Why Nothing Grows North of This Line

Across the top of the world there is a line, and north of it no forest grows. It is not the Arctic Circle and it is not any line of latitude. In Siberia the trees reach past 72 degrees north; on Hudson Bay in Canada they give up below 59. What the line follows is summer. Over a century ago Wladimir Koppen noticed that wherever the warmest month averages under about 10 degrees C, forest does not grow, and one of the standard definitions of the Arctic is exactly that. The Gulf Stream pushes the line north up the coast of Norway; a Hudson Bay full of ice until midsummer drags it south; the Mackenzie River carries spruce almost to the Beaufort Sea. It is not the cold that stops a tree. The same Dahurian larch that forms the northernmost forest on Earth, at Ary-Mas on the Taymyr Peninsula, survives winters below minus 70 in Yakutia. What fails is building: at 5 degrees a conifer can still photosynthesise at about half its full rate, but its cells effectively stop dividing, and trees at the tree line turn out to be full of sugar they never got to use. Christian Korner and Juerg Paulsen measured the soil at 46 tree lines from 68 north to 42 south and found the same number almost everywhere: a growing season of roughly 6.5 to 7 degrees at the roots. That is why the same line climbs every mountain, from 2,000 metres in the Alps to nearly 5,000 in the Andes. Then the exceptions: Iceland, which people cleared; Greenland's one forest; the southernmost tree on Earth, on Cape Horn, where the wind draws the line; and a tree line that is now moving north, more slowly than the warming. WHAT THIS VIDEO WILL NOT DO The tree line drawn on screen runs through measured points and is labelled approximate. Koppen's 10 degree rule is presented as a rule of thumb, with its exceptions named. SOURCES Koppen climate classification; Arctic Portal and GRID-Arendal on the 10 C July isotherm Korner and Paulsen, Journal of Biogeography (2004); Korner (2003), Journal of Ecology Ary-Mas and Larix gmelinii; Churchill Northern Studies Centre; the Mackenzie Valley treeline Polylepis tarapacana on Sajama, Bolivia; Mountain Research and Development (2007) on the Tibetan juniper treeline IBFRA on the boreal forest; JGR Earth Surface (2021) on permafrost extent Icelandic Forest Service on Iceland's woodland; Qinngua Valley, Greenland Ecography (2020): the southernmost tree Biogeosciences (2019) on Siberian treeline advance; Harsch et al., Ecology Letters (2009); Nature Communications (2020) on Arctic greening Imagery: NASA, USFWS and Wikimedia Commons contributors credited on screen Terrain, coastlines and borders: Natural Earth CHAPTERS 0:00 The line 0:52 Summer, not latitude 3:07 Not the cold 4:38 The same line on every mountain 6:31 The forest below it 8:13 The line is moving 9:28 Put it on one map New geography every week. The terrain always wins.