The Giving-Out Trees: Drought-Stressed Sequoias and Blue Oaks May Start to Vanish

By Sabine Bergmann

Todd Dawson’s research has taken him to forests, savannas, and deserts all over the world. But his recent investigations close to the UC Berkeley campus have taken him to the edges where ecosystem types transition.

The professor of Integrative Biology has found that the blue oaks (Quercus douglasii) at Berkeley’s Blue Oak Ranch Reserve near San Jose and the giant sequoias (Sequoiadendron giganteum) in the Giant Forest of the Sierra Nevada are so drought-stressed that they may begin to disappear from the landscapes they currently define.

The current California drought is the most severe drought on record and is thought to be aggravated by human-caused climate change. Global climate change has the power to shift climate conditions so quickly that even species that are adapted to severe events like drought may not be able to adapt in time.

Are the blue oaks in trouble?

What we’ve seen with the oaks is unprecedented [in our 15 years of research at the Reserve]. Soil moisture levels are extremely low even for drought-tolerant species. It may be going below what these plants can tolerate. They are dropping leaves like crazy and we could be pushing them to a tipping point. There’s no way to know what will happen until it happens, but we could see a conversion to non-native shrub lands—and not just in the oak groves but in other ecosystems too. We’re rolling the dice here. In a way we’re measuring history right now.

What about the redwoods?

Where the giant sequoia is in the Sierra Nevada is a system that’s driven largely by snowpack. Those guys are suffering big-time this year because the snowpack was only 40 percent of normal. They’re seeing water stress levels that we’ve never measured before. We may lose them. Not right away—they live a long time, they’re incredibly tenacious and very drought tolerant. But again, are we going to reach one of these tipping points where our snowpack gets so low that the groundwater falls below the root systems of these trees and they begin to die?

The blue oaks and giant sequoia are just another example of species that are endangered by climate change. Is this kind of human-driven ecosystem shift unprecedented?

Think about where forests used to be. What about present-day Iraq? The Tigris River runs through there. When that was Mesopotamia, those were cedar forests and they cut them all down. They modified the landscape so dramatically that it became a desert. You would never have thought of that by looking at today’s map of forests. There are those kind of transformations in past history, and you wonder: Are we entering into some more of those when we start seeing these extreme events? Will we tip again?

But that wouldn’t happen in a single year, would it?

Probably not. But something had to get it going. It could be these severe drought kind of years that push it into a new state. It could be that it takes another decade or 50 years for (these forests) to really go through a massive transformation into a whole new ecosystem or a whole new appearance.

Can anything be done?

We could intervene. We could go to Oregon and set aside land and say, “This is where redwoods will be in the next 500 years.” We may have to physically move organisms to new locations. I think that we need to talk about this because climate and land-use change is happening at such a rapid pace that human hands are going to have to come in at some stage if we don’t want to see things go extinct. We’re never going to save the polar bears because we can’t create ice caps but we might be able to save forests and the species living in those forests. We’re not going to lose redwood trees, but we may have to cope with their living in a new location.

