Height competition between Quercus petraea and Fagus sylvatica natural regeneration in mixed and uneven-aged stands

The mid-successional sessile oak (Quercus petraea (Matt.) Liebl.) and the late-successional European beech (Fagus sylvatica L.) are two major species of temperate forests. According to the literature, in mixed stands, large canopy openings should promote the growth of the light-demanding oak over the shade tolerant beech. Nevertheless, foresters who manage mixed oak and beech forests with continuous-cover silviculture in Western and Central Europe, face difficulties to promote the regeneration of oak. In the Belgian Ardennes, we monitored the mixed advanced regeneration of 7-13 year old oak and beech trees scattered across 23 sites to test the hypotheses regarding the relative advantage of the two species across the light gradient, the outcome of interspecific competition exerted by neighbouring saplings, the effect of direct and diffuse radiation and the site effect. We adjusted a set of mixed non-linear models of the height growth of saplings for the two species and selected the best model for the two species. In contrast to expectations, we found that beech saplings had the highest height growth rate at all light levels. Beech saplings reached an optimum growth at transmittance of 10%, whereas oak saplings needed more than 20%. The two species responded positively to soil richness but only oak saplings responded to direct radiation and micro-climatic variations. These results indicate that oak saplings are systematically outcompeted by beech saplings across the light gradient. Thus, the control of canopy opening is not sufficient to promote the natural regeneration of oak beneath a stand also containing beech in the Belgian Ardennes.