REHABILITATION OF CHILE'S OVERGRAZED, OVERFARMED ESPINALES
Dana Blumenthal
INTRODUCTION
Chile contains an astounding array of natural diversity,
including the extreme arid conditions of the far North, the high spine of
the Andes to the East, and temperate rain forest and large portions of
the Southern Ice-cap to the South. In the middle of all this lies
Chile's fertile central valley, home to both the majority of the
population and the majority of its agriculture.
The vegetation of a large portion of the central valley (> two
million ha) is dominated by a single tree species, the espino (Acacia
caven). The ecosystem is consequently referred to as an espinal. The
climatic zone occupied by espinales ranges from semi-arid (100 mm
rainfall/year) in the north to semi-humid (1,000 mm rainfall/year) in the
south. Although espinales were not the historically dominant vegetation,
they do have considerable value for human populations. Espinales occur
primarily in areas of extensive (i.e. unirrigated) agriculture; the
ecosystems are typically used for continuous pasture or wheat production
with long periods of fallow.
Unfortunately, the majority of espinales are at some stage of
desertification. Overgrazing and crop production have led to a loss of
vegetation, soil compaction and erosion, and consequent loss of
productive capacity. The authors of the studies reviewed in this paper
(Ovalle et al. 1990, Aronson et al. 1993) have suggested and begun to
implement a number of studies aimed at reversing desertification in order
to regain/sustain the productive capacity of the espinales. This work
can be seen as a case study only in a very large scale sense. It is an
example of an attempt to develop the understanding and techniques
necessary to restore and/or rehabilitate a particular type of ecosystem
spread out across an array of landscapes.
ORIGIN OF THE ESPINAL
Prior to the arrival of Europeans in the sixteenth century the
dominant vegetation of the central valley was presumed to have been
matorral, an ecosystem resembling the chaparral of California. It was
dominated either by clumps or continuous canopies of sclerophyllous
shrubs, such as Lithraea caustica, Quillaja saponaria and Colliguaya
odorifera (Fuentes et al. 1986). Historically the espino, a native
species (at least within the last 10,000-1,000,000 years), was probably
restricted to disturbed habitats. Its spread and growth over the last
400 years appears to be due primarily to two factors: the introduction of
domestic livestock, which serve as dispersal vectors, and increased
disturbance, in the form of clearing of the matorral for firewood,
grazing and crop production (Fuentes et al. 1989). Additional factors
that may have led to its present dominance of the region include its
nitrogen fixing capacity (Aronson et al. 1992) and the drought tolerance
of its seedlings.
With the spread of the espino came numerous other invasions,
particularly of exotic annual grasses (e.g. Bromus mollis, and Vulpia
dertonensis), but also of a variety of native shrubs (e.g. Baccharis
linearis and Trevoa trinervis among many others). The presence of the
espino appears to select for an herbaceous understory: It produces
leaves late in the season, providing little competition during the early
part of the growing season; its open canopy allows sufficient light
through to sustain herbaceous growth, and yet keeps enough moisture in
the system to prolong the growing season; lastly the nitrogen fixed in
association with its roots is made available to shallow rooted species as
its deciduous foliage decomposes.
PRODUCTIVE USES AND RESULTING PROBLEMS
The desertification of the espinales can be attributed primarily
to a long history of unsustainable crop production and grazing. Crop
production without the use of fertilizers quickly reduced soil
fertility. Both shallow cultivation and heavy grazing have caused soil
compaction and consequent reductions in infiltration. Loss of fertility
and soil compaction, in turn, reduce vegetative growth, which leads to
further losses of productive capacity, degradation of soil structure, and
so on in a downward spiral.
While espino trees have the potential to slow or reverse
desertification (see below) they are usually treated as a woody weed. In
cropping systems, they make up a brief succesionary stage, resprouting
from stumps, and are cut prior to the next cultivation. In pasture
systems they are sometimes left in place, either as trees or shrubs; most
espinos, however, are being lost to woodcutting. As supplies of large
trees become scarce the local population increasingly turns to shrubs
(espinos and other species) for energy needs.
