Managing Forest Stands for Wind and Ice/Heavy Snow Damage – two threats likely to increase with climate change
By Si Balch
(click to download a one-page synopsis of this article)
This Bulletin will be delivered in two parts; Part 1 provides background on stand and tree vulnerability to wind damage, Part 2 will cover management practices to enhance resiliency to wind damage with the goal of stands capable of withstanding 70 MPH wind gusts.
PART 1 – BACKGROUND & CONCEPTS
Forests are made up of stands that are, in turn, made up of individual trees, and we can think about the topic of building wind resistance at both of these scales. Forest stands can be managed to be windfirm, and the trees within those stands can also be managed to be windfirm. Similar principles apply to resistance against ice/snow damage as well.
There are two key areas of wind damage risk in trees: one is the tree’s grip on the ground and the other is tree stem strength. Crown breakage from snow and ice is ugly, but unless the main stem is broken, most trees recover pretty well. In fact, studies show that most hardwoods can lose up to 75% of their crown and recover. Of course, the downside is the increased threat of new internal decay and grade loss.
While there are certainly steps you can take to reduce the risk of windthrow, no stands or trees can be reasonably expected to withstand extreme winds over 95 MPH that are produced by derechos (line storms), downbursts (micro & macro), tornadoes and category 2+ hurricanes, regardless of the level of management. The greatest risk may be from derechos, which occur in warm weather along lines of severe thunderstorms that develop in humid air in the boundary between warm and cold air masses. These conditions are becoming more common in the Northeast, resulting in the potential for an increased frequency of derechos. Of course, some trees will withstand extreme winds over 95 MPH, but managing for that level of resistance does not seem reasonable in a forest situation, although it may be a suitable approach in parks.
Forest stands and individual trees may have few defenses against wind speeds over 95 MPH, but the relevant metric for forest management is critical wind speed, which is defined as the speed at which trees begin to blow over. It can be pretty low, depending on site and species. Shallow rooted trees on wet sites with saturated conditions may begin to topple at 40 MPH. Wind speeds over 50 MPH begin to damage many susceptible trees. The wind scales you normally hear referred to have the following wind speed associations:
· Beaufort Wind Scale defines hurricanes as having winds over 73 MPH.
· Saffir-Simpson Hurricane Wind Scale Category 1 hurricane is from 73-94 MPH
· Europeans define extreme storms as having wind gusts from 75-80 MPH
· Fujita Tornado Scale defines an F0 storm at 40-72 MPH and an F1 storm at 73-112 MPH
With an increased risk of strong storm events under future climate scenarios, the important question is: How do we develop stands and trees with sufficiently strong ground grip and stem strength to withstand 70 MPH gusts?
What do we know?
These characteristics are known to increase the risk of wind damage, by either blowdown or severe breakage.
· Tree height – Taller trees regardless of age, species or any other factor are more susceptible. This is simple physics, a longer lever (height) exerts more force at the fulcrum (root collar).
· Stem taper is an indicator of susceptibility. Trees with little taper are more susceptible, particularly small diameter trees. This is about stem strength because thicker stems are stronger. Trees grown in the open, where they are exposed to wind, have very tapered trunks. This makes them windfirm, but they are of little commercial value. The trick is to find the right combination of strength and value.
· Height diameter ratio – This relationship is expressed in like units: A 60 feet tall tree that is 10 inches DBH has a ratio of 72; (60X12=720/10=72). Trees with ratios between 60 and 80 tend to be stable, while trees with ratios over 100 are at high risk of damage.
· Shallow or restricted roots – Whether caused by thin soil, wet soil or species characteristics, shallow or restricted roots are a disadvantage because they reduce the tree’s grip on the ground. A related characteristic is small root area or lack of spread. In several big storms, poplar seemed particularly vulnerable. The root balls at the bottom of these trees were noticeably smaller than one might expect. Also planted trees whose roots were constrained in the pot may never grow a widely spread root system.
· Saturated unfrozen soils are physically weaker so the tree’s grip on the ground is diminished.
· Trees weakened by decay. Anytime the roots, trunk or branches have internal decay the structure is weakened and less able to withstand the bending force of wind or ice or snow.
· Trees with branching patterns that have proven to be susceptible. These include co-dominant stems and branch joints that include bark in the joint.
· Old trees, simply because older trees tend to be taller and have a higher likelihood of decay.
· Trees in recently thinned stands. Trees that have grown in fully stocked conditions, supported and constrained by adjacent trees, are susceptible when those adjacent trees are removed. This is a dilemma. Common thought is that it takes about five years for trees to adjust to more open conditions, by expanding their roots, crowns and stem configurations.
How trees grow and respond to physical stresses.
