Tree Risk - notes

 Notes from Tree Risk Assessment Qualification (TRAQ) class taught by Kevin Eckert for the Arizona Community Tree Council.  October 2022 

About 25 people are killed each year by falling trees in U.S.

Removal should only be an option when mitigations are not sustainable.  We have an ethical obligation to do no harm.  The benefits of trees are important.  

A risk is meaningless without a time frame.

Imminent - if a tear in the tree is weathered, the failure is not Imminent

Probable - failure would likely occur during normal (expected/seasonal) extreme weather.

Possible - failure would likely occur during abnormal (hurricane, tornado, derecho, etc) extreme weather.

Wounds

Thicker sap trees tend to compartmentalize better

If sap is oozing, touch and smell it:

Sticky = ok, its sap

Sour = bacteria

Slimy or foamy = fungus

Wet = just water

Drought

Tree dying from bottom up is over-irrigation

Tree dying from top down is drought (under-irrigation)

Failure

Soil failure vs root failure:  roots in air or roots break and stay in ground

Codominant is not a fault.  

Included bark is a fault because weak area doesn't hold stems.  

Aspect ratio near 1:1 for codom stems is a fault because don't have enough wood to wrap around union.  This is common cause of failure in pines, especially CA fire investigations.

Decay

Worse on compression side and at the base.  Hollow poles are strong. Most stress in tree is around the shell, so sapwood carries most weight, not heartwood (which is more brittle).

Heartwood - least concern

Sapwood - will start seeing tip dieback. 2nd worst.

Root - will also see tip dieback.  Not too bad if not structural; crown dieback will balance root dieback.

Basal - worst.  

Types of Response growth:

Reaction wood from tension/compression

Flexure wood from movement

Wound wood - e.g. lip around wound. More growth shows more stress

Likelihood of failure increases/decreases based on load (e.g. wind exposure) / response growth

Stress

Reproduction is sign of stress in trees (plants):  orchard trees cut to fruit more, but decreases lifespan.

Vigor/health does not always reduce tree failure risk

Adding N can increase leaf area and stem elongation, but that can increase load on stem and if stem doesn't thicken then can increase tree failure risk, even while increasing tree "vigor".  Sometimes reducing growth (ie use growth regulator) can improve mitigations.

Sheer plane: where tension and compression meet

Cracks: transverse (fiber failure) or longitudinal (with the grain)

Age

Old trees: retrenchment = natural process of aging, shortening and widening = more stable

Trees naturally move with wind.  Mass damping shows importance of inner branches and leaves.  Helps tree stems "undulate like snake in the wind".  Sharp bends or lack of inner leaf area can inhibit natural movement.

Construction

New concrete can leach lime/calcium oxate which raises soil pH and can harm some plants if too much

Adventitious branches = "water sprouts" (from topping = non ANSI cuts) = epicormic.  Poor attachments can rip out.

Shigo said:  "don't err on the side of safety….just do it right!"  :: Do the best you can based on your education, training, and experience.  

Restoration in hyper-arid environments?

 Is is possible to plant trees and restore grassland savanna in a hyper-arid desert?  

In Al Baydha it sometimes doesn't rain for 26 months, and apparently there are trees that can survive and thrive in that environment.

The Story of Al Baydha: A Regenerative Agriculture in the Saudi Desert.

Logging to Save a Forest from Climate Change

A good article about logging in the forest where I did my graduate research in northern Michigan.  Researchers are using funding from a timber contract with Louisiana-Pacific to cut the aspen trees on part of the experimental forest.  They will study the soil and water impacts of cutting the trees, as well as looking at species composition changes.

In the article, a researcher is quoted as saying that it would be irresponsible not to cut the trees because of climate change.  "Aspen in the Great Lakes region are considered “climate change losers,” according to Nave, and are not expected to fare well as the region’s climate continues to warm in the coming decades."

"The high-emissions scenario projects an 11.2-degree Fahrenheit summer temperature increase in the assessment area by the end of the 21st century. At the same time, summer precipitation is projected to decline by 3.8 inches under that scenario. "

"It will take a decade or more to know which of the aspen-management treatments was most effective, Nave said. It is expected that future generations of Biological Station researchers and students will carry on with the work, he said."

Bark Beetles

The Threat

Records dating back as far as 1750 indicate a consistent record of intermittent outbreaks of aggressive bark beetles in various forest types across the West. Over the past 10-15 years, however, the frequency, severity, and extent of bark beetle outbreaks have increased.

