Northern slopes are especially good for spring-blooming fruit/nut trees in areas prone to spring freezes which can kill the blossoms and thus that year's fruit crops. Trees on the north slopes break dormancy later than those on south-facing slopes, so a freeze at certain times will catch the trees on the south slopes in a vulnerable state but not the ones on the north slopes.
Thanks for your replies. The Santa Cruz mountains to the South are very wet and pines and redwoods grow everywhere. The Oakland Hills to the North are pretty dry.