During a visit to Ashland, I noticed the Cascade Mountains across Interstate 5 are nearly bare, while the Siskiyous are richly forested? Why is there such a big difference between two mountain ranges that are so close together?

During a visit to Ashland, I noticed the Cascade Mountains across Interstate 5 are nearly bare, while the Siskiyous are richly forested? Why is there such a big difference between two mountain ranges that are so close together?

— Ammar A., Ashland tourist

The contrast in the vegetation between the Cascades and Siskiyous that you see in Ashland stems from multiple factors, chiefly from the direction the mountains face, according to our two sources.

The phenomenon is called solar aspect, and here's how it works:

The dry, sparsely vegetated foothills of the Cascades face southwest, apparently the best direction to face in the afternoon if you want a serious suntan.

The lush hills of the Siskiyous face the north to northeast, the coolest direction.

"The snow stays the longest on the northeast slopes," said Jeff LaLande, archaeologist and historian. "The shadows in the winter are the deepest."

The cooler temperatures and additional moisture on the Siskiyous allow for more vegetation to thrive. In contrast, the seedlings on the Cascades are often roasted up before they have a chance to mature.

Southern Oregon's Mediterranean climate accentuates the solar aspect, LaLande said.

"The summers are bone dry, and precipitation tends to happen in the winter," he said. "In other climates where you have more summer precipitation, you don't get the dramatic distinction based on solar aspect."

A difference in soils between the Cascades and Siskiyous also plays a role in what vegetation each range has to offer.

The Cascade foothills that are visible from I-5 and Ashland are made from decomposed volcanic clay soils. When the soil heats up, it dries out and cracks, not such a great nursery for little seedlings.

The Siskiyous have granitic soil that better contains moisture.

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