Robert Sanders, Media relations|June 24, 2019
You might expect that plants hoping to thrive in California’s boom-or-bust rain cycle would choose to set down roots in a place that can store lots of water underground to last through drought years.
But some of the most successful plant communities in the state — and probably in Mediterranean climates worldwide — that are characterized by wet winters and dry summers have taken a different approach. They’ve learned to thrive in areas with a below-ground water storage capacity barely large enough to hold the water that falls even in lean years.
Surprisingly, these plants do well in both low-water and rainy years precisely because the soil and weathered rock below ground store so little water relative to the rain delivered.
“The key point from our study is that, in many sites on the North Coast, the storage capacity is small relative to how much it rains,” said Jesse Hahm, a graduate student at the University of California, Berkeley, and one of two first authors of the study. “Because the capacity for the subsurface to store water over the wet season is small, it still rains enough, even in the dry years, to replenish the water supply. The limited below-ground storage capacity is the key mechanism that decouples the plants and how much water availability they have in the summer from big swings in winter rainfall.”
As a result, these plants are much more resilient in drought years, as evidenced by California’s relatively unscathed North Coast during recent droughts that killed hundreds of millions of trees in the Sierra Nevada.
“Because the subsurface water gets replenished even in drought years, in the summer these plants feel the same amount of water supply below ground, no matter how much rain fell during the winter,” Hahm said. “They don’t really know if it rained a lot or a little, because they have the same amount of water stored below ground each summer.”
On the flip side, plants growing today on ground that can soak up as much water as the winter rains can provide are hosting plants that will have to deal with the state’s increasingly drier climate, putting them at risk as the climate changes. This may be a problem for Sierra Nevada plant communities that are relying less on a persistent snowpack and increasingly on stored subsurface water to last through the dry summer.
Hahm and David Dralle, the other first author and a former Berkeley graduate student who is an assistant professor at Sacramento State University, describe their findings, along with their colleagues, in a paper recently accepted by the journal Geophysical Research Letters and now posted online.
While most people think plants rely only on water stored in the topsoil, Berkeley’s William Dietrich, professor of earth and planetary science, and recent graduate Daniella Rempe, an assistant professor at the University of Texas, Austin, recently discovered that water stored in fractured and weathered rock underneath the soil plays an equal or greater role. What Dietrich and Rempe call “rock moisture” can amount to a significant proportion of what plants rely on annually.
A major implication of the new study, Dietrich says, is that global climate models need to incorporate rock moisture into their calculations to accurately represent and predict the impacts of drought or heavy rainfall. In recent years, drought- or heat-killed trees have fueled catastrophic wildfires in California, Spain, Greece, Australia and many regions with a dry, Mediterranean climate.
“Understanding how water is stored deep within the weathered bedrock and how variations in that water supply and in rainfall affect plant water supply in that zone is extremely important in a seasonally dry climate,” Hahm said.
In their study, the researchers looked at 26 sites statewide. All were below the snow belt, so that winter rain stored below ground was the dominant source of water for the plants during the summer dry season. Using rainfall data and U.S. Geological Survey stream flow data to calculate the amount of water stored annually underground, they were able to assess the below-ground storage capacity of the soil and the weathered rock.
Of the 26 sites, only seven — all in the Northern Coast Ranges — had limited subsurface water storage capacity and fared well during the state’s recent protracted drought, between 2011 and 2016. These sites ranged from grass and oak savanna and chaparral to dense Douglas fir forests, but all were characterized by low subsurface storage relative to average annual rainfall in the area, which tends to be high. The excess water that the subsurface couldn’t store in the winter ran through the soil and fractured bedrock and ended up in the streams.
The other sites, including most sites in Southern California, suffered in the drought, with vegetation die-offs and less healthy, less green plants. All were characterized by below-ground storage that is sufficient to sop up most of the rainfall that falls yearly, but that had been left depleted in drought years.
Using satellite images to gauge the productivity and health of the vegetation at each site, the researchers concluded that the sites with high relative storage capacity were the ones that varied the most between wet and dry years in how green the plants were. Sites with low below-ground storage capacity relative to average annual precipitation fared better, remaining similarly green and healthy in drought years and wet years alike.
