Tom's Blog

Friday, March 7, 2014

Burning a south-facing slope; effect of slope angle

In posts from earlier years, I have discussed solar radiation on steep slopes and its effect on snow melt, vegetation, and burns.

Because the sun and earth constitute a predictable system, it is possible to use a computer model to calculate solar energy input to any slope. The best paper I have seen dealing with vegetation is by Holland and Steyn published in 1975 in the Journal of Biogeography (Vol. 2, pp. 179-183). However, there is a vast literature from the solar electrical/heating field that might be relevant.

Four factors influence the energy input to a soil surface:

  • Time of year; the sun angle changes as the year progresses
  • Latitude; the effect of slope on warming will be greatest at 45 degrees; the effect toward  the equator or north pole is negligible (since the latitude of the Madison area is 43 degrees, the effect of slope on warming will be almost maximal)
  • Slope angle; the most warming effect in the critical early spring period will probably be 45 degrees
  • Aspect; N, S, E, or W. The most warming will be with south-facing slopes

The situation is obviously complex, although formulas exist that permit calculation for any of these four factors.

Because of  the increased warming (and hence increased snow melt) on slopes, it is possible to burn south slopes even when snow is still evident elsewhere on the site. 

We did an early burn when snow was all around in year 2002, in the early years of our restoration work. We burned the whole slope as a head fire, lighting along the bottom of Pleasant Valley Road. The fire roared up to the top and then went out as soon as it reached the more level area where snow was still present. Although this was an easy burn to do, in retrospect it was a mistake to do it as a head fire, as flame heights were too high, especially in the bur oak savanna near the top. If we were to do this burn again in the future, we would do the upper part of the slope as a back burn.

Graphic from Weaver and Clements; 1929; Plant Ecology; McGraw-Hill


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