February 25, 2004

Effects of climate variation and vegetation changes.

  1. 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).
  • 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:
  • 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.
  • Increase forage quality (by removing dead plant matter and sometimes increase crude protein content of green leaves).
  • Facilitate management (reduce or prevent the establishment of woody plant thickets).
  • 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).
  • Reduce wildfire hazards (remove accumulated fuel loads or prevent the establishment of high fuel loads).
  • 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.