The Trees of Texas 15 
DISTRIBUTION OF TREES IN TEXAS 
Within 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 m 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, imelude 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 
ehecked 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 