The current bark beetle outbreaks differ from previously recorded infestations because of:
Their intensity — bark beetles are killing trees in larger numbers, at a faster pace, and over longer time periods
Their extent — bark beetle outbreaks are occurring in numerous ecosystems from Alaska to northern Mexico
Their synchroneity — bark beetle outbreaks are occurring concurrently across western North America


The Characters: all beetles in the subfamily Scolytinae

Figure from Bentz, 2005.

Dendroctonus sp (Bark beetles) Outbreaks are usually associated with drought (Page, 1981). The beetle also responds to fire-damaged trees, but not those killed by fire.

Ips sp. (Pine engraver beetles): Species within the genus Ips, such as the piñon ips and
Arizona fivespined ips, also can kill their hosts, although typically they are not considered major disturbance agents. In recent years, however, elevated population levels of a number of Ips species have coincided with drought, resulting in large areas of mortality, particularly in piñon and ponderosa pine forests of the southwestern U.S.

The above beetles are distinguished from Ambrosia beetles as they only attack and consume live trees, whereas Ambrosia beetles can bore into heartwood even after a tree is dead and dry.

Ambrosia beetles (several species): Classed as wood borer beetles that attack weakened, dying, and
recently cut or killed trees. They can attack freshly cut lumber and lumber in decks before it is dried, and they can cause pinhole defects and dark staining in the outer wood. Galleries are formed in the sapwood or heartwood and damage the wood. Because ambrosia beetles tunnel into the wood, they are considered wood borers rather than bark beetles.


Management Considerations

When live trees are blown down, their phloem can remain suitable for bark beetle development up to a year later.  Stressed pine trees emit volatile compounds (terpenes). Bark beetles have evolved to detect these compounds and use them to identify suitable host trees. 

Bark beetles are also attracted to freshly cut wood.  Freshly processed chips emit the same volatile compounds (terpenes) as susceptible host trees. These chips will attract bark beetles during their active periods. The bark beetles cannot utilize the chips as a food source, but the attracted bark beetles may then colonize suitable host trees adjacent to the chips piles.

Fresh dead and down material (slash) can be a refuge for bark beetles, allowing them to breed, and possibly colonize nearby healthy trees. Lop and scatter can let slash dry out, killing any beetles in the bark and preventing new beetles from feeding on it.

Wood that has been debarked is not suitable for colonization by bark beetles. Only freshly cut, logs or slabs that have not been debarked are at risk of colonization.

Dead trees that do not have bark beetles in them and that do not pose a safety hazard can be left in the forest to be used by wildlife.  Dead trees do not necessarily pose more of a fire hazard than live trees.

Figure from Bentz, 2005.

Additional Resources

Diana L. Six, Ryan Bracewell, Bark Beetles, 2015

Bentz, Barbara.  Bark Beetle Outbreaks in Western North America: Causes and Consequences.  2005.

The Theory and the Reality of Shelterbelt Afforestation Projects

The Theory of Shelterbelts

The Reality

Introduction:  dust bowl, us efforts

Following the dust bowl years in the U.S. the government planted 220 million trees in a strip 100 miles wide, stretching 18,600 miles from Canada to the Brazos river.  1935-1942  Today, the growth and vigor of many trees has declined due to close spacing, age, and invasion of undesirable short-lived trees.  Wikipedia.

There are currently two major afforestation programs, one in China, and one in the Sahel.

Great Green Wall in China. 

This project aims to afforest 90 million hectares and eventually contain 100 billion trees in a 4500km belt.

A recent paper by Tan (2014) found decreased dust transport due to the plantings so far.  But independent Chinese media reported in 2013 that dust storms were increasing:  For centuries in northern China, annual sandstorms, called the Yellow Dragon, have been ripping through the city.  Wind erosion is obvious and most pronounced in spring, when sandstorms are common and the vegetation is still absent or dormant after severe winter temperatures. Sandstorms have increased in the last few years, calling into question whether the Great Green Wall is working.

Liu Tuo, head of the desertification control office in the state forestry administration, is of the opinion that there are huge gaps in the country's efforts to reclaim the land that has become desert. At present there are around 1.73 million sq kilometers that have become desert in China, of which 530,000 km2 are treatable. But at the present rate of treating 1,717 km2 per year, it would take 300 years to reclaim the land that has become desert.  

Background
In early times, Korqin was not a semi-desert, but savannah-type woodland, in transition between dense forest and the steppe zone. The rolling sand-sheet was deposited during the last glacial period (12000 years BP). During 10,000 years of vegetation growth, thick dark topsoil developed. Since historical times, the region has gone through several cycles of man-induced desertification and subsequent recovery, when human pressure lessened. Fertile dark topsoil vanished and extensive dune fields gradually build up.  Overgrazing (by cattle, goats, sheep, camels, horses), clearing of land for agriculture and over-cutting of trees and shrubs in this vulnerable ecosystem have resulted in an increasingly severe land degradation and desertification.

