Relationships between forest structure, understorey light and regeneration in complex Douglas-fir dominated stands in south-eastern British Columbia

A quantitative understanding of the effects complex residual forest structure has on the understorey light environment is useful in guiding strategies that facilitate regeneration in irregular stands. In this study we evaluate several methods for estimating the understorey light environment in an uneven-aged mixed conifer forest in the Interior Douglas-fir zone of south-eastern British Columbia, Canada. Measurements were collected 15 years after selection harvesting, which was designed to create four different levels of residual basal area (8 m2/ha, 16 m2/ha, 24 m2/ha and uncut). Understorey light availability was highly variable across all levels of residual basal area with estimates of stand structure, including density (N), basal area (G) and stand density index (SDI) explaining 37.2–52.1% of population level variation. While relatively large trees effectively reduced light levels below 40% open sky at a stand level; intermediate and smaller sized trees were found to have the greatest influence on light attenuation at the micro-site scale. Stand variables that heavily weighted the influence of larger trees on light attenuation were weak predictors of understorey light at the micro-site scale. Incorporating variables describing composition and inequality of tree size increased the predictive power of the models resulting in the ability to explain 55% of the variation in light levels. Small plot sizes used in this study are likely limiting the ability to describe variation in light levels and we recommend the use of larger plots (with a radius between 1.0 and 1.7 times tree height) for future studies of this nature. The abundance and growth of natural regeneration was strongly related to both light (as influenced by overstorey) and understorey vegetation in these stands.

► Examined relationships between forest structure, understorey light and regeneration.
► Density, basal area and SDI explain 37.2–52.1% of the variability in light levels.
► Adding composition and tree size influences increased R2 to 55%.
► Abundance and growth of regeneration related to light and understorey vegetation.