Fire vs. Nutrient Availability: Successional Relationships of Peatland Communities

Several community types occupy large domed peatlands and are often arranged somewhat concentrically. In the interiors of these peatlands, low pocosin occupies the deepest most nutrient poor peats. High pocosin is intermediate in nutrient status and peat depth. On the outer portions of these peat domes, where peat is most shallow, are either bay forests, pond pine woodlands or Atlantic white cedar forests or other swamp forests (Weakley and Schafale 1991). Herbaceous communities can be scattered throughout. Similar community types can be found on smaller peatlands. It is clear that fire, hydrology and nutrient availability play roles in which communities develop where, but their exact roles and interactions are not well studied.

There are two opposing theories of succession in large domed peatland communities. One theory (Wells 1928) proposes that fire is the controlling factor, where frequent fire promotes low pocosin, intermediate frequencies result in high pocosin and very low frequencies result in bay forests. The other theory (Otte 1981 cited in Sharitz and Gresham 1998 and Richardson and Gibbons 1993) suggests that nutrients are the controlling factor. There is a gradient of nutrient availability (phosphorus being most limiting) from very limited nutrient availability in low pocosin to high pocosin to more available nutrients in bay forest (Richardson and Gibbons 1993). In the development of these peat domes, as peat accumulates and becomes deeper, nutrients become more and more limiting as plant roots fail to reach underlying mineral soils. Thus, according to Otte’s theory, the successional trajectory is from marsh to swamp to bay forest to high pocosin to low pocosin, reflecting the build-up of peat and the decreasing availability of nutrients.

Others have recognized that nutrient limitations on domed peatlands may be a special case, and where nutrients are not so limiting, fire frequencies are more critical in determining community type (Christensen 1981, Frost 1995). Frost (1995) described the pre-settlement occurrence of these community types with respect to fire frequencies, soil fertility and organic matter depth.

Despite lack of study and different theories some patterns do seem clear. Atlantic white cedar forests develop under a very specific fire regime, where seed sources are available after infrequent, catastrophic, high intensity surface fires (crown fire), when water tables are high and peat doesn’t ignite or doesn’t burn deeply. Small Atlantic white cedar does not tolerate even low intensity fire, thus for stands to exist fires must not be frequent (Schafale and Weakley 1990). In the absence of fire Atlantic white cedar forests are believed to become bay forest, pond pine woodland or other types of swamp forest.

Relatively more frequent fire favors pond pine woodland rather than Atlantic white cedar forest or bay forest (Schafale and Weakley 1990). On sites where nutrients are not as limiting as in the high and low pocosins of large domed peatlands, the vegetation sequence goes from pond pine woodland to high pocosin to low pocosin with increasing frequency of fires (Frost 1995). High and low pocosins on large domed peatlands tend to maintain vegetation structure and composition even in the absence of fires thus lending credence to the nutrient limitation theory.

Severe fires that consume peat in any of these communities can result in an herbaceous community (Hungerford et al. 1998). The magnitude of soil consumption required for the development of an herbaceous community is unknown and likewise weather it is soil consumption per se that is the cause is unknown.

The prolonged absence of fire in communities except the most nutrient limited (high and low pocosin of large domed peatlands) is thought to promote succession toward bay forests (Weakley and Schafale 1991). Within the landscape mosaic, bay forests are situated where, by chance fire frequency has been lowest, or where landscape features create areas that are not likely to burn, for instance along drainage ways that develop at the edges of domed peatlands (McKevlin 1996). Most bay forest species are known to resprout so after some non-severe burns bay forests may regenerate themselves rather than becoming either Atlantic white cedar forests, or pond pine woodland or an herbaceous community (Schafale and Weakley 1990).

Many possible successional pathways have been suggested for each of these communities (reviewed in McKevlin 1996). It seems that the successional pathway a community takes is dependent on the frequency, intensity and severity of fires along with soil fertility and hydrologic conditions during and after the fire (McKevlin 1996, Frost 1995). Hydroperiod may be very important because of its effects on fire frequency and peat build-up. Peat build-up in turn affects nutrient status. Fire reduces peat formation and also creates a temporarily more fertile environment by releasing nutrients stored in organic matter. Water table depth during a fire contributes to depth of peat consumption.

These communities may be perpetuated by fire or may become other community types because of fire. With more study, these relationships may become clearer. Current research mapping community types with remote sensing technology (Bucher and High 2000) coupled with mapping of fire footprints and other parameters related to fire frequency, intensity and severity may help reveal more information on successional relationships of these communities.