Unravelling the influence of light, litter and understorey vegetation on Pinus pinea natural regeneration

Pinus pinea (L.) is one of the most valuable species used in the Tunisian reforestation programme, with about 21 000 ha of plantations. In the coming decades the oldest of these stands will begin their regeneration stage. However, little is known about the factors that control the natural regeneration of this species. It is reputed to be a strict shade-intolerant species and so needs light to regenerate satisfactorily. Regeneration can also be influenced by understorey vegetation and litter, both correlated with light availability. The aim of this study was to quantify the respective importance of these three factors in P. pinea regeneration. Live seedlings were counted in 90 plots (500 m2 each) in three forests of P. pinea located in the coastal dunes in north Tunisia, and related to light availability, biomass of understorey vegetation and litter. In addition, the influence of litter was experimentally manipulated by creating 1 m2 regenerating areas free of litter, with a light litter layer or the natural thickness. The density of 1-year-old pine seedlings was controlled mainly by litter biomass, whereas light availability increased the growth of older seedlings. Understorey vegetation did not appear to play a significant role in P. pinea regeneration in these Mediterranean climatic conditions. Management for natural regeneration of P. pinea should include scarification to reduce litter thickness and heavy thinning to significantly increase light availability