Re: Contemplation
Nan Sterman wrote:
>
> Hi folks!
>
> I'm taking a mental break from my work and happily contemplating my
> navel as I ponder the conundrums of life (forgive me for mixing
> metaphors, it is just too much fun!:). Following Jan Smithen's
> excellent (if a bit esoteric!) question about the Golden Rectangle (or
> was it the Golden Rule?), I think it's time for another lively
> discussion on medit plants about the nature of the universe. Whatta
> you say?
>
> Here is one for you: How might you explain the fact that there are so
> many more sun-loving flowering plants than shade-loving flowering
> plants?
>
> My mind immediately goes off into theories of energetics and
> photosynthesis -- i.e. it takes tremendous amounts of energy to
> reproduce (flower), which requires a tremendous production of energy
> (photosynthesis). The more sunlight, the higher potential rates of
> photosynthesis, hence the more energy for reproduction and over time,
> the increased success of flowering plants that sprout in sunny
> locations.
>
> However, this kind of simple-minded explanation begs the question as
> to why any plant would be able to reproduce successfully (in Darwinian
> terms), in the shade of, for example, the forest. But come to think
> of it, temperate forests at least are not terribly floriferous -- or
> at least, not in terms of the very showy flowers our culture treasures
> for our gardens.
Nan
Thanks for proposing this very interesting topic for discussion.
I think the question must indeed be looked at largely in terms of light
energy available. as with most reproductive processes whether in plants
or animals successful seed production is certainly a high-energy
activity.
Of course all Angiosperms potentially flower, as "flowering plants" it
is their main survival strategy, and so wherever possible they will
place
themselves in situations where they can get enough sunshine for this
operation. With the major trees in forests this is largely a problem of
being tall enough to reach the main canopy level, but smaller plants may
have to use a number of strategies to get their fair share of light.
We have to first distinguish between evergreen and deciduous forests.
Evergreen broadleaf forests are commonly called rainforests. I have
never visited
a tropical rainforest, but live in a land where temperate ones are the
norm. As the canopy is equally dense at all times of year anything
smaller than a mature tree has to either manage its reproduction with
very low illumination or devise some strategy to get to the light.
In the main, in areas of dense unbroken canopy the undergrowth in NZ
forests is almost exclusively ferns, mosses and their close relations.
If you
want to see flowering plants other than trees you will mostly have to
look up, and right up in the canopy you will find either the heads of
climbers which started on the forest floor (and tend to dangle around at
eyelevel as the long leafless plant ropes known as lians) or else
perching plants, either epiphytes or parasites, such as astelias,
orchids and mistletoes festooning the higher branches.
Few trees grow alone and some are a veritable garden of species from
ferns climbing up the stems to huge clumps of Astelias in the upper
branches looking like vast untidy birds nests, and even quite sizeable
broadleaf shrubs such as the Puka.
I should perhaps mention the Rata vines (Metrosideros spp) which are a
particularly good example of what plants will do to get their share of
the light. These start on the forest floor, but soon attach themselves
to the trunk of a suitable host tree and use that to climb up to the
sun. Only when they get to the top will they produce their spectacuar
fluffy red or orange flowers very similar to those of their non-vining
relative, M robusta which some of you seem to know. However the rata is
not content just to get to the sun, It has greater ambitions still and
continues to send up more and more stems until the unfortunate host is
entirely encased and dies. What one is then left with is a Rata "tree"
of great size and magnificence with a far-spreading crown right at the
top of the canopy and able to compete with anything in the forest for
light energy to reproduce. Often in very old specimens the host rots
away completely leaving the rata with a hollow "trunk".
When a host tree gets old, and especially if it dies, the load can
become so heavy it pulls the old thing over. This is an important event
in the life of the forest, as it lets the sunshine into a a piece of the
forest floor and for a few years may allow some smaller plants to
succeed in the gap, though what one mostly finds is young of the canopy
species, whose seed has been stimulated to grow by the increased light
and thes will all be racing to try and be the one to make it to the
heights and smother the others out.
There are a couple of places where there is more light and one may find
a few flowering plants at ground level - along streams and along tracks.
Of course many tracks these days are man-made but in most parts of the
world even if humans were not around there would still be game tracks.
Along our tramping tracks (trails) however, one finds only a very few
small native flowering plants but more often introduced weeds, such as
foxgloves and dandelions, brought in on people's clothing no doubt.
To sum up, because the main trees in a rainforest are evergreens, the
ground is shaded much the same all the year round and offers few
opportunities for plants to flower unless they have some means of
getting at least a part of their shoot system up above the enveloping
canopy.
A deciduous forest however offers much more oppportunity to flowering
plants without them having to go to great lengths to grow up into the
light. Instead they need to flower at a particular time of year when the
leaves are off the trees. This window of opportunity is also governed by
the severity of the local winter which may not allow a very early start
in spring.
Some deciduous forests have quite a show of spring wildflowers and also
many understory shrubs which bloom in spring. Under deciduous trees one
may find a great many spring bulbs, for instance such as bluebells
(Scilla), Celendines (Ranunculus ficaria) and other plants which bloom
early and then go back to sleep like Trilliums, Hepatica and Bloodroot.
Also one may find late winter or spring-flowering shrubs like
Rhododendrons and Azaleas, Pieris, Camellias and Magnolias. Once the new
leaves unfold though the flowering slows down and vey few plants will be
found which flower in the denser summer shade. However, deciduous
woodland does quite often have open spaces or "rides" and here one will
find grasses and the summer flowers which go with them and along the
borders fruits which enjoy dappled shade such as raspberries. (Maybe
currants too, though I haven't had the opportunity to see these in the
wild).
Most of the plants which grow under deciduous trees in nature do
appreciate a _partly_ shaded site in the garden, but the dense permanent
shade (of buildings for instance) may allow them to grow only but not to
flower. I have several times had enquiries from people whose Azaleas,
for instance, regularly fail to flower and the solution is often to move
them to a less shaded situation.
As to the sun-lovers, not only will they fail to flower if one grows
them in too much shade, but their whole growth and often their
appearence may be affected. Leaves may be thinner than usual and stems
long and straggly, varigations often change to plain green, as does
golden foliage and silver leaves become a dull bluish-grey. Annuals in
particular grow poorly and suffer many diseases and insect problems. The
majority of grasses really hate shade, fail to flower and will often
gradually fade away.
Shade is a real challenge to most sorts of flowering plant and only a
very few specialized species have learnt to flower at really low light
levels. One that springs to mind from my African experiences is the
genus Impatiens, many of whose members are forest dwellers and do
succeed pretty well in flowering in the shade.
My 2 cents (or maybe a bit more <G>)
Moira
--
Tony & Moira Ryan <theryans@xtra.co.nz>
Wainuiomata (near Wellington, capital city of New Zealand)
