1. Introduction
In the field of plant ecology, the allocation of biomass among different organs is a core issue [
1] that not only reveals the strategies of plant adaptation to environmental conditions [
2] and also reflects the distribution of photosynthetic products among various organs [
3]. The optimal partitioning hypothesis suggests [
4] that plants prioritize resource allocation to organs that acquire limiting resources to maintain the highest growth rate [
5,
6,
7]. For example, under limited light conditions, biomass tends to be allocated to leaves and branches, whereas in environments limited by nutrients or water, biomass is often allocated to roots [
8]. However, given that the proportion of biomass allocation changes with plant size [
9,
10], the application of the optimal partitioning hypothesis is limited. Allometric growth theory offers a solution to this problem [
11,
12]. It considers the effects of genetic traits and environmental factors on biomass allocation, revealing the intrinsic laws of biomass distribution among plant organs [
13]. This theory has been widely applied in biomass allocation research [
14], and scholars have suggested integrating the optimal partitioning hypothesis with allometric growth theory. The biomass of plants and its allocation among various organs reflect the plant’s response to environmental resource utilization and are the basis for intraspecific differentiation strategies [
15]. Although current research has mostly focused on different species or provenance levels [
16,
17,
18,
19], studies on intraspecific different site types remain scarce. Therefore, further exploration in this field will provide new insights to plant ecological research.
Allometry refers to the phenomenon wherein the relative growth rates of two traits in an organism are disproportionate and determined by the genetic characteristics of species and constrained by environmental factors [
20,
21]. It not only reveals the intrinsic laws of biomass allocation among plant organs, it also plays an important role in describing the structural and functional characteristics of biological components [
22,
23]. Existing studies have shown that allometric growth indices vary between different plant organs; this variation may be related to the growth patterns and structural characteristics of each organ [
24,
25]. Therefore, analyzing the allometric relationships among plant components is important for understanding the adaptive strategies of plants when they face environmental stresses [
26]. Plant allometry is closely related to morphological changes, biomass allocation, resource utilization, and morphological adaptation to heterogeneous environments [
27]. The allometric relationship between plant height and diameter plays an important role in estimating the maximum heights of different functional groups of plants, constructing biomass estimation models, and deriving allometric relationships between individual size and density [
28]. Existing research covers a range of fields from individuals [
29] to populations [
30,
31] and from ecosystems to evolutionary ecology, molecular biology, and theoretical physics, providing new perspectives for understanding the evolutionary importance of life. However, studies have concentrated on tree species, with allometric growth research based on the scale of shrub fascicular branches [
32], especially for sandy shrublands, being lacking.
The Hunshandake Sandy Land, one of the four major sandy lands in Inner Mongolia, not only lies a mere 180 km straight-line distance from China’s capital, Beijing, it also plays a crucial role in ecological and environmental protection [
33]. This vast sandy area is the frontline of sand prevention and control in Northern China, wherein the success of ecological restoration directly influences the ecological security and climate stability of Beijing and the North China region [
34].
Salix gordejevii Y.L. Chang & Skvortsov, endemic to Mongolia and Northen China, a xerophytic shrub, can rapidly generate adventitious roots and form new branches when buried by shifting sands [
35], becoming the pioneer or dominant species in mobile and semimobile dunes [
36]. Given its outstanding performance as a windbreak and in sand fixation, it has become an indispensable living sand barrier in improving the ecological environment of sandy areas and reversing desertification [
37,
38]. Studying the allometric growth characteristics of
S. gordejevii in extreme environments, such as the Hunshandake Sandy Land, is of great importance for ecology and environmental science [
39]. Allometric growth, as a key indicator of plant adaptability and biomass allocation, can reveal how
S. gordejevii adjusts its growth strategies in resource-limited desert environments [
40]. By thoroughly understanding the growth patterns of
S. gordejevii, we can predict and manage the restoration of desert ecosystems, thus providing a scientific basis for desertification control [
41]. Furthermore, research on the allometric growth and biomass allocation of
S. gordejevii can help us understand the adaptation mechanisms of plants against the backdrop of global climate change, hence offering important references for the formulation of ecological protection and sustainable development strategies. Therefore, the study of the allometric growth of
S. gordejevii is not only an exploration of the ecological adaptability of a single species, it is also a comprehensive contribution to maintaining biodiversity and ecological balance.
This study focuses on S. gordejevii in different habitats of the Hunshandake Sandy Land. It employed allometric growth theory to analyze the allometric relationships between the branch length and base diameter of S. gordejevii fascicular branches, examined their component biomass and allocation patterns, and discussed the allometric relationships between component biomasses. Its aim is to reveal the resource allocation strategies of S. gordejevii fascicular branches and hence provide a scientific basis for ecological restoration in sandy lands.
