Mangroves: Where's the Understory?

Abstract

With one exception, every pristine tropical forest more than a few metres in height has understory vegetation that contains numerous vascular plant species that grow and reproduce there, but never become canopy members. Likewise with one exception, all pristine lowland tropical forests have numerous species of vines (ljanas) (e.g. Gentry 1983, Pefialosa 1984, Putz 1983, 1984a, b). The exception in both cases is the mangrove swamp forest (Figure 1). Mangrove forest the world around is conspicuous for the absence of reproducing understory herbs and shrubs, and for the absence of the vine life form. If you grow rooted to the soil in a mangrove swamp, you are a tree and the reproductive members of your population have their crowns in the canopy of the forest. Equally conspicuous is the total absence of any reference to either this phenomenon or its possible causes in the voluminous mangrove forest literature (e.g. Chapman 1976, 1977, van Tine & Snedaker 1974, Watson, 1928). It is almost as though ecologists are so pleased with not having to deal with an understory that its remarkable absence is passed over with relief. The evolutionary move into the intertidal mud-flat successional habitat has occurred numerous times in numerous plant families. What has kept herbs and understory shrubs from likewise evolutionarily moving into the mangrove forest understory? What has kept vines out of the canopy? Why is the understory occupied at best by a few stunted juveniles of the habitat overhead, such that the view through a mangrove forest understory (Figure 1) is largely of sundappled and shaded trunks, stilt roots, pneumatophores and little foliage? Even the succession in a mangrove tree fall is carried out entirely by canopy-member species. The mangrove fern, Acrostichum spp. (Gomez 1983), is the closest thing to a mangrove understory herb, but even this halophile seems to be dependent on direct sunlight to be fully reproductive on salty substrates and usually grows in full sunlight.