biomass. We can find that with the increasing intensity of the hydrotropism of the vegetation root, the average density of vegetation increases. However, as the intensity of vegetation to grow toward water increases, the interval of plants also becomes larger, which means that the stability of the ecosystem in the region is also reduced.
Owing to the fact that water at the location of the vegetation itself is not enough to meet the needs of its own growth, the vegetation has to extend to the humid area to obtain water resources through the roots, which makes the root system of the vegetation exhibit hydrotropism. For different vegetation, the degree of development of the root system is different. When the root system of vegetation is developed, vegetation has higher hydrotropism effects, which means it can absorb more water and other vegetation cannot obtain enough water resources because of limited water resources in these dry regions. A large amount of vegetation died due to the lack of water, resulting in vegetation degradation. In other words, increasing the intensity of the hydrotropism of roots in a range can increase the average biomass of vegetation. However, excessively increasing the intensity of the hydrotropism of roots will lead to desertification.

Conclusion
In our paper, we mainly analyze spatial dynamics with hydrotropism effects. First, on the basis of Klausmeier's system, a spatial system is established considering the hydrophilic effect. Then, the existence and stability of the system equilibrium point are analyzed. Second, through the method of multi-scale analysis, the amplitude equation of the system and its coefficients are derived [38][39][40]. Finally, we numerically simulated the system to show the influence of vegetation root hydrotropism on vegetation growth distribution.
Bases on the numerical results, we find that the hydrotropism of vegetation roots can induce pattern generation. Furthermore, Frontiers in Physics frontiersin.org with the enhancement of the root hydrotropism effect, pattern structures change as follows: the spot pattern, mixed pattern, strip pattern, and gap pattern. Moreover, we also study the relationship between the root hydrotropism effects and average vegetation biomass. We find that increasing the intensity of the hydrotropism of roots in a range can increase the average vegetation biomass which is consistent with the results in [35,41]. However, excessively increasing the strength of root hydrotropism, the spacing between the vegetation pattern also increases, which means that the ecosystem stability is reduced and prone to land desertification. We all know that many factors will affect vegetation distribution in the real world, including climatic conditions (such as temperature, light, and carbon dioxide concentration), topographic conditions (such as mountains and plains), and human activities (such as deforestation and grazing) [41][42][43][44][45][46][47][48][49][50]. However, in our system, we only consider rainfall and vegetation root hydrotropism effects. Therefore, in order to better protect vegetation, prevent land desertification, and improve the stability of the ecosystem, we hope to establish a more realistic vegetation dynamic system, including climatic conditions, internal growth mechanism of vegetation, human activities, and other factors, to better study the dynamic mechanism of spatial vegetation and reveal relevant factors affecting the structure of the vegetation pattern.

Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.