Editing IPCC:AR6/WGII/Chapter-2 (section)

==== 2.4.3.9 Observed Changes in Polar Tundra ====

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Warming at high latitudes, documented in both AR4 and AR5, is leading to earlier snow and sea ice melt and longer growing seasons ( [[#IPCC--2021a|IPCC, 2021a]] ) which are continuing to alter tundra plant communities ( ''medium evidence'' , ''high agreement'' ) ( [[#Post--2009|Post et al., 2009]] ; [[#Gauthier--2013|Gauthier et al., 2013]] ). Woody encroachment and increases in vegetation productivity, observed in both AR4 and AR5, are widespread and continuing. Both experiments and monitoring indicate that climate warming is causing increases in shrub, grass and sedge abundance, density, frequency, and height, with decreases in mosses and/or lichens ( ''robust evidence'' , ''high agreement'' ) ( [[#Myers-Smith--2011|Myers-Smith et al., 2011]] ; [[#Bjorkman--2018|Bjorkman et al., 2018]] ; [[#Bjorkman--2019|Bjorkman et al., 2019]] ) ''.'' Shrub growth is climate-sensitive and is greater in years with warmer growing seasons ( [[#Myers-Smith--2015|Myers-]] [[#Smith--2015|Smith et al., 2015]] ). Plant species that prefer warmer conditions are increasing ( [[#Elmendorf--2015|Elmendorf et al., 2015]] ; [[#Bjorkman--2018|Bjorkman et al., 2018]] ), plant cover is increasing and bare ground is decreasing in long-term monitoring plots ( [[#Bjorkman--2019|Bjorkman et al., 2019]] ; [[#Myers-Smith--2019|Myers-Smith et al., 2019]] ). Animals such as moose, beavers and songbirds may already be responding to these vegetation changes by expanding their ranges northward or upslope into shrub tundra ( [[#Boelman--2015|Boelman et al., 2015]] ; [[#Tape--2016a|Tape et al., 2016a]] ; [[#Tape--2016b|Tape et al., 2016b]] ; [[#Tape--2018|Tape et al., 2018]] ).

In addition to direct warming, indirect effects of climate change, first found in AR4 and AR5, continue, such as thawed permafrost, altered hydrology and enhanced nutrient cycling, and these processes are causing pronounced vegetation changes ( ''medium evidence'' , ''medium agreement'' ) ( [[#Schuur--2009|Schuur et al., 2009]] ; [[#Natali--2012|Natali et al., 2012]] ). Soil moisture status influences temperature sensitivity of plant growth and canopy heights ( [[#Myers-Smith--2015|Myers-]] [[#Smith--2015|Smith et al., 2015]] ; [[#Ackerman--2017|Ackerman et al., 2017]] ; [[#Bjorkman--2018|Bjorkman et al., 2018]] ). In tundra ecosystems, permafrost thawing can decouple below-ground plant growth dynamics from above-ground dynamics, with below-ground root growth continuing until soils re-freeze in autumn (Cross-Chapter Paper 6) ( [[#Iversen--2015|Iversen et al., 2015]] ; [[#Blume-Werry--2016|Blume-Werry et al., 2016]] ; [[#Radville--2016|Radville et al., 2016]] ).

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