Edaphic and pyric influences on two contiguous savannas of East Texas
Streng, Donna Rae
Harcombe, P. A.
Master of Arts
Vegetation, soil, history (land use and tree age), site favorability (tree growth rates), and flammability were examined in four contiguous East Texas communities, two of which are savannas. The data suggest that woody plant densities are edaphically limited on one of the savannas (sedge meadows), and fire limited on the other savanna (pine-bluestem savanna). The edaphic savanna (meadows) has acidic soil with a shallow claypan, and these features are apparently responsible for its distinctive vegetation, low sapling densities, and slow tree growth rates. The soil characteristics (profile, texture, pH) and the tree composition of the fire savanna are similar to a neighboring forested upland. It is inferred that fire has reduced tree density since tree growth rate and sapling densities are higher than in the forest, and it appears that logging was not responsible for low tree densities. Furthermore age histograms from this savanna reveal low hardwood recruitment from 1953-196. 1955-1956 was a period of drought in East Texas and age structures from all communities examined have a noticeable gap during this period with the savanna's being more extensive. This age gap suggests that drought reduces tree recruitment (increases mortality) and in some communities this reduction is further accentuated by fire. Also, since both savannas are more flammable than the wooded areas (as measured by Rothermel's (1972) model of fire spread) they probably bum more frequently. Thus, they are maintained by a positive feedback cycle -- i.e. burning increases flammability by favoring herbs which increases the probability of a recurring fire. On the other hand, one of the wooded communities (west side) was a former savanna (as revealed by aerial photographs), which through chance or the intervention of man has remained fire-free long enough to develop a densely packed, nonflammable fuel bed. The present nonflammability of this community suggests that it may have effectively escaped from the influence of fire.