From the Winter 2014 Gender Assumptions issue of California.
Filed under: Science + Health
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Your article claims: “The current California drought is the most severe drought on record and is thought to be aggravated by human-caused climate change. Global climate change has the power to shift climate conditions so quickly that even species that are adapted to severe events like drought may not be able to adapt in time.” I am not sure that our drought “is the most severe drought on record,” but I am more concerned about your attribution to “human-caused climate change.” The IPCC technical report does not find a global trend in drought, and so to assert that California’s drought is “aggravated by human-caused climate change” is inconsistent with those studies. The IPCC report “In summary, the current assessment concludes that there is not enough evidence at present to suggest more than low confidence in a global-scale observed trend in drought or dryness (lack of rainfall) since the middle of the 20th century, due to lack of direct observations, geographical inconsistencies in the trends, and dependencies of inferred trends on the index choice. Based on updated studies, AR4 conclusions regarding global increasing trends in drought since the 1970s were probably overstated. However, it is likely that the frequency and intensity of drought has increased in the Mediterranean and West Africa and decreased in central North America and north-west Australia since 1950.” Also, consider: Little change in global drought over the past 60 years, Justin Sheffield, Eric F. Wood & Michael L. Roderick Nature 491, 435–438 (15 November 2012)doi:10.1038/nature11575 “More realistic calculations, based on the underlying physical principles that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years. The results have implications for how we interpret the impact of global warming on the hydrological cycle and its extremes, and may help to explain why palaeoclimate drought reconstructions based on tree-ring data diverge from the PDSI-based drought record in recent years.” The California Magazine has an unfortunate bias to accept without discussion or careful analysis, various claims that anthropogenic co2 emissions have caused or will cause substantial harm. If you read and think carefully about this subject, and if you are not fully invested in the partisanship and advocacy that has arisen, pro and con, about “global warming”, you will see it is very difficult to scientifically establish that there have been or will be substantial effects from anthropogenic co2 emissions on climate. The co2 effects (e.g., 2-3 wm2) appear to be very small compared to large, known drivers of climate change (e.g. 100 wm2 Milankovitch cycles), but the issue is far from resolved. How about a more enlightened article next time?
Climate change is changing weather patterns globally and may make California’s natural cycles much more varied and severe. Giant sequoias have withstood extreme drought before and yes with some significant die back, but they did so without the human overdraft on the aquifers. With a four foot root system tied to shallow ground water, these behemoths have survived many of natures cycles, but with groundwater wells and stream diversions, I am concerned that the sequoias may not survive the enormous impacts of a population that shows no sign of slowing its exponential growth. As for blue oaks, soil type seems pretty restrictive but they are stress deciduous, so it is hard to tell until the next spring whether a tree has died or not. Ecosystems are dynamic and we need to stop managing nature and concentrate on managing people, that is the only way for nature to survive the constant abuse by humanity.
Alison may have a good point: human over drafting of water from the aquifers may have an effect on the sequoias and perhaps oaks. I think over drafting has more direct effects on agricultural crops since irrigation becomes more difficult and expensive. Accordingly, I am unsure about the effect on mountain or costal Redwoods but I suppose it is possible. But that was not a contention of the article. Over drafting of water is not associated with “global climate change.” Instead, it is AGHG (anthropogenic greenhouse gas) emissions that are considered causes of adverse global climate change, and the case for AGHGs causing the California drought is very weak. Not to pile on, but in addition to the IPCC and Nature studies I recited above, this month, the Climate Program Office of NOAA released “Causes and Predictability of the 2011-14 California Drought”, Seager et al ( The report finds: “The current drought, though extreme, is not outside the range of California hydro-climate variability and similar events have occurred before. Although there has been a drying trend in California since the late 1970s, when considering the full observational record since 1895, there is no appreciable trend to either wetter or drier California winters.” Also the report found that “that human-induced precipitation changes to date across North America are small compared to natural interannual variability.” The reachers looked at predictions from the coupled climate models (CMIP5) - the ones used to support the “alarmist” view of the effects of AGHG emissions. This study also found that “The circulation anomalies during the recent California drought are therefore also not consistent with (CMIP5) model projections of human-driven circulation anomalies. The radiatively-forced reduction in precipitation for the current decade is less than 0.1 mm/day, an order of magnitude smaller than the anomalies that occurred in California in the recent drought, and also smaller than the drying forced by SST anomalies.”
Eloquent disinformation at its best. The overwhelming scientific consensus is that we are witnessing the growing impacts of a rapid shift in global climate patterns driven by the unrelenting burning of fossil fuels. This is just the beginning.
Ned, Yes I know this is an old post, but I wanted to correct your facts nontheless. You quoted an reference in the Climate Program Office of the NOAA’s website, but you conveniently ignored the last sentence of the report: “Nonetheless, record setting high temperature that accompanied this recent drought was likely made more extreme due to human-induced global warming.” So while it may be true that the burning of fossil fuels didn’t directly cause the recent California drought - it exacerbated it. In the central valley trees that otherwise might have survived a natural but severe drought (e.g. blue oaks, valley oaks, live oaks) did not. Rises in average and record temperatures will have a devastating effect on the landscape of California - this will be due to humans not natural hydro climate variability.
Hello Ossa, Below is the full passage from the report. There is no finding that the drought is due to anthropogenic co2 emissions. The last line does not address the drought, but makes a claim not part of the study about the very complicated issue of attribution of the cause of rising temperatures. In fact, you will see the models predicted greater precipitation, and there was less. So the same models are used for attribution of the claimed small increase in average temperature, apx 1 degree per 100 years, to increases in co2 apx 100 parts per million over 200 years. The simple inference is that the models are not accurate in predicting precipitation and hence we should be cautious in believing they are accurate in claiming that small increases in co2 (vs. all GHGs) cause small increases in average temperature, especially where the temperature changes may not be unusual for an interglacial period. The more interesting issue to me is the beneficial effects of increases in atmospheric co2 on plant growth, even when there is a drought and even with higher or lower temperatures. There are many well controlled studies that show increased co2 have significant beneficial effects, but again, this is off the specific topic of attributing drought to co2 which has very little support. “The current drought is not part of a long-term change in California precipitation, which exhibits no appreciable trend since 1895. Key oceanic features that caused precipitation inhibiting atmospheric ridging off the West Coast during 2011-14 were symptomatic of natural internal atmosphere-ocean variability. Model simulations indicate that human-induced climate change increases California precipitation in mid-winter, with a low-pressure circulation anomaly over the North Pacific, opposite to conditions of the last 3 winters. The same model simulations indicate a decrease in spring precipitation over California. However, precipitation deficits observed during the past three years are an order of magnitude greater than the model simulated changes related to human-induced forcing. Nonetheless, record setting high temperature that accompanied this recent drought was likely made more extreme due to human-induced global warming.” Thanks for the interest in the subject. Folks should civilly discuss such matters.

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