Hi rcmin,

Judging by your photos, your oak appears chlorotic. Iron chlorosis is a common problem here on the Front Range, often seen in oaks. Basically, our highly alkaline soils prevent some more susceptible plants from uptaking enough iron from the soil and, as a result, they are not able to produce enough chlorophyll (hence, why the foliage turns yellow). Chlorophyll acts as a sunscreen for the leaves, so when there is not enough chlorophyll due to iron chlorosis, the leaves can get sunburnt and brown along the margins. Here is a resource with additional information about iron chlorosis and its management (due to our soils, the only options for management are repeating injections of iron into the soil or tree). While I suspect iron chlorosis is the culprit of the browning foliage, I can not rule out other causes through photos. If you'd like, you can bring in a 12-24' branch sample to our plant clinic for further evaluation and to make sure there isn't something else going on with your oak. You can find more information, like location and fees ($7), here.

Thanks!