Our study quantified differences in understory vegetation coverage, biomass, diversity, and species composition for different plantations, and expounded the main environmental driving factors for these differences between different stand types and ages. The restoration effect of the
R. pseudoacacia plantation (RP) on understory vegetation coverage and biomass was higher than other plantations (
Figure 3). However, the understory vegetation species diversity of the
P. tabulaeformis plantation (PP) fluctuated the most with stand age (
Figure 4), which reflected the drastic environmental changes under the forest, and we propose that this relates to stand density decline as forest age increases (
Table 1). Understory vegetation species in all plantations tended to be close with increasing stand age, which was particularly obvious in the mixed plantation (MP,
Figure 5). These differences are closely related to environmental drivers (
Figure 7). Here, we discuss these issues, focusing on the potential environmental drivers of understory vegetation coverage, biomass, species diversity, and species composition in different stand types and ages, and briefly discuss the relationship between plantation management strategies and ecosystem functions.
4.1. Potential Drivers of Changes in Understory Vegetation Coverage and Species Diversity
Understory vegetation coverage and biomass of three plantation types first decreased and then increased with varying degrees during the middle stage (20 to 30-year age class) of vegetation restoration (
Figure 3). Additionally, it is noteworthy that, in
Figure 3, the lines (10 to 20-year age class) start higher than natural secondary forest (SF), and with increasing stand age, the lines of PP and MP fluctuate near the average value of SF. One possible reason is that in the early stage (10 to 20-year age class) of plantation restoration, due to the shaded environment and minimal human interference, soil erosion and high-temperature damage are significantly reduced. The abandoned farmland then quickly develops into shrub and herb communities, which then greatly improves the ecological suitability underneath the forest canopy [
23,
60,
61]. However, with increasing plantation age, under the limited soil water carrying capacity of the vegetation (SWCCV) in arid and semi-arid areas, tree growth further reduces the growing space for understory vegetation and thereby hinders the development of the understory community [
62,
63]. Therefore, the understory vegetation coverage and biomass for the three plantations decreased significantly in the middle of recovery (20- to 30-year age class,
Figure 3). Finally, after species competition, tree renewal, and elimination, a reasonable stand density and structure enables understory vegetation coverage and biomass to recover gradually, and ultimately closely resembles SF. In contrast to the other two plantations, the lowest point of vegetation coverage and biomass under RP was higher than SF, and the highest point (at 40- to 50-year age class) was more than double that of SF. Compared with other plantations, the lowest stand density (
Table 1) of RP in each age class was the main driving factor, which greatly reduced interspecific competition, and this understory vegetation had the most sufficient growth space [
37].
Similar to the changing trend in coverage and biomass, species diversity indices (Species richness, Shannon–Wiener, Pielou, and Simpson index) of the four stand types also showed a trend of first declining and then increasing. However, as stand age increased, the species diversity of vegetation under the three plantation types fluctuated and gradually became unified (closely resembling SF). Moreover, the fluctuation of PP was the highest. Zhang, et al. [
64] showed that stand density affects the understory vegetation diversity index by limiting soil phosphorus availability in the Loess Plateau, which is consistent with the results of this study (
Table 1). It is also noteworthy that the lowest inflection point of the four species diversity indices appeared mostly in the pure plantations (RP and PP) with a stand age of 20 to 30 years, and although the lowest inflection point of MP appeared in 30 to 40 years, it was maintained at a high level. As shown by Cavard, et al. [
65] and Gosselin, et al. [
66], mixed tree species increase stand structural heterogeneity and habitat diversity compared to pure forests, and allow more plant, animal, or fungal communities establish. However, this also requires mixed forest understory vegetation to have a longer interspecific competition and alternating adaptation period as the ecological environment in the forest changes (mainly the change in dominant tree species).
