Leaf Area Index (LAI) Change Detection Analysis on Loblolly Pine (Pinus taeda) Following Complete Understory Removal

The confounding effect of understory vegetation contributions to satellite-derived estimates of leaf area index (LAI) was investigated on two loblolly pine (Pinus taeda) forest stands located in Virginia and North Carolina. In order to separate NDVI contributions of the dominant-codominate crown class from that of the understory, two P. taeda 1 ha plots centered in planted stands of ages 19 and 23 years with similar crown closures (71 percent) were analyzed for in situ LAI and NDVI differences following a complete understory removal at the peak period of LAI. Understory vegetation was removed from both stands using mechanical harvest and herbicide application in late July and early August 2002. Ikonos data was acquired both prior and subsequent to understory removal and were evaluated for NDVI response. Total vegetative biomass removed under the canopies was estimated using the Tracing Radiation and Architecture of Canopies (TRAC) instrument combined with digital hemispherical photography (DHP). Within-image NDVI change detection analysis (CDA) on the Virginia site showed that the percentage of removed understory (LAI) detected by the Ikonos sensor was 5.0 percent when compared to an actual in situ LAI reduction of 10.0 percent. The North Carolina site results showed a smaller percentage of reduced understory LAI detected by the Ikonos sensor (1.8 percent) when compared to the actual LAI reduction as measured in situ (17.4 percent). Image-to-image NDVI CDA proved problematic due to the time period between the Ikonos image collections (2.5 to 3 months). Sensor and solar position differences between the two collections, along with pine LAI increases through multiple needle flush, exaggerated NDVI reductions when compared to in situ data.