By Si Balch
(click to download a one-page synopsis of this article)
This CSLN Bulletin provides a synopsis of management actions that can be employed to maximize the resiliency of forest stands to damage by wind, ice and heavy snow. The goal is the development of forest stands and trees that can withstand 70 MPH wind gusts. Part One of this Bulletin provides background on windfirmness.
PART 2 – RECOMMENDED MANAGEMENT PRACTICES
Silvicultural Techniques for Wind Firmness
The best way to promote wind firmness is to reduce stocking levels. Lower-stocked stands have less standing inventory at risk, as well as bigger trees that add more value and are more efficient to harvest. The open quality of these stands also allows for more air movement, which creates a drier environment, so there is less fungal growth; interrupted small insect movement, such as bark beetle flight; and better conditions for birds to hunt insects.
Thinning is a better tool for building wind firmness in young stands, but it is possible to increase windfirmness in older stands if it is done very carefully. Below are more detailed summaries of what to consider when thinning stands of different ages.
Thinning Young Stands – sapling (conifers) to small pole (hardwoods)
This develops more drought resistant trees and increases the economic value of residual trees through better trunk growth caused by swaying (See Part 1- How Trees Grow and Respond to Physical Stresses). Young stands that are thinned have a much better chance of developing windfirmness than older stands. This is another reason to start new cohorts of trees across the ownership, so they can be thinned for greater windfirmness.
Things to consider:
- Soil Moisture
- Put your efforts on deeper, better drained sites. Shallow rooted or frequently saturated sites are poor candidates for thinning to develop windfirmness. On these sites, dense stands managed on an even-aged basis are probably a better way to reduce risk.
- Wet soils are weaker at holding roots, even if it is deep soil. Thus, soils that are saturated and unfrozen for longer periods of time are likely to have more blowdown.
- Crown Class
- Remove suppressed and intermediate trees that are the same age as overstory trees. Trees in the upper canopy are already expressing both wind firmness and good growth.
- Maintain high crown ratios. Conventional wisdom was that 30% crown was sufficient, new wisdom is that ratios in the 50% range are preferred.
- Space residual trees so that the crowns have space to sway some, but also have some support from neighboring trees near the likely limit of “sway stability”.
- Sufficient spacing allows swaying that causes trees to increase diameter growth in those parts of the trunk where the stress of that movement occurs. The trick is to manage it so most of that growth occurs on the commercially valuable section of the tree. This will vary by species, site, and tree form.
Thinning Intermediate or Multi-aged Stands
It is difficult and risky to develop a truly windfirm stand when starting with an unmanaged, middle-aged forest. Working with forests in this condition is likely to be disappointing because you cannot simply do a typical partial harvest and have a guaranteed increase in windfirmness.
Forests that have been managed are better candidates for windfirmness development, but they also present their own set of challenges. These stands may have had a series of partial harvests over time and thus have a variety of age classes. The classic “uneven-aged” forest is really a forest with several individual age cohorts. Managing these for windfirmness, or any other purpose, requires knowing which age cohort the trees you are managing fall into. Treat the old trees, intermediates, and young trees appropriately to each development stage. Crown shape and bark character are normally good ways to identify age.
Despite the challenges, there are a number of thinning practices that can result in significant benefits to windfirmness and other forest values. Thinning also increases the economic value of residual trees through better trunk growth caused by swaying (See Part 1- How Trees Grow and Respond to Physical Stresses).
The spatial structure of the forest canopy can affect wind dynamics. In particular, dense crowns create a smoother surface and deflect wind as a continuous front. Wind flows up and over this tree front and then curls into a roll. Significant damage often starts 2 to 3 tree heights back from the edge, where this roll comes back into the crowns. Understory growth, regeneration, shrubs etc, do absorb some of the stress of wind, as well as provide habitat for more bird species, which prey on insects.
- Trees growing at the edge of a forest.
- These individuals are more windfirm, having grown in windier conditions. Be very cautious about thinning these and disrupting their unified front.
