Wind and Trees 101: To Touch a Tree

The root of the problem, to which we hope Stormwise can be a solution, is the tree/wind dynamic. Wind is the force that sets a tree in motion. A tree set in motion begins to sway with an amplitude and frequency dictated by its own physical properties – its size and shape – as well as the strength and cadence of the wind. Swaying is a tree’s natural method for the dissipation of the energy exerted upon it by the wind, but there exists the dangerous possibility of achieving an amplitude of sway greater than the tree’s elastic capacity to return upright. That’s when trees fall down. That’s when we have tree/utility interaction problems.

We might ask ourselves ‘why does this not happen more often?’ When a big windstorm hits, why don’t more trees succumb to the forces? As it turns out, all plants, trees included, adapt to the patterns of physical stimulation they experience as they grow. Physical stimulation can be anything from wind or sprinkler action, animals brushing the leaves, or being shaken by well-meaning scientists. As a plant experiences these physical agitations, it develops a capacity to withstand them. Dr. M.J. Jaffe, in 1973, coined the term to describe this phenomenon in a paper entitled “Thigmomorphogenesis: the response of plant growth and development to mechanical stimulation.”

Thigmomorphogenesis (thigma being the greek word for touch or handle) has been studied since at least the time of Theophrastus. The Greek philosopher lived around 300 BCE and wrote such works as On Plants and On the Causes of Plants, wherein he describes the stunted height and “rough” or knotted condition of trees growing in exposed areas, versus the erect and undistorted trees in sheltered forests. Since then, much scientific inquiry has been directed at the specific results of thigmomorphogenesis. Just to name a few:

  • Ashby et al. (1979) found that shaking sweetgum and maple saplings caused a lessening of total height but not of dry weight of the stem, suggesting that they weren’t truly stunted, but that resources we reallocated.
  • Jaffe, in 1980, showed that rubbing the stems of bean plants produced the same responses (shorter, thicker stems) as exposing them to wind. (Fig 1)
  • A study on sweetgum saplings tested the effects of staking and shading and found an immobilized, mostly shaded tree allocated more carbon to height growth and in some cases failed to develop even the capacity to remain upright in the absence of supports (Holbrook et al., 1989). (Fig. 1)
  • Ostler et al. (1995) showed the effects of bending on developing eucalyptus saplings to include development of stem taper, reaction or tension wood cells, and a bushy branching structure.
  • Chehab et al. (2009) highlighted a number of thigmomorphogenic responses including alteration of flowering times, senescence, chlorophyll and hormone levels, pith and stomata structure, phloem transport, electrical resistance, and resistance to various stressors, as well as physical stature of the plants.

 

Figure 1:

How is this botanical phenomenon useful to the Stormwise project?

Thigmomorphogenesis is the mechanism by which plants develop the capacity to handle the force imposed on them by the wind. We call this developing wind-firmness. The most obvious wind-firm features our northeastern trees develop when exposed to a stronger wind regime include a widened or buttressed base, increased root anchorage, a more tapered stem of lesser height, shorter branches, thicker branch junctions and smaller leaves. This gives exposed trees an overall compact, bushy shape compared to tall slender forest-grown trees (Fig. 2). Some trees (white pines and willows among others) can respond by streamlining, a developmental process that reduces drag on the crown, often by allocation of the heaviest branching to the side opposite the prevailing wind direction. Less noticeably, physical stimulus also triggers changes in cellular arrangement and allocation resulting in greater density and flexibility of the wood in an exposed tree.

Figure 2

The intent of Stormwise is to advocate for the adaptation of a forest management plan that fosters wind-firmness in our roadside forests. Every forest parcel is different and requires a tailored plan, but typically we can approach the plot for thinning, which is a common treatment in timber stands to remove less desirable trees so that resources can be allocated to more desirable ones. In this thinning, we remove any hazardous trees, select for trees with straight stems and symmetrical crowns, and give those the space and exposure they need to develop the wind-firm features we want to encourage. In this manner, we leverage well-used forest management techniques, and the natural processes of the trees themselves, to achieve our rather novel management goals.

