In January, I went to New England to see friends and relatives, and as always when I make that transition I was stunned by the differences between forests there and the forests of the Olympic Peninsula. We have the size (and one might argue, the beauty). New England has greater tree diversity.

A Northeastern forest is a checkerboard of species—hop hornbeam here, sugar maple there, basswood over there. Red oak, green ash, mountain oak, red maple, butternut, shagbark hickory, American beech, yellow birch, paper birch, white pine, eastern hemlock, red spruce, balsam fir, tulip poplar and more. All in a single forest. And, of course, none of them larger than an adolescent Douglas fir from the Olympic Peninsula.

Here a mature forest is heavy to enormous specimens of a few species—Douglas fir, red cedar, western hemlock, and—near the coast–Sitka spruce. In some habitats red alder and big leaf maple will make a run to challenge the big conifers, but those are noticeable exceptions. What accounts for these differences between New England and the Pacific Northwest?

Scientists who have contemplated the forests of the Pacific Northwest attribute the dominance of a relatively few species of conifers to competitive advantage under our historic climate.

Conifers retain their needles, allowing them to photosynthesize through the fall and even in the winter, when water is relatively abundant.

Those same needles are more resistant to water loss during the hot parts of the summer, when dry soils often make a water limiting resource.

Growing to a large size allows trees to secure nutrients and water from a relatively large area around their trunk, by shading out the potential competition. In short, our forests are dominated by a few species of very large conifers because that’s what has provided the greatest evolutionary advantage over the last several millennia. Climate is destiny, in forests and beyond. Next month we’ll explore how that destiny changes with a changing climate.

For those interested in a deeper dive, I recommend the seminal 1979 paper on the topic by the great forest ecologists Jerry Franklin (of the University of Washington) and Richard Waring (of Oregon State University): Link