Canopy structure and spatial heterogeneity of understory light in an abandoned Holm oak woodland

Understory light is crucial to understand forest ecology but there is scant information for Mediterranean forests. Understory light of an abandoned Holm oak (Quercus ilex L.) woodland was studied in central Spain by means of hemispherical photographies in a 30 x 30 grid of 1-m(2) points. Canopy height, stem density and basal area had a significant influence on understory light. Height exhibited the most significant correlation, with indirect light. However, its potential as a predictor of understory light was low due to the large fraction of unexplained variance. Sunflecks contributed to half of the understory light; they were intense and long (25 min), and 10 min shorter at the herb than at the shrub layer. Mean light availability in the understory was half of that in the open and it exhibited a significant spatial heterogeneity. Spatial grain was significantly coarser for indirect than for direct light; it was also coarser at the herb than at the shrub layer, indicating that while a single individual shrub exploits light heterogeneity via phenotypic plasticity at the shrub layer, different individuals or micropopulations exploit it at the herb layer. Abandonment of traditional management of Holm oak woodlands leads to a decrease in both the availability and the spatial heterogeneity of understory light.