Effect of gap size created by thinning on seedling emergency, survival and establishment in a coastal pine forest

Abstract

It is desirable and necessary to preserve the continuity of a coastal forest through reasonable management because it can provide many shelter benefits through altering the wind behavior along the shore. Thinning is an undoubtedly important measure for the continuity of forests as it provides suitable conditions for natural regeneration; however, thinning increases the risk of wind damage immediately after thinning in the coastal areas. Therefore, few thinning study related to regeneration in a coastal forest has been made. In order to test whether coastal forest of Japanese black pine ( Pinus thunbergii Parl.) requires a specific gap size created by thinning for regeneration and to compare seedling establishment among four thinning treatments, observations of emergence, survival and establishment of P. thunbergii seedlings, together with soil water content, litter, wind and light regime were made. The observations were conducted over four growing seasons in three sizes of circular gaps (the gap diameter to stand height ratios for the four gap sizes were 0.5, 1.0, 1.5 and 0.0, the control) corresponding to the four thinning treatments in Niigata shore, Japan. Results indicated that density of seedlings older than 1 year increased with gap size or canopy openness (OP). Seedling establishment was greater in 1.5 gap sizes than in any other gap sizes, while seedlings peaked near the west and north edges of the gaps but not in the gap centers exposed to direct solar radiation. Seedling growth in 1.5 gap sizes was also significantly higher than that in any others. A tendency of seedling height increasing from east to west edge and from south to north edge across the gap was observed. Only 1- or 2-year-old seedlings occurred in gap sizes of 0.0 and 0.5, therefore establishment in both gap sizes was considered as failing. The results imply that although P. thunbergii seeds can germinate in small gaps, even in under canopy, the seedlings are unable to survive. The seedlings apparently require a minimum gap size ≥1.0, or OP >30% in order to survive, and may require at least gap size ≥1.5, or OP >40% for further development into sapling. These results can be explained by the changes of microclimates, i.e. increase of light, soil water and airflow exchange, decrease of litter and canopy cover, and alleviation of the competitions for water in gaps created by thinning. Therefore, thinning strategy, especially patch-pattern thinning is potentially a viable silvicultural measure in management for the coastal pine forest. These results provide references for establishment and management of coastal P. thunbergii forests.