Cavitation in trees is when trees get air (or gas) bubbles in the xylem (which is similar to their blood vessels and carries sap up and down the trees). The cavitation can block the xylem so the tree can not get any fluid from that xylem path.
It's a common event with the changes of season, but it also happens under extreme stress- like this drought.
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This is related:
The destructive power of sound waves2013
Researchers at the University of Arizona College of Engineering have come up with a novel way to help the U.S. Air Force dispose of stockpiles of dangerous chemicals – using nothing more than sound waves.
Manish Keswani, an assistant professor in the Department of Materials Science and Engineering, and Reyes Sierra, a professor in the Department of Chemical and Environmental Engineering, have been awarded a $243,000 contract by the Air Force Civil Engineering Center to destroy the chemicals using a novel sonochemical process,
which uses sound waves to break down complex and toxic molecules into nothing more than carbon dioxide and water.
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Sonolysis relies on the process of cavitation for its success," Keswani said. "Under certain conditions, sound waves cause the formation of small bubbles that rapidly implode and release an intense shock wave that
produces enormous amounts of heat energy and a variety of highly active radicals, which can completely destroy adjacent material."
Cavitation is used in certain medical procedures and is also found in nature. Shock-wave lithotripsy relies on cavitation to destroy kidney stones, and mantis shrimps use cavitation when hunting their next meal. The shrimps strike with such velocity – about the speed of a bullet after it's fired – that they generate cavitation bubbles in the water between themselves and their target. Even if they don't make a direct strike, the resulting shock waves are enough to stun or kill their prey.
The heat energy unleashed by cavitation breaks down the bonds that tie large molecules together, such as the perfluoroalkyl sulfonates and carboxylates, or PFCs, found in fire-fighting foams. These toxic PFCs are hard to break down and tend to persist in the environment, and in body tissue, which is why the Air Force will be investigating cavitation as a cost-effective method of producing temperatures in excess of 10,000 degrees Fahrenheit, more than enough to incinerate the problem chemicals.
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Now more on trees...
Cavitation in trees2006
Sap is transported under tension (i.e. negative pressure) in trees, according to the tension-cohesion theory. Since water is physically instable under negative pressure, a risk of cavitation is possible. Techniques have been developed during the past two decennia to study cavitation in trees. Trees appear remarkably immune to cavitation events.
Cavities form only when extreme water stresses occur or when sap freezes. Nucleation is heterogeneous in trees, presumably caused by the aspiration of air bubbles through conduit walls. Threshold xylem pressures for cavitation vary greatly between species, in concordance with the great functional and ecological diversity of trees
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Researchers measure cavitation noise in trees2013
A team of researchers from Grenoble University in France has found that under experimental conditions, roughly half of the noise created by drying wood is due to cavitation. The team made this discovery while studying the noises trees make in drought conditions.
(Phys.org) —A team of researchers from Grenoble University in France has found that under experimental conditions, roughly half of the noise created by drying wood is due to cavitation. The team made this discovery while studying the noises trees make in drought conditions. They presented their findings at last month's American Physical Society meeting.
Scientists have known for many years that trees make noise, and not from just the creaking that occurs as wind pushes them back and forth. Trees also emit noise that is too high in frequency for the human ear to hear. Past research suggests that the noises trees make change if they're not getting enough water, and at least some of that noise is likely due to cavitation.
Cavitation occurs when air bubbles form in the tubes (xylem) that run up and down tree trunks, preventing water from being pulled upward—in some cases it causes the tree to die. What has remained a mystery, however, is how much of the noise coming from trees during times of drought stress is due to cavitation, and how much from other sources, such as cell breakage.
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Results of our meta-analysis confirm that
poplars are among the most vulnerable woody species to drought-induced cavitation (mean P50 = -1.44 and -1.55 MPa across pure species and hybrids, respectively). Yet, significant variation occurs among species (P50 range: 1.43 MPa) and among hybrid crosses (P50 range: 1.12 MPa), within species and hybrid crosses (max. P50 range reported: 0.8 MPa) as well as in response to environmental factors including nitrogen fertilization, irradiance, temperature and drought (max. P50 range reported: 0.75 MPa). Potential implications and gaps in knowledge are discussed in the context of poplar cultivation, species adaptation and climate modifications. We suggest that poplars represent a valuable model for studies on drought-induced cavitation, especially to elucidate the genetic and molecular basis of cavitation resistance in Angiosperms.
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TL;DR:
Cavitation can be a normal event. Trees can self correct in a normal environment.
Under drought conditions, trees are more prone to cavitation. (Basically tree strokes). When the cavitation/ embolism breaks (pops), this causes some of the tree noises you hear in a drought ridden forest. The sound wave, and subsequent shock wave, release a burst of heat energy. Multiple heat energy events like this raise the temperature of the forest. It's a continuous cycle until fire or bugs or rain destroy or save the trees. 