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Understanding and improving wind risk assessment for farm forests in Ireland


Project Description

Strong winds can result in tree uprooting (windthrow, the most common one) and/or breakage (windsnap) causing considerable damage and leading to major financial losses for forest owners. The annual loss to the forest industry due to windthrow is estimated at approximately €1.3 million [1]. Damage to timber is one of the main consequences of storm damage. Fitzpatrick [2] determined that during the period 1971 to 1998 15.1% of the total volume of timber sold was windthrow timber. One key issue is that after major storms the price of timber often falls[3] arising from an oversupply of timber onto the market and/or a reduction in the quality of the timber salvaged [4].

The current approach to understanding and managing these factors in Ireland follows a two-pronged approach. Firstly, attention is focused on the management of tree stability through cultivation techniques aimed at promoting tree stability[8,9], and secondly, via an empirical model which relates tree stability to site factors and wind induced forces to predict the probability or occurrence of windthrow [9,11]. Whilst much success and progress has been achieved through these approaches, these methods have limitations in that they have been developed based on general wind conditions and represent endemic windthrow in forest stands. Therefore, their usefulness to quantify the effects of severe winds and storms (i.e. catastrophic windthrow) is more limited. Hence, considerable uncertainty exists within climate change scenarios of increased frequency and severity of winter storms and their likely effects on Irish forest and whether new/different management approaches are required [12] and the influence of forest design to mitigate windthrow (e.g. minimising brown edges, direction of cultivation, etc.). New methods and technology now exist in which to incorporate the characteristics of wind and root anchorage in response to soils characteristics which allows a greater understanding of the potential effects of catastrophic windthrow.

Windthrow/windsnap arises as a result of the interaction of the wind with the tree(s) and the corresponding wind-induced force where windthrow occurs as a result of the force exceeding the root anchorage strength of the tree, while windsnap occurs when the strength of the stem is not sufficient to withstand major storms. The turbulent nature of the wind results in a time varying wind-load on the tree, which in turn introduces the dynamic motion of the tree(s). A simple engineering analysis shows that a dynamic force of a certain value can significantly exceed its static counterpart. Thus, the current empirical models which are based on static tree pulls in order to establish tree strength and give some indication of the effect of forest management on stability [13] may not fully represent the range of forest or environmental conditions which trees are subjected to (e.g. tree pumping, rotating motion, effect of severe gales, transfer of kinetic energy among tree crowns) especially the uncertainties associated with the predicted increasing incidence of severe winter gales associated with climate change. For example, the sequencing of extreme events where extremely high rainfall resulting in waterlogged soils is followed by severe wind gusts predisposed forests to windthrow in storm Darwin [12]. The effect of the forest canopy on wind dynamics including high level turbulence and generating wind shear is poorly understood and this research hope to provide a deeper understanding on how wind interacts with the forest canopy and whether this in itself is increasing windthrow and research which simulates how trees react to windstorm conditions may be of increased value [14].

The research will combine the expertise of biologists, arboriculturist, foresters and engineers in order to investigate windthrow/windsnap in trees. An idealised numerical model of the tree will be developed covering both the stem and root system [15,16]. Previous work on crops has shown that it is the latter which is key to understanding the issue and as such, a physical model will also be developed which will enable calibration of the numerical model. The research aims to use innovative methods to include wind as including physical testing in a wind tunnel to simulate different windspeeds and directions and a variety of turbulent conditions with tree models in order to predicted realistic windthrow/windsnap velocities for different soils and as a function of tree height. Combined with the numerical modelling, the aim is to provide insight on the terminal height of trees in response to wind loading and the influence of forest design on the vulnerability of forests to windthrow.

Funding Notes

Funding is secured at EU/Home rate. International students are welcome to apply as long as they can cover the overseas fees.

References

[1] (https://www.teagasc.ie/crops/forestry/research/reducing-windthrow-losses-in-farm-forestry/). Last accessed 8th June 2019.
[2] Fitzpatrick, P. J. (2000) Timber volume and value losses associated with catastrophic winthrow. Unpublished MAgrSc Thesis, University College Dublin, Ireland. (cited in http://www.coford.ie/media/coford/content/publications/2018/SM23ManagingWindthrow160418.pdf)
[3] Schuck, A., Schelhaas, M.J. 2013. Storm damage in Europe – an overview. In: Gardiner, B., Schuck, A., Schelhaas, M. J., Orazio, C., Blennow, K., Nicoll, B. (eds.): Living with Storm Damage to Forests. What Science Can Tell Us, 3, 15–23.
[5] Peltola, H. 2006. Mechanical stability of trees under static loads. American Journal of Botany, 93(10), 1501-1511.
[6] Moore, J. R., Manley, B.R., Park, D., Scarrott, C.L. 2013. Quantification of wind damage to New Zealand’s planted forests. Forestry, 86, 173-183.
[7] McInerney, D. Barrett, F., Landy, J., and McDonagh, M. 2016 A rapid assessment using remote sensing of windblow damage in Irish forests following storm Darwin. Irish Forestry 73. 161-179.
[8] Hendrick, E. 1989. The effect of cultivation method on the growth abd root anchorage of Sitka spruce. Irish Forestry 46.
[10] Ni Dhubhain, A., Walshe, J., Bulfin, M., Keane, M and Mills, P (2001) The initial development of a windthrow risk model for Sitka spruce in Ireland. Forestry, Vol. 74, No.2, 161 – 170.
[11] Ni Dhubhain, A., , Bulfin, M., Keane, M and Mills, P., Walshe, J. (2009) The development and validation of a windthrow probability model for Sitka spruce in Ireland. Irish Forestry, Vol. 66, 51 – 74.
[12] Ni Dhubhain, A., Farrelly, N. 2018. Understanding and managing windthrow. COFORD Connects silviculture/Management note No. 23. COFORD, Dublin.
[13] Gonzalez, G.F. 2017. The influence of thinning on tree stabiloity in Sitka spruce. MSc thesis, UCD, May 2017.
[14] Pivato, D., Dupont, S., Brunet, Y. 2014. A simple tree swaying model for forest motion in windstorm conditions. Trees 28: 281-293.
[16] Rodgers, M., McHale,J., and Mulqueen, J. 2006. Stability of Sitka spruce on mole-drained and ploughed surface water gley soil. Irish Forestry 63.
[18] Gardiner, B.A., Stacey, G.R., Belcher, R.E. and Wood, C.J. 1997 Field and wind tunnel assessments of the implications of respacing and thinning for tree stability. Forestry 70(3), 455 234–252.

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