Drought and Fire Effects on Evapotranspiration From Sawgrass in Florida

Drought and fire are recurring natural processes in the extensive sawgrass (Cladium jamaicensis) marsh areas of Florida. An understanding of the effects of these processes on evapotranspiration (ET) is critical for evaluation of the hydrologic flow system. ET fluxes were measured using eddy correlation techniques for a two-year period (2000-01) at a site within a sawgrass peat marsh in east-central Florida before, during, and after two droughts. During the second drought, wildfires consumed the sawgrass canopy at the field site. The impact of these droughts and the fire on ET are most easily described through the use of a value of ET normalized for the available radiant energy - the evapotranspirative fraction (EF). The impact of drought on the EF was dependent on the annual life cycle of sawgrass. During the growth stage (spring-summer), EF (averaging about 80 percent) apparently was independent of the water table depth and ET was unaffected by drought conditions. During senescence (fall-winter), EF dropped to as low as 40 percent as the water table declined. Apparently, during the growth stage, root growth allows sawgrass to seek soil moisture, thus preventing moisture limitation on ET during droughts, despite a declining water table. However, during senescence, the absence of vigorous root growth prevents sawgrass access to moisture deeper in the soil profile as the water table declines. In addition, the density of green, transpiring leaves decreases as partial browning of the canopy occurs. The non-expanding root distribution and reduction in green leaf density during senescence reduces the fraction of available energy consumed as ET and increaseds the fraction that generates sensible heat flux from the canopy. A fire in the study area in February 2001, following three months of drought, removed the sawgrass canopy, but spared the roots. Sawgrass sprouts appeared within a few weeks and the canopy reached a height of two meters within six months of the fire. Despite a continuation of the drought for almost five months after the fire, EF increased from a pre-fire value of about 40 percent to a value of about 90 percent within three months after the fire. The increase in EF can be attributed to the burn-stimulated transition from senescence to growth stage and the associated development of a green canopy, largely free of non-transpiring, dead biomass.