ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...

SOILS, FIRE, AND THE POST OAK SAVANNAH/INTRODUCTION 111
2002a). However, the dominant aspect of Post Oak Savannah soils is their sandy nature. As
noted in the following discussion of the soil-dependent fire frequency hypothesis, these
sandy soils are an important determinant of the plant communities that occur on the Post
Oak Savannah.
THE SOIL-DEPENDENT FIRE FREQUENCY HYPOTHESIS AND THE DISTRIBUTION OF
THE POST OAK SAVANNAH
The distinctive historical vegetation pattern of alternating bands of tall grass prairies on clay
soils and oak woodlands/savannahs on sandy soils in North Central and East Texas has been
described for more than a century. From west to east, the prairie bands are the Grand Prairie,
the main belt of the Blackland Prairie, the San Antonio Prairie, and the Fayette Prairie, while
the woodland/savannah bands are the West Cross Timbers, the East Cross Timbers, and three
bands of Post Oak Savannah (Fig. 66). Many authors (e.g., Hill 1887; Tharp 1926; Allred &
Mitchell 1955) have attributed this striking pattern to relatively high levels of soil moisture
available for tree growth on areas of sandy soil, and conversely, inadequate levels of soil moisture
for tree growth on clay soils. However, this explanation is not consistent with present-day
observations of rapid invasion of clay soils by woody vegetation on many areas of the Grand
and Blackland Prairies (Fig. 67). A different explanation seems needed to account for the discrete
areas of woody versus prairie vegetation observed by early explorers and settlers.
Diggs and Schulze (2003) proposed an alternative hypothesis, the soil-dependent fire frequency
hypothesis, which postulates that the distribution of prairie and oak woodland/savannah in
presettlement times was not due to insufficient moisture for tree growth on clay soils, but
rather to differences in fire frequency on different soil types, with the higher fuel quantity on
clay soil associated with increased likelihood and intensity of fire and the resulting suppression
of tree growth. Prairie fires are fueled primarily by grasses, as opposed to forbs or woody vegetation,
so an increase in fuel quantity would require an increase in grass biomass. The
hypothesis further predicts that grass biomass is typically higher on clay soils, due to better
moisture and nutrient availability at the rooting depth of grass plants. This situation would
represent two alternative positive feedbacks. High fuel quantity on clay encourages fire,
which stimulates subsequent grass growth (since grasses are fire-adapted), thereby maintaining
high fuel quantity. Low fuel quantity on sand reduces the chance of fire, which fosters
invasion by trees that then further suppress grass biomass and the subsequent likelihood
of fire. These alternative feedbacks lead to alternative stable states, prairie and oak woodland/savannah
(Fig. 68).
The key assumption of the hypothesis is that the difference in grass biomass on the
prairies versus the oak woodlands/savannahs was of sufficient magnitude to substantially
raise the likelihood, frequency, and intensity of fire on the clay soils of the prairies compared
to the sandy or rocky soils of the Cross Timbers or Post Oak Savannah. In addition, the
hypothesis leads to the prediction that patches of open grassland on sandy soil are rare except
in instances of active management (e.g., suppression of woody species) or immediate proximity
to clay soils (which would result in more frequent fires due to closeness to the fireprone
prairie vegetation).
This particular hypothesis for the distribution of prairies and oak woodland/savannah is
consistent with the more general conclusions of Scholes and Archer’s (1997) review of treegrass
interactions. They write that, “Moist fertile environments [e.g. Blackland Prairie] support
a vigorous grass growth that, if not grazed, leads to frequent intense fires…. Semi-arid
environments on sandy, low fertility soils [e.g. Post Oak Savannah or Cross Timbers] are
seldom treeless.”
The present invasion of the prairies by trees can be explained by a lack of fire that has
resulted from intentional fire suppression, plus numerous and extensive firebreaks that have
been created by human activities (roads, cultivated fields, overgrazed areas). Moreover, once