Stability of the ecosystem gross primary productivity increasing in Chinese forestry ecological engineering area

Gross Primary Production (GPP) is an important component of global carbon cycle. In the context of ecosystem construction, ecosystem GPP is significantly increasing, however, the stability of the increasing remains unclear. In this study, the nonlinearity of GPP increasing and its stability were studied with Ensemble Empirical Mode Decomposition method in Chinese forestry ecological engineering (FEE) areas during 1982-2018. The results showed that (1) although linearly increasing trend of GPP occupied 50.83% of the area, it implied different nonlinear trends as monotonic increasing, decreasing to increasing, and increasing to decreasing, accounting for 25.30%, 9.03% and 7.76%, respectively. Meanwhile, for insignificantly linear changes of GPP (41.19%), among them, 5.99% and 5.41% showed trend reversal from increasing to decreasing and from decreasing to increasing, respectively. (2) the trends in all FEE areas were dominated by monotonic increasing, especially for shelterbelt programs of Middle Yellow River (YR). Meanwhile, the trend from decreasing to increasing also has high area percentage (15.75%), especially for YR and coast shelterbelt program (CS), indicating early degradation and later recovery. However, there are large areas (>10%) of the trend shifts from increasing to decreasing in most FEE areas, especially in the shelterbelt programs of Huaihe River and Taihu Lake (HT), Taihang Mountain(TH) and upper and middle reaches of Yangtze River (YZ), indicating high risk of future degradation. (3) Although GPP changes are mainly dominated by interannual variations except YR, contribution of trends to GPP changes are high in HT and TH; For monotonic increasing and decreasing to increasing, the trend contributions to whole GPP changes were > 33%, especially in YR and TH, indicating high stability of ecosystem improvement. For monotonic decreasing, increasing to decreasing and insignificant changes, the contributions were low, indicating low stability but high recovery potential or degradation risk;(4) Both agriculture reclamation and ecological engineering not only favored GPP increasing, but also enhanced its stability. Our results highlight the importance of the nonlinearity of trend and its stability, which provides scientific implications for evaluating the effectiveness of ecological engineering.