Grassland canopy composition and spatial heterogeneity in the light quality

Both vertical and horizontal heterogeneity in red light levels and in the red/far red ratio were measured in a mountain grassland and correlated with the vegetation structure. Vertical change in the red/far red ratio was measured in three communities; canopy structure and density was investigated by the point-quadrat method and biomass harvesting. Quantities of both grasses and dicots were significantly correlated with R/FR ratio and red light intensity at ground level. In dicots, percentage cover determines their effect on light levels; in contrast, no single parameter suffices to capture the effect of grasses: grass biomass, number of hits (cover) and the mean height (distance from the soil surface) of grass hits are of importance. This indicates strikingly different geometry and optical properties of both plant groups.The fine-scale horizontal heterogeneity was assessed by means of a grid of 3.3×3.3 cm cells (plots of 8× cells) in the species-poor community. Both red light and R/FR ratio were determined at soil level in all the grid cells; fine-scale vegetation recording was done by counting all rooted stems (leaves for large rosette plants) in these cells. Spatial autocorrelation (Moran's I) of fine-scale heterogeneity in light levels, biomass per cell and individual species occurrences revealed many significant autocorrelations. Light levels (particularly red light) show autocorrelations at the distance of 1–2 cells (3.3–6.7 cm); biomass shows little autocorrelation. A multivariate analysis (redundancy analysis) showed that cellwise densities of two species had significant and systematic correlations with the light levels (both red light and R/FR ratio): a grass species with wide, mainly horizontal leaves, Anthoxanthum alpinum, was negatively correlated with light levels; Nardus stricta, with upright and narrow leaves and stems was positively correlated with light levels.