YEE-HAW! (PONDEROSA PINE)
This week we head ‘out west’ and marvel at the cartoon cowboys of the pine world, the ponderosa pine (Pinus ponderosa). We dive into one of the most interesting weather phenomena in the world, the rain shadow effect, and chat about how seemingly unrelated patterns of geology and weather change who grows where. Then, as a surprise to no one, we ruminate on the nature of sasquatch.
Ponderosa Pine
(Pinus ponderosa)
It’s safe to say that the ponderosa pine (Pinus ponderosa) is the most iconic tree of the American west. Of course, there are other contenders, like maybe aspen or Douglas-fir, but only ponderosa pine seems to have captured the whole essence of western North America in a mythological sense. It’s the tree that people think of when they imagine the vague place of ‘out west,’ and it’s often the defining characteristic people notice that tells them they’re not in Kansas anymore.
That last part is actually literally true: though planted in Kansas, ponderosa pine is native only north and west of Kansas. But where it is native, it is often a dominant tree. The reason ponderosa pine is such a star out west is that it is one of the most drought tolerant and hardy species of pine native to North America. It grows tall, straight, and big, with a stem that can reach over four feet in diameter covered in unforgettable bark that breaks up into rounded puzzle piece-like shapes and ranges in color from stunning honey-amber to breathtaking burnt orange. A big one really is a sight to behold; it’s no wonder they are such icons.
Though Oregon and Washington are known around the country as wet and rainy places, it’s not quite a fair description. Western Washington and western Oregon, on the other hand, are very deserving of this description with annual precipitation averaging around 75 inches (190.5 cm). Western in this context is delineated by the Cascade Mountains, a chain of high volcanic peaks that are the tectonic result of the subduction of the oceanic Juan de Fuca plate under the continental North American plate. East of the Cascade Mountains, the stereotypical wet landscape of the Pacific Northwest sharply gives way to a much more arid, much more classically ‘western’ landscape. This is where ponderosa pine makes its forests.
This disparity is the result of a unique set of circumstances that come together in a definitive, almost textbook way in the Pacific Northwest. Commonly we call it the rain shadow effect, but scientifically it’s the orographic effect or orographic lift. This phenomenon occurs when a mountain or range of mountains, simply by their presence, forces warmer, moisture laden air upwards where it cools and subsequently drops much of its moisture as precipitation. Once the air passes the high point, it moves down and warms, and stops dropping water, making for more arid conditions as compared to the other side of the slopes.
From the Rockies west, most of the moisture systems come off the Pacific Ocean moving eastward. These systems are chock full of water evaporated off the ocean where the air is comparably warm. As they move ashore, they are confronted by huge masses of earth, first the Coast Range then the Cascades. These systems drop huge amounts of water on the western slopes of these mountains and leave the areas to the east high and dry. Where it’s wet along the western sides, many moisture loving trees grow; where it’s dry, ponderosa pine grows.
This effect is largely responsible for the distribution of the drought tolerant ponderosa pine and many other trees in the west. By the same token, it’s responsible for grasslands and prairies often devoid of trees due to lack of water. In Oregon, the Willamette Valley lays in the rain shadow of the coast range and has historically been a savannah dominated by open grass prairie, Oregon white oak, and—you guessed it—ponderosa pine. The rain shadow of the Cascades extends hundreds of miles east from their crest and only gives way when new mountains force the air to rise and relinquish its water. The rain shadow of the Rocky Mountains is even too much for ponderosa pine and leaves only savannah and grasslands over much of the center of the continent, the famously tree-poor area we call the Midwest.
The rain shadow effect is one of the most significant weather phenomena to occur in North America. Or perhaps it’s more accurate to say that the mountains are one of the most significant features of the west, affecting the weather and subsequently what trees and plants grow where. So, it feels in fact quite appropriate that the ponderosa pine is the definitive icon of the west because it is a living manifestation of the complex interactions that take place here. Ponderosa pine is the living language in which the mountains and wind converse.
Completely Arbortrary is produced and hosted by Casey Clapp and Alex Crowson
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Cover art by Jillian Barthold
Music by Aves and The Mini-Vandals
Episode cover photo by Dcrjsr
Additional Reading:
The Ponderosa Pine (Pinus ponderosa)
The Orographic Effect
The Winds of the United States