Laurel oaks are starting to drop their leaves and live oaks won't be far behind. Over the next few weeks, it will be fall in the Bay area, as leaves come down by the bucketfuls.
Their appearance prompts many homeowners to unwittingly break a life cycle that has been going on since trees first inhabited the earth: leaf drop, decay and leaf re-emergence.
After leaves reach a certain age, they die and fall to the ground, where they decay. The elements they contain - nitrogen, phosphorus, potassium, etc. - then become available to the root systems of the same trees that dropped the leaves. The roots suck them up and they're used by the tree in the creation of new leaves.
Uninterrupted by man, this cycle repeats itself annually. This is why trees in a natural setting do just fine without people fertilizing them. The organic, carbon-based compounds in the decaying leaves also help maintain the structure and water-holding capacity of the soil.
For the sake of tidiness, or the mistaken belief that fallen leaves will build up on the lawn and prevent grass from growing, people rake up their nutrient-loaded leaves, stuff them into plastic bags and send them to the dump.
Fallen leaves do not prevent grass from growing. What prevents turf from becoming thick and lush under shade trees is the shade. Turf grasses are full-sun plants - they generally need to be in a high light environment to thrive. Of course, the answer is not to over-prune or cut down your valuable shade trees. The answer is to grow shade-tolerant shrubs and ground covers under the tree's canopy.
If you're bothered by the messy look of brown leaves scattered over your lawn, periodically run your lawn mower over them. The mower will break up the leaves and the leaf fragments will quickly disintegrate into your lawn. The same holds true for grass clippings. If there are areas of your lawn where leaves tend to build up, they can be collected and scattered in shrubbery beds, where, again, they will quickly break down and add fertility to the soil.
So save those plastic bags, which are probably made from imported oil, and let nature complete the cycle.