Multiple resource limitation model and species reproduction
synchrony
The trade-off in reproductive allocation between the dominant and
subordinate species resulted in high reproductive asynchrony within a
community (Fig. 2). However, community-wide reproductive asynchrony
decreased as the dominant species lost its competitive advantage under
belowground resource limitation due to drought stress. Reduced
competition and potential facilitation by dominant species have been
identified as critical factors that increase the overall performance and
biomass stability of subordinate species (Douda et al. 2018).
Accordingly, we expected a parallel increase in synchrony among
subordinate species as the competitive effect of the dominant species
was reduced (Williamson & Ickes 2002). This expectation was met, as
synchrony of subordinate species increased under drought stress regimes,
and increased further when the dominant species was removed under low
water treatment (Fig. 2). Compared to viewing reproductive synchrony as
simply the covariance of species’ responses to fluctuations in
belowground resources (Monks et al. 2016; Pearse et al. 2016;
Bogdziewicz et al. 2020), this explanation provides a more realistic
perspective on the reproductive dynamics of plant communities,
emphasising the importance of plant-plant interactions. It shows that
reduced competition during periods of stress can trigger reproductive
synchrony throughout the community.
A previous simulation study has proposed that resource fluctuations tend
to favour species with more acquisitive traits at the expense of those
with more conservative growth strategies (Doak et al. 1998). In our
previous experiment, we showed that during droughts, the dominant
species suppressed the growth of the clonal species C. canescens ,
which spreads via a network of stolons, while favouring the
tussock-forming species C. elongata and D. cespitosa ,
which have conservative growth patterns (Doudová & Douda 2020). The
establishment and maintenance of a stolon network in C. canescens is likely to be more resource demanding than tussock formation.
Therefore, C. canescens is more negatively affected by the
dominant species under drought conditions due to limited resources
available for stolon development (Fig. 1a). Consequently, under
conditions of drought stress, C. elongata and D. cespitosa showed synchronised reproductive allocation, regardless of the presence
of the dominant species (Fig. 3). In contrast, C. canescens synchronised its reproductive investment with other subordinate species
only in the absence of the dominant species, probably due to its higher
resource requirements for stolon formation (Fig. 3).
While interspecific synchrony in reproductive allocation increased under
drought stress treatments, intraspecific synchrony in both the dominant
species and the subordinate species (C. elongata ) decreased (Fig.
4). For the C. elongata , this decrease was only observed when the
dominant species was removed. This may reflect the different
evolutionary importance of synchronising factors and the processes
driving trait divergence between conspecifics and between species within
the community. Intraspecific synchrony in flowering is a critical
mechanism for enhancing reproductive success by increasing the
likelihood of cross-pollination among wind-pollinated individuals
(Nilsson & Wästljung, 1987; Kelly et al., 2001). According to our
results, individuals of species appear to synchronise flowering in this
way mainly at their favourable sites where generative reproduction is
more efficient, i.e. dominant species at wet sites, whereas subordinate
species at drought stress treatments with reduced competition. Increased
asynchrony after dominant removal may result from increased
intraspecific competition for belowground resources when the
facilitating effect of dominants is removed under stress conditions.
In summary, our study highlights the importance of interactions between
above- and belowground resources in shaping reproductive strategies in
clonal plants. Contrary to the null hypothesis, we found trade-offs in
reproductive allocation between dominant and subordinate species within
herbaceous communities. Optimal belowground resource availability
critically determines the reproductive success of dominant species,
consistent with evolutionary models of reproductive investment during
long resource rich periods. Subordinate species showed shifts towards
seed reproduction under combined drought stress and absence of light
limitation, while emphasising clonal growth under optimal conditions.
Furthermore, dominant-subordinate species interactions significantly
influenced community-wide reproductive dynamics, with reduced
competition of dominant species increasing community-wide flowering
synchrony in response to belowground resource limitation. Overall, our
study sheds light on how clonal plants respond to environmental changes
and competitive interactions within communities.