Neotropical coastal mangrove forests are usually “zonal,” with certain mangrove species found predominantly in the seaward portion of the habitat and other mangrove species on the more landward portions of the coast. The earliest research on mangrove forests produced descriptions of species distribution from shore to land, without exploring the causes of the distributions.
The idea that zonation is caused by plant succession was first expressed by J. H. Davis in a study of Florida mangrove forests. According to Davis’ scheme, the shoreline is being extended in a seaward direction because of the “land-building” role of mangroves, which, by trapping sediments over time, extend the shore. As a habitat gradually becomes more inland as the shore extends, the “land-building” species are replaced. This continuous process of accretion and succession would be interrupted only by hurricanes or storm flushings.
Recently the universal application of Davis’ succession paradigm has been challenged. It appears that in areas where weak currents and weak tidal energies allow the accumulation of sediments, mangroves will follow land formation and accelerate the rate of soil accretion; succession will proceed according to Davis’ scheme. But on stable coastlines, the distribution of mangrove species results in other patterns of zonation; “land building” does not occur.
To find a principle chat explains the various distribution patterns, several researchers have looked to salinity and its effects on mangroves.
While mangroves can develop in fresh water, they can also thrive in salinities as high as 2.5 times that of seawater. However, those mangrove species found in freshwater habitats do well only in the absence or competition, thus suggesting that salinity tolerance is a critical factor in competitive success among mangrove species. Research suggests chat mangroves will normally dominate highly saline regions, although not because they require salt. Rather, they are metabolically efficient (and hence grow well) in portions of an environment whose high salinity excludes plants adapted to lower salinities. Tides create different degrees of salinity along a coastline. The characteristic mangrove species or each zone should exhibit a higher metabolic efficiency at that salinity than will any potential invader, including other species of mangrove.
According to the passage, the earliest research on mangrove forests produced which of the following?