The Impact of Fire on Native Vegetation –
The Example of the Cool-Temperate Forest in Patagonia, Argentina
(IFFN No. 23 – December 2000, p. 62-68)
Fire plays and will play an important role in forestry and in most forestry projects. However, it is often difficult to determine precisely the role of fire in environmental development. On one hand, regular natural catastrophes including natural fires happen, on the other hand, since thousands of years, men are interfering with natural processes. The current nature presents itself as a result of an adaptation to those external effects. The overlapping of different elements makes it often impossible to define the single cause and its effects. In order to achieve a better understanding of the reciprocal relationship “fire and vegetation” it is advisable to use a model.
One of the few areas on earth, where fire and people were closely interrelated first in the last 200 years, is the southern part of Patagonia. The here presented small section of one great region of temperate southern forest in South America contains approx. 300,000 ha forest and is situated around 42°S latitude in the Andean mountains of Argentina. Of course, there were fires in the past but it generally occurred as disasters like for example due to volcanic eruptions or earthquakes. There were hardly any fires caused by lightning. Between each disaster were long periods of uninterrupted succession. The situation in the northern part of Patagonia and the far south Tierra del Fuego (Fireland) is different from the region described here. The settlement activities in the described region began at the end of the 19th century. This led to fire activity in until this time unknown dimensions. In a few years, blazes affected millions of hectares of native forest.
The weakness of this model is, that the area was grazed. Livestock suppressed regeneration frequently. Nevertheless, it is possible to recognize the human interference, the results of fire abuse and the regeneration dynamics and tendencies in the native forest.
The aim of this article is neither to submit scientific proof nor to show the complex relation between ownership (legal frame) and fire, but to impart a perception of human interference on the forest structure. In order to be brief, relations are presented in a simplified manner. The described process is based on observations and some research but the whole should be understood as a hypothesis.
The climate is characterized by a particularly extreme distribution of precipitation:
The west-to-east precipitation gradient. Within only 60 km, the annual rainfall declines from 4000 mm in the West (Andean mountains) to 350 mm in the East (steppe).
Mediterranean-type rainfall seasonality. Winter is the rainy season (70% of the annual rainfall). The summers are very dry. Drought periods of six months are no rarity.
The weather conditions in the region have crucial impacts on the behavior of fire
Permanent strong western winds (>30 km/h).
Minimum relative humidity during drought drops frequently below 10%.
Maximum temperatures rise over 30°C.
These extreme climatic conditions and rare natural catastrophes (i.e., volcanic activities, earthquakes) in this particular area are certainly responsible for the dominance of few main tree species in a strange stripe form (Fig. 1a). The boundaries are defined by the amount of precipitation and the temperature. Because of the simple structure, the region is just right for the model of fire impact.
The vegetation cover 150 years ago is presented in Figure 1a. The distribution of Ciprés (Austrocedrus chilensis) reflects its ability to withstand xeric conditions, it occupied frequently the drier regions (steppe range). This species is growing close to the settlements and is an important supplier of timber.
Lenga (Nothofagus pumilio) forms the timberline at 1000 m a.s.l. in this area. To the south the distribution area falls to sea level. This species supplies an excellent furniture wood and is economically interesting despite of many rot problems.
Between the habitats of Lenga and the Ciprés a transition zone of Ñire (Nothofagus antarctica) is observed frequenty. This species is much smaller than the other two. It reaches often only shrub size. But the Ñire occupies an ecological niche, for the Ciprés it is already too cold, for Lenga too dry. The economic benefit of this species is low because it provides firewood only.
First extensive wildfires
The circumstances of land reclamation in Patagonia takes a similar course like in other parts of the world, e.g. in Australia (Pyne 1991). The first extensive fire took place at the end of 19th century. Pastoralism was the economic and ecological engine of European settlements. The pioneers were looking for new pasturelands and used the power of fire to clear of the land. They knew that the fire could, immediately, transform marginal bush land into pasture. They believed that fire could reduce weeds, control pests and slow down the re-invasion of scrubs.
