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The vegetation of the western cross timbers



The vegetation of the western cross timbers



Ecol Monogr 18(3): 325-376



The boundary of the western cross timbers of Texas and its major divisions are shown. The area includes a main belt of sandy soils with gentle relief developed upon Cretaceous outcrops covering 2,436,000 acres, and a fringe of rocky and gravelly soils with rough relief developed upon Pennsylvanian outcrops covering 1,680,000 acres. Both divisions have a sparse overstory of Quercus stellata and Q. marilandica. Basic knowledge of the climate, geology, and soils of the area is summarized. Grazing influences, cultivation, crops, and erosion are considered in relation to use of the land and vegetation since settlement. Sample plots at 350-yard intervals along 76 miles of cross-country transects, as well as hundreds of plots on scores of tracts selected for special study, were analyzed during 9 yrs. They provide data on coverage by species. These data are supplemented by those acquired at monthly intervals. In the order of decreasing relative coverage, the spp. or groups are ranked as follows annual forbs, 19%; Buchloe dactyloides, 9%; the 2 oaks, 7%; annual Aristida spp., 6%; Bouteloua hirsuta, 5%; and some 20 other spp., each composing less than 5 but over 0.5% of total coverage. Buchloe dactyloides is 4 times as abundant in the latter. The oaks, Paspalum ciliatifolium, annual Aristida spp., Bouteloua hirsuta, Andropogon saccharoides, and Smilax bonanox are far more abundant in the main belt. Some 30 spp. with coverage of over 0.3% in one division show different or virtually no coverage in the other. Physiognomy of the vegetation is associated with certain classes of soils. Floristically there are 4 broad types of vegetation. They are the Quercus-Smilax type of podzolic soils, the Quercus-Prosopis type of immature reddish prairie soils, the Prosopis type of mature reddish prairie soils, and the old-field type of podzolic soils that have been cleared, cultivated, and abandoned because of erosion. Coverage of over 0.2% is reported for some 50 spp. occurring in one or more of the types. Species and their relative coverage differ greatly by types. Climax or original vegetation, as determined from relicts, consisted of the mid grass, Andropogon scoparius, as the major dominant and with the 2 tall grasses, Sorghastrum nutans and A furcatus, as lesser dominants. Species composition of relict original vegetation is reported. Differences between vegetation of the different soils are much greater now than they were originally. Andropogon scoparius was a universal dominant of the climax but in the grazing disclimax the oaks predominate on podzolic soils, annual forbs on immature reddish prairie soils, and Buchloe dactyloides on mature reddish prairie soils. Grazing by domestic livestock was the primary cause of the modification of cross timbers vegetation. Grazing coactions were determined by monthly studies of both grazing animals and range vegetation under 3 intensities of grazing. Different spp. composed an important part of the diet of livestock on lightly, moderately, and heavily stocked range land. Broad implications in range management are discussed. Autecological studies of 14 grasses are reported. Included are studies of Andropogon scoparius, Sorghastrum nutans, Bouteloua hirsuta, B. curtipendula, Sporobolus asper, Buchloe dactyloides, Aristida spp. of the Group Purpureae, Paspalum ciliatifolium, Stipa leucotricha, Andropogon saccharoides, Cynodon dactylon, Panicum scribnerianum, 5 perennial spp. of Eragrostis, and Andropogon ternarius. The course of range degeneration in the main belt and its effects on floristic composition of both overstory and understory were detd. Secondary succession on abandoned fields is descr. and shown to vary depending upon degree of grazing disturbance, proximity of a source of seed of successionally higher spp., and the degree of erosion at the time the field was abandoned. A subsere on an old field with ordinary degree of erosion, protected from grazing, and adjoining a source of seed, reached the final stage in 14 yrs. There are typically 4 stages, namely, the weed stage, the annual Aristida stage, the Andropogon ternarius stage, and the A scoparius or final stage. The A. ternarius stage is sometimes omitted. Subseres on old fields protected from grazing are commonly arrested in the Aristida stage because seeds of the next stage are lacking. This was proven by simply scattering seeds of climax grasses in bands several yards wide. On such bands the climax grasses completely eliminated annual Aristida spp. through moisture competition within 4-7 years and volunteered in the adjacent unseeded bands of Aristida. Included are notes on: 13 spp. that did not go dormant in winter at this latitude and 10 that did; 9 with prominent winter rosettes and 5 common winter annuals; time of germination of 22 spp; time of flowering of 133 spp; and time of mature fruits on 39 spp. The "climax-problem" of the transition area between forest and prairie is discussed in relation to climate, soils, grazin g, and fire. It is concluded that the area is within a grassland climate though density of the stand of oaks has increased under overgrazing to a point where the landscape now approaches that of forest or woodland rather than the original savannah.

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Accession: 025974581

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DOI: 10.2307/1948576


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