CSIRO PUBLISHING www.publish.csiro.au/journals/ajar Australian Journal of Botany, 2007, 55, 261–272 Integrating demography and fire management: an example from Florida scrub Eric S. Menges Archbold Biological Station, PO Box 2057, Lake Placid, FL 33862, USA. Email: [email protected] Abstract. In this work, I have used life-history and demographic data to define fire return intervals for several types of Florida scrub, a xeric shrubland where fire is the dominant ecological disturbance but where fire suppression is a major issue. The datasets combine chronosequence and longitudinal approaches at community and population levels. Resprouting shrubs, which dominate most types of Florida scrub, recover rapidly after fires (although their limits under frequent fires are not well known) and also increasingly dominate long-unburned areas. These dominant shrubs can prosper over a range of fire return intervals. Obligate-seeding scrub plants, which often have persistent seed banks, can be eliminated by frequent fire but often decline with infrequent fire. Population viability analyses of habitat specialists offer more precision in suggesting ranges of appropriate fire return intervals. For two types of Florida scrub (rosemary scrub and oak–hickory scrub), plant-population viability analyses narrow the interval and suggest more frequent fires than do previous recommendations, at intervals of 15–30 and 5–12 years, respectively. Variation in fire regimes in time and space (pyrodiversity) is recommended as a bet-hedging fire-management strategy and to allow co-existence of species with disparate life histories. Introduction Replicated experiments have advantages in controlling for With so many of the world’s habitats having fire as a factors other than the manipulated components of fire regimes, dominant ecological disturbance (Pyne 1997; Bond and Keeley but they can rarely be done at the landscape scale over 2005), management of these habitats is crucial to maintaining which fire operates (but see Andersen et al. 1998 for an biodiversity. Rare plant species, in particular, often require example of a landscape-scale, replicated fire experiment). periodic fire to maintain their populations (Owen and Brown Fire intensity, fire patchiness and species responses are scale 2005). Fire management is, however, more than simply applying dependent, and large burns diverge from small burns in fire to a landscape. With a potentially infinite number of these and other ways (e.g. Turner et al. 1997). In addition, combinations of the components of a fire regime (e.g. fuel unplanned burns (or inability to accomplish burns on time) can consumption, fire spread, fire intensity, fire severity, fire complicate or ruin otherwise elegant experimental designs. Even frequency or fire return interval, time-since-fire, fire season, fire large-scale replicated fire experiments, e.g. the Munmarlary patchiness), and the possibility that many factors can affect post- fire experiments, which varied frequency and season in fire vegetation responses (Keeley et al. 2005), fire managers may Australian Eucalyptus savannas, can ‘substantially fail the test of have difficulty fixing reasonable targets. Even the concept of a management relevance’ (Russell-Smith et al. 2003). Replication single-target fire regime is suspect, because key species in a of large-scale fires is an oxymoron as fire regimes (and individual landscape may be adapted to various fire regimes (Keith et al. fires) are inherently variable (Whelan 1995). Use of unburned 2002). reference sites and considering data from multiple fires as Several different research approaches can inform fire replicates are approaches that help deal with pseudoreplication management. For example, studies can be synchronic in fire ecology (van Mantgem et al. 2001). (using chronosequences), diachronic (longitudinal) or involve Combining these potentially complementary approaches to replicated fire experiments (Whelan 1995). Taken alone, studying fire can provide valuable information. Chronosequence each of these approaches has its own shortcomings. patterns can be treated as hypotheses to evaluate more intensive Chronosequence comparisons risk attributing community or long-term studies at fewer sites, and fire experiments (or population differences to the chronological variable, when site studying variation within individual fires) can help answer differences may be causing spurious patterns. For this reason, mechanistic questions. Longitudinal studies at multiple sites can understanding demographic patterns in relation to time-since- help untangle the effects of fire, weather and site variables on fire requires careful choice of study sites that are comparable demographic or community changes. Replicated experiments in soil characteristics, land-use