Woody plants in fire‐frequented ecosystems commonly resprout from underground organs after fires. Responses to variation in characteristics of fire regimes may be a function of plant physiological status or fire intensity. Although these hypotheses have been explored for trees in southeastern longleaf pine (Pinus palustris) savannas, responses of other life forms and stages have not been studied. We examined effects of fire season and frequency, geography, habitat, and underground organ morphology on resprouting of shrubs. In 1994, we located replicated sites, each containing two habitats, upslope savannas and downslope seepages, in Louisiana and Florida. Each site, which contained quadrats located along transects within a 30 × 60 m plot, was burned either during the dormant or growing season and then reburned similarly two years later. Maximum fire temperatures were measured, and densities of shrub stems were censused in quadrats before and after fires. Shrubs collectively resprouted more following dormant than growing‐season fires, regardless of habitat or geographic region. After repeated dormant‐season fires, collective densities in seepages of both regions and densities of root‐crown‐bearing shrubs in Florida seepages were greater than those initially and after repeated growing‐season fires. Shrub responses were generally unrelated to fire temperatures, supporting the hypothesis that resprouting of shrubs may be more dependent on their physiological status at the time of fires. There was, nonetheless, an inverse relationship between collective and root‐crown‐bearing shrub densities following repeated fires and maximum fire temperatures in Florida seepages. Anthropogenic dormant‐season fires over many decades may have resulted in increases in shrub densities in longleaf pine savannas, especially seepages. Repeated growing‐season fires, however, neither increased nor reduced densities of established shrubs. Long‐term shifts in characteristics of fire regimes, even in fire‐frequented habitats, may produce effects that are not reversible in the short term (<10 yr) by simply reintroducing prescribed fires that resemble those that occurred naturally during the growing season.

Woody plants in fire-frequented ecosystems commonly resprout from underground organs after fires. Responses to variation in characteristics of fire regimes may be a function of plant physiological status or fire intensity. Although these hypotheses have been explored for trees in southeastern longleaf pine (Pinus palustris) savannas, responses of other life forms and stages have not been studied. We examined effects of fire season and frequency, geography, habitat, and underground organ morphology on resprouting of shrubs. In 1994, we located replicated sites, each containing two habitats, upslope savannas and downslope seepages, in Louisiana and Florida. Each site, which contained quadrats located along transects within a 30 ϫ 60 m plot, was burned either during the dormant or growing season and then reburned similarly two years later. Maximum fire temperatures were measured, and densities of shrub stems were censused in quadrats before and after fires.Shrubs collectively resprouted more following dormant than growing-season fires, regardless of habitat or geographic region. After repeated dormant-season fires, collective densities in seepages of both regions and densities of root-crown-bearing shrubs in Florida seepages were greater than those initially and after repeated growing-season fires. Shrub responses were generally unrelated to fire temperatures, supporting the hypothesis that resprouting of shrubs may be more dependent on their physiological status at the time of fires. There was, nonetheless, an inverse relationship between collective and root-crownbearing shrub densities following repeated fires and maximum fire temperatures in Florida seepages. Anthropogenic dormant-season fires over many decades may have resulted in increases in shrub densities in longleaf pine savannas, especially seepages. Repeated growing-season fires, however, neither increased nor reduced densities of established shrubs. Long-term shifts in characteristics of fire regimes, even in fire-frequented habitats, may produce effects that are not reversible in the short term (Ͻ10 yr) by simply reintroducing prescribed fires that resemble those that occurred naturally during the growing season.

No abstract

In pyrogenic ecosystems, responses of resprouting woody vegetation may depend more on fire season than on intensity. I explored this hypothesis by examining fire season and intensity effects on response of Prosopis glandulosa, a resprouting shrub in Chihuahuan desert grasslands of the south-western United States. Clipping as well as low and high intensity fires (natural and added fuels, respectively) were applied during the 1999 growing season and the 2000 dormant season. Both fire season and intensity affected shrub responses. Numbers of resprouts increased 16%, and heights increased 8% after dormant season versus growing season treatments of fire and clipping combined. Height and resprout number decreased with increased fire intensity. Fire season and intensity effects on canopy area and stem growth were generally not detected. My results do not support the above hypothesis. Instead, fire season and intensity influence shrub responses in different ways via different mechanisms. Prosopis glandulosa has the potential to respond more after dormant season than growing season fires, perhaps as determined by carbohydrate availability in underground organs at the time of fire. However, realization of this potential is contingent on fire intensity as influenced primarily by fuel amount. In turn, fire intensity will determine the amount and duration of heat penetration into soils and thus the amount of damage to growing points of under-ground organs.