Why Don't East Texas Savannas Grow Up to Forest?

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Abstract
To explain the low density of trees on 2 adjacent east Texas savannas (designated as bluestem savanna and grass-sedge meadow), several indicators of disturbance history, site favorableness for woody plants and site flammability were compared among the savannas and a neighboring upland forest. The upland forest was previously a savanna as shown on 1930, 1941, 1952 and 1956 aerial photographs. However between 1956 and 1968 tree density increased in the upland forest but not on 2 present-day savannas. Tree age distributions from the 3 communities have a noticeable gap during 1956, which was a drought year, suggesting a climatic limitation on tree regeneration. The gap is particularly extensive (1951-1956) for the bluestem savanna. Widespread fire in conjunction with this drought may have more severely limited tree regeneration in this community. The grass-sedge meadow occurs on acid soil with a high clay content, and this edaphic condition is apparently responsible for its distinctive vegetation, low sapling and tree density and slow tree growth rates. In contrast, tree regeneration on the bluestem savanna does not appear to be edaphically limited since soil properties and woody species composition are similar to those of the adjacent forest. In fact, both rapid tree growth and the abundance of saplings indicate that the bluestem savanna is currently a more favorable habitat for trees than the forest and provide further support for the role of fire in limiting tree density in this community. Flammability estimates based on measured fuel bed characteristics suggest that both savannas are more flammable (especially under humid conditions) and consequently likely to burn more frequently than the forest. Although fires have occurred in both the savannas and the forest (as indicated by widespread soil charcoal), the upland forest has remained fire-free long enough to develop a densely packed moisture-retentive fuel bed. The present nonflammability of this forest and older upland stands, in contrast to that of the savannas, suggests that some areas may effectively escape the influence of fire.