[garg]

https://phys.org/news/2018-08-global-forest-loss-years-offse...

"They further report that much of the new growth came about due to efforts by humans (such as reforestation efforts in China and parts of Africa) and because of global warming—warmer temperatures have raised timberlines in some mountainous regions, and allowed forests to creep into tundra areas. Other areas of new tree growth resulted from large farm abandonments in places like Russia and the U.S. The researchers report that their calculations showed that human activities have directly caused approximately 60 percent of new global tree growth. They suggest their technique for monitoring tree cover could be used to predict tree cover changes in the future due to global warming."

So reforestation efforts led by humans has contributed to this change. AND global warming caused by humans has caused the tundra to warm up enough that forests have moved up there.