Small fires, on the other hand, are less intense, and therefore cause less damage to the pine. The low air temperature in many areas of shortleaf pine growth help the heat of the fire dissipate, and therefore, more fire is required to raise the temperature of the plant cambium to the point of killing the tree. Also, if debris on the ground is only dry on top, but has moisture underneath, the fire is unable to spread to the base cambium, saving the pine (Little, 1978).
On the other hand, the frequency of fires in shortleaf pine areas also has an effect.
Young shortleaf pines sprout at the root if the crown of the tree is badly damaged, as mentioned. This ability, however, is confined to trees up to 8 inches in diameter, or the trees most likely damaged in a fire. Many of the sprouts on even these trees die, leaving only one to three stems developing. Additionally, shortleaf pines can develop taproots at an early age. If damaged, the seedling is able to grow a new one. If a taproot growth is not possible because of terrain type, lateral roots grow near the soil surface (Murphy, 1986).
The result of this is that frequent small prescribed fires tend to keep the area covered with small sprouts. These fires do not kill the pine, but burn off the debris under the trees, and burn off existing sprouts. Since the trees are able to regrow sprouts quickly, these small fires are beneficial. However, frequent large wildfires tend to completely kill the pines in a given area. While they are able to regrow sprouts, frequent large fires kill off these sprouts before they can grow, resulting in the elimination of the species (Little, 1978).
The shortleaf pine is somewhat dependant upon fire. Without prescribed or mild wildfire, the oak leaf layers build up, resulting in a layer of materials the small pine seedlings are unable to penetrate. Additionally, the shortleaf pine is unable to grow in shade. Fire, as mentioned, kills off the crown of shortleaf pines, and the foliage of surrounding oak trees. This allows the shortleaf pine to absorb the full sunlight of a given area (Gilmore, 2007).
However, care must be taken when using prescribed fire with shortleaf pines. As mentioned, the shortleaf is able to resprout following a fire, due to the dormant buds at the root collar of the pine. These sprouts act as a thermal barrier for the dominant leader in the event of fire. However, smaller shortleaf pine stems are highly susceptible to fire, because of a lack of the thicker bark seen on more mature trees (Gilmore, 2007).
It should also be noted that a recent study of the effects of prescribed burning on the shortleaf pine showed limited results. In the study, the researchers examined the effects of a single dormant season fire on the vegetation in the Conasauga River Watershed of southeastern Tennessee and northern Georgia. They examined this area to determine if a dormant season fire could restore the shortleaf pine in an area. Of the six areas within the Watershed, four were burned, and two were left as controls. The vegetation of each layer, consisting of the overstory layer (trees ? 5.0-cm DBH), the midstory layer (woody stems < 5.0-cm DBH and ? 0.5 m height), and the ground flora layer (woody stems < 0.5-m height and all herbaceous species). The areas were then sampled prior to the burn, and following the burn. The fires were low severity and moderate intensity (Elliot, et al., 2005).
The results of the study were somewhat surprising. The researchers found no significant change in the layers or diversity of species following the burn. There was not regeneration of the shortleaf pine seedlings, either. The fire did reduce the basal area of the neighboring hardwood species, and undesirable species' such as the Pinus strobus, was reduced by 20%. The researchers determined that the prescribed fire was not of a high enough intensity or a high enough severity to sufficiently reduce the basal area, prepare a seedbed for successful shortleaf pine germination, affect the diversity of vegetation, or to promote bacteria recruitment. Thus, the researchers suggested additional fire treatment would be needed to promote the growth of the shortleaf pine (Elliot, et al., 2005).
The USDA Forest Service agrees with the researchers. According to their data, fires to reduce the understory layer should be done when the shortleaf pine becomes abundant enough to make up over 25% of an area's vegetation. Additionally, they recommend prescribed fires every two years in order to maintain the tree species. They do not recommend the burning of shortleaf pine stands because, as mentioned, these trees tend to be younger and thus less resilient to fire (see table below) (USDA, 2000).
Further recommendations include only using prescribed fire in dormant seasons, in an effort to reduce the effect of the burn on neighboring hardwood trees. However, other research studies have shown that the season of burning also has an influence on the effect on shortleaf pines. According to Sparks (et al., 2002), burning in dormant seasons results in lower amounts of fuel, but a greater total fuel load. Thus, fuel consumption and total fuel consumed was greater in dormant season fires, resulting in higher fire line intensity, higher heat per area, and a higher rate of spread. As a result, if a lower intensity heat is desired, growing season prescribed burns are now recommended (Sparks, et al., 2002).
Fuel beds for dormant season fires were characterized by lower amounts of live fuels, higher amounts of 1-hr time lag fuel and a greater total fuel load than growing season fires. Fuel consumption and percent of the total fuels consumed was greater in dormant season fires than in growing season fires. Fire line intensity, heat per unit area, reaction intensity, and rate of spread were greater in dormant season fires than in growing season fires.
It is clear that fire has a profound effect on any ecosystem, but that the shortleaf pine has characteristics that make it less susceptible than some other forms of vegetation. From thick bark to higher crowns, a resilient growth system, and smaller moisture and soil nutrient needs, the shortleaf pine can actually benefit from both wildland and prescribed fires. Caution is needed, however, to ensure that prescribed fires are performed properly. If burned properly, the shortleaf pine can thrive from the fire's effects in limiting undergrowth, opening neighboring canopies, and reducing competitive vegetation.
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