Californian climates are impressively diverse. However, most climates here are chaparral or desert climates, or similar to such climates. That means that almost all rain occurs during the rainy winter season, and that almost no rain occurs during the dry summer season. Vegetation that is native to California is very familiar with the climates here. Therefore, it does not expect rain during summer. This should be obvious.

Some native species can adapt to irrigation during summer. Some may grow faster than they normally would without irrigation, but compromise their natural life span in the process. This is acceptable within some landscapes. Many riparian species are not bothered by irrigation.

However, some species adapt less efficiently. Some do not adapt at all. Some that can adapt while young can not do so while mature. Oaks are a common example of this.

These coast live oaks with a few blue oaks were installed as shade trees within a formerly minimalistic landscape that did not include turf. They were happy with annual rainfall for half a century or so. Then, this generously irrigated lawn was installed around them. They can not adapt to the frequent irritation that the lawn requires. Their roots are succumbing to decay, so will eventually be unable to sustain their foliar canopies. As they deteriorate, decay may eventually compromise their structural integrity and stability. Dieback is already evident within their upper canopies, with lower watersprouts attempting to compensate.

I suspect that the trees will be removed before they deteriorate enough to become hazardous. I also suspect that new trees will systematically replace them prior to their systematic removal. Their eventual demise is likely an acceptable loss, and perhaps expected. Turf with adapted shade trees is likely more important within this particular situation than preservation of these middle aged oaks.