What I Learned in the Forests of France: About Biogeography

France, part 4.

On July 14, I went hiking in the Vosges Mountains of eastern France with the Gennetay family of Strasbourg. This was easily one of the greatest days of my life. It is always fun to take a hike with such a happy family (the family of my son-in-law). And it is always exhilarating for a gentleman of a certain age, myself, to be able to hike for seven hours and survive. But in the next two essays I will describe some of the scientific and environmental things I learned on this hike.

First, about biogeography.

The forests of France are just similar enough to those of eastern North America to feel comfortable, but just different enough to feel exotic.

Some of the weedy species were the same, such as yarrow (Achillea millaefolium), mullein (Verbascum thapsus), foxglove (Digitalis purpurea), and cow parsnip (Heracleum sphondylium). The bracken fern (Pteridium aquilinum) and the fireweed (Chamerion angustifolium) live all over the Northern Hemisphere. And one of the tree species, widespread at forest edges, is actually a native of North America, and was introduced to Europe in 1601: the black locust Robinia pseudoacacia.

Bracken fern (Pteridium aquilinum, foreground)

Foxglove (Digitalis purpurea)

Fireweed (Chamerion angustifolium)

But in most cases the forests consisted of the same general types (genera) of trees, but different species. Examples of European trees similar to but not the same species as those in America include the following. I provide scientific names for the European species, and French names.

Pines: Scots pine (Pinus sylvestris) (le pin)

Firs: Picea abies (le sapin)

Maples: Acer campestre and A. platanoides (les erables)

Birches: Betula pendula (les bouleaux)

Oaks: principally Quercus petraea (les chênes)

European maples (probably Acer platanoides)

European birches (Betula pendula)

The same was true of the shrubs:

Dogwood (Cornus mas) (le cornouiller mâle)

Hazelnuts (Corylus avellana) (le noisetier)

Elderberry (Sambucus nigra) (les sureaux)

Mountain ash or rowan (Sorbus aucuparia) (le sorbier)

The principal reason that European forests resemble North American forests but not Southern Hemisphere forests is that the Atlantic Ocean began to separate Europe and North America only about 70 million years ago, just long enough for new species but not for new genera to evolve. In contrast, Northern and Southern Hemisphere forests have been separate for about 120 million years; not only do Southern Hemisphere forests have different species, but different genera, of trees and shrubs. Evolution is the reason that the forests of France felt both comfortably familiar and enchantingly different from those I know in North America. I was standing right in the middle of a lesson in biogeography, a field of study largely invented by Darwin’s fellow-discoverer of evolution, Alfred Russel Wallace.

Here are some other interesting biogeographical differences:

But you generally won’t see maples, birches, dogwoods, hazelnuts, or elderberries in Southern Hemisphere forests. You may see trees similar to them, but not the same genera. You will find a few oaks and elderberries in South America and Southeast Asia, and only one maple species out of 128 lives in the Southern Hemisphere.

The forests of France contain no poison ivy or poison oak (genus Toxicodendron). Both Europe and North America have nettles (les orties) but they appear to be more abundant in Europe. European nettle species (Urtica dioica and U. urens) are different from those in North America.

We also saw chestnuts, Castanea sativa. “Sativa” means cultivated. Although these trees grow in the wild, they are the source of edible chestnuts. The sprays of male flowers produce very little nectar but, according to a beekeeper in the family, bees can make it into a particularly pungent kind of honey. This rarely happens, however, since it takes hundreds of flowering trees for the bees to produce just a little bit of honey. In America, most of the native chestnuts (Castanea dentata) have died; a few of them re-sprout before they are old enough to reproduce. This is because of a fungus introduced in the early twentieth century from planted European chestnut trees. Although you can see occasional American chestnut resprouts in the northeastern United States, you will never again see chestnut forests.

Chestnuts (Castanea sativa)

Everybody loved this wonderful hike through the forest; but as a botanist, I experienced a whole extra dimension of enjoyment by seeing the cousins of my plant friends from America.

My Fun Evolution Trip Part Four: East Meets West in the Black Hills

The Black Hills is also a good place to study biogeography. While no place is as good as Wallace’s line in modern Indonesia, you might find the Black Hills of South Dakota to be more accessible.

The Black Hills (so named because the ponderosa pines look black from a distance) are a small mountain range that formed separately from other nearby ranges and hills, such as the Bear Lodge Mountains, Bighorns, and Bear Butte. They are entirely separate from, and a hundred miles east of, the Rockies. The Rockies form a rough dividing line of eastern from western species: they have mostly western species. But the Black Hills has a mixture of eastern and western species. This is a perfect illustration of biogeography over evolutionary time. The species that migrated from the west include:

·         Ponderosa pines (Pinus ponderosa)

·         Limber pines (Pinus flexilis)

·         Many wildflower species

·         Violet-green swallows

·         Bighorn sheep

·         Mountain goats

It is obvious from this list that I know more about plants than about animals.

But other species migrated from the east. Western North America has tree species such as the Fremont poplar and many of its own species of oaks, but in the Black Hills you will find:

·         Bur oak (Quercus macrocarpa)

·         Eastern cottonwood (Populus deltoides)

This makes sense because bur oak can survive in prairies and is the most likely oak of all eastern species to have migrated westward across the prairie, and cottonwoods migrated from the east along creeks and rivers.

Other species migrated in from the north:

·         White spruce (Picea glauca)

·         Paperbark birch (Betula papyrifera)

·         Quaking aspen (Populus tremuloides)

·         Twinflower (Linnaea borealis)

I can scarcely refrain from posting all of the dozens of birch bark photos I took.

Some tree species can be found in virtually every state, such as boxelder (Acer negundo).

Some of these species are mere remnants of larger populations in other areas. For example, other mountain goat populations are not found within about a hundred kilometers of the Black Hills.

Not only have species migrated to the Black Hills, but some of them can only be understood as remnants from the last ice age. Thousands of white spruces persist in the Black Hills, but mostly in creek bottoms and on north-facing slopes in the higher, northern part of the Hills. As the weather became warmer and drier after the last ice age, the spruces retreated uphill while the ponderosa pines moved in. The limber pines are found in only a tiny population near Harney Peak, the highest point in the Hills, also retreating from the warming, drying climate.

The Black Hills is also a good place to observe microclimates. The wet birch forests along the creeks have lots of ferns and violets. Immediately upslope, you can find ponderosa pines on the dry rocky slopes. In thin soil on top of the rocks you can find cactus, while in the moist cracks of the very same rocks you can find small ferns. Perhaps best of all, in the bogs near Black Fox campground, as in many other acidic bogs in coniferous forests of northern North America, you can find a little birch shrub with small leaves, the bog birch Betula glandulosa.

You can also notice geographic patterns within species in the Black Hills. I am accustomed to seeing three to five leaflets on Oklahoma box elders, but in the Black Hills they mostly have five to seven leaflets.

Biogeography is like a mystery novel in which you try to figure out where everything came from over evolutionary time. I recommend the Black Hills for a biogeography adventure.