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).