In northwestern Minnesota and in northern Dakota the oaks are
near their northern limit, but even there the burr oak drags on a bare
existence among the pines and spruces. In the Black Hills, in Dakota,
poor, forlorn, scrubby burr oaks are scattered through the hills among
the yellow pines. In Colorado we find them as shrubs among the pines
and Douglas spruces. In New Mexico we find them scattered among the
pinons. In Arizona they grow like hazel bushes among the yellow pines.
On the Sierra Nevada the oak region crosses the pine region, and
scattering oaks reach far up into the mountains. Yet oaks will not
flourish between the one hundredth meridian and the eastern base of
the Sierras, owing to the aridity of the climate. I recently found
oaks scattered among the redwoods on both sides of the Coast Range
Mountains.
Darwin has truly said, "The oaks are driving the pines to the sands."
Wherever the oak is established--and we have seen that it is already
established whereever it can endure the soil and climate--there it
will remain and keep on advancing. The oak produces comparatively few
seeds. Where it produces a hundred, the ash and maple will yield a
thousand, the elm ten thousand, and many other trees a hundred
thousand. The acorn has no provision for protection and transportation
like many tree seeds. Many kinds are furnished with wings to float
them on the water and carry them in the air.
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