4. Discussion
4.1. Allometric Growth Relationship between Fascicular Branch Length and Basal Diameter
We observed a balance between vertical and horizontal growth directions from the perspective of S. gordejevii fascicular branches across different site types. This balance indicates that an allometric growth relationship exists between branch length and basal diameter. Multiple tests of the slopes revealed that except for mobile dunes, the slopes of other site types were significantly lower than those of mobile dunes, and the slope of mobile dunes was significantly greater than the theoretical value. Slopes did not significantly differ between semifixed and fixed dunes nor from the theoretical value. The slope of interdune lowlands was significantly lower than the theoretical value and lacked a significant difference from those of semifixed and fixed dunes.
The results suggest that when the
S. gordejevii population in mobile dunes is in the growth phase of its life cycle, young fascicular branches have a high individual count, and their rapidly elongating branches can effectively perform photosynthesis and expand the range of seed dispersal. Although many studies have assumed that consistent with the geometric self-similarity model [
44,
45,
46], the growth rates of plant height and basal diameter are similar in the juvenile stage or small individuals, our study found the allometric growth index to be significantly higher than the theoretical value, likely because
S. gordejevii is a typical pioneer sand-fixing plant that is characterized by sand burial tolerance, easy asexual reproduction, and rapid growth. Although the Hunshandake Sandy Land is located in an arid region, it has abundant water resources and good moisture conditions [
47]. Therefore, the allometric growth index was significantly higher than the theoretical value. This result indicates that mobile dunes provide suitable growth conditions and good genetic resources to
S. gordejevii, enabling its rapid growth and development.
The slopes of semifixed and fixed dunes did not significantly differ from the theoretical value (
p > 0.05), indicating an isometric growth relationship. The allometric growth index of the fascicular branch length to basal diameter of
S. gordejevii in semifixed and fixed dunes reduced relative to that in mobile dunes. This reduction may be related to a delay in vertical growth or an acceleration in horizontal growth. The delay in vertical growth could be due to genetic constraints or factors, such as drought or freeze–thaw cycles that cause embolism or cavitation in conduits, thus affecting growth [
48]. Horizontal growth might have accelerated because the fascicular branch populations at these two types of sites are in a stable phase; individuals are large; and the transition from primary to secondary growth has occurred, enhancing radial resistance to adverse factors. Given that stem growth is indeterminate and height growth is generally incremental and determinate, this situation might lead to accelerated lateral growth, thus reducing the allometric growth index.
In interdune lowlands, the slope between the fascicular branches and basal diameter of S. gordejevii was 0.519, which essentially aligns with the elastic and fractal network models. We likely obtained this result because trees resist morphological distortion caused by their own weight or require large diameters for support to maintain an erect and upright form. Additionally, as the thickness of the transport cross-section increases and transport distance shortens, resistance can be minimized during resource transport, thus facilitating resource delivery.
This study shows that the relationship between the branch length and basal diameter of S. gordejevii fascicular branches changed with the increase in the moisture gradient of sandy lands across different site types. In mobile dunes, the growth rate of branch length exceeded that of basal diameter; in semifixed and fixed dunes, the growth relationship between branch length and basal diameter was isometric; and in interdune lowlands, the growth rate of branch length was slower than that of basal diameter. In mobile dunes, S. gordejevii favors a vertical growth rate to perform photosynthesis and expand the space for seed dispersal. In interdune lowlands, S. gordejevii favors a lateral growth rate with the changes in soil conditions to balance morphological deformation caused by its own weight, maintain a vertical and straight form, and minimize resistance to resource transport. The adaptive responses in semifixed and fixed dunes are intermediate between those in mobile dunes and interdune lowlands. These findings are valuable for understanding how S. gordejevii adapts to different sandy land environments.
4.2. Biomass of Fascicular Branch Components and Allometric Growth Relationship
The biomass allocation of various components of S. gordejevii fascicular branches showed significant differences at different types of sites. Compared with that in fixed dunes, the biomass allocation of components in mobile dunes had a lower proportion of allocation to branches, a higher proportion of allocation to stems, and little difference in allocation to leaves. A similar allocation phenomenon was observed in fixed dunes and interdune lowlands. However, in semi-fixed dunes, the biomass allocation of components was balanced. This characteristic is conducive to the rapid growth of S. gordejevii. Compared with interdune lowlands, semi-fixed dunes exhibited a lower proportion of allocation to branches but a higher proportion of allocation to leaves with little difference in allocation to stems. The analysis results demonstrate that under poor moisture conditions, the component biomass of S. gordejevii tends to increase the biomass of stems and leaves, thus reducing allocation to branches. Furthermore, the fully sclerified, thick cuticle leaf anatomical structure of S. gordejevii may be closely related to its biomass allocation strategy, indicating that S. gordejevii growing in mobile dunes has strong drought resistance.
