2.1 EP data
The EP data was derived by using the growing season integrated annual
normalized difference vegetation index (INDVI) data in the circumpolar
vegetation dynamics product of global change study (Gonsamo & Chen,
2016). This is the first set of phenological index based plant dynamics
product for the entire circumpolar North during 1999–2013, with the
spatial resolution unified to 4km×4km (0.03571428°×0.03571428°). This
product was generated based on the SPOT VGT data, and its validation
showed good agreements with the ground-truth data from many eddy
covariance flux tower sites of North American and European deciduous
broadleaf, evergreen needle-leaf, and mixed forests, and wetlands
(Gonsamo & Chen, 2016). Overall, for the Arctic circumpolar region,
this vegetation dynamics product is an improvement to the often-used
global MODIS combined land cover dynamics MCD12Q2 product (Gonsamo &
Chen, 2016).
Specifically for the derivations of the EP parameters for the lower-EP
(LEP), higher-EP (HEP), and all-cases-EP (AEP) NH ecosystems, the
average of the lowest 30%, the average of the highest 30%, and the
average of the INDVIs for the matrix of 14×14 geo-grids corresponding to
any a geo-grid (0.5°×0.5°) in the used SP data (Chen et al.,2016) were calculated by following Equation (1), (2), and (3),
respectively.
(1)
(2)
(3)
where denotes the lowest 30%, indicates the highest 30%, is the
considered year, and is the number of the geo-grids satisfying the
definitions of the related EP parameters for any a 14×14 geo-grid
matrix. The theoretical foundation of applying such vegetation indices
for characterizing macro-scale vegetation productivity has already been
validated in a plenty of previous researches (Wilson & Meyers, 2007;
del Castillo et al., 2018). As regards to the setting of 30% in
defining the EP of the LEP and HEP ecosystems, such a relatively larger
threshold can effectively avoid the impacts of the outliers in some
4km×4km geo-grids (Gonsamo & Chen, 2016) and, hence, can better reflect
the productivity of vegetation at the ecosystem level.