- The stand edge perpendicular to the prevailing wind.
- Well-formed dominant or co-dominant trees, i.e. trees with
- Crown ratio of over 40
- Balanced crown
- Straight and vertical
- Suppressed and intermediate trees under chosen crop trees.
- Patches of trees between chosen crop trees that are not competing with crop tree crowns.
- Trees likely to fail due to:
- Poor Form
- Co-dominant stems
- Branches with included bark in the joint
- Heaved roots
- Height:diameter ratios over 90
- Taller than many surrounding trees
- Poor Health
- Stem decay
- Root decay
- Large dead branches
- Poor Form
- Species either prone to blow down or to develop internal decay.
- Less windfirm or more rot likely species from the list below.
- Trees competing with crowns of chosen crop trees.
- Remove trees whose crowns touch the crop tree crown. The desired outcome is more space for the crop tree crown to sway, but at the limit of that sway to meet a neighboring tree crown and get some support. This increased but limited sway space will result in the crop tree becoming more wind resistant and adding value through stem thickening in response to the added stress of swaying.
Note: In even-aged stands of spruce/fir, harvesting in patches results in less wind damage than uniform thinning.
Recently thinned stands are more vulnerable to wind and ice damage. It takes time for the residual trees to develop the extended root systems and the stronger, more tapered stems needed to withstand the increased motion cause by more wind within the stand. The commonly expressed time is five to ten years, but there is little research to back this up.
- There is a difference in susceptibility between recently thinned and un-thinned stands, but it is not a very large difference. Even though a thinned stand may suffer a higher % loss, it may actually lose fewer trees than an un-thinned stand because it has fewer trees to start.
- Recently thinned stands have a more irregular crown surface and therefore generate more turbulence in the wind flow. This leads to a higher wind load on individual trees and consequently more swaying. More swaying can lead to more trunk breakage and windthrow. This increased risk will last until the trees in the stand have acclimated to their new spacing.
Consider Setting a Terminal Height
Terminal height may be a concept worth introducing to tree management. Taller trees regardless of age, species, or any other factor, are more susceptible to windthrow, so setting a target terminal height may be a valid strategy.
Relative Wind Firmness (listed from most to least windfirm)
- Yellow Birch
- Sugar Maple, Beech, Hemlock
- Red and White Oaks
- Red Maple
- White Pine
- Other Conifers
- Live Oak
- Bald Cypress
- Southern Red Oak
- White Oak
- Sugar Maple
- Longleaf Pine
- Loblolly Pine
- Slash Pine
- Red Cedar
- Water Oak
- Yellow Poplar
- Red Maple
Other Methods for Reducing Risk of Wind/Ice/Snow Damage
- Get Insurance against loss from wind, fire and possibly other threats.
- Insurance is available from Outdoor Underwriters, Inc. of Columbia SC. The policies come through Lloyd’s of London and each insured property is evaluated based on location, a questionnaire, and assessment of the management plan. As an example, a 54 acre tract in Maine with $25,000 dollars of timber, can be insured against wind and fire for $500/year with a $5000 deductible.
- Put equipment on site if you know a big storm is coming.
- Weather forecasting is pretty good, so if you think there will be significant damage and road blockage, then putting machinery at key locations may be a good idea. Winds coming from abnormal directions are particularly damaging.
Bibliography, references, and credits:
- 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
- Michael Greenwood – Univ. of Maine – personal correspondence
- Frank Telewski – Univ. of Michigan – personal correspondence
- Tim Scott – USFS – Forest Products Lab – Madison Wisconsin – personal correspondence
- Dr. Claus Mattheck – Germany – tree biomechanics – website http://www.mattheck.de/english/english2.htm
- JuliaSchofield, 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 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.
- Storm Damaged Trees: Prevention & Treatments, Kim D. Coder, Professor Silvics/Ecology
Warnell School of Forest Resources , The University of Georgia, March, 1995