THE NEED FOR RESTORATION
The pressing need for restoration and rehabilitation of this
ecosystem arises from the fact that a rural population of approximately
300,000 people relies on it for its livelihood. The current situation is
described by the authors as "low input, low output", which means that
natural inputs into the system, water and sunlight are being used
inefficiently. If vegetative cover, soil structure and soil fertility
can be recovered, the region could potentially support the rural
population on an ongoing basis.
SUGGESTED RESTORATION
Aronson et al. (1993) have developed a comprehensive scheme for
restoring and rehabilitating Chile's espinales. Having surveyed a 23,000
ha transect to determine patterns of land use, they suggest the most
appropriate type of land use for each type of site. Their plan runs the
gamut from restoration to reallocation: somewhat degraded matorral can be
restored to healthy matorral; land that has been under cultivation for a
long period of time can be reallocated (i.e., converted to a type of land
use that may bear little resemblance to the original ecosystem) to
agricultural use. Between these extremes are suggestions to restore
those espinales that are in good condition to the condition commonly
found 100 years ago, and rehabilitate those in poor condition to regain
the ecosystem functions necessary to sustain productivity.
This focus on the restoration of an ecosystem that is itself a
result of human disturbance may at first seem odd. It stems from the
fact that espinos coexist well with an herbaceous understory, allowing
for sustained levels of high (harvestable) productivity in a semi-arid
climate. The authors believe that with proper management an espino
system may have great value as a sustainable pasture or agro-forestry
system and yet at the same time preserve some fraction of the
pre-disturbance biological diversity.
The methods suggested depend on the present state of the
ecosystem and the desired final state:
Restoration per se - Methods for restoration of matorral are not
discussed. As they are suggested only for areas that currently contain
significant amounts of matorral vegetation, this restoration would
probably entail protection from grazing and woodcutting. However,
Fuentes (1986) studied the natural recolonization and recovery of
disturbed matorral vegetation and found both processes to be extremely
slow, limited by, among other things, healthy populations of European
rabbits.
Restoration of espinales - Approximately 40% of the espinales
are considered suitable for restoration, i.e. they contain sufficient
soil fertility and plant diversity to be returned to a productive state
largely through altered management of existing vegetation. This
management entails the protection of larger espinos and the thinning of
dense stands of shrubby espinos. It is hoped that this will favor the
presence of native perennials over exotic annual weeds that are of little
forage value.
Rehabilitation - The remaining 60% of espinales are thought to
need rehabilitation. A number of plant breeding endeavors are suggested
to aid in the rehabilitation of severely degraded espinales. These
include the isolation and comparison of multiple strains rhizobium
bacteria in association with both the espino itself and an annual legume,
Medicago polymorpha. It is hoped that these efforts will lead to
increased efficiency of nitrogen fixation and consequent increases in
soil fertility.
Reallocation - For those lands currently under wheat production
the authors suggest the use of crop rotations, including the sowing of
annual legumes for pasture.
In order to accomplish these ends, substantial government assistance
would likely be necessary. The authors mention that there is a Chilean
law providing land managers with 75% of the cost of planting trees.
Incentives might also be needed to increase the short-term profitability
of maintaining soil fertility, and keeping rather than cutting larger
trees and shrubs. Lastly, the introduction of plant species to the
system will require additional research and substantial extension efforts.
INITIAL RESULTS/EVALUATION
Two of the above approaches have been implemented and compared to
existing land use practices. First, a comparison has been made between a
rehabilitated espinal, espinales in various stages of degradation, and
intact matorral. Rehabilitation apparently leads to dramatic increases
in species richness, total plant cover, above-ground phytomass, soil
organic matter, soil water availability, and length of water
availability. For most of these traits, the rehabilitated espinal takes
on the characteristics of largely undisturbed espinales. The exception
to this rule is annual and perennial species richness, which in the
rehabilitated espinal was only 3/4 and 1/2 half that found in undisturbed
espinales, respectively. In order to better evaluate the potential for
restoration/rehabilitation of espinales, it would be interesting to know
both the relative abundance of native vs. non-native species in different
types of ecosystem, the relative abundance of the different species, and
details of the management strategies used in the rehabilitation.
Unfortunately, detailed reports of pre- and post-rehabilitation states
and management strategies are not available.
