[Chap. Llll THE VEGETATION OF NORTH AMERICA 757 



Because these areas are subject to frequent droughts they are more sub- 

 ject to fires than other forest sites, and the resinous leaves that accumu- 

 late beneath the pines add to the fire hazard. Recurring fires that destroy 

 the ground cover and young trees restore the pioneer character of the 

 habitat; hence succession to oak and other broad-leaved deciduous 

 trees may be delayed or prevented. Pine communities if undisturbed by 



Fig. 383. An oak-hickory forest in Ohio. 



fire are relatively short-lived and are succeeded by oak and other com- 

 munities. 



Tovv^ard the west the deciduous forest is limited by the decreased 

 precipitation — 35 inches annuallv in Texas to 25 inches in Minnesota — 

 and by the frequency and longer duration of droughts. The dry condi- 

 tion of the atmosphere during the summer is often accentuated by 

 periods of hot winds from the southwest. The upland forest border is 

 characterized by open groves of small trees with a ground cover of 

 grasses, or by scattered clumps of dwarfed oaks. Where the soils have 

 more water in them than that derived from direct rainfall, as in river 

 valleys, the deciduous forest extends farther into the grasslands. 



Northward the deciduous forest species are limited by extremely low 



