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Plants grow where the geology is right

The keen observer has likely noticed horizontal bands of different vegetation along the hillsides northeast of our valley. Their presence is in part determined by the underlying geology.

Many factors determine where vegetation grows but all depend on adequate water and availability of useful nutrients. For instance, when nutrients are scarce or toxic, such as in serpentine soils, the variety and type of plants is restricted.

Soils ("rocks gone bad") that develop from the underlying bedrock contribute to the type of plants growing on them. Permeable rock allows water to sink in, forming groundwater reservoirs. Clay-rich soils are derived from easily weathered volcanic tuff (ash), debris flows or, low on the hillsides, from poorly permeable mudstone of the Hornbrook Formation.

During rainstorms, water doesn't soak very far into clay-rich soils and tends to run off. That's analogous to water coursing off concrete as opposed to sinking into a lawn. Annuals, such as grasses with short root systems, make use of near-surface water. They tend to die back early during summer, giving us golden-colored stripes along the hillsides.

In our Mediterranean climate, where summers are long, hot and mostly rain-free, groundwater is extremely important for perennial plants like trees and shrubs.

One might think that lava flows wouldn't store water, but because lava shrinks and cracks when it cools, fractured lava aquifers fill with water. That water is available during the dry summer months and is accessible by plants with deeper root systems. Look at the hillsides, especially opposite Ashland, where bands of trees and brush alternate with grassy slopes. Bands with trees commonly develop on fractured lava flows or permeable sandstone, although some vegetation strips that cut across those bands follow permeable faults or fractures. Landslides deposit broken rock over clay-rich soils, forming water traps between rock fragments that facilitate growth of perennial vegetation. Such vegetation-rich landslide material resembles witches' gnarled fingers.

The observer also will notice that trees and shrubs are localized in valleys along the hillsides. That's because during the summer groundwater continues to slowly percolate downhill into those centralized areas. Valleys that aren't always in direct sunlight retain water better than exposed hillsides. The southwest side of our valley is underlain by quite different and harder rocks that produce different soils. Too, they are shielded from intense sunlight during a portion of the day, allowing prolific growth and survival of vegetation.

Now don't you appreciate those hillsides just a little more?

Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at rockit@dishmail.net.