COS 6-7
Size structure and stand density in south Florida slash pine stands: Influence of recent disturbance history

Monday, August 10, 2015: 3:40 PM
320, Baltimore Convention Center
Michael S. Ross, Department of Earth and Environment, and Southeast Environmental Research Center, Florida International University, Miami, FL
James R. Snyder, Southeast Ecological Science Center, US Geological Survey, Ochopee, FL
Joseph J. O'Brien, Southern Research Station, Center for Forest Disturbance Science, USDA Forest Service, Athens, GA
Nilesh Timilsina, Forestry, University of Wisconsin-Stevens Point, Stevens Point, WI
Jay P. Sah, Southeast Environmental Research Center, Florida International University, Miami, FL

In pine forests of the southeastern US, stand structure, which reflects the occupancy of space and allocation of site resources among individuals of different size and species, is an important determinant of tree regeneration. Disturbances have the effect of reallocating resources by, for instance, removing a subset of the trees or altering seedbeds, thereby modifying the conditions for regeneration.  Across the south Florida region, Pinus elliottii var densa-dominated forests occupy a broad range of environmental conditions and have experienced variable histories of fire, hurricane, and logging. In this study, we investigated how disturbance history and site conditions within the region combine to influence stand density and tree size distribution, including the abundance of seedlings and saplings. During 2008-2014, we completed comprehensive tree surveys in seventy-nine plots, ranging in size from 0.5 to 1 hectare, in four climatically and physiographically distinct south Florida sub-regions.  For each stand, we determined Reineke’s (1933) Stand Density Index (SDI) and the form of the diameter distribution (e.g., negative exponential, unimodal, bimodal, rotated sigmoid) for trees > 5 cm DBH.  Pine regeneration was sampled in sub-plots, and the history of fire, hurricane, and logging was reconstructed from park records and other sources.        


SDI varied widely among sites (by a factor of ~4X), but did not differ significantly among sub-regions. In contrast, sub-regional variation in size distribution was notable: negative exponential distributions characteristic of uneven-aged forests prevailed in the Florida Keys and the southwestern Big Cypress National Preserve (BICY), and unimodal distributions usually associated with even-aged stands were the rule in Everglades National Park (ENP) and northeastern BICY. Among the sub-regions, pine regeneration was most abundant in ENP where fire has been largely absent during the last decade.  In P. elliottii var densa stands in south Florida, size distribution appear to be controlled by broad climatic and edaphic conditions, while site occupancy (as measured by SDI) and pine regeneration are more sensitive to recent disturbances, especially fire. These results suggest that incorporating variation in fire frequency into a prescribed burning program can create opportunities to rapidly restock stands lacking pine regeneration.