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.