Large contribution of woody plant expansion to recent vegetative greening of the Northern Great Plains

Aim: Extensive portions of high-latitude grasslands worldwide have
recently experienced increased vegetative productivity (i.e., greening)
and have undergone a rapid transition towards woody plant dominance via
the process of woody plant expansion (WPE). This raises the underlying
question: To what degree are WPE and greening spatiotemporally linked?
Given that these vegetative changes are predicted to continue, we seek to
understand how recent changes in vegetation extent and productivity have
interacted under recent climate change and anthropogenic disturbance to
provide insights surrounding the future trajectory of temperate grasslands
broadly. Location: Northern Great Plains (NGP), North America Taxon: Woody
plants Methods: Greening was measured as the significant increase in three
metrics between 2000 and 2019: leaf area index (LAI), annual maximum
normalized difference vegetative index (NDVI), and annual mean NDVI. WPE
was measured as the significant proportional increase in percent tree
cover change between 2000 and 2019 in grasslands. We then examine these
variables across a host of 26 potential driving variables. Results: We
show that average proportional greening increased by 0.2-1.3% yr -1
(depending on metric), and proportional WPE increased by 6.9% yr -1 since
2000 across the NGP. Both changes are largely driven by the absence of
wildfire and changing climate. Furthermore, WPE was spatially coherent and
positively associated with a large component of recent greening, as
revealed by their coupling across 34.1-40.6% of grassland area and as
evidenced by the 9.7-19.7% of the variability in greening explained by
WPE. Main conclusions: WPE and greening are spatiotemporally coupled
across large portions of the NGP. Under continued climate change and
wildfire suppression, WPE and greening are likely to continue across large
swathes of grasslands globally. Furthermore, our results show that using a
single greening metric may be insufficient to capture the large-scale
vegetative changes such as the expansion of woody vegetation.