Analysis of wlndthrow factors in windbreaks: In the Case of Bibai, Hokkaido by typhoon no.18 in 2004

Abstract

Windthrow factors in windbreaks in Bibai, Hokkaido, which was damaged by typhoon No.18 (max wind speed 21 m/s) in 2004, were analyzed. The windbreaks investigated were plantations of Picea abies, Larix leptolepis, Betula platyphylla, or Fraxinus mandshurica. Diameter at breast height (DBH) and the type of damage (e.g., "uprooted", "snapped", or "undamaged") were determined for each tree in 21 quadrats. The vertical profile of soil hardness was also measured using a simplified dynamic cone penetrometer in each quadrat The maximum moment of resistance against uprooting was measured by pulling the trunk of a standing tree laterally for each species to determine why the sensitivity to windthrow differed. The sensitivity to windthrow decreased in the order L. leptolepis >P. abies >B. platyphylla >F. mandshurica. The sensitivity to windthrow had a bell-shaped distribution with a peak at DBH = 30 cm. L. leptolepis and B. platyphylla were more likely to be "uprooted", while P. abies was more likely to be "snapped". Soil hardness was not closely related to the sensitivity to windthrow. The moment of resistance against uprooting was higher for F. mandshurica than for the other three species. The sensitivity to windthrow might be related to tree height and leaf mass, in addition to the moment of resistance against uprooting.