Measurements and modeling of canopy architecture in high latitude, non-random forests: tales from BOREAS

Professor John M. Norman has had a monumental impact on the study of canopy architecture and radiative transfer over the past several decades. Here, I reflect on some of his pioneering work on instrument development and remote sensing during his tenure at the University of Wisconsin-Madison. My career path crossed with Prof. Norman as we took part in the Boreal Ecosystem Atmosphere Study (BOREAS) in the mid-1990s. Our primary goal as part of a larger terrestrial ecology research team was to quantify carbon budgets for contrasting boreal forest ecosystems. To do so, improved estimates of leaf area index (LAI) and new ways to numerically account for the non-random spatial distribution of foliage elements and their influence on radiative transfer was needed to accurately scale photosynthesis from the leaf to regional scale. As part of this task, Prof. Norman initiated the design and construction of the highly portable Multiband Vegetation Imager (or MVI), which was comprised of a 16-bit charge coupled device (CCD) digital camera, outfitted with a filter exchange mechanism that allowed for two-band (VIS/NIR) images of plant canopies to be captured in rapid succession. This small-scale remote sensing instrument allowed us to indirectly quantify LAI and canopy gap size distributions in both coniferous and deciduous canopies, estimate leaf angle distributions, and identify and measure specific canopy components (i.e. sunlit and shaded leaf area, and branch area). The precision with which the canopy gap size distribution was quantified allowed us to determine canopy clumping indices that were used to improve indirect measures of LAI and modify Beer's Law, making it more applicable for use in heterogeneous plant canopies. In the years to follow, the MVI was also used in corn canopies and also flew aboard a single engine Cessna aircraft in central Wisconsin on multiple occasions. To date, the five key publications related to MVI work have generated greater than 350 citations, influencing canopy architecture research and carbon budget studies in many regions of the globe.