Other Approaches?

There are many who do not believe the Green Wall is an appropriate solution to China’s desertification problems. Gao Yuchuan, the Forest Bureau head of Jingbian County, Shanxi, stated that “planting for 10 years is not as good as enclosure for one year,” referring to the alternative non-invasive restoration technique that fences off (encloses) a degraded area for two years to allow the land to restore itself.  Soil fertility, already critically low, has shown a sharp decline as all organic residues from crops are removed for fuel and fodder during wintertime. Willow and poplar stands are pollarded in autumn, before leaf fall, for the same purpose. The continuous removal of potential nutrients to the soil is not balanced by the relatively small amounts of manure and inorganic fertiliser applied to crops.

Problems

 Jiang Gaoming, an ecologist from the Chinese Academy of Sciences and proponent of enclosure, says that “planting trees in arid and semi-arid land violates [ecological] principles”.The worry is that the fragile land cannot support such massive, forced growth. Tree growth in Korqin is largely dependent on the presence of a high groundwater table, fed by percolation and inflow from the western and southern mountainous areas. The long-term trend of a decreasing depth of the groundwater table is due to an increasing demand for water to irrigate crops and for human and industrial needs. If the trees succeed in taking root, they could soak up large amounts of groundwater, which would be extremely problematic for arid regions like northern China.  For example, in Minqin, an area in north-western China, studies showed that groundwater levels have dropped by 12–19 metres since the advent of the project.

Progress So Far

As of 2009 China’s planted forest covered more than 500,000 square kilometers (increasing tree cover from 12% to 18%) – the largest artificial forest in the world.However, of the 53,000 hectares planted that year, a quarter died. In 2008 winter storms destroyed 10% of the new forest stock, causing the World Bank to advise China to focus more on quality rather than quantity in its stock species.  FAO report

But the program’s widespread tree planting campaigns typically allot only one or two species of tree to an area. Professor Jiang wrote in a 2009 Epoch Times article, “In Ningxia, for example, 70 percent of the trees planted were poplar and willow. In 2000, one billion poplar trees were lost to a disease (Anoplophora), wiping out 20 years of planting efforts.”  FAO report followup

More criticisms:  Wikipedia.

Great Green Wall in Africa - the Sahel

The Great Green Wall initiative is much more nuanced than simply planting a belt of trees across the continent: “Behind the name or the brand ‘Great Green Wall,’ different people see different things. Some people saw just a stripe of trees from east to west, but that has never been our vision,” he says. “In Niger, Mali, and Burkina Faso . . . natural regeneration managed by farmers has yielded great results. We want to replicate and scale up these achievements across the region. It’s very possible to restore trees to a landscape and to restore agroforestry practices without planting any trees. This is also a sustainable way of regenerating agroforestry and parkland.”

But it should be noted that the Great Green Wall is not designed to prevent the Sahara Desert from expanding. “We are not fighting the desert,” he says. “In the majority of the areas we are working in these 11 countries, the desert is not advancing. The [Sahara] Desert is a very stable ecosystem. Of course, there are some areas on the margins—for instance in Senegal, Mauritania, and Nigeria—where there are some sand movements. But from a geographic perspective, over time the desert has been relatively stable in this area.” (Source)

But some authors advocate  "a shift from planting trees in the GGW to utilizing shrubs (e.g., Leptospermum scoparium, Boscia senegalensis, Grewia flava, Euclea undulata or Diospyros lycioides), which would have multiple benefits, including having a faster growth rate and proving the basis for silvo-pastoral livelihoods based on bee-keeping and honey production.” (Connors and Ford, 2014 Sustainability)

Biogenic VOC emissions

Trees and natural vegetation release air pollutants (biogenic), just like people (anthropogenic).

Figure from a great website by Dr. Wilson from Duke University, with lots of information.

The overall amount of this pollution can be modelled:

http://www.sciencedirect.com/science/article/pii/S1352231005000907

Its important to put this into perspective: human (anthropogenic) emissions of VOCs can be 40 times greater than the highest modelled biogenic emissions.  But the compounds plants produce may be more effective in generating hazardous compound Ozone.  Atmospheric chemistry is complex, and it is ironic that the highest biogenic pollution emissions occur in the same areas that high anthropogenic pollution also occurs, compounding the problem.  

