Understanding Disturbance
By David A. Bainbridge ©2000
Associate Professor
United States International College of Business
Alliant International University
San Diego, CA 92131
The puzzle we are trying to solve in planning for restoration is
understanding both above and below ground structure (species, arrangement,
sizes, architecture) and function (hydrologic cycles, nutrient cycles,
energy flows, competition, symbiosis, etc). Water and nutrient limited
arid and semi-arid ecosystems tend to be very brittle and easily damaged,
and even minor disturbance can lead to profound and long lasting changes.
Understanding the changes that have occurred is essential to plan a
successful restoration project.
Developing a site history is one of the most useful steps in understanding
disturbance for a specific site. We can develop a site history by looking
backwards with: aerial photographs, interviews and oral histories, books,
photographs, property records, newspapers, illustrations, diaries, land
grants, journals, archeological research, tree rings, sediment and pollen
records. A reference site is desirable for any restoration study; but in
many areas of the Southwest there are no undisturbed areas.
Characterizing disturbances on sites
Disturbance effects on sites are many and pervasive. Because the goal of
many restoration projects is to recreate the plant cover and species
composition of the site prior to disturbance (or a similar undisturbed
site), accurate data on the vegetation composition of a reference site is
desirable, but there are often no good reference sites. Sampling
procedures must be related to the data needed, expected variation over
time, and the degree of precision needed. Photo points and photo
interpretation are very useful, and they should be a regular part of any
monitoring effort, with prints, transparencies and electronic format
images.
While California's soils may appear lifeless much of the year, living
organisms, from bacteria to animals and plants, strongly influence their
fertility, structure, and response to disturbance. Small organisms such as
ants (which function in many drylands like worms do in more moist soils),
bacteria, fungi, microarthropods, nematodes, springtails, protozoans,
termites and yeasts play important roles in soil nutrient cycling, soil
development and plant establishment. Many of these little noticed
organisms are easily disturbed or destroyed by human activities and their
elimination can lead to undesirable changes in soil moisture relations,
soil structure and fertility, and plant and animal communities.
Changes in physical, chemical, and biological factors have been addressed
in several studies and key changes include: increased compaction and soil
strength, reduced water infiltration and soil fertility, increased
erosion, and reduced biological activity. Assessing these factors can be
done with an impact penetrometer, infiltrometer, and chemical and
biological assays, such as spore counts or mycorrhizal and bacterial
infectivity assessments.
Soil strength often is dramatically increased by disturbance. As soil
strength increases water can no longer enter the soil, so more runs off
and erosion increases. The increased strength of the soil may also inhibit
or stop root growth and kill established plants and new seedlings. The
soils in the dry desert Southwest tend to have low nutrient levels, but
the low levels of macronutrients are generally not a problem.
Micronutrient deficiency or imbalances are more likely to be important,
and in pollution corridors nitrogen pollution from dryfall of nitrogen
compounds may be a serious problem, encouraging weed growth and
disadvantaging native seedlings.
Disturbance also increases microclimate stress by increasing wind speed at
the ground and sand blast. Natural herbivory appears to be a common factor
limiting survival of young plants, and bare and exposed sites make plants
very vulnerable to rabbits and other herbivores. Herbivory management may
be critical on restoration sites.
Understanding the integrated effects of disturbance
The potential interplay between these types of disturbance on plant
establishment and long-term survival is still not well understood, but it
is clear it is interactive and in may in many cases be synergistic. The
following formula is a preliminary attempt to assess these interactions
and to relate them to plant establishment.
Revegetation potential = [ (d1) (d2) (d3)
(d4) .... (dn) ]
D1 increased soil compaction and reduced
infiltration, where
D2 reduced soil fertility, macros and micros
D3 more severe microclimate, increases in
windspeed, sandblast, temperature,
radiation
D4 increased herbivory, less cover, more succulent
plants
Dn other disturbance effects
Why damaged areas don't recover by themselves
None of our studies have evaluated all of these factors at one site and
related them to establishment. However, if we combine data from several
studies with our best estimates of feedbacks we find:
| D1. soil compaction and infiltration |
= 0.3 |
| D2. soil fertility |
= 0.2 |
| D3. more severe microclimate |
= 0.7 |
| D4. increased herbivory |
= 0.8 |
Revegetation potential = (0.3) (0.2) (0.7) (0.8) = 0.03
This suggests that this disturbed desert site has roughly 3% of the
predisturbance potential for plant establishment. If these preliminary
projections are close to correct, plant establishment is almost fifty
times more difficult after an off road vehicle trail or road is created
and abandoned. If a plant species may return to an undisturbed site once
in twenty years, it may be able to return only once in 1,000 years to the
disturbed site, unless it is restored or treated. This appears to fit the
current understanding of how long natural recovery takes for disturbed
sites in the arid Southwest, where there is no frost heave to help break
up compacted soil.
Treating these damaged sites can be costly. For full treatment with soil
preparation, container planting, plant protection, and irrigation and
maintenance it can cost $5,000 an acre or more. A full size sport utility
vehicle blasting through undisturbed desert areas can damage many acres in
a day, creating an astronomical repair bill. At Joshua Tree National Park
the increasing use of restoration and more accurate costs have enabled
rangers to take trespassers to court and recover significant fines for
damage to government property.
The most important lesson from these studies is obvious, but often
neglected - we must minimize the area, intensity, and frequency of
disturbance to arid and semi-arid ecosystems.
Further Reading:
Burcham, L.T. 1957. California Range Land: An Historico-ecological
Study of the
Range Resource in California, California State
Division of Forestry, Sacramento,
CA 260 p.
Charley, J.L. and S.W. Cowling. 1968. Changes in soil nutrient status
resulting
from overgrazing and their consequences in plant
communities of semi-arid
areas. Proc. Ecological Society of Australia
3:28-38.
Iverson, R.M., B.S. Hinckley, R.M. Webb and B. Hallet. 1981. Physical
effects of
vehicular disturbance on arid landscapes. Science
212:915-917.
Lovich, J. and D.A. Bainbridge. 1999. Anthropogenic degradation of the
Southern
California desert ecosystem and prospects for
natural recovery and
restoration--a review. Environmental Management.
24(3):309-326.
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