Shrub expansion into historically herbaceous communities is emerging as a key component of global change in terrestrial ecosystems.  Expansion of shrubs into grasslands is often accompanied by a substantial reduction in understory light and an associated repression of shade-intolerant plants.  Light in shrub thickets is often reduced to levels below that found in adjacent forests; however, little work has been done to describe the structure of shrub canopies and determine how canopy structure affects understory light.  Our objective was to determine which characteristics of canopy structure most influence light attenuation and spatial/temporal dynamics of sunflecks in shrub thickets.  We measured canopy characteristics at multiple levels of organization (leaf, whole-plant and stand) for a variety of native and introduced shrub species that have a history of encroachment.  Measurements included leaf orientation, foliage clumping, branch bifurcation ratios, leaf area index (LAI), leaf area density (LAD) and stem density.  We also measured understory PPFD every 1 s during mid-day using an array of nine quantum sensors. We used stepwise multiple regression to determine which canopy characteristics best predicted total light attenuation and the duration, intensity, and relative contribution of sunflecks. 

Results/Conclusions

The best predictor of light attenuation and the relative contribution of sunflecks for all species was LAD.  Stem density was also a good predictor of sunfleck activity but stem density and LAD were highly correlated, especially within species.  Leaf-level measurements were highly species-dependent.  Both leaf angle and leaf clumping increased with increasing LAD indicating that maintenance of high foliage density is dependent on the spatial organization of foliage.  For the species examined thus far, our results suggest that light attenuation and sunfleck activity are most strongly driven by stand-level characteristics that vary with species, thicket age and environmental conditions.  More importantly, we show that the relatively short stature of shrubs, coupled with LAI values as high or higher than adjacent forest,  results in a dense canopy that may reduce understory light to <1% of incident light.  This further explains the dramatic decline in herbaceous diversity when shrub abundance increases and could aide in management and restoration efforts aimed at shrub removal.