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

Coping With Foreign Invasive Species

"It's [emerald ash borer larvae] basically girdling and strangling the tree. It [the tree] can't move water. It can't move nutrients. It's like the circulation system of the tree. By the time you see this, when it's gone all the way around the main trunk, the tree can't survive. It kills 99 percent of the trees [it infects] and most of it happens before you even know it's there."
"Those [introducing parasitoid wasps as biological control agents] are the only real options at this point. Otherwise it's a matter of removing what you can't save, replanting and moving on. It [the beetle] is still a pretty devastating insect."
"It's a very long-term project. It will take a long time for populations of wasps to build up to levels where we can see any impact. But in its native range in China, the [beetle] has a range of parasitoids that keep it under control."
"They're not a kind of [insect to] sting or bite [ash borer-parasitic wasps] you, or [likely to] really want [to have] anything to do with you."
Krista Ryall, ecologist/entomologist, senior research scientist, Canadian Forest Service

Bilberry Creek Ravine forest, Orleans, Ontario

Anyone who ventures into the forest can see what's happening to ash trees. They've been dying; once infected it doesn't take that long. And it's beyond sad. An important part of the forest is being killed off and everywhere one looks in an urban setting or in a woodland there are gaps being created where dead trees abound. Ashes begin to lose their bark when they've been attacked by the emerald ash borer even as boughs distinguish themselves by dead foliage.

Injections used by botanists to try to extend the life of ash trees are expensive, quite an investment if a property owner wants to save a treasured ash shading property and beautifying it, yet the effect is temporary, it wears off and the property owner is back to square one. It's a shoulder-shrugging admission that the temporary solution is just that; temporary, and a waste of time, energy and money, leaving little option but to allow the tree to die.

The oobius agrili parasitic wasp is very small and doesn't have a stinger. It lays its eggs within emerald ash borer eggs, killing the host egg. (Houping Liu/Michigan State University)

But Dr. Ryall and her colleagues from the Canadian Forest Service have embarked on a much larger experimental study which they hope will present a solution in time. They have taken possession of adult wasps grown in ash trees at a U.S. Department of Agriculture station in Michigan. Short sections of trees called mini-bolts are hung on still-living ash trees which have been infested with the beetle larvae in the hope that the mature female wasps will respond to emerald ash borer eggs or larvae, to lay their eggs upon.

The project is four years into its process at about a dozen test sites in Ontario and Quebec. There are two types of wasps, one that attacks the eggs of the ash borers and another which feeds on its larval stage, both native to China, where the borer itself comes from. Site examination has thus far revealed that the egg-eating wasps appear to be doing the work anticipated, while it is as yet unclear whether the larvae-eating wasp has been equally successful in adapting to the work in Canada they are known for in China.

Barry Lyons, a forest entomologist for Natural Resources Canada, stands next to two "oobinators" full of oobius agrili eggs. (CBC News)

 

The emerald ash borer's existence was first noted in 2002 in North America. It has since spread in both provinces and throughout 27 eastern American states. Eggs of the small green beetle are laid in crevices in ash tree bark with the larva burrowing into the tree, cutting channels under the bark that serve to choke off the flow of water and nutrients. The result has been the death of tens of millions of North American ash trees.