Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
5.4.2 ASPECT
Aspect is the direction a slope is facing and is one of the predominant site characteristics
affecting evapotranspiration. South and west aspects receive more solar radiation during
the day and are warmer and drier, with higher ETs than north and east aspects. Soils on these
south and west aspects dry out faster than north and east slopes. In spring, during seed
germination, south and west aspects can dry very quickly between rainstorms, reducing the
rates of germinating seeds. As seedlings emerge and grow through spring and early summer,
temperatures on the south and west slopes continue to rise to very high levels, creating very
unfavorable conditions for seedling establishment. Even with planted seedlings, high surface
temperatures can damage stems near the ground line, severely affecting seedling survival
and establishment (Helgerson and others 1992).
In climates where moisture is not the limiting factor, south and west slopes are often very
productive and have greater cover. Warmer soil and air temperatures create a longer growing
season, offsetting the effects of moisture stress on plant growth.
On high elevation sites, north and east aspects are cool, with much shorter growing seasons,
compared to the south and west aspects, resulting in very different compositions of species.
At high elevations, soils on south and west slopes stay warmer longer in the fall, providing an
opportunity to plant in the late summer in time for seedlings to become established before
winter arrives. During spring, at all elevations, south and west slopes warm up much sooner
than north slopes, resulting in earlier seed germination and plant growth. The difference in
soil temperatures between north and south aspects is a consideration for determining when
to plant and what species to use.
Aspect – How to Assess
Aspect is measured in the field by facing the fall line of the slope (the imaginary line a ball
would roll) and taking a compass bearing downslope. A “northeast slope,” “northeast aspect,”
“northeast exposure,” or “northeast-facing slope” all refer to an aspect with a compass bearing
facing northeast.
Aspect can also be measured from topographic maps by drawing an arrow perpendicular
to the contour lines and pointing the tip of the arrow downslope. Aspect is often a factor
for delineating one revegetation unit from another due to the strong influence it has on the
growth and survival of seeds, seedlings, and cuttings.
Soil and air temperatures differ greatly between aspects, and taking temperature measurements
can be important for assessing the effects of aspect on revegetation. There are many types
of recording devices available on the market, but only equipment that can download data to
spreadsheets for analysis and graphing should be considered. Some equipment has become
so inexpensive that more than one unit can be purchased (Figure 5-24).
Mitigating for South and West Aspects
For most sites, any treatment that will shade vegetation on the south and west slopes from
intense solar radiation should increase survival and growth of establishing plants.
Overstory Vegetation
Keeping overstory trees at a minimum density of one tree per tenth acre is a rule of thumb for
reducing soil temperatures below lethal levels on south aspects (Helgerson and others 1992).
Shade Cards
Shade cards can significantly increase seedling survival on south aspects (Hobbs 1982; Flint
and Childs 1984) (see Section 10.4.3, Shade Cards). They must be placed close to planted
Figure 5-24 | Temperature
recording device
Temperature recording technology has
become smaller and very inexpensive.
The iButton® shown next to the nickel
can record more than a year of temperature
data.
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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