Hahm noted that many plants in the Sierra Nevada rely on the snowpack to quench their thirst during typical rainless summers. But as temperatures rise with global warming, winter precipitation will increasingly occur as rain.
“In a way, this is a glimpse into the future,” Hahm said. “As the climate warms, and as the snowline elevation increases in these mountain ranges, more and more places will switch from being reliant on snowpack to being reliant on water stored in the subsurface. Understanding how this storage capacity limitation will impact plants across the state in high montane areas needs to be explored more.”
The insights about rock moisture emerged from a long-term project at the Angelo Coast Range Reserve in Northern California, part of the UC Natural Reserve System, where scientists at the Eel River Critical Zone Observatory followed water from the sky through vegetation, soil and rock into the streams and back up into the atmosphere via evaporation and transpiration to chart the life cycle of water in the environment. Primary funding for the observatory, which Dietrich directs, comes from the National Science Foundation (EAR 1331940).
Other co-authors of the study are graduate student Alexander Bryk and Todd Dawson, professor of integrative biology, both from Berkeley, and Sally Thompson of the University of Western Australia.
- Low subsurface water storage capacity relative to annual rainfall decouples Mediterranean plant productivity and water use from rainfall variability (Geophysical Research Letters)
- Lithologically Controlled Subsurface Critical Zone Thickness and Water Storage Capacity Determine Regional Plant Community Composition (Water Resources Research)
- Hidden ‘rock moisture’ may be key to tree survival during drought (February 2018)
In a new paper in the journal Proceedings of the National Academy of Sciences, paleontologist Robert DePalma and his colleagues, including Walter Alvarez a Professor of the Graduate School and Professor Mark Richards from University of California, Berkeley Earth and Planetary Sciences, describe the site, dubbed Tanis, and the evidence connecting it with the asteroid or comet strike off Mexico’s Yucatan Peninsula 66 million years ago. That impact created a huge crater, called Chicxulub, in the ocean floor and sent vaporized rock and cubic miles of asteroid dust into the atmosphere. The cloud eventually enveloped Earth, setting the stage for Earth’s last mass extinction.
“It’s like a museum of the end of the Cretaceous in a layer a meter-and-a-half thick,” said Mark Richards, a UC Berkeley professor emeritus of earth and planetary science who is now provost and professor of earth and space sciences at the University of Washington.
Richards and Walter Alvarez, a UC Berkeley Professor of the Graduate School who 40 years ago first hypothesized that a comet or asteroid impact caused the mass extinction, were called in by DePalma and Dutch scientist Jan Smit to consult on the rain of glass beads and the tsunami-like waves that buried and preserved the fish. The beads, called tektites, formed in the atmosphere from rock melted by the impact.
Read the full article here
(Graphic courtesy of Robert DePalma)
Edwards Lab heads to sea to collect particulate lipid samples from the San Pedro Basin, halfway between Long Beach and Catalina Island. These samples will be used to study the microbial biology of the area and their impacts on ocean biogeochemistry. Learn more at the lab's website, https://www.bethanieedwardslab.com
Photos courtesy of Will Kumler, Edwards Lab Manager.
CIDER is a yearly interdisciplinary research incubator made possible by @BerkeleySeismo's Dr. Barbara Romanowicz, where Earth dynamicists collaborate on interdisciplinary new ideas like this:
Explore EPS summer classes! UC Berkeley is an open university during summer please review our schedule here.
This summer, you can gain an overview of the water supply that controls our natural ecosystems and human civilization in EPS 3 "The Water Planet."
Ever wonder what are our planets made of? Why do they orbit the sun the way they do? Why do some bizarre moons have oceans, volcanoes and ice? You can take a tour of the mysteries and inner workings of our solar system in EPS W12 "The Planets", an online course offering.
A very popular course, EPS 20 "Earthquakes in Your Backyard" gives students an introduction to seismology and geological tectonics, with particular emphasis on the situation in California.
EPS N82 "Introduction to Oceans" teaches students the geology, physics, chemistry and biology of the world oceans. The course will apply oceanographic sciences to human problems to explore topics such as energy from the sea, marine pollution, food from the sea, and climate change.
