Pathogen native to U.S. but had not infected pines until recently
Fungal pathogens that cause die-back in grape, avocado, citrus, nut and other crops has found a new host and is infecting conifer trees causing pine ghost canker in urban forest areas of Southern California.
The canker can be deadly to trees.
Scientists from University of California, Davis, first spotted evidence that the pathogens had moved to pines during a routine examination of trees in Orange County. Over four years, they found that more than 30 mature pines had been infected in an area of nearly 100 acres, according to a report in the journal Plant Disease.
Akif Eskalen, a professor of Cooperative Extension in the Department of Plant Pathology at UC Davis, suspects drought and other stress conditions brought on by climate change weakened the tree species, making it more susceptible to new threats.
“We have been seeing this on pine trees for the last several years,” he said. “Our common crop pathogens are finding new hosts.”
Pine ghost canker – caused by the fungal pathogens Neofusicoccum mediterraneum and Neofusicoccum parvum – usually infects the lower part of a tree's canopy, killing branches before moving on to the trunks. This dieback in some cases can be deadly.
Points of entry
The pathogens infect a tree by entering through wounds caused by either insects such as red-haired pine bark beetles or pruning – meaning trees in managed or landscaped areas could be at risk. Another route is via tiny natural openings known as lenticels that fungi can make their way through, said Marcelo Bustamante, a Ph.D. candidate in Eskalen's lab who is first author on the paper.
Spores from the fungi can disperse and the higher the prevalence means an increased chance of transmission. Rain, irrigation water and humidity by fog can trigger the right circumstances for the spores to spread, he said.
“The detection of these pathogens in urban forests raises concerns of potential spillover events to other forest and agricultural hosts in Southern California,” Bustamante and others wrote in the report.
Dead branches can indicate a canker. Detecting the fungi is not an emergency but “people should keep an eye on their plants when they see abnormalities,” Eskalen said.
Cankers are localized areas on stems, branches and tree trunks that are usually dead, discolored and sunken. On bark, the spores can look like strings of discolored dots.
The lab has posted a brochure bout how to best manage wood canker diseases.
Tips include:
* Keep your trees healthy: Proper irrigation and maintenance will keep trees strong.
* Prune dead branches to reduce sources of infestation.
Karina Elfar, Molly Arreguin, Carissa Chiang, Samuel Wells and Karen Alarcon from the Department of Plant Pathology contributed to the paper, as did experts from Disneyland Resort Horticulture Department, State University of New York's College of Environmental Science and Forestry, UC Irvine and UC Los Angeles.
UC Davis research associate Kevin Welch, middle, takes inventory of plant regeneration a few years after wildfire in California's Lake Tahoe Basin. He's assisted by researcher Bill Stewart. Credit: Hugh Safford/USDA Forest Service
Quick Summary
Only about half of conifer trees regenerated five to seven years after wildfire in sites studied.
Study spanned 10 national forests and 4 burned areas in California.
Study presents tool to help foresters prioritize which lands to plant after a wildfire.
A study spanning 10 national forests and 14 burned areas in California found that conifer seedlings were found in less than 60 percent of the study areas five to seven years after fire. Of the nearly 1,500 plots surveyed, 43 percent showed no natural conifer regeneration at all.
The study was co-led by UC Davis and the USDA Forest Service and published December 21 in the journal Ecosphere. It presents a tool to help foresters prioritize which lands to replant immediately after a fire, and which lands they can expect to regrow naturally.
“High-severity fires are knocking out seed sources and leading to a natural regeneration bottleneck, which poses a predicament for the sustainability of our forests,” said lead author Kevin Welch, a research associate with the UC Davis Department of Plant Sciences.
For example, 10 of the 14 burned areas in the study, which include well-known wildfires like the Moonlight (2007) and Power (2009) fires, did not meet Forest Service stocking density thresholds for mixed conifer forests, making them good candidates for replanting and restoration efforts.
“Knowing that the Forest Service doesn't have the time, budget and staffing levels to restore everything, we basically want to help foresters predict what will be there five to seven years later so they can better focus restoration efforts,” Welch said.
How Does The Tool Work?
The researchers surveyed a range of elevations, forest types and fire severities –including in the Sierra Nevada, Klamath Mountains, and North Coast regions –to determine which factors promote and limit natural conifer regeneration and how different conifer species respond after a fire.
Using a simple tool developed by the research team, a manager can enter the forest the year following a fire and take a few field measurements –including distance to seed source, slope, and the cross-sectional area of living trees in the nearby forest. They can then predict whether a severely burned site is likely to meet a desired level of tree density five to seven years later.
