Estimation of forest Leaf Area Index from SPOT imagery using NDVI distribution over homogeneous stands.

International audienc

Modeling annual production and carbon fluxes of a large managed temperate forest using forest inventories, satellite data and field measurements

We evaluated annual productivity and carbon fluxes over the Fontainebleau forest, a large heterogeneous forest region of 17,000 ha, in terms of species composition, canopy structure, stand age, soil type and water and mineral resources. The model is a physiological process-based forest ecosystem model coupled with an allocation model and a soil model. The simulations were done stand by stand, i.e., 2992 forest management units of simulation. Some input parameters that are spatially variable and to which the model is sensitive were calculated for each stand from forest inventory attributes, a network of 8800 soil pits, satellite data and field measurements. These parameters are: (1) vegetation attributes: species, age, height, maximal leaf area index of the year, aboveground biomass and foliar nitrogen content; and (2) soil attributes: available soilwater capacity, soil depth and soil carbon content. Main outputs of the simulations are wood production and carbon fluxes on a daily to yearly basis. Results showed that the forest is a carbon sink, with a net ecosystem exchange of 371 g Cm– 2 year – 1.Net primary productivity is estimated at 630 g C m–2 year –1 over the entire forest. Reasonably good agreement was found between simulated trunk relative growth rate (2.74%) and regional production estimated from the National Forest Inventory (IFN) (2.52%), as well as between simulated and measured annual wood production at the forest scale (about 71,000 and 68,000m3 year –1, respectively). Results are discussed species by species

Modeling annual production and carbon fluxes of a large managed temperate forest using forest inventories, satellite data and field measurements

We evaluated annual productivity and carbon fluxes over the Fontainebleau forest, a large heterogeneous forest region of 17,000 ha, in terms of species composition, canopy structure, stand age, soil type and water and mineral resources. The model is a physiological process-based forest ecosystem model coupled with an allocation model and a soil model. The simulations were done stand by stand, i.e., 2992 forest management units of simulation. Some input parameters that are spatially variable and to which the model is sensitive were calculated for each stand from forest inventory attributes, a network of 8800 soil pits, satellite data and field measurements. These parameters are: (1) vegetation attributes: species, age, height, maximal leaf area index of the year, aboveground biomass and foliar nitrogen content; and (2) soil attributes: available soilwater capacity, soil depth and soil carbon content. Main outputs of the simulations are wood production and carbon fluxes on a daily to yearly basis. Results showed that the forest is a carbon sink, with a net ecosystem exchange of 371 g Cm– 2 year – 1.Net primary productivity is estimated at 630 g C m–2 year –1 over the entire forest. Reasonably good agreement was found between simulated trunk relative growth rate (2.74%) and regional production estimated from the National Forest Inventory (IFN) (2.52%), as well as between simulated and measured annual wood production at the forest scale (about 71,000 and 68,000m3 year –1, respectively). Results are discussed species by species

Modelling carbon and water cycles in a beech forest Part I : model description and uncertainty analysis on modelled NEE

A forest ecosystem model (CASTANEA) is developed with the aim to bridge the gap between soil–vegetation–atmosphere (SVAT) and growth models. A physiologically multi-layer process-based model is built, completed with a carbon allocation model and coupled with a soil model. CASTANEA describes canopy photosynthesis and transpiration, maintenance and growth respiration, seasonal development, partitioning of assimilates to leaves, stems, branches, coarse and fine roots, evapotranspiration, soil heterotrophic respiration, water and carbon balances of the soil. Net primary productivity (NPP) is calculated as the difference between gross photosynthesis and plant respiration. The net ecosystem exchange (NEE) between soil-plant system and atmosphere is calculated as the difference between gross photosynthesis and total respiration (soil + plants). The meteorological driving variables are global radiation, rainfall, wind speed, air humidity and temperature (either half-hourly or daily values). A complete description of the model parameterization is given for an eddy flux station in a beech stand (Hesse, France). A parametric sensitivity analysis is carried out to get a classification of the model parameters according to their effect on the NEE. To determine the key input parameters, a +10% or −10% bias is applied on each of the 150 parameters in order to estimate the effect on simulated NEE. Finally 17 parameters, linked to photosynthesis, vegetative respiration and soil water balance, appear to have a significant effect (more than 2.5%) on the NEE. An uncertainty analysis is then presented to evaluate the error on the annual and daily NEE outputs caused by uncertainties in these input parameters. Uncertainties on these parameters are estimated using data collected in situ. These uncertainties are used to create a set of 17,000 simulations, where the values of the 17 key parameters are randomly selected using gaussian random distributions. A mean uncertainty of 30% on the annual NEE is obtained. This uncertainty on the simulated daily NEE does not totally explain the discrepancies with the daily NEE measured by the eddy covariance technique (EC). Errors on daily measurements by EC technique and uncertainty on the modelling of several processes may partly explain the discrepancy between simulations and measurements

Modelling carbon and water cycles in a beech forest. Part II : validation of the main processes from organ to stand scale

A forest ecosystem model (CASTANEA) simulating the carbon balance (canopy photosynthesis, autotrophic and heterotrophic respirations, net ecosystem exchange, wood and root growth) and the water cycle (transpiration, soil evaporation, interception, drainage and soil water status) is tested with data from a young beech forest (Fagus sylvatica L.). For this purpose, the model validity is assessed by comparison between net CO2 and H2O fluxes simulated and measured by the eddy flux technique over one year. In addition, most of the sub-models describing the processes mentioned above are tested using independent measurements from the same forest stand: tree growth, branch photosynthesis, wood and soil respirations, sap flow and soil water content. Most of the input parameters (both weather and plant characteristics) are measured in the same experimental site (i.e. Hesse forest) independently of the validation dataset (none has been fitted to match the output data, except rainfall interception parameters); some are from other beech sites or from literature. Concerning the radiative transfer, the model reproduces the measured exponential PAR extinction and provides a good estimate of the net radiative budget, except during winter. At the branch scale, simulated photosynthesis and transpiration of sun-leaves are close to the measurements.We show also, using model simulations, that the seasonal decrease of measured net photosynthesis at the branch level could be explained by a decrease in leaf nitrogen content during the leafy season. At stand scale, a good correlation was obtained between simulated and observed fluxes both on a half-hourly basis and on a daily basis. Except at the end of the leafy season, the model reproduces reasonably well the seasonal pattern of both CO2 and H2O fluxes. Finally, even if there are some discrepancies between model estimations and fluxes measured at stand scale by eddy covariance, the model simulates properly both annual carbon and water balances when compared with the sum of the measured local fluxes. The remaining differences question the scaling up process when building such a model and the spatial footprint of eddy fluxes measurements

Effect of aggregating spatial parameters on modelling forest carbon and water fluxes

International audienc

Effect of spatial parameters aggregation for modelling carbon and water fluxes in forest ecosystems.

International audienc