Biologist Greg Asner first heard the numbers in April, but they did little to prepare him for what he saw.

The Forest Service had estimated that nearly 12.5 million trees in the state’s southern and central forests were dead. But as Asner peered down upon the same forests from his airplane at 6,000 feet, he saw something far worse.

California’s drought-parched landscape was poised for a radical transformation. Much of the low-elevation forests near Mt. Pinos in the Los Padres National Forest and in Pinnacles National Park were going to disappear if trends continued.

A scientist with the Carnegie Institution for Science, Asner has a practiced eye for forest health, and with instruments aboard his plane that give him X-ray eyes into the foliage, he is able to assess not just dead trees but trees so stressed by the drought that their death is likely.

There is no saying which trees will die, but by his estimation the count statewide could be close to 120 million — as much as 20% of the state’s forests.

To understand how Asner’s instruments work, you have to first step inside a leaf. There amid the busy factory of photosynthesis, water molecules are bending, stretching, rotating and vibrating.
These motions resonate into the atmosphere as reflected light, which is picked up by an on-board spectrometer that divides it into 480 bands from ultraviolet to shortwave.

Much like star light reveals a star’s distant chemistry, these bands are analyzed for their chemical content. Water is the primary focus: The more water in the leaves, the less reflected light, and the more reflected light, the drier the foliage.

The spectrometer works in conjunction with a laser that fans out beneath the aircraft, creating a 3-D image of the forest below.

By marrying the data from the spectrometer and the laser, Asner creates topographic images that show the condition of the forest. Healthy trees are blue, and drought-stressed trees run from mild (yellow) to severe (red).

The images help him to correlate terrain and tree stress. Higher tree stress, for example, often occurs on steep slopes and near meadows.

To explain what 120 million trees dying across the state might mean, Asner paints a picture of California’s ecological diversity and size. He then takes out his calculator.

He estimates there are 585 million to 1.6 billion trees in the state’s forests and apologizes for not being more precise. An accurate census, he says, has never been conducted, but 120 million represents 7% to 20%. Under normal circumstances, forests lose between 1% and 1.5% of their trees annually.

“At what point will the forest change into something else? We don’t know,” Asner says. “We don’t know when the lack of rain will lead to runaway conditions where the forests are beyond repair.”

Such a transition, especially in the lower elevations, is already underway in parts of the West, where nearly 6 billion trees — 13% of the area of western forests — died from 1997 to 2010 because of drought and the bark beetle.

Yet as grave as the effects of the drought have been, Asner insists there is hope.

Among other things, aerial images can help land managers identify vulnerable terrain and consider how to strengthen stressed trees and protect healthy ones.

Jeffrey Hicke, an associate professor in the department of geography at the University of Idaho, said that regardless of current tree mortality rates, the state will not lose its forests entirely. But he adds, based on the observations, the low-elevation forests are in greatest jeopardy.

Beyond this year’s drought, as climate change brings warming, tree species will migrate, Hicke said. Older trees will die, and younger trees will take root.

“Species will march uphill as the climate warms,” Hicke said. “Sequoia forests might become ponderosa pine or oak. Oak forests might become grasslands. There won’t be a wholesale conversion of forest to non-forest, at least not initially.”

http://www.latimes.com/local/california/la-me-dying-forests-20151020-story.html