less exposed areas than Reds, but are able to thrive in
oxygen- starved sediment. They are bushy- topped trees
that can reach heights of up to 20 m (66 ft). Leaves
are oval, leathery, and downy white underneath. The
flower is yellow and tubular, the fruit green and ovate.
Seedlings float and, like adult trees, are tolerant of
low oxygen levels. The most notable feature of Black
Mangrove is its root system. Shallow horizontal roots
anchor it in thick, smelly, anaerobic mud, but these
roots send up vertical aboveground shoots, called
pneumatophores. Lenticels on the pneumatophores
feed into wide air passages connecting with the
underground roots, providing a means for air transport
to the oxygen- starved root system.
Two other common mangrove species are White
Mangrove (Laguncularia racemosa) and Buttonwood(Conocarpus erectus). White Mangrove is less tolerant of
prolonged immersion in the sea and tends to grow at
slightly higher elevations than Red and Black Mangroves.
White Mangrove has scaly reddish- brown bark and
greenish- white flowers that turn into small (less than 2.5
cm/1 in) reddish- brown fruits with longitudinal ridges.
It grows from about 9 to 18 m (approx. 30– 60 ft) tall.
Buttonwood resembles White Mangrove but occurs only
well away from daily flooding by salt water. It is the least
salt tolerant of any of the four common mangroves and
was once not considered to be a mangrove. As salinity
declines, such as at higher elevations or with distance
from the coast, pure stands of Red or Black Mangroves
give way to mixed stands of several species, not all of
which are mangroves. The transition from mangal
community to upland community is generally gradual.Plate 12- 67. The interior of a mangrove swamp is dense with roots that stabilize sediment and help expand the forest. Photo by
John Kricher.chapter 12 cruising the rivers to the sea 231