Shade and salt tolerance of rare, common and invasive mangroves

Background/Question/Methods

One explanation for plant distributions is the trade-off between competitive ability and stress tolerance in plants—poorer competitors may tolerate more stressful environments, resulting in narrower distributions. In groups of species adapted to stressful environments, variation in ability to cope with harsh conditions may have consequences for patterns of distribution. In harsh environments, are widespread species more stress-tolerant than narrow endemics?

Mangrove forests offer an ideal system to test this question, as they provide a physically challenging habitat for plants—the tidal influence results in flooded, hypoxic and saline soils. Mangroves were thought to be resistant to invasion because of their stressful habitat—most plants cannot survive in saline, flooded soils, especially when combined with high temperatures or low light. However, nonnative mangroves already adapted to these harsh conditions may be able to replace native species.

I examined the effects of shade and salt on survival and growth of four mangrove species. I grew seedlings of one rare, two common, and one invasive species in outdoor tubs at the Smithsonian Marine Station in Fort Pierce under varied shade and soil salinity levels.

Results/Conclusions

Tolerances of stressful environmental conditions were associated with whether species are rare, common, or invasive. The common mangroves, Avicennia germinans and Rhizophora mangle, were tolerant of all treatments. Analysis of variance showed that shade did not affect plant height for either common species, though heights of both species were affected by salt level. The invasive species Lumnitzera racemosa was tolerant of all treatments and thrived in the most stressful conditions (full sun and hypersaline). The rare species, Pelliciera rhizophorae, was intolerant of the most stressful light level (full sun), but tolerated a range of salinity levels. In addition to contributing to our understanding of plant distributions, the results from this study may aid management efforts to control the newly invasive species Lumnitzera racemosa.