Sunday, July 24, 2011

I saw the VM's face in the willows, part 1

Tomorrow I leave for a month-long field project studying vegetation, specifically Black Hills montane grasslands.  When I tell friends I’m leaving town, often I have to explain how one “studies” vegetation.
 The online World English Dictionary says vegetation is “plant life as a whole, esp the plant life of a particular region”, e.g. the vegetation of California.  The Oxford American Dictionaries dashboard widget defines vegetation as “plants considered collectively, esp. those found in a particular area or habitat”.  From the perspective of my work, this definition is spot on.
Next, what are vegetation types, plant associations and plant communities (the more common names, there are others)? These are distinctive assemblages of plant species found consistently in specific types of sites.  This is fairly intuitive; we know different types of vegetation grow in different areas.  There aren't tropical rain forests in the Great Plains of North America.  Hardwood forests thrive in New England but not in Texas.  Willows grow on river banks.  Etc.
Some vegetation types occur in very specific and localized habitats.  For example, the pebble plains unique to the San Bernardino Mountains of southern California are underlain by Pleistocene lake sediments that yield clay soils covered with quartzite pebbles.  These sites support a unique plant association -- a rare and diverse assemblage of plants, a number of which are themselves rare (four rare butterflies too).
Alpine vegetation above tree line consists of species adapted to short, cool growing seasons.  Going from mountain to mountain, range to range, we see the same alpine plants repeatedly -- as well as others restricted to a given range or region.  Patterns like these lead us to conclude that plants occur in communities -- distinctive assemblages in specific habitats.
But beware!  Vegetation has repeatedly lured human minds into seeing patterns.
For example, in the late 19th century C. Hart Merriam introduced the concept of Life Zones based on striking and consistent changes in vegetation with change in elevation and latitude.  Merriam’s specific system had problems, but the general concept was widely embraced, and vegetation ecologists still consider environmental gradients important in explaining vegetation distribution.
Henry Chandler Cowles formalized the concept of Plant Succession, and his colleague, Frederic Clements, popularized his own view that in a given habitat a predictable series of plant communities would develop over time, culminating in a final stable climax community.  He felt the process was so predictable that one could know the climax community even if a site were still in an early successional stage.  For years afterwards (and even today in some situations) vegetation ecologists classified sites based on “potential natural vegetation” rather than what was growing there.
However, “knowledge imposes a pattern, and falsifies” (T. S. Eliot, 1940).  Eliot was not a vegetation ecologist, but he knew the human mind.
[next ... the mind’s obsession with patterns ... what is a vegetation ecologist to do?]

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