Biomedical subjects

Frédéric Raulier

Publications and source records attributed to Frédéric Raulier.

3 recordsLinked to original sources

PubMed2002

Structural differences and functional similarities between two sugar maple (Acer saccharum) stands.

The spatially inexplicit or functional multilayer models used to predict canopy transpiration or photosynthesis are based on the assumption that closed stands show less functional variability than structural variability, because foliage tends to arrange itself in space to optimize the capture of light. To validate this assumption, we compared the structural and functional properties, and the measured and modeled transpiration fluxes of two sugar maple (Acer saccharum Marsh.) stands of comparable leaf mass but differing in height and diameter distributions. One stand was characterized by a well-developed single-layer canopy, whereas the other stand had a multilayered canopy and a stem diameter distribution of the classical inverse-J shape. Stand differences in height and diameter distribution, and canopy gap fraction, were highly significant. There were minor but significant differences in leaf mass and leaf mass per unit leaf area (LMA) distributions. We found no differences in tree-level relationships between basal area and either transpiration flux or sapwood area. We compared measurements of stand transpiration with transpiration estimates obtained from a multilayer gas exchange model, in which only the nonspatial inputs, leaf area index and LMA frequency distribution described stand structure. For both stands, modeled values of daily transpiration closely followed measured values (r(2) = 0.94). These results support use of the nonspatially explicit approach to estimating canopy gas exchange, especially if the intent is to scale-up to larger portions of the landscape.

Acer↗

PubMed2000

Modeling the influence of temperature on monthly gross primary productivity of sugar maple stands.

A bottom-up and a top-down model were used to estimate the effect of temperature on monthly gross primary productivity (GPP) of sugar maple (Acer saccharum Marsh.). The bottom-up model computed canopy photosynthesis at an hourly time step from detailed physiological sub-models of leaf photosynthesis and stomatal conductance. Leaf mass per area was used as a covariable to integrate photosynthesis through the canopy. The top-down model used a radiation-use efficiency coefficient to relate canopy gross photosynthesis to absorbed photosynthetically active radiation at a monthly time step. The parameters of the top-down model were estimated from simulations with the bottom-up model. Forty single-year simulations were made using records of daily maximum and minimum temperatures from weather stations selected within the natural range of sugar maple in the province of Québec, Canada. Leaf area index was randomly varied between 4 and 10. Within a broad range of values, temperature had a minor effect on predicted monthly canopy-level GPP and its contribution to explaining the variability of GPP was low, both through its direct effect on photosynthetic processes (1.1%), and indirectly through the effect of relative humidity on stomatal conductance (4.0%). This result was unchanged when key parameters relating photosynthesis to temperature and stomatal conductance to atmospheric humidity were changed in the bottom-up model. An increase in time step from hourly to monthly resulted in a downward shift in the optimum temperature range for photosynthesis, from 30 degrees C for a leaf at saturating irradiance to 22 degrees C for the canopy at a monthly time scale.

Journal Article↗

PubMed1999

Canopy photosynthesis of sugar maple (Acer saccharum): comparing big-leaf and multilayer extrapolations of leaf-level measurements.

A comparison is made between a big-leaf model (i.e., without details of the canopy profile) and two multilayer models (i.e., with details of the canopy profile) to estimate daily canopy photosynthesis of a sugar maple (Acer saccharum Marsh.) stand. The first multilayer model uses the distribution of leaf area by leaf mass per unit area (LMA) classes, the observed relationships between the parameters of a photosynthesis-irradiance curve and LMA, and the relationship between relative irradiance and LMA to estimate canopy photosynthesis. When compared with this model, the big-leaf model underestimates daily canopy photosynthesis by 26% because of an assumed proportionality between photosynthetic capacity and relative irradiance, a proportionality that is inconsistent with our data. The bias induced by this assumption is reduced when the big-leaf model is compared with the second multilayer model which, in addition to the assumptions made for the first multilayer model, accounts for the sunlit and shaded fractions of leaf area. The residual bias is almost eliminated when the big-leaf model is run using a weekly averaged irradiance. It is likely, however, that this is the result of a compensating bias that, in this particular case, compensates for the initial bias introduced by the proportionality assumption. It is also shown that canopy photosynthesis can be represented by spatially inexplicit multilayer models that use leaf mass per area as a covariable to describe leaf characteristics and environment. Such models represent an interesting alternative to the biased big-leaf approach.

Journal Article↗