© 2007 Heron Publishing—Victoria, Canada
Linking leaf and tree water use with an individual-tree model
Belinda E. Medlyn (1, 2, 3), David A. Pepper (1), Anthony P. O'Grady (4) and Heather Keith (5, 6)
1. School of Biological, Earth and Environmental Sciences, University of NSW, Sydney 2052, Australia / 2. Present address: Department of Biological Sciences, Macquarie University, NSW 2109, Australia / 3. Corresponding author (bmedlyn@bio.mq.edu.au) / 4. School of Plant Science, University of Tasmania, Private Bag 55, Hobart 7001, Tasmania, Australia / 5. CSIRO Climate Program, P.O. Box 3023, Canberra, ACT 2601, Australia / 6. Present address: The Fenner School of Environment and Society College of Science, The Australian National University, Canberra,
ACT 0200, Australia / Received January 14, 2007; accepted May 2, 2007; published online September 4, 2007
Summary
We tested the ability of a model to scale gas exchange from leaf level to whole-tree level by: (1) measuring leaf gas exchange
in the canopy of 10 trees in a tall
Eucalyptus delegatensis RT Baker forest in NSW, Australia; (2) monitoring sap flow of the same 10 trees during the measurement week; and (3) using
an individual-tree-based model (MAESTRA) to link the two sets of measurements. Photosynthesis and stomatal conductance components
of the model were parameterized with the leaf gas exchange data, and canopy structure was parameterized with crown heights,
dimensions and leaf areas of each of the measurement trees and up to 45 neighboring trees. Transpiration of the measurement
trees was predicted by the model and compared with sap flow data. Leaf gas exchange parameters were similar for all 10 trees,
with the exception of two smaller trees that had relatively low stomatal conductances. We hypothesize that these trees may
have experienced water stress as a result of competition from large neighboring trees.
The model performed well, and in most cases, was able to replicate the time course of tree transpiration. Maximum rates of
transpiration were higher than measured rates for some trees and lower than measured rates for others, which may have been
a result of inaccuracy in estimating tree leaf area. There was a small lag (about 15–30 minutes) between sap flow and modeled
transpiration for some trees in the morning, likely associated with use of water stored in stems. The model also captured
patterns of variation in sap flow among trees. Overall, the study confirms the ability of models to estimate forest canopy
transpiration from leaf-level measurements.
Keywords:
canopy modeling, Eucalyptus delegatensis, gas exchange, scaling, stomatal conductance, transpiration.