 

Referenced:

  • Ashby, W. Clark, et al. “Notes: Effects of Shaking and Shading on Growth of Three Hardwood Species.” Forest Science2 (1979): 212-216.
  • Chehab, E. Wassim, Elizabeth Eich, and Janet Braam. “Thigmomorphogenesis: a complex plant response to mechano-stimulation.”Journal of Experimental Botany1 (2009): 43-56.
  • Holbrook, N. Michele, and Francis E. Putz. “Influence of neighbors on tree form: effects of lateral shade and prevention of sway on the allometry of Liquidambar styraciflua (sweet gum).” American journal of Botany(1989): 1740-1749.
  • Jaffe, M. J. “Morphogenetic responses of plants to mechanical stimuli or stress.” Bioscience4 (1980): 239-243.
  • Osler, G. H. R., P. W. West, and G. M. Downes. “Effects of bending stress on taper and growth of stems of youngEucalyptus regnans trees.” Trees4 (1996): 239-246.

Planting Near Power Lines

A few years ago, my mom planted a little tree in her front yard. Last summer, she had me up on a ladder pruning it back. She didn’t realize when she planted the tree that it was right underneath the power line to her house!

Trees and power lines can create problems when they occupy the same space. This can be prevented by planting trees that won’t grow near the lines (are you listening, mom?!). We call it “right tree in the right place.” This might mean planting trees away from power lines, or it could mean planting trees that will not grow tall enough at maturity to reach the power lines. These trees are referred to as understory trees because in the forest they grow under the canopy of taller trees. Some understory species are oriental cherry, crabapple, eastern redbud, hawthorn, Japanese maple, dogwood, amur maple, tatarian maple, serviceberry, silver bell, and tree lilac.

These crabapple trees will not grow tall enough to interfere with the overhead power lines. Photo credit: The Morton Arboretum

When large trees near power lines need to be removed, replacing them with understory trees can ease the loss. A study in Pennsylvania (Flowers and Gerhold, 2000) found that people generally liked the replacement understory trees, and recognized the need to replace older, deteriorating trees with smaller, more power-line friendly types of trees.

Understory trees might be perceived as an imperfect solution. People enjoy taller trees that can form a canopy over-arching a street. Taller trees provide more shade and can combat the urban heat island effect. In urban areas where there is little room for tall street trees, there can be more capacity to plant shade trees on private property.

Many people have asked me, “Why don’t they just bury the power lines?” It comes down to cost. According to Electric Light and Power, underground lines cost about $1.5 million per mile to set up, which is about five times that of overhead lines. Utility infrastructure is heading in that direction though. All new developments in Connecticut are required to build underground utility lines. It will be a long time until all the lines are underground, if ever, and in the meantime we need to focus on the trees.

More information about tree planting around power lines can be found here.

Reference: Flowers, D. E., & Gerhold, H. D. (2000, November). Replacement of trees under utility wires: impacts attitudes and community tree programs. Journal of Arboriculture 26(6): November 2000.

Dealing with Storm Damaged Trees

Much of the research and demonstration work in the Stormwise program is about creating more storm-resistant conditions in our roadside woods. Growing more storm-resistant trees and forests takes time, of course, and in the meantime the woods we have near our homes and roadsides remain subject to severe winds, downpours, lightning, snow and ice that can be part of severe storm events. The last hurricane-type event was several years ago, and another can happen any time, but more commonly and more frequently these days we are experiencing smaller but very intense storm events. A localized thunderstorm during the summer for example, can be a wicked cell that uproots and rips limbs from trees, downs powerlines and damages buildings and vehicles in a small area.

For my part, the sudden and severe nature of such winds makes me nervous about the potential for damage to my humble little house from trees and limbs. I recall one recent event during which a large limb from the top of a 20-inch white oak was ripped off and came down about 20 feet from where my car was parked. There was, of course, a mess of smaller twigs and branches as well. No real property damage, thank goodness, but it was close. The storm was over a quick as it began, and now, just like many folks around the state, I was faced with a clean-up task. In the end I was fortunate in that the broken limb was at the edge of the woods and easily made into a nice neat little pile of firewood.

For many people, however, the task of cleaning up storm-damaged trees is not so straight-forward and simple. Storm-damaged trees are fraught with abundant problems, dangers, and risks. Cleaning up and salvaging downed, partially down or damaged trees is one of the most dangerous and risky activities an individual can undertake. It cannot be emphasized enough that without a thorough knowledge of safety procedures, equipment capabilities and correct methods for dealing with physically stressed trees, an individual should never undertake this type of work on their own. The very characteristics that make the wood from trees a great structural material can turn leaning, hanging or down trees into dangerous “booby-traps” that spring, snap, and move in mysterious ways when people try to cut them, resulting in serious and life threatening injuries. Just because your neighbor or relative owns a chain saw, it doesn’t make them qualified to tackle a large tree that is uprooted or broken. Contacting a Licensed Arborist, or Certified Forest Practitioner with the right equipment, training, and insurance, is the best alternative for addressing the cleanup and salvage of storm damaged trees, and avoiding potential injury, death, liability and financial loss.

