The regenerative phase (Grime 2001
) is crucial in the life cycle of plants and, since young individuals must find safe sites to establish and survive (Harper et al. 1965
), the reproductive phase is subject to strong selective forces. Different reproductive strategies have been developed among plants by means of both asexual and sexual types of reproduction, in accordance with their life histories and the environment in which they live (Bengtsson and Ceplitis 2000
; Obeso 2002
). In general, the reproductive strategies based on long lifespan are related to low growth rate and low reproductive effort, while rapid development and high reproductive effort are linked to short lifespan (Grime 2001
; García et al. 2008
). Other traits can be related directly or indirectly to the longevity of plants, such as breeding systems, seasonality of reproduction and formation of diaspore banks (During 1979
; Grime 2001
), where the probability and frequency of reproductive events generally decrease with increasing lifespan.
Plant life-history traits change along environmental gradients, sometimes within short distances, involving growth and reproductive parameters (Hassel et al. 2005
; von Arx et al. 2006
; Hautier et al. 2009
; Milla et al. 2009
). Different strategies permit plants to explore a range of distinct habitats, but not all variations in growth and reproductive traits result in different ecotypes (Reynolds and McLetchie 2011
). It has been reported that species at high altitude invest more in growth than in reproduction (a conservative approach), while species at low altitude tend to invest more in reproduction (von Arx et al. 2006
; Hautier et al. 2009
). These findings are often related to the more severe conditions in the high mountains, with low temperatures, a period of snow cover and low productivity (Körner 2007
). We do not know if they apply to altitudinal gradients of tropical areas with less severe conditions at higher altitudes.
Tropical rainforests are extremely threatened environments with high richness and diversity of species (Gómez-Pompa et al. 1972
; Murray-Smith et al. 2009
). For instance, the Atlantic forest in Brazil currently has less than 16 % of its initial cover (Ribeiro et al. 2009
), and is restricted to small forest fragments and a few large nature reserves. Areas with complete altitudinal gradients of forest from sea level to the mountain tops are now very scarce (e.g. Atlantic Forest covering the ‘Serra do Mar’ in southeastern Brazil). The Brazilian Atlantic forest retains a large number of plant species, with a total of ~15 800 (7155 endemic species), of which 1230 are bryophytes (Stehmann et al. 2009
Bryophytes are a large and important component in tropical rainforests, covering substrates such as tree bark, leaves and decaying wood, and contributing to the high species richness and diversity (Whitmore et al. 1985
; Frahm and Gradstein 1991
). They also affect ecosystem functions (nutrient and water cycling, and habitat for micro fauna—Nadkarni 1984
; Schofield 1985
; Veneklaas 1990
; Turetsky 2003
). Since these plants (liverworts, mosses and hornworts) have a dominant haploid generation, reduced size, similar types of breeding system in common, and in general a shorter life cycle than seed plants (Glime 2007
), they are excellent models to study factors affecting sexual reproduction.
Absence of males or female-biased sex ratios are common among dioicous bryophytes (Longton and Schuster 1983
; McLetchie 2001
; Stark 2002
), though population sex ratios around 1 : 1 are also recorded in the literature (Marchantia inflexa
studied by McLetchie and Puterbaugh 2000
). Specifically, they can be used to understand life-history strategies involving the reproductive performance of, for example, monoicous and dioicous species in different habitats of the same ecosystem. Our overall aim was to test whether differences in sexual reproductive performance of bryophytes in tropical rainforests are driven by the breeding system of the species (monoicous or dioicous), or are mainly affected by the habitat.
Since monoicous species can self-fertilize and therefore tend to produce sporophytes more frequently than dioicous ones (Gemmell 1950
; Rohrer 1982
; Longton and Schuster 1983
; Longton 1992
), we expected that monoicous species have higher production of gametangia (i.e. sex organs: ♂—antheridia, ♀—archegonia), fertilization rate and sporophyte production than dioicous species. We also expected to find differences in these features within species or between closely related species among sites, since reproduction can be differently triggered by abiotic factors (e.g. light, temperature and humidity; see Chopra and Bhatla 1983
; Kumra and Chopra 1983
; Longton 1990
). We studied the production of sexual branches and gametangia, fertilization and sporophyte production of bryophytes in two contrasting sites of Atlantic tropical rainforests (montane and sea level) in Brazil. We specifically investigated the following questions:
- Do monoicous and dioicous species differ in their sexual reproductive performance?
- If so, at what stages of reproduction do the differences occur (production of male and female structures, fertilization, sporophyte production)?
- Do the same, or closely related, species differ in sexual performance between habitats?
- Is there a seasonal variation in reproduction?
- Do abiotic factors such as pH and moisture of substrates influence reproduction?