Understanding how trees respond to physical stress can lead to better management for windfirmness, ice resistance and economic value. To withstand deflecting forces, trees strengthen themselves by adding wood at the points of stress. The well-known phenomena of compression and tension wood in tree stems are exaggerated examples of this growth response. An individual tree will attempt to grow straight and balanced, in order to increase its ability to reproduce and collect carbohydrates from sunlight. This involves focusing wood growth in particular places, such as reinforcement where bending stress occurs. This differentiated growth is driven by auxin, which is the primary plant growth hormone. It originates in the buds and is distributed throughout the tree via a complex transfer system.
This additional growth occurs in the parts of the tree that most commonly experience high stress, such as points within the crown where branches move around a lot, the base of the crown where the whole moving crown meets the main trunk, and at the stump were the whole tree meets the root system. Thus, a tree grown in completely open conditions develops a very large crown that can absorb light from all directions and an exaggerated root collar to deal with the bending stress of the large moving crown. Growth is concentrated on crown expansion and stem/root stability, rather than height growth. In contrast, trees grown in tight, fully stocked stands must deal with very different conditions. In this case, there is relatively little stem bending stress, but there is strong vertical competition for light, which results in tall, relatively thin trees.
Understanding these growth patterns can be used to increase both the windfirmness and economic value of trees. These will be explored in Part 2 next month.
Bibliography, references and credits
· Dr. William Ostrofsky – Maine State forest pathologist – personal correspondence
· Greg Adams – JD Irving , Limited – Manager of research and development – personal correspondence
· Tony Filauro – Retired Great Northern Silviculturalist – personal correspondence
· Mike Dann – Retired Seven Islands Chief Forester – personal correspondence
· Dr. Michael Greenwood – Univ. of Maine – personal correspondence
· Dr. Frank Telewski – Univ. of Michigan – personal correspondence
· Tim Scott – USFS – Forest Products Lab – Madison Wisconsin – personal correspondence
· Prof. Dr. Claus Mattheck – Germany – tree biomechanics – websitehttp://www.mattheck.de/english/english2.htm
· Julia Schofield, CISR, Outdoor Underwriters, Inc, 140 Stoneridge Drive, Suite 265,, Columbia, South Carolina 29210 – personal correspondence
· Living with Storm Damage to Forests – What science can tell us – European Forest Institute – 2013 – Barry Gardiner, Andreas Schuck, Mart-Jan Schelhass, Chistophe Orazio, Kristina Blennow, Bruce Nicoll
· Predicting Stem Windthrow Probability in a Northern Hardwood Forest Using a Wind Intensity Bio-Indicator Approach, Philippe Nolet1,2, Frédérik Doyon1,2, Daniel Bouffard3 1Insitut des Sciences de la Forêt tempérée, Ripon, Canada 2Université du Québec en Outaouais, Gatineau, Canada 3Insitut Québécois d’Aménagement de la Forêt Feuillue, Ripon, Canada, – Open Journal of Forestry 2012. Vol.2, No.2, 77-87 Published Online April 2012 in SciRes (http://www.SciRP.org/journal/ojf)
· Tree Survival 15 Years after the Ice Storm of January 1998, USFS, Northern Research Station, Research paper NRS-25, February 2014. Shortle, Smith and Dudzik.
· Overview of techniques and procedures for assessing the probability of tree failure – David Lonsdale, 33 Kings Road, Alton, Hampshire GU34 1PX, UK
· wind and trees:lesson learned from hurricanes – chapter 5 – Publication No FOR 118 – Mary Duryea & Eliana Kampf – University of Florida, IFAS extension
· Are Irregular stands more windfirm? – W.L. Mason – Forest Research, Northern Research Station, Roslin, Midlothian, EH25 9SY, Scotland
· A mechanistic model for calculating windthrow and stem breakage in Scots pine at stand edge; by H Peltola – 1993 – Silva Fennica. 1993, Vol. 27 N20 2: 99-111.
· Crown structure and wood properties: Influence on tree sway and response to high winds – 2009 Damien Sellier SCION, 49 Sala Street, Rotorua 3010, New Zealand and
Thierry Fourcaud CIRAD, UMR AMAP, TA-A51/PS2, Boulevard de la Lironde 34398 Montpellier Cedex 5, France 10.3732/ajb.0800226Am. J. Bot. May 2009 vol. 96 no. 5 885-896
· Size- and Age-Related Changes in Tree Structure and Function: Size- and Age-Related Changes In Tree Structure and Function – Frederick C. Meinzer, Barbara Lachenbruch, Todd E. Dawson Springer, Jun 29, 2011
· Should Newly Planted Trees Be Staked and Tied? By William R. Chaney, Professor of Tree Physiology – Purdue – The Department of FNR-FAQ-6 FORESTRY AND NATURAL RESOURCES
· http://www.uky.edu/~jmlhot2/Resources/The%20Practice%20of%20Silviculture-Smith-ch.2.pdf The response of trees to individual thinning and pruning.