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.