Popular Science Monthly/Volume 23/August 1883/The Geological Distribution of North American Forests
By THOMAS J. HOWELL.
THE causes which have determined the present distribution of the flora of the world have occupied the minds of some of the ablest students of natural history, but no satisfactory solution of the problem has yet appeared. If we accept the theory of Raumer, that plants are limited in their northern extension by heat alone, we shall find many anomalies difficult to reconcile, as no isothermal lines limit species. Nor will De Candolle's theory, that the limits are governed by the values of heat which are useful to a plant, assist the student; for climatic causes are not the only ones which limit vegetable species, or we should then find the same species growing in every portion of any isothermal belt of a continent, where the same conditions of heat and moisture exist, which is not the case. Some species, apparently very local in their habits and confined to a very limited area, are found many miles farther north, with no intervening stations. For example: the Shizæa pusilla, a little fern, was thought to be peculiar to New Jersey, where it is confined to the pine-barren district, but it has lately been found in Nova Scotia and Newfoundland, while no intervening stations have as yet been reported.
Here we have a plant, capable of propagating itself in New Jersey, which was long thought to be its only home, reappearing several degrees farther north, where the climate is colder and otherwise different, and yet unknown west of the Alleghany Mountains, where the climate is very like that of New Jersey. Neither do we find many of the plants of the western slope of the Alleghanies growing upon the eastern side.
It is well known that if a piece of coniferous forest be cleared of its timber, in Virginia or Pennsylvania, its site will soon be covered with a growth of deciduous trees, but, if then left undisturbed, the coniferous trees of the original growth will finally reassert their supremacy, and in course of time the forest again becomes exclusively coniferous. The black-walnut (Juglans niger), which grows naturally from North Carolina to the Great Lakes, and will grow with equal luxuriance on the Pacific coast at latitude 45°, bearing fruit which will germinate if planted, has never yet been known by the writer to grow in Northern Oregon if left to itself. I have examined the walnuts in the spring in Oregon, which fell from the trees the previous fall—they were invariably rotten. Now, as one of the necessary conditions of plant-distribution is the production of seed which will grow unaided by man upon the soil which supports the parent, it follows that there is some other cause than the requisite amount of heat that prevents the black-walnut from becoming naturalized in Oregon.
In the Smithsonian Report for 1858, page 246, is an article by Dr. J. G. Cooper on the "Forests and Trees of North America," accompanied with a map of North America north of Mexico.
This map is divided into provinces and regions, according to the distribution of forest-trees, and the views herein maintained will be more intelligible to the reader who will refer to it, and compare it with a geological map of the same territory.
It will perhaps be best to describe the provinces briefly:
The Lacustrine province extends from the Rocky Mountains east to the coast of Labrador, and from the northern limit of trees south to latitude 42° at or near the level of the sea. The line marking the southern boundary curves gradually from west to northwest; commencing at the west end of Lake Erie, on reaching Lake Winnipeg it pursues a northwestern direction to the base of the Rocky Mountains about latitude 60°.
The Appalachian province comprises the Atlantic States south of latitude 43° and east of the border of the prairies; the latter, commencing at the west end of Lake Erie, forms a curve nearly parallel to the Atlantic coast, and ends at the southwest corner of Louisiana.
The Campestrian province commences at latitude 60° in the Rocky Mountains; its northern boundary extends southeast to Lake Erie; its eastern boundary extends from the latter point south to the mouth of the Sabine River. The valley of the Rio Grande forms the southern boundary. The western boundary is formed by the base of the Rocky Mountains, extending northwest from longitude 104° 30' to latitude 60°.
The Rocky Mountain province embraces the high central mountains from the Campestrian province to the foot-hills of the Cascade and Sierra Nevada Mountains.
The Caurine province begins at the northern limit of trees, on the Pacific coast, and extends east to the western boundaries of the Lacustrine and Campestrian provinces. Its southern boundary begins at latitude 48°, on a southern extension of the western boundary of the Campestrian province, extends northwest to the British line at its junction with the line between Washington and Idaho Territories, thence south to latitude 42°, and then southwest to the Pacific Ocean at latitude 38°.
