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Large Scale Climate Changes and Tree Species

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Every plant and animal species has a certain combination of environmental conditions under which it can and does live. For plant species these environmental conditions include precipitation, temperature, soil pH, soil texture, soil moisture, nutrients, wind, and so forth. How can one determine which environmental conditions favor which plant species? One of the approaches developed by forest scientists is the following graphical one.

Construct a graph based on two environmental variables, in this case annual precipitation and average July temperature. Now measure average annual precipitation and average July temperature at several thousand places in North America and plot all of these thousands of points as light gray. The resulting gray cloud represents all of the possible combinations of annual precipitation and average July temperature that occur in North America from the deepest, hotest, driest desert, to the coldest arctic tundra. Next take a tree species, a tree for example, and every time this tree is seen to live in a locale with a particular combination of average annual precipitation and average July temperature, put a black dot on the graph. The black mass now represents the combination of average annual precipitation and average July temperature the species is known to live in. Keep in mind that the black mass represents where the species is known to live, not where it could live.

This particular graph is for American Beech (Fagus grandifolia), an important tree species of the Eastern Deciduous Forest. Clearly American Beech likes high annual precipitation, but tolerates a fairly wide variety of average July temperatures.

Now construct these graphs for other tree species and compare them side by side.

The graph on the left is our old friend the American Beach and the one on the right is Bur Oak (Quercus macrocarpa), a common oak species of the northern and western portions of the Eastern Deciduous Forest. The black blotch for Bur Oak is larger than that for American Beech and, in particular, Bur Oak can tolerate areas of lower precipitation than American Beech. If we could lay the two graphs on top of each other you would see that Bur Oak is, on average, found in slightly warmer and drier conditions than American Beech. In general if you continue this process with other tree species you would find that on average some oak and hickory species like drier conditions and some other species like it wetter and cooler, such as Eastern Hemlock, Beech, and some maples. We already know that the Eastern Deciduous Forest is drier in the west than in the east, on avearge. Would some oaks and hickories dominate in the west and other species in the east? Yes, they do. Forests in the western part of the Eastern Deciduous Forest are dominated by some species of oaks and hickories. Of course the situation is not nearly as easy as all that. Every species has its own set of unique tolerances and every locality has its own set of local conditions. For example oak-hickory forest occurs in the east under local condition of rocky soils and direct exposure to sunlight. Competition between tree species also rears its head. These are just broad trends. The pictures below show some of these broad trends.

A true transition forest in Ontario containing both elements of the Northern Boreal Forest and maples from the Eastern Deciduous Forest.

A young Beech, Hemlock, Maple forest from upstate New York.

Oak, tulip poplar, and beech forest from southern Maryland.

An example of the effect of local conditions on forest type; an oak-hickory forest along the Potomac River in Maryland. The ground is rocky and derived from limestone creating conditions of low soil moisture and poor nutrients. This locality is less than 15 miles from the one on the left.

A woods in Illinois dominated by species of oaks and hickories

Eastern Deciduous Forest at its extreme; dry oak forest in Texas.