For information on these topics and other departmental course offerings, please click here.
UC Berkeley News uploaded a new video onto YouTube of Earth and Planetary Science Professor Jim Bishop explaining how he and his research team are utilizing robots to collect data on climate change.
Check out the USS Oceanus’ blog on the Lawrence Berkeley National Laboratory site: http://oceanbots.lbl.gov/.
Sarah Yang continues to update the research team's blog, so video interviews from scientists and crew will be posted on the site periodically. The scientists were very fortunate to have Sarah Yang to be at sea with them, as she was able to collect some cool videos and other materials. Remember, scientists were at sea for 10 days with ocean-going robots to measure carbon dioxide in the ocean and, hopefully, to unlock important data about climate change. For further access to Professor Jim Bishop and his team of researchers, go to the blog!
The EPS 118 Advanced Field Geology class is the capstone experience for geology majors in Earth and Planetary Science. The 2016 field camp took place at the Sierra Nevada Aquatic Research Laboratory (SNARL) in Mammoth Lakes, CA from June 3rd - July 1st. Students spend four weeks in the field integrating all of the training and knowledge gained in their geology coursework to address a large but focused question, and produce a map to answer scientific questions.
This video was taken by the EPS 118 students to show their camaraderie and experience in the field.
In fall 2016, Daniella Rempe joins the faculty of the University of Texas, Austin; she begins her tenure as an assistant professor in the Jackson School of Geosciences. Daniella is currently completing her graduate work with Professor William E. Dietrich in the UC Berkeley Department of Earth & Planetary Science.
Daniella specializes in hydrologic field observations, fluid flow and near surface geophysics. In layman's terms, she is obsessed with water; how it travels through rock; what it picks up along the way; and how water transforms the environment. She focuses on how landscapes store water in the shallow subsurface, a particularly relevant topic seeing how much of Earth's hilly regions are mantled with weathered rock. Daniella especially looks at the ecological significance of rock moisture; controls on the bottom boundary of the Critical Zone; and geophysical imaging of landscape scale patterns of weathering.
Daniella is proud to be a native Texan with the stupendous opportunity to take a teaching and research position at the flagship university. She was born in Houston, called Plano home for her secondary school years and then, obviously, lived in Austin for college. It must have been a case of serendipity for her to spend her undergraduate years in Central Texas, as Daniella credits her visits to Barton Springs as piquing her interest in water and hydrology. Barton Springs is a natural water-fed swimming hole playing host not only to sunbathers and swimmers but also to important geological processes such as faulting and the dissolution of limestone by infiltrating water. It was at Barton Springs that Daniella discovered her fascination and obsession with water.
Daniella will join the faculty of the Jackson School of Geosciences at UT as a hydrologist and geomorphologist.
For information on the Rempe Research Group, click here.
The work of Professor Barbara Romanowicz and recent graduate Dr. Scott French is highlighted on Berkeley News Center's page from earlier this month.
"University of California, Berkeley, seismologists have produced for the first time a sharp, three-dimensional scan of Earth's interior that conclusively connects plumes of hot rock rising through the mantle with surface hotspots that generate volcanic island chains like Hawaii, Samoa and Iceland."
Continue reading the full story on the Berkeley News website.
Applying a new waveform imaging methodology that takes advantage of accurate numerical seismic wavefield computations, Barbara Romanowicz's group has constructed a global shear velocity model in the upper mantle that reveals the presence of low velocity channels at the base of the oceanic asthenosphere. In a paper recently published in Science (http://www.sciencemag.org/content/342/6155/227), graduate student Scott French, former graduate student Vedran Lekic (now assistant professor at the University of Maryland) and Barbara Romanowicz show that these quasi-periodic finger-like structures of wavelength ~2000 km, stretch parallel to the direction of absolute plate motion for thousands of kilometers. Below 400 km depth, velocity structure is organized into fewer, undulating but vertically coherent, low-velocity plume-like features, which appear rooted in the lower mantle. This suggests the presence of a dynamic interplay between plate-driven flow in the low-velocity zone, and active influx of low-rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer. Hotspots are not the direct consequence of plumes impinging on the lithosphere