Life history is a schedule of key events in an organism’s life cycle, and is usually defined in terms of life-history traits calculated across individuals within a population (Oli and Coulson 2016). This life history theory asserts that vital functions such as growth, reproduction, maintenance and defense compete for every resource produced by an organism (Delph 1999). It is often observed that the allocation of resources in different traits of the organism's life history induces trade-offs (Stearns 1976; Delph et al. 1996). Such trade-offs represent the costs paid when the available resource is allocated differentially among traits (Stearns 1989; Obeso 2002). For example, male expression in dioicous species may be favored in poor soils due to the low energetic requirements of this specific sex, when compared to female of flower plants (Bowker et al. 2000; Segalla et al. 2021). Indeed, trade-offs are commonly observed in reproductive traits, showing a crucial influence in the demography and maintenance of populations (Horsley et al. 2011, Krieg et al. 2018).
Reproductive traits such as sexual expression (proportion of individuals that are expressing sex), and sex ratio (number of males and females) are important variables influencing the reproductive performance of population (Glime and Bisang 2017). In this context, many biotic or abiotic factors can affect development and reproductive traits. For instance, some species need high levels of humidity to express sexually, as during the rainy season, while others do not (Maciel-Silva et al. 2012; Maciel-Silva and De Oliveira 2016). Sexual expression plays an important role in maintaining species, and in some cases, population density is a determining factor of reproductive performance. Density dependence density has been recurrently reported in species of plants and animals (Hanski 1990; Gunton and Kunin 2009). Furthermore, sexual systems have also been linked to the reproductive success of populations, for example in Fissidens scarious Mitt., and Fissidens flaccidus Mitt., which have reproductive traits differently associated with sexual systems (Stark and Brinda 2013; Santos et al. 2020).
Sexual system is defined by Leonard (2018) as “the pattern of gender allocation that characterizes a species”. For plants, we can consider the sexual system as the classification of the distribution of reproductive structures (Leonard 2018). For example, dioicous and monoicous species sexual systems are associated with reproductive allocation in plants, so that a pattern is expected according to sexual system (Bergh et al. 2011; Stark and Brinda 2013). Plants are excellent models for understanding life history; as they are autotrophic and sessile, persistent populations can be followed for long periods of time (Bisang and Ehrlén 2002).
The simple architecture of vegetative and reproductive organs of bryophytes makes them excellent models for ecological studies (Harris et al. 2020). Indeed, bryophytes are considered models to understand the ecology and evolution of sexual systems (Suzuki et al. 2018; Harris et al. 2020), reproductive allocation (Stark and Brinda 2013; Santos et al. 2022), and reproductive cost (Bisang and Ehrlén 2002; Rydgren and Økland 2002). Among the most important traits found in bryophytes are: 1) since fitness can be quantified as the growth rate of a clonal population (Stearns 2000), and bryophytes are highly clonal (Rydgren and Okland 2003), they are considered favorable plants for this quantification; 2) due to the common regeneration of their gametophytes, it is often feasible to cultivate and monitor the development of these plants (Stark et al. 2007); 3) given the small size of bryophytes, one can determine the absolute biomass of the reproductive and vegetative structures to quantify the trade-off between the two functions and reproductive allocation (Ehrlén et al. 2000; Bisang et al. 2006). Evidence suggests that reproductive allocation is strongly related to sexual systems in bryophytes. Sexual systems of bryophytes present a gradient of distance between the sexes, and the more distant the sexes, the greater is the relative reproductive allocation (proportion of resource allocated to reproduction) in the male function (Stark and Brinda 2013).
In this study, we quantified the reproductive allocation (absolute and relative), and the following reproductive traits: sexual expression, sex ratio, reproductive success, and population density for a monoicous species that has similarities with dioicy (dioecy). Fissidens flaccidus Mitt. is a species of moss with a rhizautoicous sexual system that reproduces sexually and asexually (by clavate gemma in stem tissues). The rhizautoicous system presents individualized male and female ramets that are connected, at least initially, by rhizoids. This sexual system, therefore, functionally resembles the dioicous system, since the ramets presumably do not compete for resources for their development and formation of reproductive structures. In this context, we investigate the following questions: First, is male reproductive allocation greater than female? This is the usual pattern found in dioicous mosses (Stark and Brinda 2013), and rhizautoicous species have compartmentalization of sexual functions (functionally dioicous). Second, is there a trade-off between sexual and asexual reproduction? Since, according to life history theory, the resources available to individuals are finite, and these resources are subject to competition among different life history features or phases (Oli and Coulson 2016). Third, is the number of male ramets a determinant for greater reproductive success? Since the greater the quantity of male ramets expressing sex, the greater the quantity of male gametes and consequently the chance of fertilization of the female gametes. Fourth, is population density related to the sexual expression of ramets? As density dependence effects are recurrent in many animals and plants, we expect that population density influences the reproduction of the species.