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February 25, 2004
Effects of climate variation and vegetation changes.
4. Changes on the scale of 1 year: climate effects on production

In arid and semiarid ecosystems precipitation is both limited and variable between years.
In fact, the drier a region the more year-to-year variability there is relative to the mean
(I.e. in terms of the Coefficient of Variation, CV).

Since water is the primary limiting factor in plant growth, there is a strong relationship
between annual precipitation and annual net primary productivity (plant growth). The
French scientist Henry Le Houérou found out that in many arid ecosystems the slope of
the regression line between annual net primary production and annual precipitation is
similar and between 3 – 4 kg DM mm-1 ha-1 y-1. He also found out that the variability on
production is always greater than variability in rainfall, most likely because production
depends not only on rain but also on other variable factors (temperature, plant densities,
etc.).

When production levels are variable between years, stocking rates must be adjusted to
avoid over- or under-utilization of rangelands. In practice this is quite difficult, because a
season’s precipitation is often not known when stock rate decisions have to be made. The
greatest threat to sustainable management is overgrazing during drought. In this situation,
the land can be seriously and irreversibly degraded.

A rancher’s income varies with climate. In drought years most farmers lose money,
because of increased operating costs (e.g. hauling water, buying supplementary feed,
leasing more land), decreased gains (selling cows at lower prices), or both. Economic
survival often depends on the capacity of ranchers to buffer against economic losses in
drought years, either through personal savings or governmental support policies. Poorer
ranchers are more likely to risk overgrazing in the interest of short-term economic
survival than richer ranchers, who can survive despite one year of low or no income. In
addition, richer countries like the United States can offer more support to ranchers than
poorer countries like Mexico. The consequence is that US ranchers can manage land in a
more sustainable way than Mexican ranchers.
Prescribed burns as a tool of land management.

Prescribed burns are an attempt to improve ecosystem conditions by creating a disturbance
regime that more closely resembles that of the pre-European times. However, we have to
keep in mind that prescribed burns may be different in many ways from natural wildfires 200
years ago (burn frequency, fire intensity, spatial scale).

Fire effects on vegetation depend chiefly on two factors: 1) fire intensity (depends on fuel
quantity and quality, such as water content, particle size and compactness) and 2) fire
duration (depends on fuel quantity and the rate of combustion). An Australian study found
that there can be a positive feedback between fuel quantity and fire intensity: more fuel can
increase fire intensity, which can ignite more fuel (e.g. an overstorey species), which makes
the fire burn even hotter).
February 25, 2004

During a prescribed burn it is important to control fire intensity and the rate and direction of
ignition, as the fire line moves through the landscape. Therefore, there are strict guidelines for
when prescribed burns can take place (not too dry, not too hot, not too windy). In one of the
worst examples of a prescribed fire getting out of control near the town of Los Alamos (New
Mexico) total damage was estimated at $ 1 billion, 48,000 acres were burnt, over 350 families
lost their homes, and a major National Research Laboratory took damage of $342 Million.

The expected benefits of prescribed burns to vegetation are:
1. Manipulate plant species composition (e.g. kill woody plant species, shift competitive
balances between species on the basis of their tolerance to fire at a given time). The
timing of fire determines to a great extent which effect on vegetation it may have: species
are most vulnerable when they have just leafed out and least vulnerable when they are in
a dormant state. Late season hot fires are better at killing shrubs.
2. Increase forage quality (by removing dead plant matter and sometimes increase crude
protein content of green leaves).
3. Facilitate management (reduce or prevent the establishment of woody plant thickets).
4. Prepare land for seeding (e.g. in prairie restoration, a USDA program provides funds to
farmers who want to restore farmland into prairie. Participants in the program are obliged
to burn every few years and reseed with a mixture of commercially produced prairie
species seeds).
5. Reduce wildfire hazards (remove accumulated fuel loads or prevent the establishment of
high fuel loads).
6. Manage wild animals (many animals prefer a patchwork of habitat in various stages of
post-fire revovery, or succession to serve different needs: food, shelter, mating).

The costs and risks associated with prescribed burns are:
1. Prescribed burns are costly, between $2.78 – 33.65 per acre. The magnitude of the
benefits are not well known.
2. Negative impacts of smoke on human well being.
3. Increased erosion: burning just before an intense rainy season can increase rates of
erosion.
4. Costs to society of burns that went of out control.
The issues surrounding prescribed burns are a good example of the difficulty in assessing the
economics of ecosystem goods and services. How do we value the risk of fires burning out of
control against the benefits of a species-rich environment? How do we value the long-term costs
associated with loss of topsoil due erosion against the short-term gains through increased forage
quality? These questions highlight that sustainable land management is as much a question of
science as it is of society, culture and economics.