Tuesday, 18 November 2003: 4:30 PM
Fire Ecology of the Mountain Pine, Pinus harwegiiPoster PDF (2.2 MB)
Pinus hartwegii is found in 17 states in Mexico, from 3 000 to more than 4 000 m above sea level. It is typical of the highest portions of mountains and above 3 500 m forms pure open stands associated with grasses and some shrubs. Among the natural disturbances that are important to the dynamics of this ecosystem are high winds, lightning, pests (mostly bark beetles) and fire, the latter characterized by a frequent, low-intensity, surface/stand thinning fire regime. The fire frequency is estimated at 5-10 years. The fire season is from January to June. Anthropogenic disturbances include cattle grazing, wood and firewood extraction, and fire. P. hartweggi is one of the most fire-adapted pine species in Mexico. It exhibits six fire-traits: Regenerates well in fire-created seed beds, it has a thick bark that protects the cambium, it shows self-pruning capacity, readily recovers from crown scorch, and resprouts when young. It also exhibits different degrees of grass stage among populations. In pure stands excessive fire leads to grasslands. In lower-elevation sites, lack of fire allows succession and oaks, other pines, or true firs displace P. hartwegii. Where fire is excluded, particularly in low-density stands, tall rhizomatous grasses are abundant and represent an obstacle for the seeds to reach mineral soil, reducing establishment. The grass also increases fire danger. In some areas, surface fires may reach high intensities that kill the grasses. A project in the Distrito Federal headed by the Universidad Autónoma Chapingo (University of Chapingo) has identified the advantages of low-intensity prescribed fire no later than halfway into the dry season (March), when compared with the impacts of forest fires after that month (fire season peaks in April-May). Research recently completed and research in progress show dramatic differences in fire behavior. In March, early in the morning, against slope and winds, fire propagation reached no more than 1 m/min, with flame lengths around 0.5 m. In contrast, heading fires in the afternoon, in the same direction as slope and winds, peaked with propagation rates >60 m/min and flame lengths of 6-8 m. Another dramatic difference is the one-year mortality of young trees (2-6 m tall), which was minimal in March, with no statistical difference to an unburned control (10 and 5%, respectively), while mortality from May burns was quite significant (70%). Other advantages of early-season, low-intensity prescribed fire, in comparison with forest fires in the peak of the season, are less crown scorch, higher radial growth, and higher alpha diversity of understory grasses and forbs (21 species one year after March prescribed fire, 12 species in May confined fire). Intense fires also cause considerably more erosion and smoke, than prescribed burns. Understanding the fire ecology of this pine and this ecosystem allows forest managers to identify the positive and negative impacts of fire on the ecosystem and on the resources and benefits it produces. This information will guide the development of a more enlightened approach to managing fire in this fire-prone ecosystem where poverty and the lack of a forest culture lead to many unwanted fires. The goal of this research is to preserve the ecosystem composition (including endemic and endangered species), structure and function, while looking for a holistic approach to provide economic alternatives for the campesinos who are dependent on the land for their livelihood. An approach may be to temporarily eliminate fire in the sites where it has been present in excess, to maintain it where has been helping to preserve the forest, and to reintroduce it where has been excluded. Clearly, fire exclusion alone is not the solution.