Trade-offs in reproductive allocation between dominant and subordinate species
Our results highlighted the critical role of optimal belowground resource availability in determining the reproductive success of the dominant species, consistent with models of the evolutionary advantage of reproductive investment at resource-rich sites (Reekie & Bazzaz 1987; Kettenring et al. 2011; van Lent et al. 1995; Johnson et al. 2017). Specifically, we observed a lower reproductive allocation of the dominant species in both drought stress treatments (Fig. 1a), but no decrease in stability compared to the optimal water level (Fig. 1b). The absence of reduced stability in the permanent drought regime indicates that there is also no potential for resource accumulation for future flowering, contrary to the predictions of the resource budget hypothesis (Isagi et al. 1997; Satake & Iwasa 2000; Han et al. 2014).
Water limitation interacts complexly with the acquisition and storage of resources in plants over time (Barringer et al. 2013). Nitrogen, phosphorus, and potassium are depleted in plant tissues following seed masting (Crone et al. 2009; Sala et al. 2012), but with little evidence of nutrient storage and translocation prior to flowering (Pearse et al. 2016).
The dominant species also showed no response of RA stability to resource fluctuations (Fig. 1b), contrary to expectations of the resource-switching hypothesis. This may indicate that dominant species do not switch to reproductive investment according to annual resource dynamics, but only after several years of optimal conditions. This evolutionary strategy may also be based on plant developmental constraints, as the differentiation of the apical meristem into flowering and vegetative buds in clonal plants also depends on the environmental conditions of the previous year (Stenström & Jónsdóttir, 1997).
Following the multiple resource limitation model, the observed patterns of reproductive allocation in subordinate species highlighted their ability to dynamically adjust their strategies in response to resource availability dynamics and competitive interactions. When dominant species was removed, subordinate species experienced a release from competition, resulting in increased access to resources such as light, water, and nutrients (Liancourt et al. 2005; Gross et al. 2009; Mariotte et al. 2013; Doudová & Douda 2020; Douda et al. 2021). Accordingly, we found the highest increase in reproductive allocation, although not significant, in C . elongata and D . cespitosa following dominant removal compared to a lower response to drought regimes (Fig. 1a). This is likely due to a functional trade-off between the photosynthetic compensation point of plants better adapted to drought stress or optimal conditions (Sack 2004; Westerband & Horvitz 2017), where drought-tolerant subordinate species lack shade tolerance. The demand for photosynthesis increases with reproduction to compensate for the cost of reproduction (Reekie & Bazzaz 1987). Therefore, in optimal environments where light availability is limited by dominant species, the carbon assimilation required to compensate for the cost of flowering by subordinate species may be severely restricted.
Our results also showed that C. canescens further adjusted its reproductive allocation strategies based on the availability of belowground resources, further reflecting its ability to adapt to changing environmental conditions. Seed reproduction emerged as a cost-effective propagation strategy under combined drought stress and absence of light limitation (Fig. 1a). Seed production under stressful conditions allows efficient dispersal and establishment in new areas, potentially escaping competition or exploiting newly available resources (Gardner & Mangel 1999; Kooyers 2015; Blanco-Sánchez et al. 2022). Conversely, under optimal conditions, when resources are abundant and competition with the dominant is experimentally removed, C. canescens is likely to postpone seed reproduction and prioritise investment in clonal growth to support future reproductive efforts (Eriksson 1997).
In contrast to the dominant species, the stability of subordinate species decreased under both drought stress regimes (Fig. 1b), indicating that both a switching and a resource budget model are at play. In clonal plants, investment in non-flowering ramets prior to resource pulses may lead to a proportional increase in flowering ramets during the pulse when resources are sufficient to maintain both vegetative growth and reproductive structures (Bazzaz et al. 2000). Resource fluctuations may also act as a cue for clonal plants to coordinate seed production, rather than acting solely as a constraint (Pearse et al. 2016). For D. cespitosa , reduced stability was only apparent in the absence of dominant species (Fig. 1b), suggesting that the negative effects of low belowground resources are reduced in the presence of dominant species. This is consistent with a previous finding that subordinate wetland species can be facilitated by the dominant species under reduced belowground resources, which was also supported under field conditions (Douda et al. 2021).