By John Upton
Large, old trees sporting gaping holes in their trunks are like empty nesters who open their homes and hearts to foster kids, or to help raise grandchildren.
Tree cavities provide homes for all manner of life, including mushrooms, birds, bats and possums. They can provide respite from squalls and from scorching or freezing temperatures, sculpting microenvironments and boosting biodiversity.
But these holes are disappearing.
Tree holes normally arise over painstakingly long periods, sometimes with the assistance of woodpeckers, termites or fungus. They are most commonly found in the most senior members of a tree stand. Sometimes, cavity-pocked trees are dead trees that remain standing for decades.
Scientists warn that climate change, logging and other human activities are prematurely felling the world’s oldest trees, taking their cavities down with them. Droughts and intense fires are taking heavy tolls.
“The loss is global,” warned a team of foresters and other scientists in a paper published Friday in the online journal PLOS ONE.
“Many ecosystems worldwide are increasingly characterized by the rapid loss of large trees with cavities, a failure to recruit new trees with cavities, or both,” the scientists wrote. “Many kinds of human disturbances cause this problem, including recurrent logging, altered fire regimes, grazing by domestic livestock, and the impacts of exotic plants.”
The scientists monitored populations of Eucalytpus trees in the southeastern Australian state of Victoria, some of which were growing in areas hit hard during unprecedented bushfires in 2009 that killed 173 people. They found an alarming decline in the number of cavity-bearing specimens.
In areas severely impacted by fires, the number of cavity-bearing trees declined from 138 in 2006 to 42 in 2012, greatly diminishing the number of holes available for use by Leadbeater’s possums and other creatures. But the losses were also staggering in areas unaffected by fire: The number of cavity-bearing trees in these areas fell from 414 in 1997 to 159 in 2011.
“There was a heat-induced, drought-induced mortality spike in the deaths of large trees,” lead researcher David Lindenmayer of the Australian National University told me in an email. “These kinds of findings are seen in many kinds of forests worldwide.”
Perhaps even more alarmingly, during this period of disappearing holes, not a single new tree cavity was formed in the study area during the entire 15 years of research. It’s as if hundreds of homes were razed but no new ones were built to replace them.