That said, there are a few things a homeowner can do about trees on their property that are damaged and/or uprooted after a storm:

  • First, from a safe distance note the location of any and all downed utility lines. Always assume that downed wires are charged and do not approach them. Even the soil for some distance around a down wire can be charged! Notify the utility company of the situation, stay away and do nothing further until they have cleared the area.
  • Once you are confident that no electrocution or other physical danger exists, you can visually survey the scene and perhaps document it with written descriptions and photographs. This will be particularly helpful if a property insurance claim is to be filed. Proving auto or structure damage after a downed tree has been removed is easier if a photo record has been made.
  • Don’t forget to LOOK UP! While you may be fascinated with examining a downed limb, there might be another one hanging up above by a splinter, ready to drop at any time.
  • Take steps to flag off the area or otherwise warn people that potential danger exists.
  • Remember that even if a downed tree or limb appears stable, it is subject to many unnatural stresses and tensions. If you are not familiar with these conditions, do not attempt to cut the tree or limb yourself. Cutting even small branches can cause pieces to release tension by springing back, or cause weight and balance to shift unexpectedly with the potential for serious injury. Call a professional for assistance.
  • Under no circumstances, even in the least potentially dangerous situation, ever operate, or allow anyone on your property to operate a chainsaw without thorough knowledge of safe procedures and proper safety equipment, including, at the minimum, hardhat, chaps, eye and hearing protection, steel-toe boots and gloves.

An assessment of the damage to individual trees, or more widespread damage in a forest setting is best undertaken by an individual with professional expertise. Homeowners should contact a Licensed Arborist to examine trees in yards or near to structures, roads or power lines. A CT-Certified Forester is qualified to evaluate wind damage in the forest and advise landowners about the suitability of salvage or cleanup operations. The CT-DEEP Forestry Division can provide information about contacting a Certified Forester or Licensed Arborist. Check the DEEP Website: www.ct.gov/deep/forestry or call 860-424-3630. Contacts for arborists can also be found at the web site of the CT Tree Protective Association: https://ctpa.org/ .

While a nice tidy pile of firewood from a tree that was damaged in a storm might be the silver lining of that storm cloud, it is not worth the risk of injury to yourself or someone else when tackling a very dangerous task without the proper knowledge, equipment or preparation.

Forest Canopy Structure: What Does That Mean and Why Does It Matter?

I spend a lot of time shooting lasers at trees, but this is definitely not as exciting as it sounds! These aren’t Han Solo style blasters we’re talking about, rather I’m talking about Terrestrial Laser Scanners. This is an instrument that shoots out millions of laser pulses and records the amount of time that elapses between the initial pulse and its return to the sensor after it strikes an object. In reality, rather than running around blasting things, the work mostly involves me sitting next to the instrument making sure that I’m not blocking the laser and ending up in the scan myself (see photo 1). Using this instrument we determine the distance of objects from the scanner and make 3D maps of the surface of just about any object – including the canopy of a forest. You may be thinking, “Why would you spend time doing that?” Well, I’m glad you asked! Let me tell you a bit about how important canopy structure is in a forest.

Forest canopy structure can mean a lot of different things. Often scientists are interested in the total quantity of leaf area in a forest, because those leaves are the photosynthetic system of the trees and thus determine how much they grow and how much carbon they pull out of the atmosphere. This is usually described as Leaf Area Index – which is the amount of leaf area per unit ground area. Because it would be pretty time-consuming to go out and cut down all the leaves and measure their total area, this aspect of forest canopy is usually estimated either by collecting leaves in “litter traps” (of a known area) as they fall off the trees or using hemispherical (or “fish-eye”) photographs pointed up to look at the canopy (see photo 2).

Another way of thinking about canopy structure is in terms of the arrangement of leaves and branches in the canopy in 3D. Knowing how these canopy elements are arranged can tell us a lot about how the forest will function and the values that the forest can provide to ecosystems and human society. The legendary ecologist Robert MacArthur was one of the first scientists to delve into this topic. He created a metric he called “Foliage Height Diversity” – basically the vertical stratification of canopy elements. His research showed that this feature of a forest was very strongly related to the habitat value of the forest for birds and that different bird species used forests with different types of canopy structures (e.g., dense subcanopy vs. tall canopy and open subcanopy) due to needs for nesting, foraging, hunting, etc. Work by my lab and a number of collaborators has illustrated that the 3D structure of the canopy is also very important in how productive forests are and, thus, how much carbon they are able to remove from the atmosphere. We currently have funding from the National Science Foundation and US Department of Agriculture to assess the generality of this finding across the eastern US in a range of forest types and with different management strategies.

