Influence of thinning on the shelter effect of windbreaks as clarified by a wind tunnel experiment

Abstract

We measured the shelter effect of model windbreaks in a wind tunnel before and after adjustment thinning based on methods for managing overcrowded windbreaks. One of the aims of this experiment was to determine whether or not the actual thinning done on an actual windbreak would sustain the shelter effect. The shelter effects were evaluated based on the critical wind speed and the shelter distance. Six model windbreaks were set to model a windbreak after the thinning (AT1–6) and one was set to model it before the thinning (BT). In models AT1–3, the trees in the middle part of the windbreak were cut and the trees in both the windward and leeward parts were left. AT1 and AT2 were models that reproduced the actual windbreak after the thinning. In the other three models (AT4–6), which were set as additional thinning models based on AT3, trees in both the windward and leeward parts were cut. The thinning ratios of these models ranged from 21% (AT1) to 84% (AT6). The shelter effects of AT1–3 were sustained or only slightly decreased compared with that of BT. The shelter effects of AT4 (thinning ratio of 63%) and AT5 (79%) were the highest among all of the models. From these results, it was confirmed that the thinning done on an actual windbreak was able to sustain the windbreak's shelter effect. If the trees at the edges are not felled, the shelter effect of the windbreak can be sustained or improved, even if the thinning is heavier than AT2.