Studying the accumulation rates of the stem, branch, leaf, and total biomasses of S. gordejevii fascicular branches under different site conditions is an important method for revealing its ecological adaptation mechanisms and investigating its biological characteristics. Through SMA regression analysis, we found significant positive correlations between S. gordejevii fascicular branches across different sites and traits. The only exception was the lack of correlations between leaf and stem biomasses in semifixed and fixed dunes. Additionally, the allometric growth index between different traits was related to site type. Without considering site type, the accumulation rates of branch–stem biomass were similar and followed the isometric growth rule. With a certain input of stem biomass, the input of branch biomass was smallest in mobile dunes and largest in interdune lowlands. Its value in semifixed and fixed sandy lands was intermediate between those in mobile dunes and interdune lowlands. However, the accumulation rates of branch and stem biomasses were greater than the total biomass accumulation rate, indicating the existence of allometric growth between total stem and branch biomasses, with the theoretical value being greater than the allometric growth coefficients. These traits are all related to site type. In mobile dunes, leaf–stem biomasses followed the isometric growth rule, whereas in interdune lowlands, the accumulation rate of leaf biomass was less than that of stem biomass, indicating an allometric growth relationship. In mobile and semi-fixed dunes and interdune lowlands, leaf–branch biomasses followed the isometric growth rule, whereas in fixed dunes, the accumulation rate of leaf biomass was less than that of branch biomass, showing an allometric growth relationship. In mobile dunes, the accumulation rate of leaf biomass was greater than that of total biomass. By contrast, in interdune lowlands, the accumulation rate of leaf biomass was less than that of total biomass. Meanwhile, in semifixed and fixed dunes, the accumulation rates of leaf and total biomasses demonstrated an isometric growth relationship.
The allometric growth relationships between different site types and other traits of S. gordejevii are closely linked. Overall, the biomass allocation strategy of S. gordejevii can be effectively adjusted by effectively controlling the accumulation rate of leaf biomass. That is, the main regulatory factor of the biomass allocation strategy of S. gordejevii fascicular branches is the accumulation rate of leaf biomass. The accumulation rate of leaf biomass was high in mobile dunes; low in semifixed and fixed dunes; and lowest in interdune lowlands. S. gordejevii fascicular branches adopt a growth strategy of increasing the accumulation rate of branch biomass in fixed dunes and increasing the accumulation rate of stem biomass in interdune lowlands to balance the reduction in the accumulation rate of leaf biomass. In this way, S. gordejevii can regulate the accumulation rate of leaf biomass, thus achieving improved growth results.
5. Conclusions
Coordinated development among different organs during the life cycle of plants is an important field of study. This physiological characteristic is influenced by various environmental factors, such as light intensity, moisture, and nutrients. Researching the allometric growth of plants in the vertical (i.e., plant height) and horizontal (i.e., basal diameter or crown width) directions is crucial for understanding the growth mechanisms of plants themselves. The study of the allometric growth relationships of biomasses among plant components is a hot topic. By investigating the allometric growth relationships of biomasses among plant components, we can not only enrich and optimize existing theories of resource utilization but also provide a scientific basis for multiscale biomass estimation. Notably, the size and number of leaves of annual branches directly determine the canopy morphology and growth mode of plants, which in turn affect the photosynthetic efficiency and carbon utilization capacity of plants. The balance between the size and number of leaves plays a vital role in revealing the differences in leaf size among various plants in nature as well as in the coexistence and maintenance of diversity of different species within the same habitat.
This study, by using S. gordejevii at different sites of the Hunshandake Sandy Land as material, analyzed the allometric growth characteristics and biomass allocation patterns of S. gordejevii fascicular branches. Understanding the adaptability of S. gordejevii to environmental changes and its role in the carbon cycle is of great importance. This study found the following: (1) The growth characteristics of S. gordejevii fascicular branches are significantly influenced by the soil moisture gradient, wherein the growth relationship between branch length and basal diameter demonstrates allometric growth in mobile dunes and interdune lowlands but isometric growth in semifixed and fixed dunes. (2) Biomass allocation exhibits a positive correlation with the soil moisture gradient across different habitats. In particular, in environments with low moisture, S. gordejevii tends to increase biomass allocation to stems to adapt to arid conditions. When moisture conditions improve, allocation to branches increases. (3) The total biomass allocation of S. gordejevii fascicular branches indicates that the biomasses of leaves and stems show an isometric growth relationship in mobile dunes, whereas they have an allometric growth relationship in interdune lowlands. This result suggests that S. gordejevii adapts to environmental changes by adjusting the biomass allocation of leaves and stems in different habitats.