Ecological Consequences of Sexual versus Asexual Reproduction in Dicranum flagellare and Tetraphis pellucida

Abstract

To test hypotheses of the relative advantages of sexual and asexual reproduction, the reproductive ecology ofDicranum flagellare is compared to that ofTetraphis pellucida. Both species share a habitat preference for decaying logs of the forest floor, but they differ in reproductive mode. In D. flagellare asexual reproduction predominates, whereas T. pellucida produces both sexual and asexual propagules. The ecological consequences of reliance on asexual reproduction are explored: dispersal, colonization, and spatial distribution. The asexual brood branches of D. flagellare suc- cessfully colonize gaps produced by disturbance, but only when D. flagellare is present in the nearby vegetation. The local distribution ofD. flagellare is limited to spatially aggregated substrates, whereas T. pellucida is widely dispersed. Dicranum flagellare invests significantly more biomass in asexual reproduction, but the overall reproductive effort of T. pellucida exceeds that of D. flagellare. Brood branches of D. flagellare germinate significantly more rapidly than Tetraphis gemmae. Dicranum flagellare persists longer than T. pellucida in log communities, perhaps as a result of accumulation of perennial shoot increments. The distribution of bryophytes through a wide range of transient microhabitats reflects the evolu- tion of diverse reproductive strategies. Bryophyte life histories range from fugitive species that rely on sexually produced spores to species whose repro- ductive effort is entirely asexual. The adaptive sig- nificance (if any) of these reproductive patterns is not well understood (Mishler 1988). It has been suggested that bryophyte reproduction involves a balance between the long-term advantages of sexual reproduction and the more immediate benefits of asexual propagation (Cummins & Wyatt 198 1). The nature of this balance may shift with changing en- vironmental conditions--in particular, the distur- bance regime. Recent studies demonstrate that ad- aptation to disturbance frequency may be a strong selective pressure on the reproductive ecology of the forest floor bryophyte Tetraphis pellucida (Kim- merer 1991a,b, 1993).