Modelling understorey light for seedling regeneration in continuous cover forestry canopies

Continuous cover forestry (CCF) aims at enhancing stand structural diversity and favouring natural regeneration. To give guidance on how to manage a CCF stand to achieve seedling growth below canopy, an estimate of light transmittance is required. So far, in the UK, only stand-level parameters have been used by managers to predict the understorey light in CCF stands. We assessed a UK Sitka spruce stand undergoing transformation to CCF and measured canopy transmittance using hemispherical pictures. Stand-level characteristics were found to be highly stand specific and not appropriate to predict seedling growth in CCF stands. We parameterized a detailed light model (4C-A-RTM) and a simple one-layer turbid medium model (BL). A sensitivity analysis was performed to test the effect of key stand structural parameters on the modelled transmittance. Measured transmittance from hemispherical photographs was used to validate the models. Both models tended to underestimate canopy transmittance but were positively related to current-year growth of the below canopy seedlings (R-2 = 0.92, P < 0.001). Comparison of the two models showed that the 4C-A-RTM provided a better estimation of light transmittance across observed canopy structural differences. Furthermore, the inclusion of stand characteristics in the 4C-A-RTM is likely to confer greater applicability across stands.