Effects of relative irradiance on the leaf structure of Fagus sylvatica L.seedlings planted in the understory of a Pinus sylvestris L. stand after thinning

Beech seedlings were established in the understory of a Pinus sylvestris pl antation close to one of the southernmost populations of beech in Europe, t he beech-oak forest of Montejo de la Sierra. Four years later, the overstor y was partially reduced by removing pine trees. Solar radiation in the unde rstory was evaluated by hemispherical canopy photographic technique and the effects of relative irradiance increment on the leaf anatomy of beech seed lings were analyzed during the two years after opening the stand. The incre ase in specific leaf mass (SLM) in seedlings during both years runs in para llel with the increase in relative irradiance estimated by the global light factor (GLF) which expresses the proportion of global radiation relative t o that in the open. There were significant relationships between the light index as a surrogate of light environment and the morphological and anatomi cal characteristics of the leaves. In the first year, SLM increase was more related to total blade thickness. In the second year, thickness of palisad e parenchyma (PP) appears more relevant than that of spongy tissue (SP) as indicated by the absence of significance in the relationship between SP and SLM. Moreover, stomatal density was also higher according to increasing re lative irradiance. The shift response of beech seedlings to the overstory o pening makes evident their capability of acclimatization to light increase through changes in leaf anatomy.