We simply compared the global GPP(g C m−2 yr−1) generated by FORCCHN with other studies related to seven light use efficiency(LUE) models’ outputs of five forest types during 2000−2010 (ref. 9) and seven estimations of forest ecosystem during 1993−2006 (ref. 1). Our model yielded comparable GPP per unit area as other methods on a global scale(Fig. 2). For example, FORCCHN generated a GPP of 841.4 and 2748.7 g C m−2 yr−1 for evergreen needleleaf forest (ENF) and evergreen broadleaf forest (EBF), which is ~12% and ~11% higher, respectively, than the average value of the seven LUE models (750.4 and 2479.1 g C m−2 yr−1). Meanwhile, the smallest discrepancy between our result and ref. 9 occurred in mixed forest (MIF) with GPP values of 1140.4 and 1144.3 g C m−2 yr−1, respectively and a corresponding difference of only −3.9 g C m−2 yr−1. In terms of forest ecosystem GPP, there were no particularly obvious discrepancies among the seven estimations, with a range varying from 1156.7 g C m−2 yr−1 for the LUE method to 1484.7 g C m−2 yr−1 for the MIAMI method. Meanwhile, the GPP generated by FORCCHN for the period of 1993−2006 was 1329.1 g C m−2 yr−1, which is much closer to the average value of the seven estimations (1305.4 g C m−2 yr−1).

To further investigate the rationality of applying the FORCCHN model to evaluate the GPP per unit area of forest ecosystem on a global scale, a comparison concerning the global spatial distributions of averaged GPP (g C m−2 yr−1) derived from FORCCHN and ref. 8 over the 1982−2011 period is shown in Fig. 3. Here, we extracted and recalculated the 30-year averaged GPP from ref. 8 at a resolution of 0.5° latitude by 0.5° longitude to match the forest distribution of IGBP-DIS classification (Fig. 1a). Fig. 3 shows that both of the spatial distributions of GPP are similar with regard to the largest values occurring in the equatorial tropics followed by monsoonal subtropical regions (e.g. south and east Asia) and humid temperate regions in eastern North America and western Europe. Boreal forests show a clear longitudinal gradient in northern Eurasia where GPP in boreal zone decreases toward the east, where the main forest type is deciduous needleleaf forest (DNF). Note, however, that GPP derived from FORCCHN in tropical rain forest (20 °S−20 °N) was generally ~300 g C m−2 yr−1 smaller than that of ref. 8, except for parts of Africa. Conversely, compared with FORCCHN outputs, ref. 8 captured an obviously lower estimate in south-central Africa with a difference of approximately −900 g C m−2 yr−1; presumably in response to the reclassification of tropical savanna into C3, C4 and C3/C4 types in ref. 8.

As analyzed above, our model generated comparable GPP per unit area as other related studies on a global scale; thus, it is convincing enough to detect the impacts of coverage rate on global annual GPP of forest ecosystems using the FORCCHN model. Here, we computed two GPP results based on two circumstances of forest coverage rate. Calculation I, termed ‘100% coverage’, is a traditional assumption that each grid is 100% covered by a given forest type and is generally adopted by most carbon flux studies at regional or global levels. Calculation II, termed ‘realistic coverage’, is a new strategy that takes the IGBP-DIS coverage rate into account to reflect the actual forest area of each grid. Because some land cover grids not categorized as forest might also have a certain amount of forest area, both realistic forest coverage of forest grids (Fig. 1b, 4) and non-forest grids (Figs. 1c,4) are taken into account in Calculation II.

The global spatial comparsion of annual GPP (Tg C yr−1 grid−1) derived from two calculations during 1982−2011 is given in Fig. 5.It is evident that the difference between ‘100% coverage’ and ‘realistic coverage’ for forest grids is similar with the spatial distribution of IGBP-DIS coverage rate to some extent. For example, the overestimated GPP in most forest varies within the minimum range of 0−0.5 Tg C yr−1 grid−1 (Fig. 5d) when the corresponding coverage rate is more than 90% (Fig. 1b). Because of the lower coverage rate (40%−70%) along the periphery of EBF between 20 °S and 20 °N (Fig. 1b), the largest overestimation of GPP is found with a value of 3−6 Tg C yr−1 grid−1 (Fig. 5d). Note, however, that the ‘100% coverage’ does not always overestimate annual GPP at each forest grid compared with the ‘realistic coverage’. It is particularly true for some DNF and ENF girds in the Northern Hemisphere (Fig. 5d), where the difference between two calculations has been inversely underestimated by 0−0.1 Tg C yr−1 grid−1. This phenomenon can be attributed almost entirely to the fact that each grid of DNF and ENF consists of ~14% and ~15% MIF, respectively (Fig. 4). The relatively higher GPP per unit area of MIF might counteract the effects of non-forest coverage and increase total GPP of these DNF and ENF grids. With respect to the difference between ‘100% coverage’ and ‘realistic coverage’ for non-forest grids, the underestimated GPP in most grids varies within the range of 0−0.5 Tg C yr−1 grid−1 (Fig. 5c,d) when the corresponding forest coverage is less than 5% (Fig. 1c). Meanwhile, due to the higher forest coverage (20%−50%) along the periphery of forest ecosystems (Fig. 1c), the largest underestimation of GPP is observed with a value of 1.2−4.5 Tg C yr−1 grid−1 (Fig. 5c,d).

The long-term changes in GPP estimates are consistent between the two calculations. Both calculations show a significant GPP increase (p < 0.001) from 1982 to 2011 regardless of forest type (Fig. 6), which is broadly consistent with the trend of terrestrial vegetation productivity given by 18 CMIP5 earth system models over the 1986−2005 period10. In terms of averaged annual GPP in the ‘100% coverage’ calculation, the forest types with the greatest carbon dioxide uptake are EBF, MIF and ENF, with annual GPP of 39.58 ± 3.49 (mean ± 1 standard deviation), 10.84 ± 0.96 and 4.25 ± 0.72 Pg C yr−1, respectively (Table 1). Moreover, the global GPP estimates of forest ecosystems is 58.83 ± 5.61 Pg C yr−1, which is comparable to the observation-based estimations of 52.61−67.54 Pg C yr−1(ref. 1) and satellite-based simulations of 37.59−59.77 Pg C yr−1(ref. 9). As far as ‘realistic coverage’ calculation is concerned, the corresponding annual GPP estimates in EBF, MIF and DNF decrease to 35.13 ± 3.5, 9.65 ± 1.02 and 1.23 ± 0.13 Pg C yr−1, respectively, while ENF and deciduous broadleaf forest (DBF) inversely increase to 4.51 ± 0.75 and 3.19 ± 0.47 Pg C yr−1 and gross carbon dioxide uptake by global forest ecosystems is reduced with an annual GPP of 53.71 ± 4.83 Pg C yr−1, which is comprised of 46.19 ± 4.24 Pg C yr−1 for forest grids and 7.52 ± 1.59 Pg C yr−1 for non-forest grids(Table 1). Overall, compared with the ‘realistic coverage’, the ‘100% coverage’ calculation, which was generally adopted by most previous studies, has overestimated annual GPP across all forest types with exception of ENF and DBF. In terms of global forest ecosystems, the annual GPP is approximately overestimated by 5.12 ± 0.23 Pg C yr−1, accounting for ~8.7% of the global GPP estimates in the ‘100% coverage’ calculation.