Can understorey native woodland plant species regenerate under exotic pine plantations using natural succession?

Abstract

Forestry industry in many European countries has begun to focus on sustainable forest management practices, and consequently, a greater emphasis is now being placed on the restoration and enhancement of native woodlands in places where intensive forestry is nowadays not highly profitable. In this context, we evaluate the natural regeneration of native oak woodland vegetation under cultivated stands of Pinus radiata in the Biscay region, Northern Iberian Peninsula. We compared vegetation composition and diversity on 60 stands representing the three commonly observed habitats: regenerating Quercus robur woodlands, old-growth native Q. robur woodlands, and their adjacent P. radiata plantations at different successional stages. The aim was to assess the potential of natural successional processes to restore the native oak woodland species under pine plantations, determining whether natural regeneration is sufficient or some management interventions are needed. The results reveal significant differences in understorey species composition between pine plantations and oak habitats. However, these understorey compositional differences were reduced during natural successional process (from young to old age plantations), being especially important in the case of tree and fern growth-forms. The successional trends are driven by an increase of tree, fern and native species cover during pine plantations succession, although the richness was always higher in plantations mainly by the presence of a great number of generalist and opportunistic species. Nevertheless, some typical woodland species, such as Ulmus minor and Lamiastrum galeobdolon, did not appear in plantations. Here, the natural successional process produced a slowly convergence in understorey species composition between plantations and oak habitats. However, the old pine plantations and oak habitats still differed considerably in understorey composition, suggesting that using only natural succession a much longer time frame is needed to achieve our ecological restoration objective. Natural succession could be used to achieve the restoration objectives at relatively low costs almost for tree and fern growth-forms, although in the case of ancient woodland species especial actions would be needed. The reorientation of pine plantations towards species compositional states that are more similar to native oak habitats could be faster using adaptive forest management practices (e.g. single tree selection).