The Nevadian province lies south of the Caurine, between the Rocky Mountain province and the Pacific Ocean.
The Mexican province lies south of the Rocky Mountain province, between the Campestrian on the east and the Nevadian on the west.
Now, comparing this forest-map with a geological map of the territory embraced, it will be found that the provinces and regions of the former coincide with the geological formations to a remarkable degree.
Beginning with the Lacustrine province it will be found that the formation is mostly granite, or what is popularly known as such, with some beds of Silurian and Devonian; also a few patches of tertiary rocks along the coast. Hence, the three regions comprising this province are really but one; and, accordingly, it has no trees not found south of it in the Alleghany Mountains, which are an extension of the granitic rocks. This province is characterized by its great numbers of coniferous trees; while some of the trees peculiar to the valleys west of the Alleghanies grow on the Silurian and Devonian beds.
The Appalachian province is composed of all the geological formations of North America, and its regions are very distinct.
The Alleghany region, comprising the eastern slopes of the uplands, and the lower Alleghanies, terminating in a point of latitude 34° in Georgia, is mostly granitic, but has streaks of Silurian and Triassic running through it. We find the same class of trees in it that grow in the Canadian region (Canada), with a few added which are perhaps limited by heat. These, with a few oaks and hickories, which are more prevalent on the Triassic formation than elsewhere, form the bulk of the forest-growth.
The Ohio region, embracing the eastern uplands of the Ohio Valley, east of the prairies and north of latitude 38°, is composed geologically almost wholly of Silurian, Devonian, and carboniferous beds, covered in places with drift from the north. It is marked by its large number of deciduous trees, no other country boasting of so many fine oaks, hickories, and walnuts. It is, however, very poor in coniferæ, and, but for a few stragglers, might be said to have none. Allied species are found to be plentiful in the tertiary formation nearly across the continent, indicating that this class of trees at one time reached from the Alleghanies to the Rocky Mountains, the middle of the belt having been destroyed by the more recent changes of the physical conditions of the earth's surface. As they have never returned since the glacial epoch, the inference is that the conditions of soil and climate have been so changed that the country west of the ninety-seventh degree of longitude is not capable of supporting these trees.
The Tennesseean region is a southwestern continuation of the Ohio region. It is composed of the same geological beds, with a few spurs of the granite ridges of the Alleghanies running into it, and therefore contains more coniferæ than the Ohio region. Still, the bulk of its timber is of the same class of broad-leaved trees that are found north of it, the only differences being such as climate alone makes.
The Carolinian region borders on the Atlantic coast between the Alleghany Mountains and the ocean from Middle Georgia to Long Island. It is composed of cretaceous and tertiary beds, with a strip of Triassic along the western edge. In the northern portion are some beds of drift of granite from the north. Here we have a distinct class of coniferæ on the cretaceous beds that are peculiar to this region, and another class on the drift that are also found growing farther north. Arthur Hollick, who has made observations on the flora of Staten Island, says: "We have on Staten Island two well-marked geological formations: the drift, which covers about two thirds of the entire island, nearly all of the northern part, and extending as far south as Prince's Bay; and the cretaceous, which occupies the remaining small area in the southern and western part. This latter is a continuation of the New Jersey clay-beds. The geological line of separation between the two formations is not always very distinct, but the limits of the different species of plants mark it in unmistakable characters. The two floras are remarkably distinct. That one belonging to the cretaceous is well represented by Arctostaphylos, Uva-ursi, Aster concolor, Pinus inops, Quercus Phellos, Quercus nigra, Lycopodium inundation, var. Bigelowii, and many more of the pine-barren plants. Thus far I have never found any of these species to have crossed the line of the drift, but in their stead will be found Pinus nigra, Quercus alba, Quercus rubra, etc., and the majority of those plants which grow in the vicinity of New York Island and up the Hudson."