4.2. Potential Drivers of Species Composition Differences
Both stand type and age significantly affected the species composition of understory vegetation (
p < 0.01,
Table 3). NMDS analysis evaluates the interrelationships between vegetation communities [
67]. According to the NMDS results (
Figure 5), the understory vegetation composition of RP, PP, and SF has a relatively independent distribution space (especially for low-age stands). In contrast, the MP site occupies the area close to the center of the NMDS ordination diagram (
Figure 5) and coincides with the distributional area of the three other stand types, which is consistent with the results discussed in
Section 4.1. The diverse environment of a mixed forest enables the understory vegetation of other pure forests and natural secondary forests to survive and develop. A common law also exists among the three plantation stands (RP, PP, and MP), which is that as stand age increases, the understory vegetation composition unifies (the high-age stand sites move toward the center in
Figure 5).
As stand age grew, the change in stand microhabitat significantly affected the understory vegetation composition [
68,
69]. During the growth of RP, the significant change in canopy affected the ecosystem temperature and light quantity, which is one of the main driving factors for species compositional change under the stand [
70,
71]. In this study, the above two conclusions were further confirmed according to the changes in indicator species under the forest (
Table 7). The superior light conditions in the early stage of vegetation restoration enabled the growth of many pioneer species that require high lighting and are drought tolerant, e.g.,
Wikstroemia chamaedaphne Meisn. and
Prinsepia uniflora Batal. With the canopy density increase, the survival rate of shade-loving species increased [
72], and shade-tolerant species (e.g.,
Pistacia chinensis Bunge and
Digitaria sanguinalis (L.) Scop.) gradually occupied the central position in the understory vegetation communities. Woody vines promote forest regeneration through direct competition with trees [
73,
74]. With a further increase in stand crown and shrub density, the limitation of soil nutrients (SOC, TN, and TP) on understory species is further enhanced. To compete for limited light and absorb nutrients, woody vines (e.g.,
Asparagus cochinchinensis (Lour.) Merr. and
Wisteria sinensis (Sims) Sweet) begin to appear in the late stage of RP.
The stand density of PP in the Loess Plateau often affects soil properties, thus changing the diversity and composition of understory vegetation [
64,
75]. The response of the shrub layer to stand density is usually higher than the herb layer [
75,
76]. The density of PP in this study area generally decreases with increasing stand age (
Table 1). Excessive forest density and fierce intraspecific competition are the main driving factors in the earlier stage (stand age ≤ 30 limited by SC,
Figure 7). Therefore, we note that the indicator species are mainly herbs (e.g.,
Pilea sinofasciata C. J. Chen and
Potentilla chinensis Ser.) in the early stage (stand age ≤ 20) of vegetation restoration, while shrubs (e.g.,
Rhodotypos scandens (Thunb.) Makino and
Artemisia argyi Levl. et Van) and tree seedlings (e.g.,
Amygdalus davidiana (Carrière) de Vos ex Henry and
Celtis sinensis Pers.) occur mainly in the middle and late stage (stand age > 20) of vegetation restoration (
Table 7). Combined with the high fluctuation in understory vegetation species diversity of PP in
Section 4.1, with the sudden stand density decline, the difference in the microclimates (e.g., heat radiation, air humidity, and temperature) in the forest caused by EL has become the new main driving factor (
Figure 7) [
33].
The understory vegetation diversity of MP in the study area is significantly higher than that of a single plantation. Gong et al. [
77] showed that the main reason was that the response rate of understory vegetation diversity increases with increasing stand age. Due to niche differentiation and rotational cycle differences, multispecies forest structure often affects renewal by affecting light, water, nutrient use efficiency, and forest heterogeneity [
78,
79]. The multispecies MP structure usually leads to forest light heterogeneity, resulting in the coexistence of shade-intolerant and tolerant species [
80,
81]. We found an interesting phenomenon: the indicator species under MP implied the process of community succession. With increased stand age, it gradually changed first from light-loving pioneer shrubs (e.g.,
Elaeagnus pungens Thunb. and
Artemisia stechmanniana Bess.) to light-loving tree seedlings (e.g.,
Platycladus orientalis (L.) Franco,
Ailanthus altissima (Mill.) Swingle, and
Betula albosinensis Burkill), and finally to moisture and water sensitive tree seedlings (
Quercus wutaishansea Mary) at an age of 40 to 50 years (
Table 7). Additionally, it is emphasized here that
Q. wutaishansea is the main tree species in SF (
Table 1). Therefore, we deduced that the understory vegetation diversity and species composition of MP not only indicate future understory species composition but also the direction of species competition among trees (this indicates that there may be more numbers and species of trees from SF coming into MP). The understory environment of MP has, therefore, obviously gradually developed into a habitat for some of the main tree species of SF. Therefore, we believe that it will further approach the developmental direction of SF.
