Environmental Engineering Reference
In-Depth Information
2.4 Analysis of Climate-Carbon NEE Interactions
The long time series of Carbon NEE, meteorological variables and ecosystem status
characteristics measured by the Italian network of Eddy Covariance sites can be used
to analyze the interactions between climate and ecosystem processes. Understanding
the driver factors controlling the capacity of terrestrial ecosystems to store carbon is
extremely important in the context of climate variability and climate changes processes.
Carbon fluxes measured at the Italian sites have been used to analyze the role of
the main meteorological variables and length of the CUP in the annual NEE across
sites and the inter annual variability.
2.4.1 Quality Criteria and Data Selection
The annual NEE has been calculated as the median of the four cumulative values of
daily NEE time series that have been routinely produced in the Carbo Italy database
using different processing approaches (gap filling and storage correction). Uncertainty
of annual NEE (NEE
max-min
) has been instead estimated as the difference between the
maximum and minimum cumulative values of the four daily NEE time series.
Two quality criteria have been used to select site years to be included in the
analysis. The first quality check excludes site years with less than 330 days of
data. The second criteria is based on the quality flags of daily NEE time series
(defined in Reichstein et al.
2005
) that indicate the percentage of high original
measurements for each daily NEE value or high quality gap filled data: in particu-
lar a site year is discarded if any of the four NEE versions has a within year per-
centage of low quality days (daily quality flag < 0.85) higher than 25 %.
The application of these quality criteria resulted in a dataset for analysis which
includes 22 cropland (CRO) site years, 26 deciduous broadleaf forest (DBF), 18
evergreen broadleaf forest (EBF), 27 evergreen needle leaf forest (ENF) and 14
grassland (GRA).
2.4.2 Relationships Between Carbon Uptake Period
Duration and Annual Net Ecosystem Exchange
The analysis of the relationship between annual cumulated NEE and the duration of
the CUP has been conducted aggregating site years according to PFT. CUP for all site
years has been computed as the number of days with daily NEE <
−
1 g C m
−
2
d
−
1
.
A linear regression analysis at PFT level has been conducted in order to evaluate
annual NEE sensitivity to CUP. Data have been weighted according to the uncertainty
estimates (NEE
max-min
) defined as described in Sect.
2.4.1
.
Results of the linear regression analysis between annual NEE and CUP are
reported in Fig.
2.5
. Higher R
2
values are obtained for EBF, ENF and GRA as
presented in Fig.
2.6
. DBF is the PFT with the lowest R
2
value probably because
