Discussion
The present results showed the complexity of the competitors (targets and neighbors) in influencing the outcomes of plant-plant interaction and supported that the biotic factors played an important role in influencing intrinsic mechanism of plant-plant interaction. Firstly, the effects of neighbor species were significant on the intensity and importance of competition, and the ω2 value of neighbor species was the highest among all predictors, which explained 29.448% and 31.591% of the total variance in the intensity and importance of competition, respectively (Table 2). Recently, more and more studies indicated the different neighbors could show different effects on the nutrition availability by direct resource depletion or changes of the soil microbial composition and activities as the indirect (Schofield et al. 2018; Suding et al. 2004; Zhang et al. 2019). Similarly, the competition intensity of different target species neighbored by the same neighbor was different from each other (Pérez-Ramos et al. 2019; Saccone et al. 2017). Secondly, only the factor of plantation condition (grown alone or in mixtures with different neighbors) significantly influenced the root: shoot ratio, which indicated that different neighbors resulted in different nutrition availability for target species or influenced the nutrition absorption of the target species, independently of the soil nutrition conditions (Uddin et al. 2020; Zhang et al. 2008). Therefore, in the present study, we found several mechanisms potentially contribute to these differential neighbor effects on different target species, as mentioned by other researchers (Bertness and Callaway 1994; Grime, 1977; Grime, 1979; Suding and Goldberg 2001).
First, when S. grandis or S. krylovii was grown in mixture with L. chinensis , the intensity and importance of competition inhibition increased with the addition of soil nutrition, supporting “stress gradient hypothesis” and Grime’s theory (Bertness and Callawa 1994; Grime, 1977). In addition, the importance of competition was significantly negative on S. grandis under both soil nutrition treatments and on S. krylovii under high soil nutrition treatment, which suggested that the competition effects of the neighbor played an important role in affecting the performances of target species (Díaz‐Sierra et al. 2016). In addition, the importance of competition was not significant on S. krylovii under the low soil nutrition treatment, suggesting that the ability of species to tolerate low resource availability could influence the importance of competition, like the results found in other researches (Delerue et al. 2018; Gaucherand et al. 2006). What is more, in mixture with L. chinensis , the relatively higher root: shoot ratio indicated that resource competition is main dominant driver, and the nutrition-addition effect was lowest for each target species among all plantation conditions and was non-significant on S. grandis (Fig. 4). That is to say, L. chinensis could inhibit the resource availability and increase the competitive inhibition on its competitors, which was consistent with the finding in the PSF experiment (Zhao et al. 2018). All these results demonstrated that as a dominant species, L. chinensis could impact resources to benefit itself or to inhibit other species. Similar performance that a plant species captures a large percentage of limiting resource in a given area and reduces that resource for other species growing nearby have been reported in many researches (Groves et al. 2003; Parker et al. 2019).
Second, when S. grandis and S. krylovii were grown together, the competition inhibition decreased with the increasing soil nutrition but it was influenced by the interaction between target species and soil nutrition condition (Fig. 2), supporting the theory of competitive reduction. In this case, high nutrition treatment could modify species associations simply but not change the community composition (Suding and Goldberg 2001). A research has indicated that light competition was the main driver in the stable or restoration community of S. grandis where community height and canopy density were high enough (Li et al. 2017). The lowest value of root: shoot ratio was found in S. grandis - S. krylovii mixture system (Fig. 1B), which indicated that the resource allocation model was in favor of aerial competition such as light and space according to the balanced growth hypothesis (Davidson, 1969). However, in this study, the resource of light or space was not limited because only two seedlings were grown in one microcosm. All these facts confirmed the results that the importance of competition was not significant on each target species inS. grandis - S. krylovii mixture system. That is to say, the relative contribution of competition among all processes could be ignored in affecting the individuals’ performances and population dynamics of S. grandis and S. krylovii (Díaz‐Sierra et al. 2016).
The biomass of S. krylovii was higher than that of S. krylovii under the high nutrition treatment when each species was grown alone, indicated that the growth potential of S. krylovii was comparable to or even greater than that of S. grandis . This could explain the reason that S. krylovii appears in the disturbance region of S. grandis communities, supporting that the gap of light niches is very important for S. krylovii and proving that recognizing the nature of competition mechanisms (here is competition reduction) is very important for ecologists to understand and explain the community dynamics (Suding, 2001). Such results also suggested thatS. krylovii was an inferior competitor for light or space but not for nutrition in the stable S. grandis communities and resulted in competition exclusion gradually in the closed canopy communities because the individuals of S. krylovii were suppressed and shaded by taller canopy species such as S. grandis . In addition, the biomass of S. krylovii was highest under the high nutrition treatment in S. grandis - S. krylovii mixture system (Fig. 1a), which supported that the presence of S. grandis could facilitate the growth of S. krylovii and enhance the nutrition-addition effect of S. krylovii (Zhao et al. 2018).
Third, when each target species was grown in mixture with A. cristatum , the competition inhibition on S. grandis decreased and that on S. krylovii increased, and the relative competitive hierarchies of S. grandis and S. krylovii changed with the change of soil nutrition condition, which supported the theory of competitive change (Suding and Goldberg 2001). In this case, the change of soil nutrition condition could not only modify species associations but also change the community composition (Suding and Goldberg 2001). Such species-specific responses and shifts in competitive priority along environmental gradients caused by the significant differences among species in their growth potentials and their tolerance under a particular condition (Hartley et al. 2005). For example, Suding et al. (2004) showed that the inhibitory effects associated withAcomastylis rossii were offset to a greater degree than those associated with Deschampsia caespitosa when N supply rates were enhanced through repeated N additions. Our findings indicated that the intensity and importance of competition inhibition of A. cristatum on S. krylovii increased significantly with the increase of soil nutrition might be caused by the significantly lower nutrition-addition effect on S. krylovii in mixture with A. cristatum than in any of the other plantation conditions or than that on S. grandis (Fig.4). In a plant-soil feedback experiment,S. krylovii grown in the soil conditioned by A. cristatumshowed the lower N-addition effect than that in the soil conditioned byS. grandis (Zhao et al. 2018). Thus, both the legacy effects and the immediate effects of plant species could strongly influence plants’ performance and nutrition availability by a similar pattern, suggesting that the outcomes of plant-plant interaction might be mediated by soil microbial community or by the combination of resource depletion and soil microbial community (Larios et al. 2015).
Our findings supported that the intensity and importance of competition and their variations along soil nutrition conditions were species dependent and proved that plant-plant interactions can have significant ecological and evolutionary influences on the niches of species (Strauss, 2014). Although some studies have indicated that the abiotic soil condition but not neighbors played more important roles in influencing the competitive hierarchy (Suding, 2001), more and more studies including ours supported the significant effects of neighbors and the interaction between abiotic soil condition and neighbor species on the competitive hierarchy (Fynn et al. 2005; Van den Berge et al. 2014). In this study, we also confirmed that the growth potential ofS. krylovii is not lower than that of S. grandis , thus, higher tolerance to unfertile condition and lower tolerance to reduced canopy openness (light) of S. krylovii determined its distribution. The resource conditions of light or space and soil nutrient could be influenced by biotic factors such as neighbors or abiotic factors such as disturbance or N deposition, which makes all possible two-species equilibria unstable in the face of the global changes.