The research of Zhang et al. [
82] on SF in the Loess Plateau shows that the percentage of Fabaceae, Poaceae, and Asteraceae vegetation increases annually, corresponding to the increase in soil C, N, and P, respectively. For instance, Fabaceae vegetation increases the soil nitrogen content during succession due to its nitrogen fixation ability [
83]. Poaceae vegetation fixes carbon in the atmosphere and returns it to the soil in the form of litter [
84]. The six indicator species under SF in the study area included two Fabaceae vegetation (i.e.,
Vicia sepium L. and
Lespedeza bicolor Turcz.), two Poaceae vegetation (i.e.,
Setaria viridis (L.) Beauv. and
Pogonatherum crinitum (Thunb.) Kunth), one Asteraceae (i.e.,
Sonchus wightianus DC.), and one Saxifragaceae (i.e.,
Deutzia scabra Thunb).
Another noteworthy point is that, with increasing stand age, the repetition quantity of atypical understory species of different stand types also increased significantly (
Table 7). This finding complements our previous explanation of the NMDS ordination analysis. The high-age stands in the center of
Figure 5 mainly result from the contribution of atypical understory species. In particular, for MP, this is one of the main reasons why it occupies the central area in the NMDS ordination diagram. More importantly, from the perspective of the whole watershed, the convergence trend of atypical understory species in different stands shows a positive and stable signal.
Finally, by comparing plantation stands and SF, under different stand ages, the repetition numbers of MP and SF atypical understory species were the highest, while PP was the opposite. This again confirms that MP understory vegetation species composition is closer to SF.
4.3. Dynamic Response of Species Composition to Environmental Drivers
Our study found that the composition of RP understory vegetation was most affected by crown development, as the age of the stand increased, it gradually shifted to competition for soil nutrients. RP as the typical deciduous broad-leaved forest in north China, low density provides more differences and possibilities for canopy growth, and many studies deal with canopy closure [
85], or gap dynamics [
86], at forest stand level demonstrated that forest canopy structure significantly affects the diversity and productivity from understory vegetation, and affected understory vegetation by changing the understory light conditions [
87] and through litter and nutrient cycling [
88]. In addition, numerous studies have shown that phosphorus (P) is the main limiting factor for its sustainable development, and show an increasing demand for P over time [
89,
90], which is consistent with the results of community succession and indicator species change under the RP. Topographical and stand structure factors (here referring to the SC and EL, respectively) have a significant impact on PP understory vegetation composition. Together with this, the SC mainly affects the understory vegetation of low stand age (10 to 30-year age) PP, while the EL mainly affects high stand age (30- to 50-year age). From
Table 1, the density of PP in the early stage of afforestation is much larger than the afforestation target range in the Loess Plateau (1100–1600 trees∙ha
−1 [
91]). The effective space between trees has become the biggest factor limiting the understory vegetation species composition, which not only affects light conditions in the vertical space but also affects competition for soil nutrients and water in the horizontal space in coniferous forests [
92,
93]. In the middle and late stage of PP restoration, the topographical factor (i.e., EL) has the greatest impact on the understory vegetation, which is related to temperature, drought sensitivity, temporal and spatial precipitation distribution characteristics, and human activities all affect the movement of PP to a higher EL. For MP, in the early afforestation stage, it is mainly affected by SBD, and changes in SBD have the greatest influence on the spatial distribution of water and the water use strategy of vegetation in the slope [
94]; with an improvement in SBD, this impact further weakens. Finally, as the reference group of this study, SF is widely distributed in the study area. Together with this, among the 12 SF sites, the understory vegetation composition is affected by multiple environmental factors, which are caused by differences in stand species composition, stand structure, soil heterogeneity, and topographical diversity. Therefore, environmental impact factors are not discussed here.
