Consequences of mating system and dispersal potential
Species with different reproductive strategies might present different sensitivity to small patch sizes and to isolation to maintain genetic diversity (Honnay et al., 2005; Obbard et al., 2006; Nazareno et al., 2013). Bisexual bryophytes are presumed to have high fertilization rates and consequently higher spore production, which ensure successful colonization (Laenen et al., 2016). In contrast, unisexual species will present lower dispersal potential as the probability of fertilization will depend on that male and female plants are found in the same colony (Snäll et al., 2004; Maciel-Silva et al., 2012; Alvarenga et al., 2016).
We observed high migration of R. flaccida among patches across the fragmented landscape irrespective of patch size. Functional connectivity between patches recovered in migration network analysis explained the ability of the species to maintain genetic diversity despite population density decline. Colonization rates of R. flaccida (unisexual), albeit low, were sufficient to curtail genetic drift as suggested by Zartman et al., (2006). On the other hand, the maintenance of genetic diversity observed in R. flaccida in small patches could be attributed to other two reasons. Firstly, genetically diverse populations have been maintained in small patches (although declining) due to their adaptative potential to cope with environmental change (Lowe et al., 2005); which is corroborated by the maintenance of linkage disequilibrium in small forest fragments (Zartman et al., 2006). Second, the standing genetic variation has not yet been lost through stochastic demographic events due to constant local recruitment from asexual propagules when sexual reproduction is limited (Honnay & Bossuyt, 2005). Limited sexual reproduction has been reported forR. flaccida majorly during the dry season, which is expected to exacerbate under future climate change scenarios (Zartman et al., 2015) affecting long-term demographic stability (Scott et al., 2021). Clonal reproduction represents an alternative recruitment strategy when environmental conditions suppress sexual reproduction in unisexual species, maintaining genotypes before fragmentation effects become apparent (Honnay & Bossuyt, 2005). In bryophytes, as observed herein for R. flaccida , asexual reproduction plays an important role in maintaining genetic diversity over time by preventing local genotype extinction (Honnay & Bossuyt, 2005) and assuring successful sexual reproduction (Alvarenga et al., 2016). Evidence from unisexual bryophytes found that genetic diversity is maintained when species stakes on asexual reproduction for their survival (Pohjamo et al., 2008; Wang et al., 2013; Holá et al., 2015; Escolástico-Ortiz et al., 2023), but contrasting evidence was found for a hornwort species (Alonso-Garcia et al., 2020).
Regarding C. surinamensis (bisexual), we expected a higher dispersal potential due to the higher probability to produce sporophytes since both androecial and gynoecia branches are produced in the same colony (Maciel-Silva et al., 2012; Laenen et al., 2016). Contrary to our expectations (H3), this species shows restricted dispersal among small patches (1- and 10-ha) where significantly lower genetic diversity and significant changes in allele frequency were observed. The stochastic genetic loss with declining populations of C. surinamensis in small patches, suggests an extinction debt has been already paid during the 40 years of patch isolation. A possible explanation for the lower dispersal potential would be that the genus Cololejeunea presents low spore output per capsule (250–900 spores) when compared to other genera in the family Lejeuneaceae (He & Zhu, 2011). Even if the sporophyte frequency of the bisexual and unisexual species studied here are similar in a population, the lower spore output per capsule ofC. surinamensis might limit the dispersal across the landscape (Snäll et al., 2004). Demographic surveys of epiphyllous bryophytes showed that species differ in the threshold size to initiate sexual expression, where higher size requirements might limit sporophyte frequency under unsuitable environmental conditions (Zartman et al., 2015; Alvarenga et al., 2016). Environmental conditions in degraded forest fragments might affect reproductive performance, by limiting sexual expression and sporophyte frequency of colonies growing in unsuitable habitat conditions (Maciel-Silva et al., 2012), which will further determine the assembly rate of individuals and genotypes (Sierra et al., 2019a; Mežaka et al., 2019).