Catestrophic Fire in the West

What if wildfire risk were catastrophically worse than we have imagined? Dr. Littell (University of Washington) walks us through just such a scenario in this USFS webinar. He starts by pointing out that the recent increase in wildfire area is not unprecedented. The traditional story has been that large "mega-fires" have increased in recent years, but Littell's reanalysis and recalculation of historical records found that the natural fire regime from 1910-1940 may have matched the last thirty years of large fires. (cf: The Big Burn) In both time periods perhaps 97% of the area burned was the result of less than 3% of the fires, the mega-fires.
Dr. Littell considers whether the dip in the middle is actually due to the triumph of fire suppression, or whether it is climatically driven:


What happens if it gets even warmer? Littell goes on to calculate fire probabilities based on temperature and precipitation and projects the results in a model of a 1 degree C climate change future:
Percentage Increase in Area Burned with +1C. White areas did not have a significant relationship between area burned and temperature.

Threatened Forests: a bug for every tree

US Forest Service Alien Pest Explorer Maps:

http://nrs.fs.fed.us/tools/afpe/maps/

Eastern Forest Environmental Threat Center:

http://threatsummary.forestthreats.org/browse.cfm?stateSearch=OH

Sudden Oak Death

http://www.oakmapper.org/resources

Agroforestry: the "Evergreen Revolution"

Dr. Dennis Garrity spoke today at SENR about how agroforestry is transforming entire landscapes in Africa. Dr. Garrity detailed a number of ways farmers are growing more crops by cultivating trees in their field. The details differ, but usually involve a leguminous tree that can provide both fertilizer for the field and fodder for livestock. One such tree, Acacia (Faidherbia), has the amazing added benefit of loosing its leaves during the wet season when farmers grow crops. This is basically the perfect tree, providing timed inputs of fertilizer, light, and forage at just the times the crops, farmer, and livestock need them.

According to Dr. Garrity, Faidherbia does not grow in the wild and is dependent on human cultivation. But until 10 years ago, no one knew how to propagate the tree: farmers tended them when they found them, but attempts to plant seeds or take cuttings had failed. Once researchers from the World Agroforestry Centre developed a cheap and easy way to propagate Faidherbia, its use has been rapidly adopted, especially in the Sahel region of Africa, where millions of hectares have been transformed.

Many of the questions from the audience focussed on possible problems and objections to this agronomic technique. What if pests wiped out the tree, or used the tree as refuges? If it was so good, why hadn't farmers adopted it before? What would happen when commercial farms replaced small holders? Some of these questions had initially occurred to me as well, but after reflection, the question I was most interested to ask would have focussed on what this example of agroforestry innovation could teach us to look for in the future. How can we learn from this example to look for other species in other parts of the world that might become essential components of new agricultural systems that can increase diversity, resilience, and production for the 21st century?

The real insight of this talk is that productivity is not set in stone: humans can and should manage their environments to improve them over and above what is "natural".

This presentation has in-depth information on the spread of agroforestry in Niger.

This is a great agroforestry organization: http://www.plant-trees.org/about.htm

Do Trees Pollute?

Plants produce VOCs as part of normal respiration. These, along with anthropogenic VOCs can produce ozone smog in the lower atmosphere.

In the South, pine plantations used for their fast-growing supplies of timber have proven to be havens for sweetgum trees, which are major producers of VOCs. Indeed, virtually every tree that grows fast -- a desirable quality for forestry production -- is a heavy emitter of VOCs.

"It's just one of those biological correlations," said Purves. "What you want is a fast-growing tree that doesn't produce a lot of VOCs, but that doesn't seem to exist."

http://www.princeton.edu/pr/news/04/q3/0927-trees.htm

The debate continues: "... trees were reported to contribute to ozone formation. This misleading fact contains only part of the truth. Most trees do emit biogenic VOCs such as isoprene and monoterpenes which can contribute to the formation of ozone and carbon monoxide. The other side of this story is that in areas with low nitrogen oxide concentrations, such as more rural areas, VOCs are believed to remove ozone. Additionally, since trees lower air temperature, the net effect of increased trees in urban areas is an overall lowering of VOC emissions and therefore ozone formation.

Trees in urban areas require energy inputs for planting, maintaining and removing. Because we burn fossil fuels (which emit CO2, SO2, N, CO and VOCs) in all these activities, we also need to factor that into the Trees + Air equation. In this case, it tips the scale a bit to the net loss side… but not for long!"

http://ecology.com/features/trees-air-quality/trees-clean-air.html

Life Support for the American Elm