The Trees of Texas 15 



DISTRIBUTION OF TREES IN TEXAS 



AY it hiii the wide domain included by the boundaries of Texas, 

 the natural conditions which go to determine the distribution of 

 trees are extremely varied in character. The chief factors which 

 have influenced distribution are rainfall, character of the soil, 

 temperature, light and winds. Of these the first two are by far 

 the most important. 



The factor which more than any other dominates the distribu- 

 tion is rainfall. This is shown by the transition which takes 

 place in the character of the tree flora from the moisture-loving 

 species of the eastern part of the state to the drought-resistant 

 varieties of the west. 



A map of Texas constructed to show the difference in annual 

 rainfall for each degree of longitude divides the state into nine 

 zones differing from each other by five inches of rainfall and ex- 

 tending in a general north and south direction across the state. 

 The first of these zones receives an annual rainfall of more than 

 fifty inches, and in this zone the trees reach their maximum size 

 and abundance. The second, with forty-five inches, extends to 

 the ninety-sixth meridian, the third, with forty inches, extends 

 to the ninety-seventh, and so on to the -one hundred and second, 

 where the rainfall is fifteen inches. These zones, passing from 

 north to south across the state, include a wide variety of soil 

 types and physiographic features, which have, to a certain extent, 

 modified the character of the vegetation. In order to understand 

 fully the factors which have influenced the distribution of trees 

 in any area, it is necessary to consider the quality of the soil, 

 the physiographic features and all the climatological factors. 



The trees which inhabit the eastern part of the state are ex- 

 tensions of the forests of the Atlantic and gulf coastal plains, 

 as well as some from the Middle and North Atlantic States. 

 These trees are all adapted to abundant rainfall and become 

 checked near the Brazos River by the dry climate of the South- 

 west. Concerning the further westward extension of this forest 

 area, Bray says : ' ' Here its vanguard is broken into straggling 

 detachments of which only the hardier varieties push onward 

 along the prairie streamways or up the deeper canyons of the 

 hills. It is a striking phenomenon this breaking and gradual 

 dwindling away of so vast and vigorous a forest. Not only in 



