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The famous drawing of Mount
Chimborazo, Mont Blanc and Mount Sulitjema by Alexander von Humboldt probably
shows the first scientific attempt of physiognomic zonation related to
elevation, clearly distinguishing between forests, shrubland and montane
savannah. In 1955, Beard described the plant communities in the vegetation
systems for the tropics on the basis of their structure and growth form and
identified a number of units he called formations. These are further subdivided
into associations or communities according to floristic composition and can
occur in more than one vegetation system. By the end of the 1960s, early 1970s
applications ranging from forestry exploitation, land-use analysis to nature
conservation, all needed descriptions and spatial delineation of vegetation
structures. The detailed Zürich-Montpellier (Braun Blanquet 1928) method
commonly used in Europe, was too slow and required too much taxonomic knowledge
from its applicants to work under conditions of high species richness. Several
renowned scientists experienced in the application of the Zürich-Montpellier
method used their experience in vegetation analysis to design a
species-independent method that could be universally applied. A variety of
physiognomic classification systems have been developed, usually in combination
with other modifiers. The most broadly accepted
system has become the “Tentative Physiognomic-Ecological Classification of
Plant Formations of the Earth”, developed under the auspices of the UNESCO. It
is a hierarchical classification system designed to compare ecological
“habitat”. The system combines physiognomic and ecological modifiers to
characterise and classify vegetation units (Muellerr-Dombois & Ellenberg
1974). The authors never seem to have had in mind to provide an exhaustive list
of possibilities, but rather provide a framework approach, that allows
customisation to the broad variability of nature. Instead, the often-repeated
instruction “Subdivisions possible”, makes it an extremely flexible and
intuitive system, suitable for all terrestrial conditions. That the system has a
sound foundation may be concluded from the fact that several later systems have
been spun off from the system, most notably the USNVC system in 1998 and the
Land Cover Classification System, LCCS of FAO/UNEP in 2000 (Di Gregorio &
Jansen 2000). Analysis of physiognomic
vegetation classification from LANDSAT images (detail of 30 X 30 m pixels for
the visible and near-infra-red bands) is possible, but it is largely deductive,
as one cannot actually observe the physiognomic structures such as trees and
shrubs or their absence. The level of detail of what can be deducted directly
from deflexion of light radiation alone is rather limited, but through
combination of indirect information, such as elevation levels, terrain patters,
seasonal leave-shedding, etc., one may deduct further detail. Experienced field
biologists may further enhance the level of detail from field reconnaissance and
prior knowledge, such as dominant species of structural classes in certain
regions of a country. Hence, detailed analysis depends on substantial field
knowledge of the analyst. As considerable as these
limitations may seem, satellite images have great advantages, such as1: ·
Each image covers a large area; ·
The per hectare cost of LANDSAT image has always been lower than
that of aerial photographs; since the launching of LANDSAT 7 in 2001, “raw”
(Un-processed) satellite images have become very cost effective2,
as they now cost a mere US $600 each (http://edc.usgs.gov/products/satellite.html
2003 pricing); ·
A new series of images of the entire earth is taken every 16 days; ·
National military institutions can’t block out the analysis of
regions by prohibition of the distribution of imagery; ·
Digital format facilitates frequent change of scale; ·
Classification is rapid and digital mapping can be done directly
by computer3,
thus speeding up the process and reducing costs. In areas with homogeneous vegetation
structures, physiognomic classification systems show rather little detail, while
there is no good knowledge to indicate whether or not more detailed spatial
differentiation of species assemblages occur. To partially compensate for this
low level of detail, biogeographical criteria should be applied whenever
reasonable assumptions for such divisions may be made. In applying such
criteria, one must use reason and logic. Rare ecosystems, which would need to be
fully included in a protected areas system, may not need further splitting,
unless biogeographical regionalisation is clearly present. It would primarily be
relevant for the more common and what shall later be defined as “typically
large terrestrial ecosystems”. 1
This list is not meant to be exhaustive but illustrative. 2 A
set of 37 images for Central America would now only cost $17.575 and
regionally purchased could serve all 7 countries of the region. 3
This is also possible for aerial photographs, but the process is far more
elaborate since each photograph must be converted to digitised format,
rectified and processed individually. |
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