Measurement of Leaf Area Index Leaf area index (LAI) is a dimensionless quantity that characterizes plant canopies.  LAI is a useful proxy variable for numerous other variables of ecological interest including plant biomass, plant productivity, forage quality, carbon balance, ecosystem energy flux, plant density, and the heterogeneity of plant cover. LAI is also used widely as a key input variable to models that seek to predict ecological processes such as carbon cycling. Regional to continental scale estimates of LAI are typically derived from satellite data, but validation of satellite data with aircraft and ground-collected data is relatively uncommon. By leveraging NEON’s aircraft and ground-collected estimates of LAI, it will be possible to develop ground-validated estimates of LAI at the continental scale. NEON uses the Digital Hemispherical Photo (DHP) system (aka fisheye photos) to estimate LAI, since it is rapid and non-destructive. DHP are collected across SRER through time: 1) data are collected on the ground every three years from a subset of Distributed Plots (n = <20) and those data are used in conjunction with NEON’s Arial Observation Platform (AOP) LAI data products to estimate LAI at the site scale; and 2) data are collected every two weeks during the growing season at a subset of Tower Plots (n = 3) that will be used to construct a time-series dataset that will complement the spatial extensive snapshot of LAI generated from AOP data.