DISCUSSION
The
dominant shrub A. ordosica had an overall neutral effect on the
biomass and richness of herbaceous understorey species in the controlled
conditions of this dune system, and neither watering nor N fertilization
significantly influenced the effect of the shrub on herbaceous species
at the community level. However, within the community, we identified
contrasting changes in species responses to the effect of the shrub
along watering and N fertilization treatments that were related to the
two measured traits, height and LDMC. When species were grouped based on
the differences in both their traits and responses to the effects of the
shrub, we found significantly contrasting changes in species group-level
responses to the effects of the shrub with watering and N fertilization,
that balanced at the community level, consistent to our main hypothesis.
Our results have strong implications, for understanding both changing in
species responses to neighbours along environmental gradients, a highly
debated issue in community ecology, and the stabilizing effects of
functional divergence and plant-plant interactions for community
responses to global change.
In
our experiment, four species groups could be clearly separated according
to their divergences in functional traits (height and LDMC) and their
changes in responses to the effects of the shrub with watering and N
fertilization. These results support former theories arguing that plant
communities include different functional groups of species having
different ecological requirements and plant traits
(Ellenberg,
1953; Whittaker, 1956; Lavorel et al. , 1997). They are also
consistent with results of pioneer climate change experiments that
showed that climate change differently affects species of contrasting
functional strategies within a single community (Chapin & Shaver, 1985;
Harte & Shaw, 1995). However, to our knowledge, this is the first
evidence that environmental manipulations mimicking global change
effects in a single community differently affect the responses to a
dominant neighbour of species having different functional trait values.
Plant height appeared to be the best predictor of plant responses to the
effect of the shrub, with the tallest species (group C) being
facilitated and the shortest ones (groups A and B) overall negatively
affected by the shrub. These findings are consistent with previous
studies showing that competitive responses are more related to plant
height than LDMC (Michalet, 2001; Liancourt et al. , 2005b;
Michalet et al. , 2008; Wang et al. , 2019b). Additionally,
Liancourt et al. (2005a) have shown that short stress-tolerant species
are more likely to be negatively affected by neighbours than tall
competitive species, since the benefit of being shaded by neighbours for
mitigating physical stress outweighs the cost for light for the latter
but not the former. Although significant, relations between LDMC and
responses to neighbours along the treatments were more complex to
understand. For example, group D including the species with the highest
LDMC values (i.e., being the most conservative; Díaz et al. , 2004
& 2016; Wright et al. , 2004), was strongly negatively affected
by the shrub in the W40N0 treatment, but facilitated in W40N60
treatment, which response is opposite to those of group C. This might be
explained by indirect interactions among different groups of competitors
having contrasting resource requirements, as shown by Michalet et al.
(2015) who assessed direct and indirect interactions at the species
level in a subalpine shrub community from the Tibet Plateau.
Interestingly,
changes in the effects of the shrub along the watering and N
fertilization treatments supported different predictions of competition
and facilitation models, depending on the species cluster groups and
resources. For group A, facilitation decreased and competition increased
with increasing N fertilization, supporting the SGH model (Bertness &
Callaway, 1994). For group B, RII did not change significantly
with neither watering nor N fertilization, a result supporting the
meta-analysis of Maestre et al. (2005) for a drought gradient and
the model of Tilman (1982) for a nutrient gradient, respectively. For
group C, facilitation was the highest in the wettest conditions and
decreased with increasing drought, with no significant competition or
facilitation in the driest conditions, which result supports the
collapse of facilitation model (Michalet et al. , 2006). Finally,
for group D, competition increased with watering in nutrient-poor
conditions, but facilitation switched back to competition with
increasing drought in nutrient-rich conditions, results supporting the
SGH and switchback to competition models, respectively (Bertness &
Callaway, 1994; Maestre & Cortina, 2004; Michalet et al. ,
2014a). Together these results suggest that apparent
inconsistencies
in results of experiments on variation in competition and facilitation
along environmental stress gradients might be explained by the
functional strategies of the target species involved in the different
studies or the type of stress gradient (Michalet, 2007; Maestre et
al. , 2009; Michalet et al. , 2014a; Liancourt et al. ,
2017) rather than methodological issues as argued by He & Bertness
(2014).
Consistent
to our main hypothesis, the contrasting between-group variations in the
effects of the shrub along the watering and N fertilization treatments
balanced at the community level, since there were no significant effects
of the treatments on RII for community biomass and richness.
Previous studies have found community-level balance of the effects of
dominant neighbours among groups of species within a community (Michaletet al. , 2015; Wang et al. , 2017). These authors have
argued that this is more likely to occur in species-rich communities of
intermediate stress conditions. Indeed, in extreme conditions of low or
high stress, functional divergence is thought to be low, consistent to
the humped-back model of Grime (1973) and, thus, communities are
dominated by either competitive or stress-tolerant species with a
negative or positive net response to neighbours at the community level
(Michalet et al. , 2006). However, to our knowledge, this is the
first experiment showing that significant changes in the effects of a
dominant shrub along environmental treatments mimicking global change
balance at the community level. These new findings are of crucial
importance for including biotic interactions in predictive models of
species and community responses to global change, a current important
goal in global change research (Tylianakis et al. , 2008; Wiszet al. , 2013; Anthelme et al. , 2014; Michalet et
al. , 2014b; Svenning et al. ,
2014). It shows that, although global change may have strong significant
effects in plant communities at species level, increasing or decreasing
competition or facilitation depending on the species functional strategy
and stress factor involved, there could be no significant changes in the
effects of dominant neighbours on community biomass and richness. We
argue that this is more likely to occur in species-rich communities form
intermediate stress conditions that exhibit a high functional
divergence, as proposed by Grime (1973) and Michalet et al. (2006) and
shown by a number of studies
(Gerhold et al. , 2013;
Duru et al. , 2014; Fryet al. , 2018; but
see
de Bello et al. , 2006). Thus, species diversity, and in
particular functional richness, might be considered as insurance for
community resistance to global change, and functional divergence and
plant-plant interactions as stabilizing factors for community response
to global change. These results do not imply that global change should
not affect community composition when community-level changes in
interactions are not significant, since there could be important changes
at the within-community patch scale. For example, from our results, we
can expect that some species will increase in abundance or biomass in
open patches relatively to below shrubs when N deposition increases
(case of group A in our experiment and stress-tolerant and ruderal
species in general), and that other species will increase conversely
below shrubs relatively to open patches when rainfall increases (case of
group C in our experiment and competitive species in general). Such
internal reorganization of the community in response to global change
has already been observed in communities with strong environmental
heterogeneity such as European calcareous grasslands
(Fridleyet al. , 2011). These results also suggest that disturbing
communities through the removal of dominant species may certainly
disrupt community stability. Indeed, this could alter these contrasting
interactions among functional groups and decrease community resistance
to global changes as shown with modelling by Losapio &
Schöb
(2017) for alpine cushion-plant communities.
To
conclude, our study showed that species of contrasting functional trait
values within a single community exhibit contrasting changes in
responses to the effect of a dominant shrub along environmental
treatments mimicking global change, and that these interactions balance
at the community level with no effects for the biomass and richness of
the community. These results suggest that functional divergence and
plant-plant interactions stabilize community response to global change.
Our results are crucial for understanding the mediating role of
plant-plant interactions for species and community response to global
change and may help reconciling the conflicting literature on variation
in facilitation and competition along environmental gradients.