O

Rhizophora type

Bruguiera type

Bruguiera type

(d)

Xylocarpus granatum type

FIGURE 3.27 Views of lateral root systems of mangrove tree species. (a) Pop root system. (b-e) Cable root systems. (From Tomlinson, P. B. 1986. The Botany of Mangroves. Cambridge University Press, Cambridge, U.K. With permission.)

Xylocarpus granatum type

FIGURE 3.27 Views of lateral root systems of mangrove tree species. (a) Pop root system. (b-e) Cable root systems. (From Tomlinson, P. B. 1986. The Botany of Mangroves. Cambridge University Press, Cambridge, U.K. With permission.)

controversy in the literature. Many workers, starting in the 1950s, studied particular sites and reported in opposition to Davis that mangroves follow silting rather than cause it. This opinion is summarized by Frank Egler (1952a) in his unique style as noted below:

As for the Rhizophora community actively "walking out to sea" (an action suggested by the way the prop roots extend out from the front of the community), and as for the fruits plunking into the mud below and planting themselves (an action suggested by

FIGURE 3.28 View of the aerial root system of the red mangrove, Rhizophoramangle. (From Scoffin, T. P. 1970. J. Sed. Petrol. 40:249-273. With permission.)

their plummet-shape and green upper parts), these stories appear to be part of the armchair musings of air-crammed minds of a century ago. I have seen sporadic instances of such phenomena: they are not impossible as accidental events. On the other hand, for its being a normal community activity accounting for the bulk of the Rhizophora belt, I place its probability — for the regions I know — on par with that of a chimpanzee bowed before a typewriter and batting out the Sermon at Benares.

Thom's work (Thom, 1967, 1984; Thom et al., 1975) is most often cited in opposition to the theory that mangroves build land (see also the review by Carlton, 1974). The modern view is, then, that "Mangroves establish once sediments have built up sufficiently by physical processes but once established, mangroves contribute to the process of accretion by accelerating the rate of land elevation, preventing erosion and by stabilizing the accretion-erosion cycle" (Lugo, 1980). This action,

FIGURE 3.29 Energy circuit model of the roles of roots in controlling erosion in a mangrove swamp. Sediment transport is shown with the box symbol, H, representing the Hjulstrom relationship.

though not as dramatic as imagined by Davis (1940), represents another kind of self-building behavior. In this case mangroves help to build their own forests by stabilizing and accreting sediments. Figure 3.29 depicts this mechanism for a tidal flow situation. Mangrove biomass is shown with an aboveground storage that drives primary production along with two root storages that play roles in erosion control. Aerial roots reduce current velocity causing sediments to deposit, and fine roots reduce the loss rate of the sediments once deposited. The self-building feedback is shown in the pathway from sediments to the production work gate. This represents the necessary role of sediments in providing substrate for the mangroves to grow on. The early view (i.e., Davis, 1940) and the modern view (i.e., Thom, 1984) of mangrove sedimentation agree that this self-building process occurs but they differ in the extent of new land that has been created. This disagreement is partly due to the complication of the rising sea level that has been occurring globally since the end of the last ice age. Humans are apparently accelerating sea level rise through the greenhouse effect, and an interesting literature is developing on impacts to existing coastal wetlands (see Chapter 5).

The goal of ecological engineering is to actively utilize the erosion control potential of mangroves and other types of coastal vegetation. Carlton (1974) provides a view of this use (Figure 3.30), which combines hard engineering (bulkhead seawall) with soft bioengineering (mangroves) for the best coastal development. This kind of action would provide by-product value through the detritus food webs and habitat

Best Development
Worst Development Mangroves and Seagrasses
FIGURE 3.30 Comparisons of shoreline designs. (A) A natural mangrove shoreline. (B) A developed shoreline with bulkhead and mangroves. (C) A developed shoreline with dredging and bulkhead protection. (From Carlton, J. M. 1974. Environmental Conservation 1:285-294. With permission.)

values that mangroves provide to local fisheries, while controlling erosion and loss of human property.

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