The Mississippi region embraces the lowlands bordering the Gulf of Mexico from Middle Georgia to Texas, and extending up the Mississippi and its branches to latitude 30°. It is a continuation of the Carolinian region, its characteristic trees growing from the Gulf of Mexico to the coast of Maine. It is composed of the same tertiary and cretaceous beds of the Carolinian region, with a few patches of alluvial deposit along the coast.
The Florida region is well marked and peculiar, being entirely coral alluvial. It has the peculiar flora of that formation found all over the world. Some of its plants are found farther north; but small beds of alluvial are not uncommon along the coast as far north as New Jersey.
The Campestrian province might be considered as one region, but Cooper has divided it into five. The Saskatchewan region, embracing all north of latitude 49°, together with the basin of the Red River of the North, has some spurs of the Canadian region running into it, and consequently some of the Canadian species are found on them and on the adjacent Silurian and other formations. This region has no characteristic trees of its own. The Illinois region lies between latitudes 46° and 38°, running west to longitude 101°; on the east it is bounded by the forest provinces. It is a continuation of the Ohio region, being underlaid with the same beds of Silurian and carboniferous deposits, with cretaceous and tertiary beds on the west. But here a new feature enters into the geological characteristics. The loess or lacustrine deposits which cover the whole province from four to one hundred and fifty feet, though devoid of trees, have a peculiar flora, composed largely of compositæ, and being one of the latest of geological deposits, they furnish the most recent botanical species of the composite. None of the compositæ have yet been found in any of the fossil flora; hence it has clearly appeared upon the earth since the Tertiary period. Another remarkable fact is that on the eastern side of the Illinois region where the loess-beds are not very thick, and, therefore easily cut through by streams, wherever they are thus eroded, we find dense groves of oak, walnut, hickory, and other trees characteristic of the Ohio region. With these exceptions however, the whole region is prairie; hence it would seem that the loess is not capable of sustaining forest-growths for any length of time, for it evidently was timbered during the time that part of it was covered by lakes and marshes. But when the great rivers cut their beds down to nearly their present level, the timber gradually died out; not being burned, as some suppose, but disappearing because the geological formation will not retain moisture enough to sustain forest-growth.
The Texan region, lying south of the Illinois region, and extending west to 101° of longitude on the Rio Grande, is a continuation of the Mississippi region, and is underlaid with cretaceous and tertiary deposits. It is covered in many places with loess. It, therefore, has the characteristic trees of the Mississippi region wherever trees grow, and the characteristic loess flora on the prairies.
The Comanche region, lying south, and the Dakota region, north of latitude 38°, are nearly destitute of timber. The former is underlaid with triassic and the latter with cretaceous and tertiary beds; but they are covered with loess from ten to one hundred feet or more, and hence the loess flora predominates.
The mountain-region of the Rocky Mountain province is composed of granite, but has enough trachyte and other volcanic rocks to modify its flora to some extent. It also has some beds of Silurian on the eastern border, and here, strongly corroborating our views, we find some of the Eastern flora mixed with the Western and Southwestern that lie next to it. Its valleys and parks are covered with loess, and are treeless.
The Saline region, comprising the remainder of this province, is underlaid with tertiary of a different epoch from that of the Atlantic coast, but is covered with heavy beds of basalt in many places. This basalt is covered with a deposit analogous to the loess, and is treeless, but has a flora very similar to that of the Dakota region. The tertiary has a flora of its own, generally known as the sage-brush (Artemisia) flora, being composed of a number of shrubs peculiar to this region.
The Caurine province is composed of basaltic rocks principally, but has some tertiary beds, and the higher mountains are granitic at their tops. As the rocks are of an entirely different character from those of the Atlantic side of the continent, we should not be surprised at finding an entirely different flora. In fact, none of the Eastern trees reach this province, nor do any of its trees appear farther east than the Rocky Mountains. The geological formation of this province being mostly basaltic, the trees are characteristic of that formation, for the tertiary beds, wherever they are of sufficient size to make an impression on the flora, are all prairie, with scattered groves of oak (Quercus gargana), and this of but one species. The Abies Donglasii, Pinus ponderosa, and Thuga gigantea, are samples of this flora, fori have never seen any of them growing on any land that was not made from the disintegration of basaltic rocks.
