Seite 7 - Einblicke 57

57 EINBLICKE 9
The rainforests and the destruction of tropical forests are
issues that through years of media reporting have become
firmly lodged in theminds of the German public. The repeated
descriptions of the biological treasures that we are losingwith
the ten million hectares of rain forest that are cleared each
year have ensured that
everyone knows about
the incredible diversity
of species in these ecosystems. This biodiversity is indeed
a source of wonder: a single hectare of Amazonian lowland
forest is home tomore types of trees that thewhole of Europe,
and just one giant tropical tree hosts more ant species than
the whole of Britain.
Aside from the formidable profusion of species, there are a
whole series of plant growth forms in the tropics which are
either rare or entirely absent in this part of theworld. These in-
clude tree ferns, strangler figs, climbing vines and “epiphytes”,
so called because they grow on other plants. Epiphytes are
a particularly vast plant group, accounting for almost ten
percent of the world's flowering plants, including over 10,000
orchids, thousands of ferns and more than a thousand plants
of the pineapple family.
Current species records provide an impressive document of
local diversity: more than 100 epiphytic species
were recorded on a single tree in a Costa Rican
mountain rainforest. The particular focus
of my work group, “The Functional Eco-
logy of Plants”, is the group of vascular
epiphytes. We travel regularly to the
tropics to conduct field research, and
to Panama in particular, where the
research stations of the Smithsonian
Tropical Research Institute provide
ideal conditions for scientists.
Our research covers a very
broad spectrum, from
comparative physiologi-
cal analysis to the
documentation
of the dynamics
of whole epiphyte
commun i t i e s , and
decade-long observa-
tion with the help of a
construction crane that
has been erected in the
rainforest. This gives us
the opportunity to collect
plants on site and send them
back to Oldenburg for
detailed physiological
experiments inenviron-
mental chambers. These
experiments can provide an-
swers to such questions as howwell
epiphytes will cope with the climate
conditions predicted for the year 2100 – by
which time temperatures and CO2 levels will presumably be
significantly higher.
Although Central Europeans tend to associate tropical forests
with permanent humidity, paradoxically the epiphytes that
grow in the tops of these trees are more likely to be exposed
to drought stress. Since they have no roots in the earth and
grow directly on the bark or the moss cushions in the forest
canopy, even less than 24 hours without rain at temperatures
of around 30 degrees celsius mean extreme water scarcity.
The plants have developed many adaptations for coping
with these arid conditions, including thick, fleshy leaves and
the formation of water-storing cavities or “tanks”. The “tank
bromeliads” form their own “flowerpots”, so to speak, from
overlapping leaf bases where water and earth collect, allow-
ing them to survive without rain for several days at a time. In
extreme cases a single tank can store up to 20 liters of water.
These tanks in turnbecome aqueous environments in their own
right in the forest canopies, providing a habitat for all sorts of
creatures. The most spectacular of these are undoubtedly the
poison dart frogs, whose tadpoles grow in these little “ponds”.
The tanks are alsohome to a host of other fauna that onewould
not necessarily associate with treetop-living, from dragonfly
larvae and woodlice to crustaceans and earthworms.
With a few exceptions, these tanks are found only in bro-
meliads; they are certainly not typical of the many other
epiphyte varieties. Some non-tank epiphytes have internal
water-storage systems. Many orchids, for example, develop
so-called pseudobulbs, thickened stems which, like the ex-
ternal reservoirs of many bromeliads, can compensate for
irregular rainfall.
Economical use of the water resource is, however, just as
important as its efficient absorption and storage. In extreme
cases some species simply shed their leaves temporarily, like
many species of plant found in areas subject to long periods of
drought, for example the countries surrounding theMediterra-
nean. Thismakes sense particularly in forests that have a regular
dry season, as is the case in many Central American countries.
Lastly, a high percentage of epiphytes use a particular method
of photosynthesis known as crassulacean acid metabolism
(CAM). This water-saving photosynthesis method was origi-
nally observed in the succulent plants found in semi-deserts
such as columnar growth cacti, paddle cacti, agaves and aloes
– which is why early descriptions of CAM in tropical epiphytes
from the 1960s were dismissed as isolated cases. By now, CAM
has been verified in so many epiphytes that is seems likely
that, globally speaking, the majority of CAM species grow
epiphytically on trees.
Our research has demonstrated the need to replace the outda-
ted imageof the “typical” CAMplant as a terrestrial cactus –with
that of a thick-leaved orchid growing on a giant rainforest tree.
So what is the trick of the CAM plants? Normal plants have to
keep the pores of their leaves, or “stomata”, wide open in day-
light hours in order to absorb the carbon dioxide (CO2) in the
air and, with the help of sun's energy, convert it into sugar in
the process of photosynthesis. This process is fundamental to
plant life and thus forms the basis of all life on this planet. The
downside to opening leaf pores is that plants inevitably lose a
Wet biotopes in the treetops