Fire Effects on Soil Biota

Plant Roots and Fire

There is a large amount of information available that details the effects of fire on plants in southern forests (see Fire Effects on Plants). These effects range from completely positive to completely negative. This range of effects depends largely on the community of plants present in a forested landscape (fire tolerant species, fire sensitive species, or a mixture), and on the intensity of the fire (low intensity prescribed fire, high intensity wildfire, or something in between). Fire almost always results in the death of some plants in a given system, and the extent to which plants are killed has a strong relationship to the effects of fire on roots. The killing of fire sensitive plants aboveground results in an input of dead roots belowground, and this input of new material has the potential to influence the decomposers (microbes) as well as the entire soil food-web at least in the short term.

Soil Microbes and Fire

The effects of fire on soil microbes in southern systems seems to be dependent to a large extent upon fire intensity. The responses of soil microbes to fires range from no detectable effect in the case of low intensity prescribed fires to total sterilization of the surface layers of soil in very hot wildfires (see Joergensen and Hodges 1970; and Renbuss et al. 1973). This early work focused primarily on the abundance of microorganisms and not their activity levels. This is interesting because workers have observed that although there is a decrease in abundance of microbes following fire, the remaining microbes can have levels of activity that are greater than that of the microbial community prior to the fire (Poth et al. 1995). These authors, working in tropical savanna systems in Brazil, found that the increased rates of microbial processes, such as denitrification and production of methane and carbon dioxide, persisted for one year following fire, but the nature and duration of microbial responses to fires in southeastern forests are not well known. (See diagram: Temperature effects on soil)

Microinvertebrates and Fire

In one of the few studies dealing with microinvertebrate responses to fire in southern forests, Metz and Farrier (1971) reported a general reduction of microarthropods with increasing prescribed fire frequency in loblolly pine stands on the coastal plain of South Carolina. In this study, the authors compared the abundance of microarthropods in plots that had either been burned every year, burned every 3 to 4 years, or left unburned for many years. The main results from this study were that abundances of mites and springtails were reduced by a small amount (~25%) by periodic prescribed fires, but that this reduction was dramatic (75-80% fewer) when prescribed fires were conducted annually. Similar studies in midwestern forests have shown similar results in that reduction of litter mass with prescribed fire generally results in reductions of microarthropod numbers (Dress and Boerner 2004; Brand 2002). The consequences of these reductions for the decomposition of new leaf litter have not been thoroughly addressed.

The response of microarthropods to fire has also been studied in many other systems including southern grasslands such as the tallgrass prairie systems in eastern Kansas and Oklahoma. These studies have generally found that microarthropods are decreased in abundance with frequent fire (Seasteadt et al. 1991). This negative effect of fire is mostly attributed to decreased habitat for mites and springtails, because many of these organisms live in decomposing leaf litter, and much of this litter is lost in fires. Other researchers in yet other ecosystems have suggested that changes in the size of the microarthropod population in soils of burned areas might serve as an indicator of fire intensity. One group working in southern California found that this type of microarthropod index worked well for estimating fire intensity when combined with other easily measured variables (Henig-Sever et al. 2001).

Macroinvertebrates and Fire

There have been few scientific studies of the responses of soil macroinvertebrates to fire in forested ecosystems of the southeastern U.S.. Of the few studies that have addressed these organisms responses, the general pattern observed is that the response is often driven by changes in habitat structure, or by changes in the amount or the quality of food resources. Thus, whenever fire affects vegetation, temperature or moisture, or the nutrient status of a soil, there is potential for impact on the soil invertebrate community. These impacts are not always predictable, as demonstrated by a study of ground and litter dwelling arthropods conducted by Hanula and Wade (2003). These authors found that the frequency of prescribed fires (plots burned annually, every two years, every four years, or unburned for 40 years) in longleaf pine flatwoods of northern Florida had dramatic effects on numerous organisms. Interestingly, most of the arthropod groups collected during the five-year study had negative responses to fire, but other groups were strongly favored by fire. For example, among 28 different spider groups that were collected, there were only four that responded positively to the frequent fires employed in the study.

Another study of litter dwelling and soil dwelling macroinvertebrates showed that the density of macroinvertebrates was significantly reduced one year after a prescribed fire in the upland forests of the Cumberland Plateau in Kentucky (Kalisz and Powell 2000).  Reduction in the number of beetle larvae accounted for a large proportion of the difference following fire, and the authors proposed that repeated fire in a single location could potentially have long-term negative effects on beetle populations and on the functions these beetles perform within the system.

Several studies have been conducted in grassland soils in Kansas that focused on the responses of soil macroinvertebrates to fire. Studies have repeatedly shown that earthworms are strongly affected by fire in tallgrass prairie soils, and the usual pattern observed is for fire to increase the abundance of earthworms in undisturbed areas (e.g., James 1982). However, in more disturbed areas (i.e. close to human habitations), fire also has the interesting effect of limiting the colonization of non-native earthworms into soils under frequently burned vegetation (Callaham et al. 2003). Results of this study suggested that the native earthworms in grassland soils are adapted to the warmer soil conditions frequently found under frequently burned vegetation, and that because fire improves the performance of grasses, the native earthworms may have strong habitat preferences for soils with abundant grass roots.

Future Directions for Research in the Southeast

Most of the information presented in this article comes from ecosystems that are not very well represented in the southeastern U.S.. We have discussed results from studies that were conducted in various systems in various geographic ranges, but there are relatively few studies available for ecosystems or states in the southeast. Clearly, there is a large gap in our knowledge of soil organism responses to fire in these systems. Basic studies of fire effects on soil invertebrates is needed. Some new studies have recently been initiated, and these should shed some light into the effects of this important land management practice on soil organisms in southern forests. Furthermore, there have been recent calls for increased effort toward identifying and understanding the linkages between aboveground and belowground organisms (Wardle et al. 2004). As such, future efforts should certainly include research on the effects of land management on aboveground vegetation and the subsequent effects on soil biota and the soil processes they influence.