Cypress was damaged strongest. Fire in the pure and dens stands of Austrocedrus chilensis (mesic forest) has an extreme behavior involving high rate of spread, prolific crown fire and spotting. However in the steppe ecotone (xeric woodlands), Ciprés occurs in open woodland but still with high accumulation of fuel (grass, shrubs), enough to create fires with lethal effect. The species reacts sensitive to fire.
However, small “islands” oftenremained only on rocky (lack of fuel) or wet sites (Fig.1b). These islands secure a fast regeneration. The species shows certain characteristics of a pioneer species, but the seedlings require mostly some protection from desiccating wind and high radiation (Meier 1992, Veblen 1995). For the beginning of regeneration more than 30 years can be necessary before a favorable microclimate has been established.
Lenga was far less damaged if the affected area will be used as parameter. The fire extends here slowly, nevertheless with a catastrophic effect (abundance of heavy fuel). All trees die after fire. It has no coppices, and the regeneration period is very long. Lenga shows in the mesic sites a sharp transition between the intact forest and pasture (= old forest area affected by fire).
Fig. 1. Distribution of the principal trees and fire disturbance in the cool temperate latitudes in the southern Andes (42° S)
Unlike the previous two species the Ñire extended its distribution area. However, this species dies but it has coppices and good regeneration ability too. Unfortunately it forms an extensive shrub land with high fire hazard.
Successive fire (fire intervals less than 30 years)
First fires can be seen as favourable if the bio-diversity is the criterion for the review of the impact of fire. The initial homogeneity was disturbed by fire; the large-areas of “monoculture” were transformed into mosaic structures.
The first colonists used fire in order to win grassland for their domestic animals. The process went haphazardly. Each new fire meant a local drama. The islands of old forest disappeared and the succession was interrupted. The evolution was set back by decades (Fig.3). Successive fires in a short sequence reduced the biodiversity, too. Therefore the subsequent fires have to be seen as negative.
The succession stage is a very crucial period and plays an important role by spreading of fire. Therefore, it should be discussed more detailed. The succession needs decades in this region. Shrubs dominate during this time. The entire increment accumulates in thinner boughs, which die in shadow conditions. In this way, 20 t/ha of dead material (fine and middle material <5 cm diameter) can gather in three decades over the ground. It is an inflammable mixture. No miracle that the subsequent fires expand fast on the regenerating areas. From there, the fire consumed the primeval forests. The primary forest area decreases with every fire, the continuous stripes dissolved (Fig. 1c-d). Less and less “islands” remained on the southernaspect (humid site).
The Ciprés area is reduced strongest by fire, only a fraction of former forest exists at present. In the transition to the steppe (xeric condition), only small, occasional islands with 10 trees on rocky hills testify an ex-distribution area.
Lenga has also been strongly decimated on the drier site (e.g. Pre-Cordillera), from the stripe remained medium-sized islands only.
Ñire is the clear winner; the species regenerated fast on the affected areas and colonized the neighbour areas. An impenetrable shrub sea stretches thousands of kilometers along the cordillera as far as Fireland. Millions of hectares cannot be fully used in an economical sense of view. The wood is suitable for firewood only, but the demand for firewood is little. However, this areas are grazed but with small profit. Approaches to replace this species through exotic ones (e.g. Pinus ponderosa) failed mostly due to the high clearance costs (> 400 US$/ha). Currently, it is cheaper to afforest the steppe.
In this context also the influence of livestock should be considered. Just on the xeric sites, the combination animals and fire is a fatal combination for the forest. The regeneration does not get on, but in the meantime fires destroy more and more native forest.
Fire and succession
Every plant society reacts differently to disasters. Most of the research in this regard was on Ciprés forests. Of course, there are great local differences. However for didactic reasons, these differences cannot be considered. The regeneration process is shown in an abstract way in Figures 2 and 3. This semi-pioneer species has frequently problems to regenerate immediately the fire affected the areas. Only if protected by other plants, it succeeds finally. 60-100 years might pass until the initial situation will be achieved again (Fig. 2).
Fig. 2. Post-fire succession in Ciprés forest
An indicator species is Radal (Lomatia hirsuta). Other abundant shrub species in early post-fire stages are Aristotelia maqui, Fabiana imbricata, Schinus patagonicus, Berberis spp. etc. (Meier 1992, Cwielong 1994).
