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Darwin-L Message Log 3:39 (November 1993)

Academic Discussion on the History and Theory of the Historical Sciences

This is one message from the Archives of Darwin-L (1993–1997), a professional discussion group on the history and theory of the historical sciences.

Note: Additional publications on evolution and the historical sciences by the Darwin-L list owner are available on SSRN.


<3:39>From mayerg@cs.uwp.edu  Sat Nov  6 11:32:24 1993

Date: Sat, 6 Nov 1993 10:22:59 -0600 (CST)
From: Gregory Mayer <mayerg@cs.uwp.edu>
Subject: Re: Teaching the historical sciences
To: darwin-l@ukanaix.cc.ukans.edu

> In addition to talking about the history and methods of the historical
> sciences I would like to include several practical exercises in which the
> students will be given some complex object or situation and will be asked to
> reconstruct the sequence of events that produced that object or situation.
	I teach an ecology course in which one element of the field
portion of the laboratory is landscape interpretation: inferring the
history of plant communities.  I do not teach this as a separate lab
exercise, but rather it is an aspect that recurs in a number of field
situations, and thus I cannot provide a "canned" lab; in any event, what I
do in Wisconsin would not be directly applicable to North Carolina anyway.
The species composition and physiognomy of the vegetation of any site tell
us much about current conditions at the site, but we can also infer much
about the history of the site, and even about its future.  Let me give
four examples to illustrate the approach.
	1) Many species of trees have different growth forms depending on
the environmental conditions around them during their period of growth.
Because trees are long lived, we can infer what conditions were like
during the earlier life of the tree.  Trees with large, spreading,
spherical crowns are often "open grown", that is grown with light reaching
them from all sides (as would be the case if they were growing in an open
field).  Finding a large open grown tree within a forest allows one to
infer that the area had previously been a field, which, in the eastern
U.S., allows the further inference that prior to the field an original
forest had been cleared.
	2) On a recent visit to a lowland prairie we noted that the
prairie had formed on an extinct lake bed (had we been geologists, we
would have determined the position of the ancient lake shore ourselves,
rather than relying on our topo maps).  A railroad ran along the ancient
shore line, and we knew, from other sources, that prairie and other fire
resistant plants often persist along railroad tracks after they have been
otherwise extirpated by development.  Along the railroad we found typical
prairie herbs and grasses and fire resistant trees, forming a narrow strip
of savanna, and evidence of fires on the trunks of the trees.  Since the
locomotives (one went by while we were there) rarely spew burning embers
and coals anymore, we speculated about the frequency of fire under current
conditions, when the fires we had found evidence of had occurred, and
whether when fires were more frequent if the strip had been pure prairie
rather than savanna, but we could not firmly answer these questions
without taking tree cores and checking railroad records.  In Wisconsin
most prairies are maintained by fires, and in the lowland prairie proper
we found much evidence of fire on the few trees and human-implanted posts
we found, and in the prairie shrubs.  From a post implanted at the
entrance of a trail we were able to infer that winds blew mostly from the
northeast during fires on the basis of the different extent of charring on
the sides of the post.  The depth of charring also gave us some idea of
the severity of the fire.  Live prairie shrubs grew from the same spot as
burned ones, showing the fire had killed above ground portions of the
plants, but that the below ground parts had survived and regrown.  In
fact, the burned portions were the same height as the live ones, so we
could infer that the last fire had occurred in the time it takes them to
grow that high, and that if fires occur with a regular frequency, we were
due for another one.  Had we cut down a live shrub at its base and counted
the rings, we could have converted the fire frequency from units of shrub-
height to years.
	3) Just as old, large trees can help us infer the past, seedlings
and saplings can help us predict the future of the site.  Although it is a
great oversimplification, it is sometimes not a bad first approximation,
and makes a good laboratory exercise, to assume that the probability of a
tree species growing into the canopy is proportional to its prevalence as
a sapling.  Based on this assumption, a very simple Markov model can be
used with data on abundance in the canopy and sapling layers to predict
the future history of the forest.  If forests of known age are available,
predictions so generated can be compared to their actual species compositions.
	4) Tree falls create light gaps that fast growing, shade
intolerant species tend to grow in, and inferences concerning the history
of a small site can be made using this knowledge.  When a spruce dies and
falls, it creates a light gap that is likely to be colonized by paper
birch, a fast growing, short-lived tree.  When you find a a cluster of
three or four mature paper birch growing roughly in a row in a spruce
forest, you know that a spruce (now decomposed) fell, and along what
compass direction it fell, several decades ago.
	These are just a few examples. To do this sort of ecological
history requires much knowledge of the natural history (sensu lato) of the
species involved: their preferences for soils, light, moisture, temperature,
their life span, their growth characteristics, etc.  It also helps to know
local geology and physiography.  I have used these historical inferences
as just part of other exercises; you could, without too much difficulty, I
think, create exercises centered on the historical aspects.
	Unfortunately, I do not know of any published works that take this
field-oriented approach to historical reconstruction of vegetation history
as their central theme.  There may well be some; I am a zoologist and
could easily be ignorant of what botanists have written.  The Peterson
Field Guide to Eastern Forests has a little bit on this, and so might the
Sierra Club regional field guides.  There is a good reference for the
Markov approach to predictive history: Horn, H.S. 1975. Markovian
processes of forest succession. pp. 196-211 in M.L. Cody and J.M. Diamond,
eds. _Ecology and Evolution of Communities_. Harvard Univ. Press,
Cambridge, Mass.  There are numerous works on the use of pollen records
for inferring vegetation history, but this is a longer time scale approach
based on the sub-fossil record.  Because it also has much on the use of
all sorts of data other than pollen for the inference of historical events, I
recommend E.C. Pielou. 1991. _After the Ice Age_., Univ. of Chicago Press.

Gregory C. Mayer
mayerg@cs.uwp.edu

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