Effects of thinning on seed rain, regeneration and understory vegetation in a Pinus canariensis plantation (Tenerife, Canary Islands)

Stand density management in even-aged coniferous plantations may increase biodiversity and enhance natural tree regeneration. Thinning of pine plantations has only recently been started on the Canary Islands, yet the ecological effects of this treatment are relatively unknown. Our objective was to investigate the impact of thinning on pine seed rain, natural pine regeneration and diversity of understory vegetation in Pinus canariensis plantations. Seven years after thinning, we sampled 60-year old plantations thinned by 60% of basal area, unthinned stands and adjacent semi-natural old-growth forests. Thinning significantly increased pine seed rain to levels similar to semi-natural stands. This is probably a result of fast crown development and enhanced cone production due to the immediate release of resources and reduction in competition. In contrast, plantations with the highest tree densities had very low seed production, limiting pine regeneration. Seed rain showed seasonal fluctuations with clear summer peaks of seed release. Thinning also significantly increased seedling, sapling and juvenile densities. However, thinned plantations still differed from old-growth forests in seedling and juvenile densities. Thinning promoted understory plant diversity, including typical endemic pine forest species, without triggering plant invasion, which is of great conservation interest. This study provides evidence that heavy thinning represents an important management tool to enhance naturalization and local biodiversity of Canary pine plantations, common in large areas of this Archipelago.