4.4. Communities Stability and Stand Management Strategy
The stability of forest vegetation communities includes constancy, resistance, resilience, and persistence [
95]. This multi-dimensional comparison makes the evaluation of community stability more complex and inconvenient. Therefore, the mathematical ecological stability method (i.e., M. Godron method) that can reflect the quantity and frequency information of all species to quantitatively the stability of different stand types and ages was adopted (
Table 9). The smaller distance between the intersection coordinate and the stable coordinate indicates a more stable community.
Our research shows that compared with SF, the coverage, biomass, and diversity of understory vegetation of RP for different stand ages are significantly improved. However, from the perspective of community stability, there is still a big gap between RP and SF. When the stand age > 40, the community stability under RP showed a slow declining trend (
Table 9). The reasons for this appearance are as follows. As a typical deciduous broad-leaved forest in the Loess Plateau, the understory vegetation species composition of RP is greatly affected by the canopy. Light availability and P element are key factors for the survival of understory vegetation and seedlings of
R. pseudoacacia [
89,
90,
96]. As the canopy closes, they may be replaced by more shade-tolerant species due to light restriction; an indicator species change confirmed this phenomenon under RP (
Table 7). Végh and Tsuyuzaki [
97] held that stand spatial structure is more important than stand age, and passive restoration often leads to poor communities. Since the light environment under the stand changes with canopy closure, it is valid to consider that the emergence of various shade-tolerant climbing species, here referred to as woody vines (e.g.,
A. cochinchinensis and
W. sinensis) and voluble herbs (e.g.,
Ipomoea nil (Linnaeus) Roth), as indicators in the later stage of RP restoration is a potential risk hindering healthy plantation development [
98]. Therefore, active canopy management is an important condition for restoring the high-quality habitat of RP. In the early stage, while preventing the height of climbing vegetation from exceeding the canopy height, expanding the canopy gap can effectively prevent the growth of shade-tolerant climbing species [
98,
99]; In the later stage, corresponding soil nutrient cycling elements, especially soil phosphorus can be supplemented [
89,
90], which can promote the growth of young trees under the forest, as well as improving species diversity.
For PP, its community stability is the worst among all stand types and has not changed significantly with the increase in stand ages (
Table 9). The drastic change in the understory environment is one of the main reasons for community instability. Understory species change caused by sudden stand density decline (
Table 1) is at the expense of species diversity to some extent, which greatly reduces the stability of resistance of understory communities. Therefore, there is poor resistance stability (juvenile and mid-aged stands) under PP and a severe fluctuation of vegetation diversity under different stand ages (
Figure 4). Expanding the effective distance between trees in the early stage of restoration is an important prerequisite for reducing resource competition and promoting the stability and succession efficiency of understory communities [
64,
100,
101]. Concurrently, in the near-mature forest at a later restoration stage (age > 30), attention should be paid to the influence of elevation on the composition of understory vegetation, here mainly referring to the possible positive or negative effects of population migration phenomena. On the basin scale in arid areas, great differences exist in the distribution of temperature, moisture, and soil nutrients, resulting in differences in vegetation growth and stability [
102].
Additionally, according to the NMDS (
Figure 5), the two pure plantations with single tree species differed greatly during the early stage, but species compositional changes under the forest also increased with increasing stand age, and a unified trend existed near the “central area”. Also, for MP, the understory vegetation for different stand ages was more obviously closer to this “central area”. More importantly, while the understory vegetation of MP maintained a higher community stability and its community stability closer to SF (
Table 9),
Q. wutaishansea, as the main tree species of SF, became the main indicator species of high-age MP, which undoubtedly implies the succession direction of MP understory vegetation and the developmental direction of tree competition. However, for MP in the early stages of recovery, SBD reflecting the maturity of soil under the stand was the dominant factor limiting the allocation strategy of vegetation growth resources (especially water use) [
94,
101]. Therefore, minimizing unnecessary human disturbance is a necessary condition for the succession of vegetation under MP to approach and eventually reach near natural secondary forest.