Thursday, January 27, 2011

Geology begets biology. North facing vs. South facing slopes

I taught High School biology for a long time, almost my whole K-12 career. Of course, that meant my biology classes had a decidedly geological slant to them. One concept I tried to instill in my biology students was that the literal underpinnings of the ecosystem was the underlying geology.

I called these explorations "Geology begets Biology"

One idea that seems simple but created problems with many students was north facing slopes vs. south facing slopes. Both sides of a hill receive approximately the same amount of precipitation, have similar soils and experience the same temperatures but can appear very different from each other. However, the south facing slopes receive more direct sunlight and snow will melt faster, soil moisture will evaporate faster creating a dryer micro climate than the shady north facing slope which might be only meters away. The slower evaporation on the north side means that plants (that biology connection) will have more available moisture. With the snow melting at a slower rate, there is more water available longer in to the spring.

Ultimately, the species of plants found on south facing slopes will be more drought tolerant while their north facing cousins less skilled at surviving with low water amounts. Here, in Colorado's southern mountains, the north facing slopes are full of large evergreen trees, while the south side has more shrubs like the ubiquitous sage brush. So, in just a glance we should be able to roughly decide on "north" even without a compass.

In the foreground, the grassy slope is facing south and west while the heavily treed area is more north and east.
Notice the scrub oak. What you can't experience is the snow depth under the oak is only a few inches.
Looking across the gully at the fully melted south facing slope. I was traveling on the north facing slope in 8-10 inches of snow from a recent storm while the other side is clear.
Again, looking across the gully at the melted out slope. I was in the shade of a large Ponderosa pine tree. The south facing side is home to a large population of scrub oak trees

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