Tested against four wildfires that were not in the study, the researchers found the tool was able to predict with more than 70 percent accuracy whether an area would likely need to be replanted or not.
The study plots were in California, but the authors suggest study results could apply to mixed conifer forests across the North American Mediterranean Climate Zone, which stretches from southwestern Oregon through California to northern Baja California and includes parts of western Nevada.
A Race For The Sun
As the research team saw while hiking through miles of dense brush, high-severity fires also stimulate shrub growth to the detriment of fire-resistant tree species that foresters try to encourage. The conifer regeneration that is occurring is heavily dominated by species that tolerate shade but not fire, such as Douglas fir, white fir and incense cedar.
Fire-resistant and drought-tolerant trees, such as ponderosa, sugar and Jeffrey pine, do not tolerate shade well. Such species are likely to better withstand the warmer, drier climates projected for California in the future.
Currently however, forest and fire conditions are not favorable for the survival of these more desirable trees. According to the study, strategies for increasing pines in California forests include reducing forest densities and fire severities while increasing overall fire occurrence (both prescribed fires and managed wildfires). They also suggest planting pines before shrubs and shade-tolerant trees crowd them out and remove their light source.
“As western forests increasingly experience warmer weather and more frequent and more severe fires, a better understanding of what conifers need to regenerate naturally after fire can help us create and manage more sustainable, resilient forests,” said co-author Hugh Safford, regional ecologist for the USDA-Forest Service's Pacific Southwest Region and a member of the adjunct faculty in the UC Davis Department of Environmental Science and Policy.
The study was funded by the USDA Forest Service and UC Davis.
This article is reposted from the California Fire Science Consortium blog. For more information go to http://www.cafiresci.org/blog/.
Something so rare and wonderful happened this week that it must be shared:
The Stephens Wildland Fire Science Lab at UC Berkeley recently hosted a departmental happy hour. To celebrate the occasion, Stephens Lab PhD candidate Anu Kramer illustrated a typical Sierra Nevada post-fire succession in the form of three cakes:
The first cake, shown above, represents a dense Sierra mixed-conifer forest. Large sugar cones with green rice crispies are large remnant pines. The smaller cones with green shredded coconut represent invading shade-tolerant fir trees. Slivered almonds represent a heavy buildup of surface fuels.
Tp represent immediate post-high-sevirty fire effects, Anu employed a highly accurate rendering of stand conditions using a dense, flourless chocolate cake.
A passionfruit cake with chocolate-coconut frosting was used to demonstrate one year post-wildfire regeneration. Note the vibrant understory regeneration with wildflowers.
How's that for effective science communication? Pretty darn good, I'd say.
Article reviewed:Stand-replacing patches within a ‘mixed severity’ fire regime: quantitative characterization using recent fires in a long-established natural fire area
By B.M. Collins and S.L. Stephens. Published in the journal Landscape Ecology and available for download.
The plot line: This study used an area within Yosemite National Park where wildfires had been allowed to burn over the past ~30 years. They looked for patterns in how often fires created large gaps (holes in the canopy) versus small ones. In other words, they measured how often fires killed a lot of trees versus just a few. They also attempted (with pretty good success) to explain the reasons why some gaps were large (fire more severe) while others were small. They found that, while most gaps were small (less than about 5 acres), there were also a few very large gaps that were created by fire- up to 230 acres! Overall, the portion of the burned areas that actually created gaps (as opposed to the fires remaining on the surface and not killing lots of trees), was about 15% over a 30-year time period. For previous fire occurrence to reduce the chance of another high severity fire occurring, the fire had to occur recently (within about 30years). This is what I call the Janet Jackson effect… “what have you done for me lately?” They conclude that, while the high severity fires that create gaps were not the dominant type of fire behavior that occurred in this case, they had a significant contribution to the mix of fire severity that occurred.
Relevant quote: “While high-severity fire represents a fairly low proportion of the total burned area (15%) stand replacing patches should be considered an important component shaping these forests.”
Relevance to landowners and stakeholders:
Studies like these that attempt to measure how disturbances shape forests are important because debates about forest management often come down to debates about what types of disturbances are “more natural” than others. If a certain treatment “mimics” a natural disturbance then it might be considered better. For example, doing clearcuts or allowing high severity fires to occur may be preferred because they are thought to be a more natural type of disturbance. On the other hand, doing light thins or only allowing low severity fires may be thought of as more natural. Mimicking a disturbance regime might be the primary objective of management, as was discussed in this post about using disturbances as a guide for management.
