Surface Fire Regimes

Ponderosa pine forests of the Southwestern US typify this regime. Productivity is sufficient to allow trees to grow fast enough to, in effect, escape the flames of surface fires. This strategy requires that trees also evolve thick bark to withstand high temperatures around those parts of the plant directly exposed to flames. In addition, trees must drop dead branches (termed self-pruning) otherwise these would carry fire from the surface into the canopy. Under natural conditions, as trees grow in size they become increasingly resistant to being completely killed by fire. However, because surface fuels heat the soil surface intensely, seeds dispersed prior to the fire are typically killed, thus regeneration from seeds in the soil prior to the fire is relatively limited.

For many of the dominant trees reproduction is poor in unburned forests because of the thick surface litter that inhibits seedlings from reaching the soil surface and because the shade of the tree canopy inhibits seedling growth. Trees like ponderosa pine require open gaps, such as produced by fire, but the parent trees must survive fire in order to provide seeds after the fire that created the gap has passed. Even in surface fire regimes small gaps are created by localized crown fires that may comprise a few to many trees being torched, and these are important sites for tree regeneration. These high light gaps are ideal establishment sites for pine seedlings and the extent to which establishment occurs is a function of the proximity of parent trees.

Gap size is very critical to successful regeneration. If gaps are very small they have a higher probability of accumulating sufficient fuels to carry a repeat fire before the saplings have developed sufficiently to withstand a repeat fire. However, if gaps are very large the probability of seeds reaching the site from trees that survived the fire is diminished, and for areas with large crown fires of thousands of hectares, natural regeneration of the original forest cover may require centuries. Of course, while foresters may consider such events as undesirable, they are a natural part of most ecosystems and there is a diversity of shrub and herb species that depend on such conditions for their long-term persistence.

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