A method is presented by which a simple physical model of radiative transfer may be used to estimate the vertical distribution of the plant area index (PAI) from measurements of average photosynthetically active radiation at various heights within the canopy and a known phytoelement inclination distribution. Eight semi-natural mountain grasslands, differing in land use, are investigated. Using a set of baseline parameters obtained from the literature, predicted plant area indices compare both qualitatively and quantitatively favourably with those determined from destructive harvesting. Model predictions are shown to be sensitive to the phytoelement dispersion coefficient and phytoelement optical properties. Due to mutually opposing effects on PAI predictions, their parameterisation is found to be critical. Predictions assuming a spherical phytoelement angle distribution are demonstrated to be not significantly different from those based on measured inclination distributions.