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Legumes are a prominent component of biodiversity in many fire-dependent ecosystems (Gill, 1975, 1981; Arianoutsou & Thanos, 1996; Hainds et al., 1999), and fire is vital to the persistence of many native legume species (Martin et al., 1975; Leach & Givnish, 1996). In frequently burned longleaf pine (Pinus palustris Mill.) savannas, native herbaceous legume species comprise more than 10% of vascular plants (Drew et al., 1998; Hainds et al., 1999). Legumes not only contribute significantly to the structure of these fire-maintained communities, but also are presumed to play a key functional role in replacement of nitrogen following frequent fires via fixation of atmospheric N2 and subsequent tissue turnover (Boring et al., 1990; Hendricks & Boring, 1992, 1999).
Although fire can cause short-term increases in N availability (Christensen, 1977; Hulbert, 1988; Dudley & Lajtha, 1993), burning leads to substantial N losses in longleaf pine ecosystems through volatilization, ash transport, and leaching (Christensen, 1981). Repeated burning can also promote the development of a positive feedback between nitrogen availability and litter substrate quality, which may diminish long-term N availability (Christensen, 1977; Ojima et al., 1994). Frequent fires lead to low-quality litter inputs, resulting in rapid decreases of soil organic N and microbial biomass (Ojima et al., 1994).
Through symbiotic N2 fixation and high-quality litter inputs, legumes may offset N losses and increase N-availability (Hendricks & Boring, 1992, 1999; Towne & Knapp, 1996). Although Hainds et al. (1999) found legume species to be widely distributed and stem densities to be uniformly high across a moisture gradient (Hainds et al., 1999), very little is known about the N2-fixing ability of perennial legumes found in the longleaf pine-wiregrass (Aristida beyrichiana) ecosystem. Because N2 fixation rates can vary considerably among herbaceous legume species, legume population size alone is not a sufficient indicator of N2 fixation activity (Hendricks & Boring, 1999).
Fire season can affect fixed N2 use efficiency in legumes by consuming leaf area of legumes and by altering carbon partitioning to tissues above and below ground, as well as to reproductive structures. Within-plant C competition for growth, reproduction, defense (Antonovics, 1980) and, for legumes, N2 fixation (Pate, 1996) is a well-established concept. Fire regime and, specifically, season of burn can significantly influence growth and reproduction in native forbs (Platt et al., 1988; Brewer & Platt, 1994; Hiers et al., 2000). The pre-Columbian fire regime in longleaf pine savannas is thought to have been characterized by frequent, low-intensity, surface fires that occurred every 1–5 yr (Christensen, 1981), and season of burn is hypothesized to have been an important element of this disturbance regime (Platt et al., 1988; Brewer & Platt, 1994). In the south-east, fires traditionally have been set from late-February to mid-April for game management (Lemon, 1949; Stoddard, 1950; Grelen & Epps, 1967). This late winter/early spring fire season, however, differs from the putative pre-Columbian fire regime when wildfires, ignited by lightning, are thought to have occurred between May and August (Robbins & Myers, 1992).
Burning seasons may influence N2 fixation activity in native legumes through a number of possible pathways (Fig. 1). Nutrient pulses of PO4− and base cations (Christensen, 1977; Gholtz et al., 1985) and increased light availability (Brunswig & Johnson, 1972; Hulbert, 1988) following fire can cause short-term increases in N2 fixation rates of native legumes (Sanginga et al., 1995). Growing-season fires may also have direct adverse effects on fixed N2 use efficiency through both loss of above-ground investment and decrease of leaf area. In annual legumes, N2 fixation is sensitive to the supply of current photosynthate, and canopy loss can lead to nodule loss (Hartwig et al., 1987; Pate, 1996), yet the role of stored carbon in perennial legumes has not been ascertained. Flowering phenology and maximum N2 fixation rates are also closely related in legumes (Trinick et al., 1976; Danso & Kumarasinghe, 1990; Twary & Heichel, 1991; Hansen et al., 1993). Because growing season fires delay flowering in many native legumes (Platt et al., 1988; Hiers et al., 2000), reproduction may be displaced later in the growing season when evapotranspiration and water stress are higher (Entrekin, 1997; Hendricks & Boring, 1999). Fine root dynamics are an important, and often overlooked, component of net primary production and carbon allocation (Hendricks et al., 1993). In frequently burned ecosystems, there is greater allocation of biomass below-ground by perennial herbs (Auld, 1987; Pate et al., 1990). In addition to canopy replacement, the maintenance and production of fine roots serve as yet another competing sink for C.
Figure 1. A conceptual model depicting the feedback mechanisms that link season of burn, plant phenology, reproduction, growth and symbiotic N 2 fixation. Season of burn could influence N 2 -fixation directly if fixation depends on current photosynthate (1) or through complex feedbacks between reproduction; (2), net primary production; (3) interactions between reproduction and production; (4) and (5) with competition for carbon and N 2 fixation.
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Trade-offs among reproduction, N2 fixation, and fine roots have been documented for crop legumes (Pate, 1996), but have yet to be investigated for perennial legumes.
To begin to assess the importance of legumes in maintaining N in longleaf pine savannas under different burn regimes, the objectives of this study are: first to determine N2 fixation rates of dominant native legume species under season of burn and unburned treatments; and second to explore the effects of fire and season of burn on N2 fixation and C partitioning in perennial legumes (Fig. 1). We hypothesize that fire season will affect fixed N2 use efficiency through tradeoffs in allocation of C among plant functions. Specifically, we predict that: first crown development (above-ground biomass) will be positively correlated with N2 fixation; second that lower N2 fixation rates will accompany increased reproduction; and that third increased fine root dynamics (increased mortality followed by replacement of roots) and canopy replacement following fire in the growing season will decrease N2 fixation rates.