With respect to fire, most people think that the “most natural” regime for the Sierra Nevadas is one referred to as mixed-severity. As the authors point out, this term is difficult to define. Very broadly defined, it simply means that when fires occur, they are diverse in terms of having some areas where lots of trees are killed but also having areas where no or very few trees are killed.
The study points out the confusion of this term, however, when it is defined more precisely. From the results, one could conclude that these fires were not of mixed severity at all because most of the canopy gaps were relatively small (i.e. it was a low severity regime). On the other hand, the portion of total area in canopy gaps was dominated by a few very large gaps (i.e. a high severity regime). Putting this fire regime into the context of other types of regimes that we see in different forest types around the world, however, I think that it is safe to say that the researchers found these fires to be of mixed severity.
Relevance to managers:
Figure 4 in this paper is very useful. Perhaps not as a broad guide for management across the Sierras, but more as a demonstration of how one could think of disturbances as a guide for management:
The graph shows that, in this case, most of the gaps that were created by the fires were relatively small. In general, there was a downward trend in the frequency of larger gaps. One could related this to management, for example, by allocating forests to either even-aged or uneven aged management in order to also achieve a downward trend in gap size. I think the total patch size area would be tremendously variable from fire to fire, so that part of the graph is less relevant. And if they had looked for smaller gap sizes (their minimum was 1.2 acres), the dots on the graph may have ended up looking more U-shaped.
Taking it another step, one could even set a rotation age based on this graph. Over a 30-year time period, 15% of the total area that burned was converted to gaps (i.e. regenerated to new trees). If one were to mimic this conversion rate into the future, it would lead to an approximate 200 year rotation age. Again, I don’t think this is useful as a broad guide until more studies like this are done, but the method may be useful for those how have an objective of mimicking what they think is a natural disturbance regime.
Critique (I always have one, no matter how good the article is):
I really like what these researchers did, and I’ve been waiting for something like this to be done for mixed conifer forests. Similar studies have been done in other forest types, but it is more difficult with mixed-severity fire regimes so there are some limitations.
In terms of being useful for management, it would have been much better to have a smaller minimum mapping unit that 1.2 acres. What we consider regeneration of a distinct cohort can occur at much smaller scales. Ideally, we would go down all the way to the scale of a single canopy tree dying. It makes perfect sense why they used 1.2 acres- it was because of technological limitations of remote sensing data. But I think it would have been useful to do some kind of sensitivity analysis. In other words, how would the results have changed if the MMU was smaller? Bigger?
While this study uses an area that is probably as good as we can find when it comes to areas where fire has been allowed to burn, the reality is that it still has not been very long. This area has only had two fires, and as this study points out, fires are tremendously variable so more will be needed in terms of having broad implications. The authors know this and point it out, but it is worth noting as a limitation- we’ll get better information as more time passes and more fires burn in these areas. It will take a while for us to overcome the 60+ years of fire suppression, a period of time that were the dark ages of fire ecology where we learned nothing! Fiat flamma!
Earlier this month, I was lucky enough to spend two weeks touring Italy. It was a glorious trip, filled with pasta, wine, more pasta and more wine. When not eating and drinking, I made a point of noticing the flora. What I noticed was fascinating: What grows wild in Italy is largely the same as what grows wild right here in Solano County. No big surprise, really. We share the Mediterranean climate, and the regions we saw — from Rome to Bologna — are studded with farms and oak chaparral. It felt just like home.
There was one type of tree that stood out, however: the quintessentially Italian stone pines (Pinus pinea). I heard them called “Umbrella Trees” by a tour guide at the Roman Forum. The oldest of these trees stand quite tall, throwing their umbrella-like profile over the ruins in Rome and farmhouses in Tuscany. The trees are most often found along Italy’s two coastlines.
Two stone pines show off their umbrellalike good looks at the Villa Borghese public park in Rome. (photo by Kathy Thomas Rico)
Stone pines are native to the Mediterranean region, and have been cultivated there for at least 6,000 years. The trees are slow growing, but can eventually reach 80 feet in height. Bushy when young, the umbrella-shaped canopy comes along in the trees’ middle age, and can be up to 200 feet in diameter. The cones of stone pines bear delicious pine nuts. Sadly, the western conifer seed bug (Leptoglossus occidentalis) was accidentally imported with timber to northern Italy in the 1990s. The pest has since spread across Europe, feeding on the sap of developing conifer cones. Its sap-sucking causes the developing seeds to wither. It has destroyed most of the pine nut seeds in Italy, and threatens Pinus pinea in its native habitats. Consequently, most pine nuts you see in stores are grown in China.