PARENT SESSION
Oral Session #4: Plant Ecology: Gas Exchange. Presiding: J. Pereira.
Monday, August 6, 2001. 8:00 AM to 12:00 PM. Hall of Ideas P&Q.

Inversion of eddy covariance CO₂ flux measurements for the estimation of canopy structure.

Hanan, Niall¹, Burba, George², Berry, Joe³, Verma, Shashi², Walter-Shea, Elizabeth², ^{1 2 3}

ABSTRACT- The structural characteristics of vegetation canopies, in particular leaf area and light interception capacity, determine the amount of radiant energy absorbed by the canopy and thus directly affect overall rates of photosynthesis and transpiration and the partition of available energy into sensible and latent heat fluxes. In models describing terrestrial ecosystem-atmosphere exchange and biogeochemistry, the fractional absorption of photosynthetically active radiation (f_PAR) is frequently a key variable. Yet direct measurement of f_PAR in most natural and agricultural systems is difficult for several reasons: adequate sampling in heterogeneous vegetation stands can be difficult, the presence of non-photosynthetic stems and dead leaves within the canopy confounds measurements by optical methods and the seasonal/phonological evolution of canopies means that green leaves may vary in photosynthetic capacity and PAR absorption through time. This paper explores a novel approach to the estimation of f_PAR that uses micrometeorological measurements of CO₂ flux and incident radiation to estimate light response parameters from which canopy structure can be inferred. Data from two Ameriflux sites in Oklahoma (tall-grass prairie and wheat sites) during a three-year period (1997-1999) are used to derive 7-day moving average inverse estimates of f_PAR. The inverse estimates are compared to long-term and comprehensive field measurements of PAR interception. Good agreement is obtained when the field-measured f_PAR is scaled by an estimate of the green fraction of total leaf area. The paper discusses the relative merits of the different f_PAR estimates and the potential value of the inverse method in ecological studies, modeling and remote sensing applications.

KEY WORDS: par interception, canopy structure, co2 flux, inversion