Giant Sequoias • Yosemite • Hidden Water • New Research
Giant sequoias can use more than 2,000 litres of water per day during California’s long dry summers. New subsurface imaging beneath Yosemite’s Mariposa Grove suggests that trees growing on dry hillsides and ridges may survive by accessing water stored more than 2 metres deep inside weathered bedrock.
A giant sequoia can drink an extraordinary amount of water.
During California’s long, rainless summers, water use by a mature tree can exceed 2 cubic metres per day — more than 2,000 litres.
That creates an obvious problem.
Some of these enormous trees grow beside drainages where moisture near the surface makes sense.
But others stand on dry hillsides and ridge tops, far from obvious streams and shallow groundwater.
So where does all that water come from?
Part of the answer appears to be hidden inside the rock beneath the forest.
Researchers have now imaged the subsurface beneath the Mariposa Grove of Giant Sequoias in Yosemite National Park and found that the underground water landscape changes dramatically depending on where the trees grow.
Near drainages, much of the water-rich material occurs close to the surface.
But beneath hillslopes and ridges, important reservoirs occur deeper down, within weathered bedrock more than 2 metres below the ground.
The findings suggest that giant sequoias may send extensive root systems deeper into the subsurface than previously appreciated.
A Giant Tree Can Use More Than 2,000 Litres of Water a Day
Giant sequoias (Sequoiadendron giganteum) are among the largest living organisms on Earth.
Their enormous crowns contain vast amounts of foliage, and keeping that foliage supplied with water creates equally enormous hydraulic demand.
According to the new study, individual giant sequoias can use more than approximately 2 m³ of water per day during the dry season.
That is especially remarkable because their natural habitat experiences a Mediterranean-style climate: wet winters followed by long, dry summers.
Rainfall declines sharply just when atmospheric demand for water becomes intense.
Yet these trees can live for thousands of years.
Their survival therefore depends on accessing water that remains available long after the surface soil has begun to dry.
Giant sequoias already belong among the extraordinary biological adaptations covered in our Trees & Forest Oddities guide.
Scientists Looked Beneath Yosemite’s Mariposa Grove
To find the hidden water, researchers studied the subsurface beneath Yosemite National Park’s famous Mariposa Grove.
Instead of digging around the roots of ancient trees, they used a geophysical method known as elastic full-waveform inversion.
In simplified terms, the technique uses seismic waves to build a detailed picture of physical properties beneath the ground.
The researchers then combined those seismic measurements with geostatistical rock-physics modelling to estimate how much water was stored within the subsurface.
The goal was not merely to find groundwater.
It was to map water stored throughout the entire weathered zone beneath the forest — including moisture held inside fractured and decomposed bedrock.
The Underground Water Pattern Changed With the Landscape
The resulting maps revealed a striking contrast.
Near drainages and low-lying areas, zones with high water content were concentrated mainly within approximately the upper 2 metres of the subsurface.
That is relatively intuitive.
Water naturally accumulates in lower parts of landscapes, where drainage and shallow subsurface flow concentrate moisture.
But the trees growing higher on the landscape told a different story.
Beneath hillslopes and ridge tops, high-water-content zones tended to occur deeper or in more isolated pockets.
In other words, the driest-looking parts of the forest may conceal some of their most important water stores deeper underground.
The Trees May Be Drinking From Weathered Bedrock
This deeper water is not necessarily a conventional underground aquifer.
Much of it appears to be what geologists call rock moisture: water retained within pores, fractures and weathered zones inside bedrock.
Bedrock near Earth’s surface is rarely perfectly solid.
Over time, chemical alteration, cracking, biological activity and physical weathering create networks of fractures and weakened minerals capable of storing water.
This weathered layer can therefore function as a hidden reservoir between ordinary soil above and fresher, less altered rock below.
The study concludes that giant sequoias growing on hillslopes and ridge tops probably access substantial volumes of rock moisture deeper than 2 metres to avoid severe summer water stress.
This Does Not Mean Scientists Saw Individual Roots Drinking From the Rock
There is an important distinction.
The geophysical images map subsurface water.
They do not directly photograph individual sequoia roots entering every water-rich pocket.
Instead, the researchers compared the distribution and estimated volume of subsurface water with the enormous water requirements of the trees.
Their conclusion is that hillslope and ridgetop sequoias probably access these deeper stores, implying that their extensive rooting systems probe deeper into the subsurface than previously thought.
So the evidence strongly supports deep water use, but it should not be interpreted as a complete map of individual roots.
Why Weathered Rock Can Hold So Much Water
At first glance, describing a tree as getting water from rock sounds strange.
But near-surface bedrock can be surprisingly porous.
Fresh granite may contain relatively little accessible water, while fractured and chemically weathered granite can contain cracks, mineral boundaries and decomposed zones that trap substantial moisture.
Winter rain and snowmelt can infiltrate these deeper layers.
