An astonishing 12.5m trees have died in California, unable to survive a harsh fourth year of drought, according to a US government study.

The news of the massive tree die-offs came this week, after the United States Forest Service, a Department of Agriculture agency, released the results of an aerial survey it undertook in April over 8.2m acres of forest. The survey was organized three months ahead of schedule.



“The special early season aerial survey was prompted by knowledge of the worsening drought situation and reports from field crews that copious amounts of new mortality had appeared after the regular survey was flown in July of 2014,” explained Jeffrey Moore, a biologist with the agency who was one of the surveyors on the expedition.

The California drought has now entered its fourth year. On 1 April, California governor Jerry Brown imposed the first set of mandatory water use restriction measures – a step never before taken in the state’s history. Brown said the drought had reached “near-crisis” levels.



The dead trees add to the flammability of a drying landscape that is increasingly threatened by large, intense wildfires. Photograph: David McNew/Getty Images

Moore, the US Forest Service biologist, called the death of millions of trees “a cause for concern”.



Aerial photos from the survey show a landscape of red, purple and brown trees – the dead ones – interspersed among the usual green. Even those green ones were sometimes lighter and yellower than usual, a sign of distress.

These are a mix of colors more usually associated with autumn in New England.

But the scenes depicted were hardly romantic. Yellow and pale beige dry-looking earth laid the scene for dead pine and oak trees, still standing among their struggling-to-survive forest companions.

The sudden prevalence of such an abnormal amount of dead trees in California’s forests – a scale of loss not seen since the 1970s droughts – has raised concern more broadly about the increased risks of wildfires.

“Increased fire risk, already extremely high, at least over the short term is also a serious concern. Freshly killed trees are dry tinder that have the potential to make fires hotter and spread faster,” Moore said.

Fire fighters battled more than 5,000 wildfires last year in California, over a thousand more than usual.

Last Saturday marked National Wildfire Preparedness Day.

Appearing before the press to raise awareness about the upcoming wildfire season, California fire department chief Ken Pimlott not only congratulated those residents living close to forest areas for not wasting water by watering their lawns, he also invited good citizens to take things one step further and rip their lawns out entirely.

Just like dead trees, dead lawns increase the temperatures and spreading powers of fires.

A lack of water is not the only reason for trees dying en masse. With trees weakened and their sticky resin defense mechanisms down, the last nails in the coffin are often bark beetles.

Bark beetles, which are individually no larger than a grain of rice, move in when trees are stressed, weakened or injured. As they eat, they finish the tree off.

With beetle colonies expanding as the availability of food does, the current California drought also means there is a risk of massive, exacerbated bark beetle activity.

According to the US Department of Agriculture, a thriving bark beetle population, emboldened and enlarged by dying-tree feed, may then be able to turn, through “mass attacks”, to completely healthy trees and successfully destroy those.

But Moore warned against too much alarmism. The worst affected areas were fringe ones – harsh sites by tree standards.

Trees are surviving in most areas and may even be healthier as a result of some of the deaths, with less competition for the available moisture, Moore said.