Windthrow Hazard Assessment and Management Terry Rollerson Golder Associates Ltd.

tr Workshop outline Terminology / windthrow damage Mechanics Factors affecting windthrow Hazard assessment / classification / risk Management strategies Monitoring and operational trials Case studies Discussion groups

tr Terminology Windthrow –rotational falls –hinge falls Breakage –stem break –stock break

tr Terminology Endemic windthrow –occurs regularly on a small scale –individual trees or groups of trees Catastrophic windthrow –occurs infrequently when exceptionally strong winds cause widespread and extensive damage to large areas

tr Terminology Windthrow hazard –The likelihood or expected severity of endemic windthrow Windfirmness –the ability of a tree to resist overturning

tr Silvicultural systems Edges dominant: –Clearcutting –Clearcutting with reserves –Group retention –Strip shelterwood Spatial pattern dominant: –Single tree selection –Group selection –Irregular shelterwood –Strip shelterwood –Uniform shelterwood

tr Variable Retention Cutting Patterns

tr Windthrow Mechanics Torque   (F i h i ) Wind force   A i C D i u i 2 Gravitational force  m i x i g

tr Terminology Sway period –amount of time required for a tree crown to move though a complete sway Sway amplitude –the distance the tip of the crown moves from the vertical to its outermost sway point Damping –dissipation of energy in a tree through movement and contact of branches, stem and roots

tr Basic truths The smaller the perimeter/area ratio the less windthrow per unit area logged Stumps don’t blow over

tr Windthrow hazard classification Wind force factors Resistance to overturning Other indicators

tr Windthrow hazard Applied forces Resistance to overturning

tr Wind force factors High hazard –topographically exposed locations –windward boundaries –tall trees –large dense crowns –heavy removal Low hazard –topographically protected –lee boundaries –short trees –small open crowns –light removal

tr Resistance to overturning High hazard –trees with low taper and no butt flare –shallow rooting (<0.4m) –shallow soils –poorly drained soils (low shear strength) –root rot areas Low hazard –trees with high taper and large butt flare –deep rooting (>0.8m) –deep soils –well drained soils –coarse soils that favor deep rooting

tr Other indicators High hazard –moderate to extensive natural windthrow present –extensive windthrow on similar logged areas nearby –pit and mound micro topography Low hazard –no natural windthrow –no windthrow in similar logged areas nearby

tr Windthrow hazard class

tr Windthrow risk assessment Risk = Hazard x Consequence –Hazard = likelihood or severity of windthrow –Consequence = life / safety erosion - stream channel damage landslide activity fish habitat damage timber damage

tr Windthrow risk rank

tr Windthrow hazard assessments Office review: –air photos, maps, local knowledge Falling boundary traverses - edges Traverses within cutting area - spatial Collection of relevant field data Synthesis of all data Development of management options

tr Windthrow hazard mitigation Minimize applied forces Maximize Resistance to overturning

tr Management strategies Select Avoid Remove Retain Modify Induce change

tr Windthrow management Minimize total edge Minimize edge on high hazard sites Feather windward boundaries Top/prune high risk stems Limit % removal on high hazard sites Edges Edges/Spatial Edges/spatial

tr Windthrow management Remove vulnerable stems and stands Maintain smooth edges and canopies Retain stable windthrown edges Plan for salvage Vary treatments locally & monitor Edges/spatial Edges Edges/spatial

tr Windthrow toolbox Field assessment Common sense Management options LoggingMonitoring Existing knowledge Research Windthrow history maps (vector maps) Current toolbox New tools

tr Research approaches Process studies Treatment and effect experiments Retrospective studies Operational trials

tr Monitoring approaches Maps and simple databases Opportunistic / systematic Stratification by: –topographic and/or physiographic zones –soil/terrain types –stand types –treatments Field observations over time