1 Effects of shrub and canopy cover on the relative growth rate of Pinus pinaster Ait. seedlings of different sizes

Background: Shrubs are recognized as important tree regeneration niches. In this study we experimentally analysed the effects of shrub presence, canopy cover (closed cover and open cover-gaps), and seedling size on P. pinaster growth. Objective: We expected that: 1) seedling-shrub interaction would depend on seedling size and would shift from positive to negative with increasing size; 2) overstorey canopy would affect seedling-shrub interaction, with stronger and positive interactions in gaps and 3) microsite factors would be affected by vegetation. Results: Social status and plant size appeared to determine the post-shrub-treatment growth response. The effect of shrub on seedling growth varied with the canopy cover. Under closed canopy, there was a shift from a positive net effect on smaller seedlings to a negative net effect on mid-size seedlings, and then back to a positive net effect on larger seedling response. Under open canopy a negative net effect was observed, with no change in the sign of the interaction as seedling size increased. Microenvironmental conditions varied in relation to overstorey and understorey vegetation, while soil conditions varied in relation to overstorey. The seedling-shrub interaction had both positive and negative net effects, which could help define the natural regeneration dynamic of P. pinaster