Tree Mortality in California’s Sierra Nevada Mountains Where Have All The Green Trees Gone?

CA Tree Mortality California is experiencing a tree die-off at an unprecedented scale 102 million trees have died across the state 95% of the trees are in the Sierras The map shows the changes in tree mortality since 2006.

“The scale of die-off in California is unprecedented in our modern history,” said Randy Moore, the forester for the region of the U.S. Forest Service that includes California. Trees are dying “at a rate much quicker than we thought.”

Tree Mortality Factors Drought Pest Infestation Tree Density Trees possess defense mechanisms that help them fend off attacks by bark beetles. When beetle populations are low and adequate moisture is available, trees have the advantage. Healthy trees can produce enough resin (pitch) to overcome attacks by “pitching out” beetles that are attempting to bore in through the bark. During drought or when trees are severely stressed by other factors (competition, diseases), they are not able to produce enough resin to defend against numerous attacks.

Extreme drought puts additional pressure on already stressed trees Drought is more severe in southern half of CA However, drought conditions in the northern half of the state continue to intensify.

Drought Influences on Trees Carbon Dioxide intake reduced Trees and plants need CO2 in order to grow This means less CO2 is being removed from the atmosphere during drier times How do species of pine respond to drought?

Drought Influences Sap production decreases Stressed trees cannot produce enough sugars due to a decrease in nutrient uptake which reduces sap production Sap production discourages pests from preying on them Consecutive years of drought have stressed pine trees throughout California, making them more susceptible to attack from pine bark beetles. When a pine is healthy, it can successfully fight off attacks from pine bark beetles by successfully drowning and flushing out the beetles that attempt to enter (pitching out). During a drought, pine trees are too water-stressed and can’t produce that defensive response to prevent bark beetles from attacking. Yet, when beetle populations are high, even a healthy tree may not be able to produce enough pitch to ward off hundreds of simultaneous attacks (a mass attack)

Drought Influences Air bubbles in the xylem Plants suck water up through their xylem by creating a pressure gradient The less water in soil…the harder they have to suck At a certain point, which varies by species and tissue, air bubbles get pulled into the xylem When enough of these air bubbles form in the xylem, they form an embolism, similar to the kind found in humans, that blocks the flow of needed water and nutrients, leading to "hydraulic failure”.

How Water Moves Through A Tree

Pest Infestations Stressed trees from drought are more susceptible to bark beetle infestations Multiplying bark beetle populations due to a warmer climate have contributed to the death of millions of trees Millions of trees in the Sierras and Central Coast forests are stressed from higher temperatures, competition for water resources during this historic drought, and multiplying bark beetles. They simply can't withstand this deadly combination of stressors and are rapidly turning orange and dying. Even with the increased rainfall this past year, stressed trees will continue to die because while green, they have been invaded by bark beetles and just don't know they're dying yet.

Tree Density Higher tree density = greater competition for resources Lower tree density = lower competition for resources In times when resources are scarce, trees in lower density stands are at less risk for tree mortality Wildfire suppression should be discussed in relation to tree density. No burn policies have created densely populated tree stands more vulnerable to drought, disease, and pests. Overgrown forests are more susceptible to insect attack and drought because there are too many trees competing for limited water and nutrients. Reducing competition by doing more restoration, such as ecologically-sound thinning and using prescribed or managed fire, can help protect our still-green forests from future drought, insects, and disease.

Wildfire Suppression and Tree Density In the past, small fires would periodically burn through the California forests reducing fuels and thinning trees Suppressing wildfires have created densely populated tree stands more vulnerable to drought, disease, and pests.

What are the next steps to combat this problem? Conclusion… Many factors have contributed to the large tree die-off in the Sierra Nevada mountains It is predicted that more trees will die due to stressed trees and a warming climate What are the next steps to combat this problem?