How does this relate to Stormwise and the resilience of forests to disturbance?

Although the effects that canopy structure has on habitat use and C uptake is obviously important, this feature of forests is also a major concern as we attempt to understand how to make forests more resilient to disturbance. The natural disturbances that cause damage to forests (and adjacent infrastructure) are generally associated with either high winds or ice/snow accumulation on branches. The potential for both of these types of disturbances to cause damage or result in tree or branch failures is strongly related to the structure of tree crowns and forest canopy. For example, the movement of wind over and through a forest stand, and it’s propensity for damaging branches or trees, can be strongly related to the structure of the canopy, both in terms of the smoothness vs. roughness of the canopy exterior and the interior structure of the canopy and how it creates turbulence within the canopy. We are endeavoring to better understand these relationships by pairing measurements of the movement of trees with mapping of all of the canopy elements in a forest stand (photo 3), both before and after Stormwise-related management activities. Also, the structure of the trees and branches in a forest could have a very strong impact on the potential for ice/snow related branch and tree failures. We are collaborating on an NSF-funded experimental ice storm project being conducted in New Hampshire at the Hubbard Brook Experimental Forest (http://www.hubbardbrook.org/research/climate/Rustad_12.shtml), to attempt to better understand both how ice storms affect canopy structure and also how different types of canopies (with different initial structure) might be affected by ice storms. Through the studies we hope to both understand how canopy structure affects the resilience of trees and forests to disturbance and use that scientific understanding to more effectively manage forests to promote resilience.

 

Terrestrial laser scanning in action
Terrestrial laser scanning in action

 

Hemispherical or “fisheye” canopy photograph
Hemispherical or “fisheye” canopy photograph

 

3D model of UConn Stormwise research site developed using terrestrial laser scanner.
3D model of UConn Stormwise research site developed using terrestrial laser scanner.

Ethics Practices in Human Dimensions Research

Human dimensions is a social science that seeks to understand how people make decisions about natural resources, and characteristics of individuals that affect those decisions. As part of the Stormwise project, human dimensions research is helping to understand public concerns about, and opportunities for, roadside tree and forest management in communities across Connecticut. Although data collection and analysis are the main foci, ethics also plays a critical role in the human dimensions research process.

Collecting research data directly from humans (i.e., human subjects) requires researchers to abide by a code of ethics similar to that of doctors and lawyers. This requirement is the result of past research misconduct, such as the Nazi experiments during WWII, Tuskegee Syphilis study, Milgram experiments, Stanford prison experiment, and human radiation experiments. In the United States, the National Research Act of 1974 initiated development of ethical guidelines for biomedical and behavioral research involving human subjects. An important outcome of this legislation was the Belmont Report (1978), which outlined three core principles for research involving human subjects: respect for persons, beneficence, and justice. Additional guidelines and recommendations were modeled after the Nuremberg Code, including voluntary consent, protection of subject identity, risk/benefit analysis, and the right of a subject to end participation in a study at any time.

In 1991, the Federal Policy for Protection of Human Subjects (45 CFR 46), referred to as the “Common Rule,” was published and integrated across 15 federal agencies. This policy mandates federal protection of human subjects from physical or psychological harm through review and formal approval of all research protocols and materials. Additional measures extend to “vulnerable” research populations including pregnant women, prisoners, and children. Other countries have similar ethical guidance authority, such as Canada’s Interagency Advisory Panel on Research Ethics and the UK’s Health Research Authority.

How does this relate to Stormwise social science research? The US Food and Drug Administration and the US Department of Health and Human Services Office of Human Research Protection maintains ethical oversight for research involving human subjects for all universities that receive federal research funding. This oversight provides extension of the Common Rule to universities through research ethics committees often referred to as Institutional Review Boards (IRB). Before researchers can initiate data collection from human subjects, all research protocols and data collection instruments (e.g., interview and focus group questions, surveys, etc.) must be reviewed and approved by the IRB. All members of a research team who will have access to data collected from human subjects must also complete a certification course focused on responsible conduct of research involving human subjects. You can read more about the UConn IRB here. You can read about ongoing Stormwise human dimensions research here.