THE DISTRIBUTION OF SAVANNA SPECIES IN RELATION TO LIGHT INTENSITY

Abstract

The distribution of savanna species in relation to light intensity was investigated during ecologic study of the prairie–forest border in Wisconsin. Four thousand intensity measurements were made in 17 stands, of which 915 were taken over sampled quadrats. Measurements were made on clear days during the midday period in July and August with a Weston photometer, and readings from different stands were considered comparable. Light amplitudes were determined by calculating the percentage of a species in the quadrats of each intensity size class. These classes were based on a log 10 scale after schemes of linear and geometric progression indicated curvilinear distributions would most likely result from a log scale. Forty-seven species distributions were calculated, 43 of which showed normal patterning with distinct modes.The log 10 scale was used to show distribution of light intensity in 16 stands along a gradient from prairie to terminal forest.Light distributions of species on two soil moisture levels showed a highly significant tendency for the lower light amplitude to occur on the drier moisture level. This tendency operated for species at all levels of the soil moisture gradient.Possible mechanisms for the species–light distributions were discussed in relation to growth–light curves and to conditions of soil moisture and evaporation which are correlated with or controlled by light intensity.