Our research goal is to assess the regional vegetation
dynamics in the Iberian Peninsula (IP). For this purpose, estimations of net
ecosystem production (NEP) from a
productivity ecosystem model, the Carnegie Ames Stanford Approach (CASA) model [Potter
et al., 1993], were compared with
local CO2 flux measurements. The CASA
calibration process aimed the tuning of efficiency scalars directly related to
net primary productivity and soil respiration calculations: maximum light use
efficiency (ε*) and temperature
effect on soil fluxes (Q10),
respectively. Local weather station data was used for climatic inputs, as well
as remotely sensed leaf area index (LAI) and fraction of photosynthetically
active radiation (FPAR) from the MODIS TERRA sensor. Firstly, NEP calculations were performed at different temporal
resolutions, ranging from monthly to daily time steps, in order to assess the
impact of temporal scales on productivity estimates. Both the calibration and
validation procedures showed significant confidence, although the main
processes behind vegetation carbon fluxes were best simulated at temporal scales
ranging from 8 days to monthly. The impact of spatial scale was also analyzed
on the NEP estimates. It was found
that results accuracy was influenced by the data spatial resolution, and,
furthermore, by the tree cover percentage of the aggregated cells. A correction
method was implemented and a reduction of the spatial aggregation error up to
10% was obtained. The long term NEP
analysis for the IP indicates statistically significant positive trends mainly
related to solar radiation positive trends. A less significant negative trend
was also found with a strong spatial autocorrelation behavior.
Author: N. Carvalhais, J. Seixas and R. Myneni (nmmc at fct dot unl dot pt)
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