Forest Investment Account

Abstract of FIA Project 6065004

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Variable Retention Windthrow Monitoring Pilot Project 2001 to 2003

Author(s): Rollerson, T.P.; Beese, W.J.; Peters, C.M.
Subject: British Columbia, Forestry, Silviculture/Forest Management Systems
Series: Forest Investment Account (FIA) - Land Base Investment Program - Innovative

Abstract

The Variable Retention (VR) Windthrow Monitoring Pilot Study ran for a two-year term.
The first year of the pilot project was carried out in the eastern portion of Weyerhaeuser's West Island operating unit in central southwestern Vancouver Island (WI-C) and in the Queen Charlotte Island operating unit (QCI). The second year of the pilot study was carried out in the Klanawa and Walbran watersheds in the western portion of Weyerhaeuser's West Island Timberlands unit (WI-W), and in the Port McNeill Timberlands (PM) and North Island Timberlands (NI) units on the north coast of Vancouver Island.
Locating the pilot study sites in these widely separated areas allowed better evaluation of the possible extremes of windthrow that may be associated with variable retention silvicultural practices.
The overall project objectives that are partially addressed by the pilot project are:
Document the amount of windthrow associated with VR.
Document the spatial distribution or patterns of windthrow associated with VR.
Document regional differences in the extent of windthrow associated with VR.
Identify the qualitative and quantitative factors associated with VR windthrow including both environmental factors and treatment effects.
Identify specific management options to control windthrow associated with VR.
Develop field indices and decision-making tools for windthrow hazard assessment by operational planners.
Communicate results to field staff to help reduce the potential for wind damage by improving harvesting layout and silvicultural treatments.
The pilot study database now consists of total of 1640 plots: 112 in the NI, 464 in PM, 193 in the QCI, and 872 plots in the WI split between 226 in the west central part of Vancouver Island and 646 closer to the West Coast. A total of 56 harvested blocks are included in the sample.
The pilot showed some definite regional differences in windthrow. The average amount of windthrow along external setting boundaries and the edges of larger retained patches varied from an average of 5% on the PM blocks to 21% in the NI blocks, with an overall average of 11% for those blocks that had experienced at least two winter wind seasons. There was a similar regional trend with windthrow for retained groups. The average windthrow in retained groups is 10% in the PM area and 39% in the QCI with an overall average of 28%. For strips of retained timber, the windthrow rates range from 14% in PM to 52% in WI-C, with an overall average of 26%.
The data indicate that windward edges on external boundaries and larger patches are more vulnerable to windthrow than other boundary exposures. There is an apparent trend of increasing windthrow with increasing fetch distance along these edges; this effect may be less pronounced at greater distances. The character of the fetch surface may affect the amount of windthrow along these edges. Boundary edges downwind of areas of retention appear to be subject to less windthrow.
There is some indication that topping and pruning treatments reduce the amount of windthrow in retained groups of timber, but as the data set for these treatments is small, this conclusion should be considered tentative. The effect of edge treatments along external edges and retained strips is unclear.
There may be some relationship between topographic location and the amount of windthrow. Topographically exposed locations tend to experience more windthrow. For both external edges, retained groups and retained strips, the amount of windthrow increases with increasing stand height.
There is a reasonably strong trend of decreasing amounts of windthrow as the size of the retained group increases. Windthrow in retained patches and groups tends to be concentrated along windward edges.
In general, external edges and the edges of retained strips are much more vulnerable to windthrow when they occur along the edges of gullies than other locations.
The distance that windthrow penetrates into a stand edge tends to be affected by some of the same factors that control percent windthrow. Average penetration distances along external boundaries range from a mean of 6 m in PM to 12 m in WI-W, with an overall average of 8 m. Penetration tends to increase as percent windthrow increases and also varies with changes in boundary exposure. Penetration distances increase with increasing exposure to wind, being least on lee boundaries and greatest along windward boundaries.
A similar relationship is seen with average fetch distance, with penetration increasing as fetch distance increases. Windthrow penetration increases with increasing stand height and increasing rooting depth.


For further information, please contact Derek Ferguson

Updated September 08, 2005 

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