Among the consequences of shrub encroachment is a decline in abundance and diversity of herbaceous species.  In some circumstances shrubs may form nearly monospecific stands because of the significant reduction in understory light.  To better understand how canopy structure affects patterns of understory light, the spatial heterogeneity of photosynthetically active radiation (PAR) was measured in the understory of three woody plant communities:  evergreen shrub thickets, an abandoned pine plantation, and a deciduous forest.  Heterogeneity was determined at 5 sites in each community during mid-winter, early spring, and mid-summer by measuring PAR every 1 sec for 15 min using a linear array of nine quantum sensors.  Data were analyzed using a variety of autocorrelation techniques to determine the maximum distance that PAR at any two points was related through time.  Shrub thickets demonstrated the lowest heterogeneity in understory light during the summer.  In winter and spring, spatial heterogeneity decreased in both forest types but increased in shrub thickets indicating that heterogeneity could not predict total understory PAR and was poorly related to leaf area index.  During summer, spatial heterogeneity in understory light is likely a function of stem density, canopy height, and foliage distribution which determine the frequency and size of natural breaks in the canopy and affect the size, frequency, and duration of sun patches.  The low heterogeneity in thickets suggests a lack of microsites with a favorable light environment and may further explain why shrub thickets often support a less diverse understory than other woody plant communities.