history and other variables. continued for many years are particularly useful. For example, Longitudinal studies can avoid the problems of substituting the Konza Prairie experiments manipulating fire, grazing and space for time, but the time scale of ecological change may dwarf mowing have found interesting interactions affecting ecosystems the length of funding cycles or even the lifetimes of individuals across several decades. Since 1988, I have worked together or institutions. with researchers in my laboratory and other collaborators, © CSIRO 2007 10.1071/BT06020 0004-9409/07/030261 262 Australian Journal of Botany E. S. Menges using a combination of approaches to understanding fire and and Purdum 1992). Fire is ubiquitous in upland Florida plant demography in Florida scrub, and we have used our research communities, with Florida scrub burning less frequently than results to infer aspects of this ecosystem’s fire regime. neighbouring plant communities that have more graminoids or In this paper, I first summarise research approaches that pines that contribute to fine fuels (Breininger et al. 2002). Very combine population biology and fire ecology in Florida scrub. frequent or infrequent fires can convert certain types of Florida I then explore the relationships between life-history variation scrub to other vegetation types (Myers and White 1987). Fires (particularly the post-burn recovery modes of resprouting and vary in intensity and patchiness (Outcalt and Greenberg 1998; seeding) and fire regimes, with analyses of life-history patterns Wally et al. 2006), creating a complex mosaic of burned and of these recovery modes in Florida scrub. Following from unburned habitats. Scrub fires can be intense crown fires burning this, I consider the extent to which these life-history patterns through the shrub canopy (Outcalt and Greenberg 1998). can inform fire regimes in various types of Florida scrub. Fire top-kills most shrubs and, thus, opens up the landscape. Finally, I examine how population viability analyses can provide Most dominants of Florida scrub respond to fire with improvements in recommendations for fire management, and resprouting; notable examples include oaks and palms. Two conclude with a short discussion on the role of pyrodiversity in species of pine can resist and survive fire and one species retains fire management. an aerial seedbank in serotinous cones (Myers 1990). Florida rosemary and many herbaceous plants form persistent soil Florida scrub and fire seedbanks that promote population pulses after fire (e.g. Johnson Structure of Florida scrub 1982; Carrington 1999; Weekley and Menges 2003). Post-fire recovery strategies affect the trajectories of vegetation recovery Florida scrub is a xeromorphic shrubland found on xeric, in different fire regimes (Abrahamson 1984). Obligate seeders low-nutrient sands, dominated by oaks, palmettos and ericads are found mainly in rosemary scrub; other types of scrubs are (Abrahamson et al. 1984; Myers 1990; Menges 1999; Schmalzer dominated by species that can resprout (Menges and Kohfeldt 2003). Herbaceous plants are secondary, but include many 1995). After fire, the recovery of diversity and cover occurs endemics, post-fire specialists and species found mainly in within 1–5 years, although height growth continues for longer gaps among shrubs (Menges and Hawkes 1998; Petru and periods (Abrahamson 1984; Greenberg 2003; Schmalzer 2003). Menges 2003). Several types of Florida scrub have been The diversity and density of herbaceous plants are highest in the described, including rosemary scrub, which retains open gaps years immediately following fire (Menges and Hawkes 1998). for decades and other types of Florida scrub which have closed Fire suppression is one of the most common and most serious shrub canopies (e.g. oak–hickory scrub, scrubby flatwoods, threats to Florida scrub. Fire-suppressed scrub stands have a oak-palmetto scrub) (Abrahamson et al. 1984; Menges 1999; lower representation of herbaceous plants, fewer gaps and lower Schmalzer 2003). diversity (Menges et al. 1993). Seedbank densities are likely to Fire ecology of Florida scrub decline. With fire suppression, shrubs may form a tall sub-canopy or canopy and be less likely top-killed by fire (Guerin 1993; The dominant ecological disturbance in Florida scrub is fire, Duncan et al. 1999). If fire-suppressed scrub is burned, post- which is typically variable in intensity, spatial coverage and fire recovery may be less dramatic than typical of frequently return time (Fig. 1). No direct, long-term fire-history data exist burned sites (Abrahamson and Abrahamson 1996). Habitat for Florida scrub. Lightning during the late-spring and summer fragmentation has added to the problem of fire suppression by months was the major ignition source historically