Introduction
It is clear that environmental changes could alter plant community
composition and diversity, and competition (plant – plant interaction)
is often considered as one of the main drivers of the consequence of
environmental changes (Grime, 1979; Tilman, 1982).
Therefore, plant ecologists have
been investing tremendous efforts in interpreting and predicting how the
intensity (amount of inhibition or facilitation in the target’s
performance as a consequence of a neighbor’s presence) and importance
(the impact of a neighbor on the target expresses as a proportion of the
total environmental impact on the target) of competition varies along
the environmental gradients especially soil nutrition gradient, and have
concluded three theory models with empirical evidences (Bertness et al.
1994; Brooker et al. 2005; Gaucherand et al. 2006; Grime, 1979; Laurent
et al. 2017; Tilman, 1982). First, it is assumed that competition
inhibition would decrease from benign (fertile) habitats to harsh
(unfertile) habitats, which is called “stress gradient hypothesis”
(Bertness and Callaway 1994; Grime, 1979). Second, it predicts that
total competition is equally important throughout soil resource
gradients, and belowground competition is most intense for soil water
and/or nutrition in unfertile sites and aboveground competition is most
intense for light and space (aerial competition) in fertile sites, which
is called Tilman’s theory (Tilman,
1987). Third, it argues that
competition importance increases
with the increase of resource availability and it is higher in fertile
environments, which is called
Grimes’s theory (Grime,
1977).
Although plenty of experiments have successfully supported these models
under different soil resource conditions, there is a growing number of
experiments that have emphasized the importance of species identities
(both target and neighbor) in influencing competition for resources and
consequently influencing the community composition and structure (Read,
2016; Suding et al. 2004; Yelenik et al. 2017). The responses of a
single target species varying based on the identities of neighbors have
been found in several studies (Kong et al. 2018; Stoll et al. 2000). For
example, Keddy et al. (2002) found that the response of a single target
species to competition was equally sensitive under two different
environmental conditions but varied with the identities of 63
experimental competitors. These experiments have also shown that the
effect of neighbor identity to competitive intensity may even surpass
that of environmental differences. Although there were not consistent
conclusions, two components of species, the effects of neighbors on
resource availability (the growth potential) and the ability of
individuals to tolerate these effects, were usually considered when
researchers tried to understand how species identities influenced the
outcomes of plant-plant interaction (Goldberg, 1996; Suding et
al. 2004). In addition, ecologists found that the co-occurring
species may compete or facilitate one another by modifying the microbial
environment to change nutrition availability (Brooker et al.2008). Several studies have found out that a species could influence the
outcomes of plant-plant interaction by immediate effect when another
species co-occurred with it, legacy effect or plant-soil feedback when
another species grew in the soil that it has been conditioned (Zhao et
al. 2018).
Suding and Goldberg (2001) addressed the processes responsible for the
patterns of species composition change after the environmental changes,
and suggested that both competitive reduction and competitive change
were two alternative hypotheses to explain the relative competitive
abilities of different target species affected by different neighbor
species in changing environmental conditions. The competitive reduction
hypothesis predicts no shift of competitive hierarchies of the target
species, suggesting environmental changes may modify species
associations simply; while the competitive change hypothesis predicts
competitive hierarchy of target species may shift with the change of
environmental conditions, thus influencing the species composition, not
just the intensity of competition (Suding et al. 2001).
Therefore, considering the species identities is very important for
understanding the community processes and intrinsic mechanisms in the
context of global changes which have altered the soil resource
conditions and community composition and structure dramatically.
Arid and semi-arid grasslands are very sensitive to the environmental
changes. In this region, nature ecosystems are being degrading to more
stress conditions because of over-grazing and drought (Huang et al.
2019; Ren et al. 2018). Simultaneously, the observed acceleration in N
cycle caused by fossil fuel combustion, fertilizer use and legume shrub
invading into grassland substantially have contributed to soil nutrition
(Chen et al. 2019; Zhou et al. 2019). The different responses (growth
potential or tolerance) of different species to such changes would
influence the outcome of plant-plant interaction, alter plant community
composition and diversity. Similar to the worldwide terrestrial
communities, the semi-arid steppe of China is suffering from global
changes (Li et al. 2012). In this region, Stipa grandis community
is the most normal, stable and representative community, and it is very
sensitive to environmental changes; and compared with S. grandis ,S. krylovii occupies dryer and more infertile habitats. The
previous experiments demonstrated that the lower tolerance of S.
grandis to drought, disturbance and infertile soil condition might be
the reason for the replacement of S. grandis by S.
krylovii (Chen et al. 2013). Recent study showed that soil conditioned
by S. grandis with high N treatment could increase S.
krylovii ’s competition superiority and N-addition effect in a
low-density controlled experiment (Zhao et al. 2018). Therefore,
exploring how the identities of neighbors influenced the competition
hierarchies of S. grandis and S. krylovii under different
soil nutrition conditions is very important for us to understand the
mechanisms of community dynamics and predict community processes.
In this study, we conducted a microcosm experiment to explore how soil
nutrition condition, neighbor
species influenced the intensity and importance of target species
(S. grandis and S. krylovii ), how plantation condition,
species identity influenced nutrition-addition effect, and how the
variation of the competitive hierarchy altered with the change of soil
nutrition when the target species were grown with different neighbors.
Especially, we conjectured that the factor of soil nutrition condition
might not be the main driver that influenced the intensity and
importance of competition on S. grandis and S. krylovii .
What’s more, we hypothesized that S. krylovii would show a higher
competitive hierarchy than S. grandis in the low fertility
condition. Such results would be very important for us to understand and
predict the community dynamic in the semi-arid steppe of China when
facing environmental changes.