This report describes the establishment of three 0.5-ha gaps and the measurement of two until the end of the 2010 growing season. Three study sites were established on eastern Vancouver Island to explore the effect of stand edge on the growth of four species of conifers with different shade tolerance rankings. One light study gap was planted at the Snowden Demonstration Forest (GS10) in 2004, another near Elk Bay (HU10) in 2006, and a third at Gray Lake (PP10) in 2009. All study sites are within 40 km of Campbell River. Funding restrictions prevented light estimates from being made at the Gray Lake site; therefore, only growth measurements have been possible at that site. At GS10 and HU10, the light environment of each seedling was characterized using hemispherical photographs. In addition, instruments were installed to measure transmittance of above-canopy light (T), temperature, rainfall, soil moisture, and vapour pressure deficit across both gaps. Data for at least five growing seasons (April 1 - October 31) are presented for both sites. Maximum T at GS10 and HU10 was approximately 0.8 and 0.7, respectively, and minimum levels in the understorey declined to less than 0.05 at both sites on the south side of the gaps. Growth on the sites was affected by both soil moisture and vapour pressure deficit. An additive model was used to incorporate the moisture variables with T estimates to model the effect of light on growth, although replication was considered insufficient for statistical comparisons between the two sites. Growth tended to increase to a maximum at T values of approximately 0.3 and 0.8 for western hemlock (Tsuga heterophylla [Raf.] Sarg.) and western redcedar (Thuja plicata Donn.), respectively, and did not reach a maximum at approximately 0.8 above-canopy light for Douglas-fir (Pseudotsuga menziesii [Mirbel] Franco) and western white pine (Pinus monticola Dougl.). Light modelling with TASS III revealed both differences and similarities between the modelled values and estimates derived from hemispherical photos.
These microclimate and light assessments suggest that conifer seedling growth is a function of a combination of light, moisture, and temperature conditions that change depending on site and proximity to the residual stand and position within gaps. These early 5-year results indicate that a group selection system with gaps smaller than 0.5 ha may not provide adequate light for less shade-tolerant species such as Douglas-fir and western white pine to outcompete more shade-tolerant species such as western hemlock and western redcedar but that moisture differences can help compensate for lower light levels. Continued monitoring of these two sites along with additional gap studies will provide valuable information from which to explore forest gap regeneration dynamics and improve science-based decision-making regarding partial cutting silvicultural systems.
Technical Report 077 (6780 KB)
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Updated January 08, 2014