Lasers on a plane! Using LiDAR to analyze spatial variations in shrub thicket canopies and light attenuation in coastal environments

Background/Question/Methods

Barrier island shrub thickets off of Virginia’s Eastern Shore are dominated by Myrica cerifera and are rapidly expanding in certain areas. Thicket canopies can reach over 7m in height and have a high leaf area index (LAI), >10, with compressed foliage. The goal of this study is to link LiDAR-based remote sensing canopy measurements to field data collected on canopy structure and light characteristics for thickets on Hog Island, VA. Full waveform LiDAR measurements were gathered via aircraft 500m above ground level and have been classified by height. Stem basal area, LAI, percent canopy, stem density, canopy height, and light patterns at the soil surface were measured in sixteen 5 x 5 m² plots established across the island’s chronosequence.

Results/Conclusions

Field data indicate strong relationships between light attenuation and the light characteristics with regard to shrub canopy features, particularly LAI, stem density and basal area. LiDAR data using comparisons with ground plots indicate close agreements with structural characteristics of the shrub thickets. There is significantly greater variation in canopy characteristics moving across the island, reflecting different ages in shrub canopies (5-70 years), the result of island accretion and shrub encroachment. In comparison, there was little spatial variation along the long axis of the island.

Variations in canopy structure and species composition profoundly influenced light patterns at the forest floor. Sunfleck patterns are analogous to potential “LiDARflecks” that reach the forest floor, creating a heterogeneous environment in the understory. The spatio-temporal variation in light was a function of both canopy composition and LAI. Size and spatial distribution of LiDARflecks varied in the different communities, with best penetration in the maritime forest. The shrub monoculture attenuated the most light, resulting in sunflecks of short duration and low intensity. However, even with dense vegetation and low light penetration; we were still able to discern terrain features below the canopy. Successfully linking LiDAR data to field measurements will allow for accurate conclusions to be drawn about locations that have yet to be ground truthed.