tion, many plants among the monocots could not be considered
trees. Palm (one of the most wellknown and widely planted tree
families in the world) and bamboo
(the tallest members of the grass
family) trees for example, possess
unbranched stems. Along with
palms and bamboos, other monocot trees from the genera Pandanus, Dracaena and Cordyline (to
name but a few) have structural
material that resemble ordinary
‘wood’ (and are even colloquially
called wood), but their structure
and composition is quite different
from that of ordinary wood. Closer
to home, the monocot Aloe dichotoma (Quiver Tree or Kokerboom)
that is indigenous to southern Africa, although bearing secondary
branches, does not have woody
stems. Having said this, we can
then surmise that the term ‘tree’
could be variably applied and even
be re-defined. If we considered
these examples, a tree could then
be defined as a perennial plant that
may, or may not, have secondary
branches supported clear of the
ground on a single main stem or
trunk, with clear apical dominance,
and that would achieve a minimum
height of 3 m at maturity.
This latter definition allows us to
consider kelp plants as trees. Compared to other seaweeds, kelp
plants are comparatively long-lived,
with individual plants of the Giant
Kelp (Macrocystis pyrifera) living for
over 10 years (and thus certainly
perennial) and attaining lengths of
up to 60 m. Not only does this latter figure satisfy the height (length)
requirement for a tree, but also the
United Nations FAO definition of
being ‘higher than 5 m’ when defining a forest. During optimal conditions, Giant Kelp can grow as
much as 60cm per day, quite easily
attaining lengths (heights) prescribed as minimum for classification as trees. As a consequence of
their rapid growth rates, kelp communities are among the most productive ecosystems on earth, supporting in situ primary production
levels of as much as ten times
greater than most terrestrial crops
and intensive agriculture. Similar to
terrestrial trees, individual kelp
plants are comprised of a root-like
holdfast, a stem-like stipe (that in
most species is unbranched), and
blades formed mostly at the distal
ends of the stipe(s). Although not
bearing an apical meristem, growth
in kelp are controlled by an intercalary meristem that is located at the
transition zone between the blades
and the stipe. These meristems produce a kind of apical dominance
that in most species of kelp results
in only a single stipe.
Ecklonia maxima occurs abundantly along the cold
temperate west and southern west coasts.
Canopy cover of more than ten
percent
While the UN refers to a forest as a
vegetation type possessing a canopy cover greater than only 10% of
trees, most South African forest
literature advocate a canopy cove r
greater than or equal to 75% of
trees that form a closed canopy. In
addition, the South African literature makes reference to a community with definite and recognisable
strata or layers, having at least
three layers, namely a canopy, subcanopy or understorey (which includes epiphytes) and grounddwellers. Even here, kelp communities quite readily satisfy the requirements.
In established kelp communities,
kelp plants are most often the only
marine algae that reach the surface
(i.e. canopy cover equivalent to
100% of kelp plants) and, unless
there are sandy gullies or barren
areas, the kelp plants will form a
closed canopy. Kelp can easily occur in densities of over 10 plants
per square metre and the closed
nature of the canopy is enhanced
by the fact that over 50% of kelp
biomass is blade material located
apically at the distal ends. Similar
to terrestrial forests, kelp communities
are
three-dimensional
(stratified) in their structure. Firstly,
the kelp plants comprise the cano-
Ecklonia radiata, a small species of kelp, is restricted to
the warm temperate to subtropical shores of the south
and southern east coasts.