The ongoing California drought has killed more than 100 million trees, according to a recent U.S. Forest Service estimate. Many of these, it turns out, are very old oaks – trees that are known to be drought resilient and have survived numerous droughts in the past.

So what happened to these oak trees?

Todd Dawson, a biology professor at U.C. Berkeley, and several colleagues investigated that issue at three sites in Central California over the past couple of years. They found that these stately, mature oak trees fell victim to severe groundwater depletion.

In short, even the very deep roots of ancient oak trees could no longer reach the aquifers that have sustained them for centuries. That’s because the groundwater shrank amid a combination of historic rainfall deficits, high temperatures and unprecedented groundwater pumping by urban and agricultural Californians.

Their study also found that the trees showed measurable signs of stress before death. The trees switched to consuming a different type of carbon in their cells, they produced dwarf leaves and some produced no leaves at all. These are all telltale signs that could be used in future droughts to protect oak trees and manage groundwater – if we’re willing to pay attention.

Dawson recently spoke with Water Deeply about the study and the future of California’s iconic oak landscapes.

It’s worth noting, by the way, that the Forest Service survey of 100 million dead trees only covers the Sierra Nevada region. It doesn’t include the Central Valley or coastal hills, where millions more trees, including oaks, likely succumbed to the drought.

Water Deeply: What do your results say about the severity of this drought on a historic scale?

Todd Dawson: The 2012-2015 drought was the most severe in the known weather station records (115 to 125 years of recorded weather). And according to tree ring investigations, the drought may be the worst in over 600 years.

Oaks are very resilient to drought. But the groundwater receded below their rooting zone, so they simply died.

The drought was the drought – the cause is unknown – but it was linked to the hottest temperatures in the history of human records, and the velocity of these temperature increases are linked to fossil fuel combustion (there is no doubt about that) that was caused by man. Period.

Water Deeply: And what about the links between groundwater and habitat? Is that a connection we’ve been blind to, in some ways?

Dawson: During the drought, groundwater recession was massive – the largest I’ve ever seen. This led to access to these deep water resources being lost by many of our native trees, and this led to mortality. If the groundwater is not restored each rainy season, this will lead to further losses of our natural resources across the entire state.

We need to strictly regulate groundwater use. Agriculture in the state has gotten a free ride with pumping this water, yet they are only one place where the water is needed. The over-extraction of this water is killing some of our state’s natural resources.

Water Deeply: You showed how some trees produced clearly visible growth changes during this severe drought. Could these observations serve as warning signs during future droughts?

Dawson: Yes, it does seem that the ability of the oaks to make very small leaves is a “early warning” sign that they are running out of water resources. And when this happens, they also fix less carbon and therefore grow smaller leaves.

This response, as well as dropping leaves altogether, is a handy way to “avoid” the stress that comes with drought. But like any response it has its limits as a coping strategy, and it didn’t prevent many trees from dying.

Water Deeply: Your study looked at two sites in the Bay Area and one near San Luis Obispo. To what extent can your results be extrapolated to oaks statewide?

Dawson: Our sites were focused around Central California, but did capture a marked rainfall and temperature gradient that represented a significant fraction of oak woodland systems across the state. With the exception of southern California, which has seen an even more severe drought than central or northern parts of the state, I think what we have seen does provide insights about tree response to drought that goes beyond our sites.

Water Deeply: Will these drought-killed oaks be able to regenerate? Or will we see a permanent shift in the extent of California oak woodlands?

Dawson: Some of the oaks, like blue oak, will not regenerate from existing root stocks. They will have to regenerate from seed if this is possible.

Other oaks, like some of the state’s evergreen oaks, may regenerate from resprouting like they can do after fire.

Water Deeply: Blue oaks seemed to be especially vulnerable in this drought, because they tend to grow on hilltops or slopes. What can be done to help them?

Dawson: The blue oaks are dying in the very edges of their range – at the driest southern end and hilltops where they have run out of water. They are also not regenerating from seed, and we don’t really know why.

Planting them could help, and it works where it has been tried. It is always a good idea to look at preservation options to protect tracts of habitat and land as key natural resources, for their ecosystem functions and services, and linked them to our rich statewide biodiversity.

Water Deeply: What changes might we see in other oak-associated wildlife, since oaks provide food and shelter to so many other species?

Dawson: Some large oak trees serve as “habitat” trees – places where birds nest, hide, eat, fledge their young. If these habitat trees are lost for species of wildlife like the acorn woodpecker, this will certainly have negative effects on the birds. This may be one impact that is least well understood.

Water Deeply: If our oak population shrinks dramatically, what is likely to replace those trees?

Dawson: Shrubs could replace the trees if conditions remain very dry. And invasive species are on the rise and could also replace many of our native plants, too – trees and shrubs.

There are many invasive woody and herbaceous species from Europe, from Australia and from Africa that could move in. The list is long.

As a result, we could see losses of diversity, increased risk of more losses in species of importance and of ecosystem services.

Water Deeply: How will this change the iconic California landscapes and scenery that have become so familiar to us?

Dawson: This is the largest unknown at this time. We really don’t have a good idea or model for what these changes will really look like.