Because the water is stored below the rapidly drying surface soil, some of it may remain available far into summer.
For a giant sequoia growing high above a drainage, that hidden reservoir could mean the difference between prolonged water stress and continued access to moisture.
Roots Do Not Stop Where Soil Ends
One of the broader lessons from the study is that the biological world does not always respect the convenient boundary humans draw between soil and rock.
Plant roots can exploit fractures, weathered minerals and weak zones beneath the soil profile.
Microorganisms and fungi can also colonize cracks in the subsurface, helping turn apparently solid geological material into part of the living ecosystem.
This transition zone between vegetation, soil and bedrock is sometimes described as part of Earth’s critical zone — the near-surface environment where rock, water, air and life interact.
Giant sequoias may represent one of its most spectacular examples.
Why Ridge-Top Sequoias Are Especially Interesting
Trees near streams or drainages have an obvious potential source of water.
Ridge-top sequoias are more puzzling.
Gravity generally moves water away from elevated terrain, and shallow soils exposed to summer heat can dry quickly.
Yet enormous trees still survive there.
The subsurface maps suggest that these trees may exploit deeper, spatially isolated pockets of moisture that cannot be inferred simply by looking at surface topography.
What appears to be a dry ridge at the surface can therefore contain a very different hydrological environment several metres below.
These Hidden Reservoirs May Help Explain Sequoia Longevity
Giant sequoias can survive for millennia.
During that time they experience repeated droughts, heat waves, fires, storms and periods of unusually low precipitation.
Thick bark and fire adaptations explain part of their resilience.
But survival also requires maintaining the continuous movement of water from roots to leaves through an organism that can reach extraordinary size.
Deep rock moisture may provide a relatively persistent buffer when shallow water stores become depleted.
That doesn’t make the trees invulnerable to extreme drought.
It does suggest that the effective rooting environment of a giant sequoia may extend much farther beneath the visible forest floor than previously assumed.
Hidden Water Beneath Forests Could Matter Beyond Yosemite
The implications are broader than giant sequoias.
Ecologists commonly measure soil moisture when assessing how forests respond to drought.
But if trees can obtain substantial quantities of water from weathered bedrock below conventional soil profiles, shallow measurements may miss an important part of the water actually available to vegetation.
That matters for understanding forest drought resistance, ecosystem water use and how landscapes respond to prolonged dry periods.
The Mariposa Grove study demonstrates how geophysics can help reveal those hidden ecological reservoirs without excavating vast areas around tree roots.
A Forest Mystery Hidden Below the Surface
Above ground, a giant sequoia is almost impossible to ignore.
Its trunk can dominate the forest.
Its crown can rise tens of metres overhead.
Its lifespan can stretch across thousands of summers.
But one of the secrets that allows such an enormous organism to survive may be almost completely invisible.
Several metres beneath the forest floor, fractured and weathered rock can preserve water long after the surface dries.
And the new study suggests that giant sequoias growing in some of Yosemite’s driest-looking locations may be reaching into those reservoirs.
The giant tree above the ground may depend on an equally remarkable hidden landscape beneath it.
Frequently Asked Questions About Giant Sequoias and Bedrock Water
How much water can a giant sequoia use in one day?
A mature giant sequoia can use more than approximately 2 cubic metres of water per day during dry summer conditions. That’s equivalent to more than 2,000 litres of water in a single day.
Where do giant sequoias get water during California’s dry summers?
New research from Yosemite’s Mariposa Grove suggests that giant sequoias growing on hillsides and ridges may access substantial rock moisture stored more than 2 metres below the surface within weathered bedrock. These deeper reservoirs may remain available after shallow soils begin to dry.
What is rock moisture?
Rock moisture is water stored within pores, fractures and weathered zones inside bedrock. Unlike ordinary surface soil moisture, some of this water can remain available deeper underground even after shallow soils have dried during a long, rainless summer.
Did scientists directly see sequoia roots inside the bedrock?
No. The researchers imaged subsurface water rather than individual tree roots. However, the location and volume of deeper water imply that giant sequoia root systems probably probe deeper into the weathered subsurface than previously appreciated.
Why do ridge-top giant sequoias need deep water?
Ridge tops and hillsides tend to lose shallow water more quickly than drainage areas. Yet enormous sequoias can survive in these apparently dry locations. Deeper pockets of moisture inside weathered bedrock may provide a more persistent source of water during California’s long dry summers.
Can weathered bedrock store large amounts of water?
Yes. Bedrock near the surface is not always solid and impermeable. Weathering and fracturing can create pores, cracks and weakened mineral zones that store substantial amounts of water beneath the conventional soil profile.
Why is this discovery important beyond giant sequoias?
The findings suggest that forests may rely on water stored much deeper than conventional soil-moisture measurements capture. Understanding this hidden water could improve scientists’ estimates of tree drought resistance, ecosystem water use and forest resilience during prolonged dry periods.
