thorny question

Andrew K. Rindsberg arindsberg at GSA.STATE.AL.US
Wed May 17 17:29:53 CDT 2000


Thomas Schlemmermeyer wrote, 'OK, Now it is getting really interest[ing]! Is
there any correlation between the toxicity and thorniness of plants?'

Indeed an interesting question. When I moved from the Southeastern US to
Colorado, I noticed that some related flora is noxious in the Southeast and
positively beneficial in the mountains of Colorado. For example, the
allergenic poison ivy (Toxicodendron radicans) of the Southeast is related
to edible bearberry (sorry, I don't know the Latin name) in Colorado. Some
of the thorny plants in the Southeast correspond to thornless plants in the
Rockies. And so on. Has anyone compiled statistics on this, rather than
anecdotes?

Another point. I once went through a book of eastern North American trees
page by page and noted the ones having limited distributions, looking for
patterns related to climatic change. Unsurprisingly, it turned out that such
trees are concentrated in the Southeastern and Pacific states, outside the
glaciated area and outside the central grasslands. The coastal strip from
Louisiana east to Georgia is particularly rich in endemic trees.

More interesting was the fact that such trees tend to have fruits and seeds
that are unappealing to mammalian herbivores and are also not winged,
sticky, or burred. Many of them reproduce by small seeds in woody capsules.
Perhaps their present limited distribution can be partially explained by the
fact that they migrate slowly compared to trees that have attractive or
easily transported fruit. The other trees succeeded in following the
retreating glaciers to the north; these did not. Of course, another partial
explanation is the general increase in plant diversity toward the tropics.

On the average, then, a postglacial forest anywhere in the world should have
a relatively high proportion of edible, winged, sticky, and burred fruits or
seeds compared to a neighboring nonglaciated forest. Comments?

Andrew K. Rindsberg
Geological Survey of Alabama




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