The Nevadian province is composed of nearly all the geological formations common to North America, and, in accordance with our views, it has a flora of corresponding variety. Nearly all the genera of Eastern forests are represented, though different species are common. It has some very local trees, no doubt confined to geological formations of a peculiar character. Of these are the two Sequoiæ—the redwood and the "big tree." The former is confined to a narrow strip along the coast, the latter to the tops of the high mountains in isolated groves. Exact data are wanting, but it appears from the geological maps at the writer's command that the redwood is confined to the cretaceous formation which extends from about latitude 34° to 40°. As this is about the range of the redwood-groves, it will probably be proved, on close investigation, that this tree is confined to the above formation.
The Sequoiæ have a peculiar interest for the students of natural history, being the only living representatives of a once large and widely distributed genus now found in the tertiary beds from British Columbia to California, and east to Nebraska. It appears to have been nearly exterminated about the glacial epoch, and is now confined to small localities that appear not to have been covered by the ice at that time.
In the foregoing pages I have made use only of trees to illustrate the affinity of plants for certain geological strata, but, should I have taken the general flora, the argument would appear still more convincing. To do this, however, it would have been necessary to divide the country into smaller regions, and to have given the geological characters more in detail than is at present practicable.
Were other proofs wanting to demonstrate the intimate relations existing between geological formations and the geographical distribution of the flora, they are close at hand in the writings of our eminent botanists. Sir Joseph D. Hooker, in a lecture on the distribution of the North American flora, treats the subject upon the theory that all plants originated from small centers of creation and spread by slow encroachment upon the adjacent territory as fast as this was in a condition to receive them, and that climatic influences alone limit their extension. He makes four general floral regions:
"1. The great Eastern forest-region, extending over half the continent, and consisting of mixed deciduous and evergreen trees, reaches from the Atlantic to beyond the Mississippi, dwindling away as it ascends the western feeders of that river on the prairies. It is noteworthy for the number of kinds, especially of deciduous trees and shrubs, to be found in it. . . . 2. The prairie-region succeeds; a grassy land, with many peculiar herbaceous American genera, including Mexican types, of which last the most conspicuous are a yucca and the cacti, which latter increase in number as the Rocky Mountains are approached, where they form a noticeable feature in the landscape. In the parks and lower valleys of the Rocky Mountains, deciduous trees are few and scattered, and the forest is an open one of conifers. . . . Higher on the mountains the coniferous forests are dense. . . . 3. Descending to the sink-region, . . . deciduous trees are very few and confined to the gullies of the mountains. . . . The hardy sage-brush (Artemisia) covers immense tracts of dry soil, and saline plants occupy the more humid districts. 4. The Sierra Nevada is clothed with the most gigantic coniferous forests to be found on the globe, among which a very few species of deciduous trees are scattered; but none of these are identical with trees of the Eastern forests."
Applying the geological charts to these four general floral regions, we find corresponding to each of them respectively: 1. The great Silurian and carboniferous beds, with their large varieties of deciduous trees, the Alleghanies on the east, with their coniferous plants, and the loess-beds on the west, with their peculiar prairie flora, and a few trees along the streams. 2. The deeper loess-beds with a peculiar flora, and the Rocky Mountains with their mixed geological characters, mainly volcanic, and with a mixed flora of Eastern and Western trees, the latter predominating. 3. The tertiary beds of the saline region, which axe different from those of the East, with their peculiar sage-brush and saline flora. 4. The Sierra Nevada region, with mixed geological characters of gneiss and lava, and a mixed Mexican and Northwest flora.
Thus, from more than one point of view, the North American flora is susceptible of being divided into three or more distinct floras, corresponding to the different geological formations which they inhabit.