Frequent, low-intensity fire is a crucial process for regulating structure and function of open-canopied forests and woodlands, and its exclusion may lead to alternation in provision of ecosystem services such as nitrogen (N) processing. We experimentally excluded fire from longleaf pine forest that was otherwise managed at a 2-year fire return interval. The study site was in southwest Georgia, on weathered Ultisols of the Southeastern Coastal Plain. Nitrogen fertilizer (150 kg/ha/year) was applied to gauge the ability of soils to process added N under the contrasting fire regimes. Litter was collected in baskets to provide information on the contribution of litter return to N supply; nitrogen in plant crowns (longleaf pine, oaks, and wiregrass) was estimated from live foliar samples and allometry, and N mineralization was measured in the upper 15 cm soil horizon at 1 – 2 month intervals depending on time of year. The study began after application of prescribed fire to all treatments, and continued for seven years.
Results/Conclusions
Fire exclusion did not affect crown content of N of any plant group, but N fertilization led to increased crown N in all plant lifeforms. Fire exclusion caused a large increase in oak (but not longleaf pine) litter production, because without regular topkill there was a rapid increase in midstory oak leaf area. Fertilization alone did not increase litterfall because oaks remained under control by fire, but fertilization of fire-excluded stands caused a synergism that resulted in 50% greater litter production than under the next highest treatment. The effects of fire exclusion were not manifest in N mineralization data until 5 years after the last burn (> 1 missed burn cycles), when N mineralization began to increase compared to the regularly burned stands. Effects of N fertilizer addition on mineralization were contingent on fire exclusion treatment: mineralization increased in burned stands but decreased in unburned stands. The study results indicate the profound importance of frequent fire on ecosystem processes in longleaf pine forest, in particular in their mediation of oak effects on N mineralization.