Convergence and divergence emerging in climatic controls of polynomial trends for northern ecosystem productivity over 2000-2018

Recent rapid warming has caused uneven impacts on the composition, structure, and functioning of northern ecosys-tems. It remains unknown how climatic drivers control linear and non-linear trends in ecosystem productivity. Based on a plant phenology index (PPI) product at a spatial resolution of 0.05 degrees over 2000-2018, we used an automated poly-nomial fitting scheme to detect and characterize trend types (i.e., polynomial trends and no-trends) in the yearly-integrated PPI (PPIINT) for northern (> 30 degrees N) ecosystems and their dependence on climatic drivers and ecosystem types. The averaged slope for the linear trends (p < 0.05) of PPIINT was positive across all the ecosystems, among which deciduous broadleaved forests and evergreen needle-leaved forests (ENF) showed the highest and lowest mean slopes, respectively. More than 50% of the pixels in ENF, arctic and boreal shrublands, and permanent wetlands (PW) had lin-ear trends. A large fraction of PW also showed quadratic and cubic trends. These trend patterns agreed well with esti-mates of global vegetation productivity based on solar-induced chlorophyll fluorescence. Across all the biomes, PPIINT in pixels with linear trends showed lower mean values and higher partial correlation coefficients with temperature or precipitation than in pixels without linear trends. Overall, our study revealed the emergence of latitudinal convergence and divergence in climatic controls on the linear and non-linear trends of PPIINT, implying that northern shifts of veg-etation and climate change may potentially increase the non-linear nature of climatic controls on ecosystem productiv-ity. These results can improve our understanding and prediction of climate-induced changes in plant phenology and productivity and facilitate sustainable management of ecosystems by accounting for their resilience and vulnerability to future climate change.

Automated summary

Recent rapid warming has uneven impacts on northern ecosystems. This study investigates the climatic drivers controlling linear and non-linear trends in ecosystem productivity using a plant phenology index. Results show the importance of climate in determining the trends and variability of ecosystem productivity.