Succession is disturbed again and again by recurring fires, the result is an artificial stage of development of biota. The seral stage (Fig. 3) is determined by fire regime (fire climax). If the frequency of wild fires occurs every 30-60 years, only shrub vegetation can develop (Fig. 3).
Close to settlements, fire occurs in shorter intervals. The vegetation often burns every 3-5 years. Under such intense fire influence, only grassland can grow (Fig 3).
Fig. 3. Fire frequency and succession in Ciprés (Austrocedrus chilensis) forest
Fire is burdened with emotion, and there are tendencies to dogmas. On one hand, there is the European point of view, which considers fire as a source of disturbance only. Whereby the “American” movement has an opposite view. Their fire ecologists see the wildfire as a component of the natural system. Every side can be right, but if one side defends an extreme interpretation it can go wrong. All depends on the local situations.
Reality is not white or black but frequently grey. Nature should be considered in a differentiated manner. The results of a fire can be locally devastating, however regionally it can also bring some advantages. Even in a modest through fire-burdened system like the here presented forest in Patagonia, fire can show advantages, e.g., humus decomposition and increase of biodiversity.
Single or sporadic fires normally support a mosaic structure and raise a variety of flora and fauna. The negative balance predominates, if it burns often and uncontrolled. Wildfires in short interval support grassland, that often forms a fire climax. Therefore not a large single fire leads to disaster but the sum of frequent uncontrolled fires. The disaster is not only ecological but often also economic.
In the here presented region, millions of hectares of native forest were lost irretrievably. In the present days, it is realized that this primeval forest would contribute more to GNP as grassland and shrub vegetation. A reasonable action to achieve financial advantage in a short time turned out to be a bad solution on a long-term basis. The pastoral explosion not only destabilized indigenous biota but also modified them in a way that made reconstruction impossible.
It is easy to transform the high-energy forest systems to grassland; wildfire like each chemical reaction follows the enthalpy gradient. However the return to the initial state is difficult and expensive. The transition form grassland to forest leads through a long period of shrub vegetation. However especially this vegetation stage is endangered extremely through fire. In presented area of Patagonia no fire should be given at least 80 years.
But to claim a fire free region is an illusion today. Three factors are speaking against this:
The frequency of the fires often depends proportionally on population density. The extension campaigns can reduce the risk significantly but not completely because of accidents and criminal intentions. Since the population increases constantly, a steep rise of wild fires can be anticipated.
The intact primeval forest areas will still be reduced and parallel the fire endangered succession areas (and secondary forests) will be extended.
Fire protection (control and prevention) is expensive. The most developing countries because of the increase of population will not be able to find necessary financial funds.
It is still a speculation, but the announced change of the climate could even aggravate the situation in some regions. All signs refer to a further increase of the fire activity.
The future problems of the forest projects will be that the classically European strategy for fire prevention (control and extension) alone will not bring the desired success. One has to think, maybe against the inner conviction, whether it is not more reasonable, to use the instruments of fire ecology to avoid larger disasters.
Cwielong P., N.Rodríguez, and G.Meier. 1994. Regeneración post incendios en bosque de Ciprés area Lago Epuyen. Anales 1991-1994, Vol. 1, 116-133. CIEFAP, Esquel.
Meier, G. 1992. Wiederbesiedlung von Brandflächen durch Ciprés (Austrocedrus chilensis) und andere natürlich ankommende Baum- und Straucharten in Nordwest-Patagonien. Diplomarbeit. Universität Freiburg, Arbeitsgruppe Feuerökologie.
Pyne, S. 1991. Burning bush. A fire history of Australia. Henry Holt and Company, Inc., New York, 520 pp.
Veblen, T.T., B.R.Burns, T.Kitzberger, A.Lara, and R.Villalba. 1995. The ecology of conifers of Southern South America. In: Ecology of the southern conifers (Enright, N.J. and Hill, R.S., eds.), 120-155. Melbourne University Press, Carlton, Victoria.
Social Forestry and Conservation Project, Forestry Division
P.O. Box 774