Mangrove understory: an expensive luxury?

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Recently, Janzen (1985) observed that mangrove forests lack a plant understory and proposed three hypotheses to explain the absence of reproducing unders ory herbs, vines (lianas), and shrubs in mangroves. Although the general observation of an absence of these plants in mangrove understory is true and had already been made by Chapman (1975, p. 13), there are, nevertheless, examples of plant understory in mangroves (e.g. Corlett 1986). Chapman lists well over a dozen vine, herb, fern, and palm species that grow in mangrove understory in Bangladesh (Sunderbans), Borneo, Malaysia, Martinique, India (Bombay), and the Philippines. All these examples are from high rainfall locations, the landward portion of mangroves, and/or ecotones characterized by low salinity soils (Chapman 1975). These examples suggest that hypotheses are needed to explain the general absence of plant understory in mangroves and why understory occurs when soil salinity decreases. I agree with Janzen's dismissal of his hypotheses 2 and 3 (however, for different reasons) but believe that his hypothesis 1 has merit. This hypothesis states that 'plants with low light resources cannot accumulate enough [sic] fast enough to meet the metabolic demands of the drain of the machinery and morphology of salt tolerance.' This formulation recognizes the need to balance the physiological and morphological cost of overcoming salinity stress with the light energy available to plants. However, there are other natural stressors and other energy sources that must be included in any evaluation of the plants growing on mangrove sites. Stressors include soils with high concentrations of hydrogen sulphide and low oxygen and loss of nutrients due to frequent tidal flushing or poor substrate conditions (Lugo 1978, Lugo et al. 1981). Oxygen to sustain root metabolism, freshwater to mitigate high salinity, and nutrients for biomass synthesis are vital energy subsidies to plants growing in mangroves, and they are all obtained, at least partially, from the substrate. Therefore, favourable conditions for root growth may be as important to plant survival on mangrove sites as light availability.