The effect of thinning intensity on the below-canopy light environment in a Sitka spruce plantation

A Sitka spruce stand in central Scotland was thinned with five interventions from a basal area of 59-15 m(2) ha(-1) before being clearfelled. For each stand density, the light environment beneath the canopy was characterised by making direct (with a ceptometer) and indirect measurements (using hemispherical photography) of the canopy transmittance. A comparison between the two methods of measurement showed that the indirect method discriminated poorly at low levels of transmittance (<0.1). Above this, there was good correlation between the two methods, although the indirect method underestimated the transmittance when compared to the direct measurements. Transmittance increased exponentially as basal area was reduced, to a value of almost 0.5 at basal area 15 m(2) ha(-1). The relationship between basal area and transmittance conformed to Beer's law (transmittance = exp(-kL), where k is an extinction coefficient and L the leaf area index), which is most commonly applied to continuous canopies. The value of k fitted to the data (0.31) was much lower than the value of 0.6 associated with closed Sitka spruce canopies, indicating the reduced light interception efficiency in discontinuous canopies. The relationship between transmittance and proportion of basal area removed could be used as guidance for managers wishing to manipulate the light environment beneath a closed-canopy Sitka spruce stand, for example to encourage natural regeneration. However, there are other factors to consider, such as the effect of thinning on the stand stability. The vigour of the original stand is also important, as it determines whether the canopy will quickly regrow, or whether the increase in light levels will be sustained. (C) 2002 Elsevier Science B.V. All rights reserved.