Wildfire ecology and the Shortleaf Pine

Shortleaf Pine

Forest fires have become a daily event in the United States, whether those fires are prescribed or wildland fires. However, the effects of those fires are often overlooked or misjudged, due to the rapid regrowth of vegetation and to the variations of effect on different species of tree life. While vegetation is certainly the life form primarily affected by fire, the entire ecosystem of the forest can also be affected. This paper will focus on Eastern and Midwestern forests, specifically the shortleaf pine species, and will examine the effects of wildland and prescribed fire on the ecosystems of these forests, and on the trees themselves in an effort to show the vast range of possible effects.

First, it is important to understand what makes up the shortleaf pine species. The shortleaf pine, scientifically named Pinus echinata, is a member of the Pinaceae family of trees. It is the most common of the southern yellow pines, with an average height between 70 and 100 feet. The trunk, on average, is between one and a half and three feet. It is considered one of the larger pines, and has a broad, open crown. The needles of the species are slender and flexible, and vary from dark blue to light green in color. They are often between two and four inches in length, with two or three in each needle bundle. The cones of the shortleaf pine are long and egg-shaped, dull brown in color, and vary from one and one half to three inches in length. The cones remain attached following maturity, but do open at that time to reveal thin, keeled scales with small prickles (Farjon, 2001).

The shortleaf pine originated in dry, rocky mountain areas, but now grows additionally in sandy areas as well as in the silt loams of flood plains and in open field. This species is often found with other pines, or with oak trees. This species can be found from southeastern New York to southern New Jersey, northern Florida to Texas, and from Texas up through Missouri. In all, the shortleaf pine is native to 21 states (see map below) (Farjon, 2001).

Distribution Map of the Pinus echinata

Source: Morin, N. (1997). Pinus echinata. in: Flora of North America, Vol. 2, pp 350.

The species has numerous important functions in today's society. It is used as lumber in construction, for millwork, and is used in plywood and veneer for containers. Further, this and other pines are used in the production of barrels, as well as in other wood products (Farjon, 2001).

Generally speaking, the effect of fire on any ecological system can be both a positive and a negative. Primarily, there are three detrimental effects of fire on an ecological system. First, fire kills many organisms in the community (Danter, 2005). According to the Florida Division of Forestry (2006), this is generally not due to direct mortality, but to the alteration of habitat. Depending on the type and frequency of fire, habitats may support fewer, or in some cases more, densities of particular wildlife species. Fire kills insects, reducing a food source for many animals. Fast moving fire can cause significant animal mortality. Small mammals may lose vital cover as well as nesting areas, forcing the species to relocate (Florida Division of Forestry, 2006).

However, fire can improve a community, as well. Pathogens, or disease causing organisms, are often killed or controlled by fire (Florida Division of Forestry, 2006). This can save both plant and animal life. Further, in some cases, nitrogen-fixing legumes and other plant life that survives on the nutrients fire provides often appear in newly burned areas. This can actually serve to increase the size of a community, and allows the return of many animals (Huggett, 2004).

In terms of shortleaf pine forests, fire can quickly alter the community. Many birds and small animals use the seeds of the shortleaf pine and the berries of plants using the shortleaf as cover as their only food source. Squirrels cut the cones and eat the seeds, as well (Halls, 1977). Further, the tops of the shortleaf pines are used as protection from wind and cold for many animals, and older shortleaf pines are used as nesting trees for the endangered red cockaded woodpecker (Scott, et al., 1977). Clearly, fire can have a detrimental affect on the animal community of the shortleaf pine forest.

A second detriment in forest fires can be that, after a major fire, the ground can be left without vital nutrients (Huggett, 2004).

In general, the rate of oxidation and decay is increased in fire. As a result, leaf and other materials decompose and oxidize quickly, releasing nutrients and minerals into the soil that enhance plant growth. However, while there is a short-term transfer of phosphorous, potassium, and nitrogen from litter to soil, the soil temperature increases, and vital soil bacteria and insect populations decline immediately. Additionally, the PH levels of the soil increase for up to two years following a fire, inhibiting some form of plant life (Florida Division of Forestry, 2006).

However, these issues are less likely to affect the shortleaf pine. Particularly in upland areas, the root systems of shortleaf pines are in mineral soil. The heating of this mineral soil enhances nitrification of organic life. Nitrification, or the biological oxidation of ammonia with oxygen to create nitrite, and the resulting oxidation of those nitrites into nitrates, is vital for the growth of the shortleaf pine.

The process is completed by the bacteria of Nitrosomonas and Nitrobacter, both of which use carbon dioxide for growth (Ripple, 2005). While these bacteria diminish immediately following fire, they increase up to ten times the original population within one month of fire (Florida Division of Forestry, 2006). The result, then, is a higher level of bacteria vital to soil nutrient for shortleaf pine growth.

A third problem often affect forests following fire is volatilization (Higgens, et al., 2006). Volatilization is the process that occurs as fire vaporizes nutrients, sending the gasses into the atmosphere. Following a forest fire, volatilization sends carbon, hydrogen, oxygen, sulfur, and phosphorous into the air. As the nutrients in the soil are altered due to heating and cooling, clay and soil fracture and dehydrate, causing a loss of hydration, as well as an increase in potassium, that can hinder plant growth (Higgens, et al., 2006).

In terms of the shortleaf pine, however, this process can be beneficial. The shortleaf pine is highly adaptable to soil changes, which accounts for its wide distribution. Generally, this species grows best in soils classified as Ultisols and Udults. These soils are low in organic matter, high in clay accumulation, and are drier than other soil types. These soils are well-drained, and often sandy or rocky, with higher potassium levels (USDA, 1975). As a result, volatilization can actually be beneficial for the shortleaf pine, since the process increases potassium, and since the species requires fewer nutrients than other types of species.

Specifically, the shortleaf pine benefits from both wildland and prescribed fire in a number of ways. First, without fire, hardwoods such as oak take over the area of shortleaf pines. The process works as follows: first, a pine stand develops. Hardwoods, specifically oaks, seed under the pines. As the pines mature, and die, the hardwoods begin to dominate, taking the minerals and nutrients the pines require for growth. Hardwoods live and grow under more shade than pines, and hardwood seeds are larger than shortleaf pine seeds. The hardwood seeds thus become established within a thick cover of dead leaves that often accumulate under unburned stands of longleaf pines. The pine seeds, however, do not grow well in such conditions (Little, 1978).

When fire is introduced, however, the shortleaf pine benefits. Oaks have thin bark, so less fire is required to kill their cambium near the base of the tree. Fire generally damages this area, and can kill the oak if the entire cambium is burned. With shortleaf pines, the pine crowns are often the first to burn. Far more heat is required to kill the base cambium, particularly in the case of large, more mature pines. Therefore, the pines generally only have damage to the foliage, or to the major buds and large branches. Shortly thereafter, the dormant buds begin to sprout new crown. Even when the entire part of the pine above ground is damaged, the tree can live if the sprouts develop (Little, 1978).

The larger shortleaf pine has even more protection against fire. Large pines have thick bark, and the crowns tend to grow father from the ground. Prescribed fires often kill only small pines, or those one to four inches in diameter. The larger pines, those with a breast height diameter of over 13 inches. Less intense wildfires tend to kill even fewer pines (Little, 1978).

To some extent, the effect of fire on the shortleaf pine depends upon the temperature of the fire, and the burning conditions of the fire. Large fires have, in the past, been known to kill 68% of pines in a single area. 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).