Artificial canopy gaps in the restoration of boreal conservation areas: long-term effects on tree seedling establishment in pine-dominated forests

Abstract

Creation of canopy gaps can be used as a restoration measure to aid in the diversification of structural variability and tree species composition in forests. However, it is not generally known how the specific properties of gaps influence restoration success. In this study, we examined tree seedling regeneration in 94 small canopy gaps that were established for restoration purposes in two protected Pinus sylvestris (Scots pine)-dominated forests in eastern Finland 10 years earlier. In particular, we assessed the effect of gap size, within-gap position, and soil preparation (exposure of the mineral soil) within the gaps (soil disturbance) on seedling establishment. We found that tree species composition in the sample plots consisted mainly of P. sylvestris (73 %) and Betula spp. (16 %) seedlings. Most Pinus and Betula seedlings were found in the disturbed plots, and only 6 % of the seedlings were found in the undisturbed plots (no soil preparation applied). The mean number of pooled Pinus and Betula seedlings per disturbed plot (40 × 60 cm) was 0.5 and 0.1, respectively. Mean height of the Pinus and Betula seedlings was 9.2 cm and 51.5 cm, respectively, which suggests that seedling development in the disturbed plots was very slow and is probably hindered by an insufficient amount of solar radiation reaching the forest floor, as well as by root competition with the trees that surround the gap. Both factors are affected by gap size, which was probably not large enough (mean diameter ≤23 m) in our study forests to reach a threshold value that meets the requirements of Pinus and Betula seedlings. We conclude that the creation of small canopy gaps does not appear to be an effective method to restore the age class structure and tree species composition in boreal pine-dominated forests. Further studies that manipulate the gap size more widely than this current study are needed.