Grassland canopy composition and spatial heterogeneity in the light quality

Both vertical and horizontal heterogeneity in red light levels and in the r ed/far red ratio were measured in a mountain grassland and correlated with the vegetation structure. Vertical change in the red/far red ratio was meas ured in three communities; canopy structure and density was investigated by the point-quadrat method and biomass harvesting. Quantities of both grasse s and dicots were significantly correlated with R/FR ratio and red light in tensity at ground level. In dicots, percentage cover determines their effec t on light levels; in contrast, no single parameter suffices to capture the effect of grasses: grass biomass, number of hits (cover) and the mean heig ht (distance from the soil surface) of grass hits are of importance. This i ndicates 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 x 3.3 cm cells (plots of 8 x 8 cells) in the species-poor community. Bo th red light and R/FR ratio were determined at soil level in all the grid c ells; 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 a nd individual species occurrences revealed many significant autocorrelation s. Light levels (particularly red light) show autocorrelations at the dista nce of 1-2 cells (3.3-6.7 cm); biomass shows little autocorrelation. A mult ivariate analysis (redundancy analysis) showed that cellwise densities of t wo species had significant and systematic correlations with the light level s (both red light and R/FR ratio): a grass species with wide, mainly horizo ntal leaves, Anthoxanthum alpinum, was negatively correlated with light lev els; Nardus stricta, with upright and narrow leaves and stems was positivel y correlated with light levels.