2.3 Vegetation productivity data
The peak biomass during the growing season was considered the net
primary productivity (NPP) because all plants in this study were
cultivated from seeds (Scurlock et al., 2002; Singh et al., 1975).
Comparing the species-level above- and belowground NPP allocation with
that from field observations of grass communities at the global scale
can help us understand the mechanisms of variation between results at
different study scales. It is also possible to further verify whether
the pattern of experimental data from greenhouse manipulations can be
extended to the field. Two types of sources for paired aboveground NPP
(ANPP) and belowground NPP (BNPP) data at the field community level were
used in this study: online databases and literature-reviewed data. The
online data were obtained primarily from the Oak Ridge National
Laboratory Distributed Active Archive Center (ORNLDAAC)
(http://daac.ornl.gov), which contains three datasets (Esser, 2013;
Olson et al., 2013; Scurlock et al., 2015). The literature-reviewed data
were obtained primarily from ISI-Web of Science (Thomson Reuters, New
York, NY, USA), Google Scholar (Google Inc., Mountain View, CA, USA) and
the China National Knowledge Infrastructure (http://epub.cnki.net).
Details regarding the extraction processes and initial screening
criteria for these data are described in Sun et al. (2020).