Florida scrub gap dynamics over time: Revisiting the relationship among species diversity, gap properties, and time-since-fire

Background/Question/Methods

Gaps are open patches within the dominant vegetation and are an important structural component in many systems.  Gaps within Florida scrub, a pyrogenic shrubland, serve as the preferred microhabitat for many endemic scrub species, including several federally endangered plants.  Scrub gaps are characterized as areas devoid of shrub vegetation >50 cm tall; they are often associated with bare sand, although litter and ground lichens increase in cover after fire.  In 2003, we investigated the role of gaps in Florida rosemary scrub across a fire chronosequence.  Although plant species diversity measures strongly increased with gap size and generally decreased in the longest unburned gaps, gap area did not show a strong pattern across the fire chronosequence.  In 2011, we revisited 280 gaps to investigate the scrub gap dynamics over the intervening eight years.  Since 2003, three prescribed fires burned a subset of our study gaps; other gaps remained undisturbed, resulting in 10 time-since-fire (TSF) regimes in 2011 (1, 3, 7, 10, 12, 14, 18, 25, 39, >50 years).  We remapped gap perimeters using a Trimble GPS, recorded species occurrences, determined dominant shrub matrix type, and noted any recent disturbances.

Results/Conclusions

Gap fate was variable; gaps survived (n=108), disappeared (n=12), split to create smaller gaps (44 split gaps created 105 new gaps), and merged to create larger “mega” gaps (24 mega gaps subsumed 90 old gaps).  Gap closure and splitting occurred across most TSF classes.  Gap merging occurred most frequently post-fire as gap-defining shrubs were top-killed; few gaps merged in long unburned areas due to shrub die-back. Gap area was highly variable within most TSF classes (range 0.82-2242 m²).  Mean gap area varied significantly with TSF.  Burned areas (≤ 7 years TSF) had larger, but fewer, gaps than unburned areas; however, there was not a strong pattern in gap area across TSF.  On average, only gaps burned since 2003 increased in size and unburned gaps became smaller.  This supports the hypothesis that gap area declines with TSF, a pattern not observed in the chronosequence data due to the high variation in gap sizes and site differences among TSF classes.  Changes in species diversity and distributions may show similar patterns as gap area when evaluated over time.  Gaps are an integral part of the structure in Florida scrub and investigating gap dynamics furthers our understanding, management, and conservation of this globally-imperiled habitat.