The mountains are steep and rugged, and the lower slopes are dominated by chaparral. This ecosystem is naturally adapted to large, high-severity fires every 30 to 100 years or more. Nearly every area within the San Rafael Mountains has burned within the last century, largely due to an increase in human-caused ignitions. In the more shaded and moist canyons and northern slopes oak woodlands, bigcone Douglas-fir, and Coulter pine are found. Gray pine is also interspersed across the chaparral-dominated slopes.