A preliminary survey of regeneration of Sitka spruce in wind-formed gaps in British planted forests

Abstract

A transect survey of wind-formed gaps was conducted in planted Sitka spruce stands in the British uplands. Observations were made of gap size (as cover and opening) and presence of regeneration. Gaps ranged in size from 4 to 7900 m 2, but the size distribution was highly skewed with a predominance of small gaps less than 100 m 2. Sitka spruce seedlings were present in 27% of the gaps, while germinants were present in 62% of the gaps. The largest seedlings most often occurred on raised positions provided by the upturned root plates. Mean gap size (cover and opening) was significantly greater for gaps with spruce seedlings than those without. Measures of gap opening provided the best guide to presence of regeneration, and no seedlings were observed in gaps with an aperture (proportion of hemisphere without canopy) of less than 0.15. This threshold is consistent with results from natural forests of Sitka spruce in the Pacific Northwest, whereas comparison of gap areas was less reliable due to the influence of tree height. The results provide guidance on the minimum size of opening required to regenerate Sitka spruce that has application in the development of new silvicultural systems.