Effects of experimental fire regimes on the population dynamics of Schwalbea americana L

Summary

American chaffseed Schwalbea americana (a perennial hemi-parasitic forb) is a USA federally endangered species which appears to thrive in habitats with frequent fires, particularly longleaf pine Pinus palustris stands of southeastern USA. Although it occurs in fire-maintained habitat there is concern about the use of fire for managing its rare populations. The purpose of this study was to examine how seasonal timing of fire and fire suppression (dormant season fire, growing season fire, growing season mowing and control, i.e., no experimental treatment) affect population demography, flowering phenology and spatial distribution; to identify modes of persistence associated with fire regimes; and to determine if summer mowing provides a management alternative to fire.

Study area: In summer 1992, four chaffseed populations (31º13'N; 84º29'W) with locally high densities (>< 100-1,000 individuals/ha) were selected for study in Baker County, Georgia, southeast USA. All sites are gently sloping areas between freshwater depressional wetlands and upland longleaf pine forest. For many decades these areas had been managed by prescribed dormant season fires (1-3 yr intervals) for maintenance of bobwhite quail Colinus virginianus habitat. The resultant forest has an open canopy, little to no subcanopy and diverse ground cover.

Treatments: Each site was divided into four treatment sections (except one site where the limited number of individuals allowed only three treatments, mowing being excluded). All sections were burned in March 1992 (to ensure an equivalent 'start point'). The four treatments were:

i) dormant season burn in March (1993, 1994, 1995);

ii) growing season burn in June (1993; 1995);

iii) growing season mow (using a 'weed eater', which cut plants close to the ground and plant litter was lightly raked off) in June 1993, and burnt in 1995;

iv) control (no treatment).

In 1994, the growing season burn was omitted due to heavy summer rainfall and lack of adequate fuel production since the 1993 burn treatment; it was resumed in 1995. Fire was controlled within treatment boundaries with a carbon-based foam spray and water.

Monitoring: Within each treatment two permanent plots (3 x 7 m) were established. Their orientation were chosen to capture the greatest range of moisture conditions within a plot. A string-gridded (0.25 m² units) sampling frame, was placed over each plot for sampling and mapping of chaffseed. In 1992, 2,611 plants were tagged (numbered aluminum disk placed on ground next to stem base), mapped and measured. Subsequently, in May, August 1993, August 1994, August 1995, August 1996) these plants were re-measured (those not relocated were noted), and new recruits tagged and measured. Leaf size classes were assigned to each individual and measurements of stem length, number of flowers, and number of fruit were recorded.

Measurements of soil moisture and relative light intensity penetrating to the ground, were also made.

In 1992, 819 plants were tagged. By 1996, 2,611 individuals had been cumulatively tagged, of which 1,282 were alive. Comparison of the two burn treatments revealed that seasonal timing appeared to have similar consequence for population structure and area of occupancy. However, burning, regardless of season, resulted in increased population density and expansion in areal extent (i.e. occupancy in plot cells). Overall mortality rates were similar between the burnt and mown plots, and higher recruitment rates may explain the population increases among burning treatments; in 1993 and 1995, there was higher recruitment in burned treatments than the mowing or control treatments.

Burning caused an immediate, but short-lived increase in the density and abundance of flowering plants. In 1992 after all plots were burnt, the percentage flowering ranged from 40–60%. In 1993, the burn treatments had higher flowering densities (approximately 52% of population) than the mown or control treatments (6–13%). In 1994 (with absence of growing season burn or mowing), the average density of flowering, and consequently fruiting, individuals in the growing season burn treatment greatly declined and was similar to that observed in the control or mowing treatment, suggesting that flower production response is more or less limited to the year of the burn. With the resumption of all treatments in 1995, the growing season burn reproductive density returned to the 1992 level. In 1996 (no treatment application) few plants flowered and whilst the percentage flowering and setting fruit was similar among burned treatments; the percentage fruit set was >50% less in the mown treatment.

Plants appeared to regress from a reproductive state to vegetative state in the absence of fire and some individuals became dormant for one or more seasons. Growing season mowing did not appear to be an adequate substitute for burning.

Moisture and light intensity: Annual patterns of recruitment in response to soil moisture and treatment were confounded by the lack of complete treatment applications every year. Seasonal differences in moisture conditions further complicated interpretation of any soil moisture and burning regime interactions. Little relationship between soil moisture and recruitment, even by site, was revealed. Similarly, any effects of light intensity were difficult to interpret.

Conclusions and management implications: The key to habitat preservation and management of S. americana appears to be that frequent fire is necessary for the maintenance of populations. An annual summer burn is probably not a realistic management option in many sites, whereas burning at 2–3 year intervals is more attainable and more reminiscent of the fire interval under which the species evolved. However, as a cautionary note, the authors highlight that results might be very different in sites that do not have a history of frequent fires. There is no evidence from this study to suggest that the season of the burn has serious consequences on population structure. This is important because many extant chaffseed populations in Georgia (and South Carolina) are on private land routinely managed by dormant season burns for quail habitat. Winter burns, particularly in the wetter habitats are probably less effective in removing competing woody species.


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