The purpose of the study was to test a simple light meter (luxe meter) as a tool for determining light conditions in the fertile beach forest understory, as well as, evaluating its usefulness in understanding the determinants of changes in the phenology of fertile beach forests undergrowth species. Field research was conducted during the period from 10 March to 10 November 2014. A permanent plot (100 m²) was located in the fertile beech forest Dentario glandulosae-Fagetum, the Świętokrzyskie Mountains (southern Poland). Phytosociological relevés were done (for seven months of the growing season; every four weeks on average) on the permanent plot using the Braun-Blanquet method. In addition, phenological phases of herbaceous species of undergrowth which occurred in every relevé were noted.

   In the centre of the permanent plot and in the nearby glade measurements of illuminance, E, were performed once a week, using a hand-held light meter. The measured illuminance (in lux) values were converted into photosynthetic photon flux density PPFD (in mmol·m^-2 ·s^-1 ) and irradiance I (in W·m^-2 ). The changes of the absolute values of the indices of light conditions and the overstory light transmission (in %) were analyzed. The illuminance values measured using a simple light meter accurately reflected changes in fertile beech forest understory light conditions depending on vegetation variability, cloudiness and wind speed. A significant negative correlation was found: r_s = -0.93 at p = 0.003 (t = -5.51) between the percentage coverage of tree layer (based on the Braun-Blanquet scale) and the overstory light transmission (obtained from the illuminance measurements). Statistically significant relations between the percentage coverage of tree layer and the number and percentage coverage of geophytes and hemicryptophytes in a particular phenophase were also found.

Keywords: fertile beech forest, luxe meter, permanent plot, percentage coverage, Poland.