They found a number of pronounced differences between espinales
in good condition and matorral: matorral consisted primarily of perennial
species while espinal had more annuals; matorral vegetation also had more
plant cover and greater amounts and duration of water availability than
did espinal; conversely, mature espinal contained approximately ten times
as much above ground biomass as did matorral.
The second approach that has been experimented with is the
agro-pastoral system of crop rotations and legume-based pasture. A four
year rotation was used, including winter wheat, and the legume Medicago
polymorpha. Wheat yields in the rotation were between two and four times
those obtained on other farms in the region. Similarly, live weight
gains for ewes grazing the medic pasture were much higher than those
typical for the region. Although data on soil fertility and structure
are not presented, the authors mention that soil fertility also increased
with the implementation of this system.
CRITIQUE
I think the overall framework constructed by the authors is
extremely useful. By determining what land is capable of being restored
to what productive state, it has the potential to make resource use
considerably more efficient in an area where resources - both natural and
capital - are scarce. Furthermore, by classifying the various management
possibilities as restoration, rehabilitation and reallocation, the
framework forces us to make a conscious decision regarding the degree of
alteration in the original landscape we hope to end up with.
The authors also take a big step in suggesting the restoration of
an ecosystem that was not present historically. As the landscapes we
live increasingly diverge from their original state(s), it will be more
and more necessary to preserve biological diversity within such altered
systems. These studies suggest one instance in which this may be possible.
My greatest reservation about the ambitious restoration efforts
suggested by these authors has to do with the failure to grapple with the
larger political and economic questions that determine land use
patterns. For restoration to occur on such a large scale it must not
only be feasible but also fit with the prevailing economic and political
system. For example, if there is a trend towards increasingly intensive
agriculture in the espinales, as is the case in many parts of the globe,
creating the capacity for sustainable extensive use of the land may a)
receive little funding, and b) have little effect on land management over
the long term. It should be noted that the authors are working with
rural economists, although the results of that work are not presented in
the above articles.
I would also, hypocritically, raise a concern about the lack of
consideration given to restoration of matorral vegetation. The focus of
these works is on the return of espinales to a productive state in which
ecosystem productivity can be harnessed for human use. This, no doubt,
is due to a consideration of precisely those political and economic
factors discussed above: resources probably don't exist to embark on
ambitious restoration projects. Nevertheless, restoration of this
ecosystem has not been tried, and might turn out to be more feasible than
the authors seem to assume.
Finally, I would cast doubt on the likelihood of reversing the
trend towards invasion by unpalatable annual exotic weeds. It seems
likely that these species have a competitive advantage largely because
cattle don't eat them. Without the removal of cattle from the system
many of these weeds would be expected to continue to compete with native
perennial vegetation.
In sum, the authors' plan to restore an ecosystem type across an
entire region is laudable and ambitious. It can only be hoped that
support for this restoration effort will increase as the low productivity
of degraded espinales becomes increasingly problematic.
REFERENCES
- This paper draws primarily from the following two sources
(Additional references are cited in the text and listed below):
Ovalle, C., J. Aronson, A. del Pozo, and J. Avedao. 1990. The espinal:
agroforestry systems of the Mediterranean-type climate region of Chile.
Agroforestry Systems 10:213-239.
Aronson, J., C. Floret, E. Le Floc'h, C. Ovalle, and R. Pontanier.
1993. Restoration and rehabilitation of degraded ecosystems in arid and
semi-arid lands. II. Case studies in southern Tunisia, central Chile and
northern Cameroon. Restoration Ecology 1:168-187
ADDITIONAL REFERENCES
Aronson, J., C. Ovalle, and J. Avedao. 1992. Early growth rate and
nitrogen fixation potential in forty-four legume species grown in an acid
and a neutral soil from central Chile. Forest Ecology and Management
47:225-244.
Fuentes, E. R., A. J. Hoffman, A. Poiani, and M. C. Alliende. 1986.
Vegetation change in large clearings: patterns in the Chilean matorral.
Oecologia 68:358-366.
Fuentes, E. R., R. Avils, and A. Segura. 1989. Landscape change under
indirect effects of human use: the savanna of central Chile. Landscape
Ecology 2:73-80.
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