Piers Sellers passed away just before the end of the year, leaving many people with the last week of 2016 reconsidering his impact on their lives and on society. I saw him speak once a few years back, at AGU, but the room was so crowded with his old friends and colleagues that I could never manage to get close to him. He looms especially large in the imagination of Earth scientists, recalling an epitaph I especially like:Here lies Marcus Aurelius Sabinus, also called “The Little Rover”. A most beloved man whose way of life outshone by far the young men of his own rank and age.
Piers Sellers set off studying the function and form of plants in Earth’s system. Far more than most scientists, his career was marked by the interplay between rigorous mathematical theory and ambitious tests of the theory. But Sellers’ genius stretched far beyond, as he instrumented entire continents and ultimately the planet itself, first as a staff scientist for NASA, later as a bonafide astronaut, and finally as Director of Earth Science of the Goddard Space Flight Center.
The earliest paper of his that I reference is from 1985, titled “Canopy reflectance, photosynthesis, and transpiration.
” This work is foundational to all work that makes use of satellite, drone or airborne data to understand plant growth. Startups like Planet Labs, Mavrx and Precision Hawk have built entire companies around interpretation of maps of the Normalized Difference Vegetation Index (NDVI). Sellers showed that remotely-sensed light absorption from the sun was quantitatively linked to plant function, that is photosynthesis and transpiration, and not just form, that is standing crop biomass.
Sellers’ work went far deeper than showing that “this spot is green” and “that spot is brown,” as much of this work is presented today. The main insight we take from this work is that (1) canopy photosynthesis and transpiration are linearly related to NDVI, but that (2) the actual amount of photosynthesis and transpiration depend on both biotic and abiotic drivers. Measuring light absorption is critical, but to get the complete picture we need to measure temperature, humidity and others, as well as biotic constraints, such as leaf photosynthetic capacity or stress.*
At Arable, Sellers shows us that an effective tool for measuring and predicting plant growth and water use must measure both the slowly-changing state of the plant canopy (that is the NDVI), as well as the time-integral of the abiotic drivers of photosynthesis and transpiration (light, temperature, humidity, and others). Bonus points awarded for measuring other biotic factors like leaf chlorophyll or water content.Together, these tell us from first principles nearly everything we need to know about the rate of plant growth, and therefore, future plant biomass.
Sellers went on to be a leader in several profoundly important scientific efforts: FIFE, SIB and BOREAS.FIFE
was an observational effort in 1987 that made use of Sellers’ theoretical predictions linking satellite measurements of vegetation with plant growth and transpiration. It’s worth thinking about how observational and descriptive ecology was at the time to understand what a profound conceptual shift it was to cordon off a 15km x 15km square in the Kansas prairie and to use the “total annihilation” approach to measurement and instrumentation: 16 weather stations, 18 flux towers, atmospheric sounders, an army of handheld devices, 4 satellites, 3 planes, and an actual Huey.