Fuel Loads in the Understory of Florida Keys Pine Forests along a Chronosequence of time since Last Fire
Jay P. Sah, Michael S. Ross, Suzanne Koptur, Chris Borg, and Hong Liu
Florida International University, Miami, FL
James Snyder
US Geological Survey, Ochopee, FL
ABSTRACT
The pine rocklands of South Florida are globally endangered ecosystems containing a rich herbaceous flora with many narrowly endemic taxa. Pine rockland forests occupy uplands in Everglades National Park (ENP), Big Cypress National Preserve (BCNP), and the Key Deer National Wildlife Refuge (KDNWR). In the pine rocklands, fire plays an important part in shaping the structure and function of ecosystems. Many rare and endemic species are found in pine rocklands and depend on conditions resulting from frequent fires. In the absence of fire, a closed hardwood canopy develops and the characteristic pineland herb flora is lost. In fact, it is the requirement of these endemic herbaceous species for fire that led to the prescribed burning program in pine rockland forests in ENP and other areas. Here we characterize fuel loads before and after prescribed fires at six separate sites in the KDNWR during a four year period (1998-2001). The sites represented a chronosequence of 30+ years since previous fire, and we take advantage of this range in stand histories to assess how rapidly fuels accumulate over a decadal scale.
Experimental burns were carried out at two times of the year: in June/July (wet season), and in December/January (dry season). Two types of pineland were treated: those with a relatively sparse shrub layer and a well-developed herb layer and those with a dense shrub layer and poorly developed herb layer. In each type of pineland, three blocks of at least 4 ha were established, each containing three plots of 1 ha each and a buffer area. Altogether there were 18 plots scattered in six blocks (2 types of pineland x 3 blocks x 3 treatments), namely Buttonwood, Dogwood, Iris, Locust Berry, Orchid and Poisonwood.
Fuel loads were measured before and after the burns to estimate fuel consumption. Shrub fuel was measured in 20 circular 50 m2 sub-plots within each 1 ha burn unit. Pre-burn low vegetation and woody fuels were collected from two 50 cm x 50 cm quadrats nested within the shrub sub-plot, and were defined as all live plant material less than 0.5 cm in diameter and all dead plant material < 2.5 cm up to 1 m above ground level. After burning the remaining low vegetation and woody fuels were collected from adjoining quadrats. Materials were sorted into live and dead fuels, and further into the following categories: forbs, ferns, grasses (including sedges and grass-like forbs), woody plants, palms, litter less than 0.5 cm, and litter greater than 0.5 cm but less than 2.5 cm, and dried at 70o C.
Significant among-block differences were detected for both shrub (p = 0.0003) and total low vegetation fuels (p < 0.0001) before fire. Shrub biomass and low vegetation fuels increased in a sigmoid pattern with time since previous fire. The three longest unburned sites, Buttonwood, Poisonwood and Dogwood, did not differ in total shrub biomass, suggesting that shrub biomass stabilized after about 11 years. Pre-burn low vegetation fuels reached highest after 12 years since last fire. Low vegetation fuel consumption ranged between 40% in Locust berry and 73% in Dogwood, but was not correlated with time since last fire. Post burn recovery of shrub and low vegetation biomass also varied among sites depending on their micro-climatic conditions. The present results suggest that fuel loads were the function of time since last fire in pine rockland forests.