Threatening Biodiversity

"It's never just one thing that brings down trees. It's always a combination. The first may weaken trees; the next stresses trees over time. Then comes a third, shutting down the trees' immune and defense systems."
"Finally, the last may come along to disrupt nutrient systems. When all this happens at once, or in rapid succession, trees are no longer able to save themselves."
David Rizzo, head, plant pathology, University of California, Davis

Large-scale tree mortality in northern Sonoma County, California. Credit: David Rizzo

Dr. Rizzo and Matteo Garbelotto, professor of environmental science policy at the University of California, Berkeley, studied the pathogen Phytophthora ramorum, discovering that it had settled in to infest coast oaks before anyone realized what was happening. The pathogen, which has funguslike characteristics, is known by the more plebeian name that aptly describes its lethal effect on certain trees, of "sudden oak death". But it wouldn't be nearly as effective at killing oak trees were it not for other, complicating issues.

As Dr. Rizzo notes, it's never one single issue that can be held to be responsible for the dreary spectacle of California's Sierra Nevada and North Coast forests crammed with dead and dying conifers and deciduous trees; oak to pine, a combination of drought, warmer winters thanks to climate change, and the presence of killer pathogens, aided and abetted by native fungi ready to take advantage of opportunity, all together manifesting challenges to the health of forests.

Climate change is no doubt involved in periods of prolonged drought, and it certainly is held to be responsible for the warmer winters that no longer freeze to death forest pestilence as they once did; bark beetles known to be deleterious to the longevity of certain trees are now able to withstand the rigours of warmer winters, reproducing in far greater numbers and laying their larvae under the bark of trees to feast on the hapless hosts, destroying their ability to thrive normally.

Add to that non-native interlopers, plant pathogens of immense potency that have infiltrated from abroad in plant specimens imported for cultivation, carrying with them nasty stowaways that thrive in the high humidity areas of the North Coast; some of them hitching rides in other ways, thanks to global trade. The forest floor, littered with dead trees, becomes susceptible to wildfires feeding on the resulting combustible woody debris.

Map shows risk of infection in 2030 under no control on left; control on and ahead of wave-front on right. Credit: Nik Cunniffe

Read more at: http://phys.org/news/2016-05-californian-sudden-oak-death-epidemic.html#jCp

Pines and other conifers are falling victim to the deadly combination of circumstances. Oh, and that "sudden oak death" interloper that has steadily been victimizing great swathes of forests has likely been around since the mid-1980s, reckons Drs. Rizzo and Garbelotto. Who estimate five million to ten million coastal trees have perished resulting from the onset of sudden oak death. Nothing new about such pathogens, a century earlier in Ireland, a cousin pathogen created the Great Potato Famine.

The wet, cool climate of the North Coast presents an ideal environment for sudden oak death to thrive. The vector of the disease spread by the pathogen is the bay laurel tree, itself immune to the deadly affect of the Phytophthora ramorum, but as a host implicated in its spread. When rain occurs the water falling on the contaminated bay laurel leaves spills over onto the forest floor enabling the lethal spores reach close-by trees such as tan oaks and coast oak.

Once the pathogen penetrates the bark, lethal spores are launched sapping the tree of its nutrients and inviting native fungi to follow, along with other pests. Their efficient invasion of a healthy tree leaves it dead within a few years. The potential for the pathogen to spread even further in Northern California's redwood forests, felling trees as it spreads, increases the risk of wildfires resulting from all that dead wood, threatening the prized redwood giants' survival.

National Science Foundation -- Pathogen that causes sudden oak death leaves redwoods vulnerable to fire

To place the issue into a greater perspective of potential harm, over one hundred species of trees and shrubs species are known to be susceptible to the threat that the pathogen presents; the situation is one of dire existentialism, threatening the biodiversity of invaluable ecosystems. That aside, in a more personal perspective, we owe much to the presence of trees and forests as carbon sinks, cleaning the air we breathe, and awing us with the beauty of their life forms, their venerable age and their importance to the survival of all living things.

Sudden oak death is known to affect over one hundred species of tree and shrub, presenting a significant risk to the biodiversity of many ecosystems

Read more at: http://phys.org/news/2016-05-californian-sudden-oak-death-epidemic.html#jCp

Sudden oak death is known to affect over one hundred species of tree and shrub, presenting a significant risk to the biodiversity of many ecosystems

Read more at: http://phys.org/news/2016-05-californian-sudden-oak-death-epidemic.html#jCp