Effects of diversity, topography, and interannual climate variability on pathogen spillover

Text from an abstract I submitted for the 6th Sudden Oak Death Science Symposium on the initial results from one of my dissertation chapters. Additional analysis is in progress.

Our knowledge of sudden oak death (SOD) disease dynamics indicate that without bay laurel (Umbellularia californica) there is seldom oak (Quercus) infection. This requirement of an alternate host species for disease transmission to oak species is an example of pathogen spillover. We developed a path analysis to test specific hypothesized relationships between physical and ecological factors affecting pathogen spillover. Path analysis enables simultaneous examination of direct and indirect effects from multiple factors, which can enhance our understanding of the multiple influences on pathogen spillover in SOD. We rooted our path model with the topographic wetness index, indicating potential soil wetness and moisture persistence, and examined the direct and indirect effects of species diversity, temperature, precipitation, and bay laurel density on potential inoculum load and infection of oak species.


Path model structure defining relationships between factors influencing pathogen spillover.

We applied 10 years of data from a long-term SOD-monitoring plot network in southeastern Sonoma County, CA. Each of the 200 15-m by 15-m plots was equipped with a temperature logger and plots were visited once per year from 2004 to 2012, and in 2014 to assess P.ramorum/SOD host species for disease symptoms and download temperature data. We inspected oak species for canker symptoms and indexed potential inoculum load by counting symptomatic leaves on each bay laurel stem for 60-seconds. We recorded the abundance of all tree species rooted in each plot during visits in 2005 and 2014 to quantify community diversity. Rainfall was measured at 15 rain gauges installed throughout the study area during this period.


A field crew member taking measurements at one of the plots.

We conducted a piecewise assessment of the path model, enabling us to account for the repeated measures structure of these data. Results from our path model of disease observations aggregated to the plot level revealed that diversity mediates the potential for pathogen spillover through a relatively strong direct negative effect on oak infection. Potential inoculum load on bay laurel had a direct positive effect on oak infection, with its overall influence moderated by temperature, topography, and diversity. Temperature and rainfall had relatively weaker influences on pathogen spillover compared to diversity and inoculum load. The net negative effect of diversity on oak infection is consistent with the dilution effect found in other studies of SOD. Topographic wetness had significant direct influence on diversity and inoculum load, where higher values of the wetness index tended to have lower values for diversity, but higher values for inoculum load. This is consistent with areas where moisture is likely to accumulate and persist providing a more favorable environment for P. ramorum sporulation.


The American Chestnut Epic


American Chestnut Trees. Source: http://goo.gl/IYekjq

The American Chestnut (Castanea dentata) is a storied tree. An American epic with books written and legends told. It was a behemoth, not necessarily by the size of individuals, though plenty were taller than the Statue of Liberty and had trunks you could hang a basketball hoop from if it were lying on its side. No, the behemoth legend of the American Chestnut lies in its versatility and pervasive presence in the lives of people living in the Eastern United States during the its reign as the dominant tree in the forests of the Appalachian Mountain range.


Original range of the American Chestnut. Source: http://goo.gl/yWSFz1

Chestnuts roasting on an open fire…” were the sweet nut of the American Chestnut. The productivity of these tress provided fodder for pig farmer’s herds at no cost of sowing and harvesting. Homes, barns, fences, furniture, and firewood were hewn from these trees. The tannin content in the bark provided natural resistance to insects and made them a valued resource of the tanning industry. “The Perfect American Tree.”


Chestnuts, leaves, and burrs. Source: wikimedia commons (Timothy Van Vliet, 2004)

Like many giants of lore, or perhaps more aptly the alien invaders in H.G. Well’s War of the Worlds, the American Chestnut was reduced to nearly nothing by less than a speck of dust. Cryphonectria parasitica, the cause of Chestnut Blight was introduced to the East Coast of the United States in the early 1900s. It took less than 50 years for it to decimate the American Chestnut throughout its range. This ascomycete (sac fungi) was brought to the United States via plant material collected in Asia, where the Asian Chestnut had evolved resistance to the pathogen. Meanwhile the American Chestnut had no resistance, or at least none that people had the presence of mind to look for. Remember, this was the 1920s, 30s, and 40s. The nuances of passive mutations and genetic variation and heritability were far from mainstream knowledge. So, the demise of the American Chestnut was hastened by preempitive harvest of healthy trees, essentially eliminating the possibility of finding a native tree with inherent resistance.


Chestnut Blight. Source: https://goo.gl/Kbx5eu

The American Chestnut still persists as a species throughout its original range, but mostly as a resprouting understory “shrub.” But the legend continues, as concentrated efforts are being made to restore this tree, if not to its former magnificence and dominance, at least to having American Chestnuts roasting on open fires once again.

*Here is a video from the University of Maine summarizing the story.

More about efforts to save/restore the American Chestnut:

News flash: genetic engineering may save the American chestnut tree

I haven’t been into genetics work since labs during undergrad, so I wonder how CRISPR might help/influence these efforts

The American Chestnut Foundation

A 95-foot tall Chestnut tree was recently found in Maine.

Note: The pathogen is also prevalent on chestnut trees in Europe. Hypovirulence of the pathogen was first discovered here. http://international-pest-control.com/hypovirulence-hope-for-management-of-sweet-chestnut-blight/


Pathogens Causing Us Pain

Ebola, flu, HIV/AIDS, malaria, are a handful of diseases that most people readily recognize as causing us pain. There are also many microbes in forest ecosystems that cause us “pain.” This pain may be economic, public safety, or otherwise. Large trees suffer mortality from disease more frequently, reducing biological diversity and beauty. While death by tree is less likely than death by ebola, it can still happen. Like human diseases, forest diseases are a natural part of the system, but control and prevention is much more difficult. Trees don’t go to the doctor when they’re sick, and detection is the number one challenge for dealing with any disease.

Video identifying ash dieback

Some diseases are introduced to new places where the local hosts have no resistance, such as sudden oak death

Sudden oak death landscape. Photo by David Rizzo, UC Davis.

Forest diseases can be managed, but it requires concentrated efforts in finding out how the disease is transmitted from one host to the next. And what environmental and human factors are affecting this transmission.

What diseases are impacting the forests where you live?

This was inspired by Richard Cobb and the talk he gave in the NC State FER Seminar series and finished due to participation in the 2015 #SciFund Outreach course