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.

Methods
All data in this dataset were obtained via remote sensing. For more information of data collection, see the manuscript.

研究目的：近期，全球范围内大片高纬度草原的植被生产力持续提升（即绿度增加），且通过木本植物扩张（Woody Plant Expansion, WPE）过程快速向木本植物主导的群落转型。这引发了核心科学问题：木本植物扩张与绿度增加在时空尺度上的关联程度究竟如何？鉴于此类植被变化预计将持续发生，本研究旨在厘清近期气候变化与人为干扰背景下，植被范围与生产力的变化如何相互作用，以期为全面解析温带草原的未来发展轨迹提供科学依据。

研究区域：北美北部大平原（Northern Great Plains, NGP）

研究类群：木本植物

研究方法：以2000-2019年间三项指标的显著提升作为绿度增加的衡量标准，分别为叶面积指数（Leaf Area Index, LAI）、年度最大归一化植被指数（Normalized Difference Vegetative Index, NDVI）及年度平均归一化植被指数。木本植物扩张则以2000-2019年间草原区域内树木覆盖比例的显著比例增长为判定标准。随后，本研究针对26项潜在驱动变量展开了综合分析。

研究结果：结果显示，2000年以来，北部大平原区域的平均绿度提升速率为0.2%~1.3%·年⁻¹（因指标不同存在差异），木本植物扩张的年均比例增幅达6.9%·年⁻¹。两类植被变化主要由野火活动减少与气候变化共同驱动。进一步分析表明，木本植物扩张具有空间一致性，且与近期大范围的绿度增加呈显著正相关：二者的耦合区域覆盖了34.1%~40.6%的草原面积，木本植物扩张可解释9.7%~19.7%的绿度增加变异。

主要结论：北部大平原的大片区域内，木本植物扩张与绿度增加存在显著的时空耦合关系。若未来气候变化持续且野火抑制政策维持现状，全球范围内的大片草原区域或将持续发生木本植物扩张与绿度增加。此外，本研究结果表明，仅使用单一绿度指标可能无法全面捕捉木本植被扩张等大规模植被变化。

方法：本数据集所有数据均通过遥感手段获取。有关数据采集的详细信息，请参阅研究论文。