Academic journal article The American Midland Naturalist

Geographic Variation in Flowering Responses to Fire and Season of Clipping in a Fire-Adapted Plant

Academic journal article The American Midland Naturalist

Geographic Variation in Flowering Responses to Fire and Season of Clipping in a Fire-Adapted Plant

Article excerpt

ABSTRACT.-

Identifying optimal fire regimes for a given species requires monitoring its responses to different fire regimes. This study examined the effects of fire during the lightning season and clipping in different seasons on the induction of flowering in a fire-adapted species, Pityopsis graminifolia, in two different fire-dependent ecosystems (oak forest edges in north Mississippi and longleaf pine savannas in south Mississippi). The two ecosystems differed in the frequency of lightning (higher in south Mississippi) and the timing of peak drought conditions during the lightning season (earlier in south Mississippi). Flowering was induced by prescribed fires during the lightning season in both regions. Flowering was greater in unburned controls in north Mississippi than in south Mississippi. These differences persisted in a common grass-dominated environment in the greenhouse, suggesting a genetically-based bet-hedging strategy with respect to fire-induced flowering in north Mississippi. Flowering of both varieties responded better to clipping during peak drought periods during the lightning season than to clipping treatments at other times of the year, but the causes of such seasonal differences (be they genetic or environmental) remain unresolved at this time and require further investigation.

INTRODUCTION

Despite widespread recognition of the occurrence of fire-dependent ecosystems throughout North America, ecologically optimal fire regimes (i.e., those to which firedependent species are best adapted) are poorly understood at best. Regional variation in fire regimes may give rise to a situation in which conditions that benefit species or ecotypes in some regions are not the same conditions that benefit species and ecotypes in others (Little and Dorman, 1952; Perry and Lotan, 1979; Givnish, 1981; Brewer, 1995; Parker et al., 1997; Gowe and Brewer, 2005). Today, fire-dependent ecosystems are often highly fragmented, lack effective fire conductivity, and thus cannot be managed by relying on pre-modern processes (aboriginal people or the unimpeded spread of lightning fires) to burn vegetation (Leach and Givnish, 1996; Frost, 1998). Finding optimal fire regimes in different regions therefore requires manipulating fire regimes and monitoring responses of fire-adapted species in different regions.

Given our incomplete knowledge of historical fire regimes, optimal fire regimes for a fireadapted species must be evaluated by assessing the relative importance of each of type of fire regime as a selection pressure today. Fire regimes fall primarily into three ignition-type categories: lightning-started or natural wildfires, accidental/uncontrolled human-ignited wildfires and prescribed or planned fires. Examinations of fire scars, direct observations and inferences from landscape features indicate that fires started during the lightning season occur more frequently in the outer coastal plain of the southeastern United States than further inland (Komarek, 1964; Christensen, 1981; Martin, 1989; Guyette and Cutter, 1991; Frost, 1998; Huffman et al, 2004). In many regions, however, both lightning-started and most human-started wildfires tend to burn the greatest amount of land at dry times of the year (Howe, 1994; Ruffner and Abrams, 1998; Brewer and Rogers, 2006; Slocum et al, 2007). Hence, using the effects of different fire seasons to distinguish between lightning-started and human-started wildfires as selection pressures may not be possible when peak frequencies of lightning and drought occur at the same time of the year (see Slocum et al, 2007, however, for evidence of human-started wildfires during dry periods outside the lightning season in the Everglades). On the other hand, humans have purposefully modified fire regimes for thousands of years throughout most of eastern North America (Pyne, 1982; Delcourt and Delcourt, 1998). In contrast to wildfires, many anthropogenic fires in the southern United States historically were and currently are intentionally set outside peak lightning-fire or dry seasons, especially (but not exclusively) in late autumn, winter and early spring (Hilgard, 1860; Cushman, 1899; Beilmann and Brenner, 1951; Cowdrey, 1996; Audubon (Dec. …

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