How could there be vegetation at the North Pole if it is always night/day there for six months?

Answer

The geological history plays no role in the current plant growth, which is based on the domestic climatic conditions. In the past, the Arctic has indeed been greener. This was due to plate tectonics, changing position of the continents, and changing air and water circulation so that the poles were ice-free and had a significantly milder climate.

Tree growth is limited by several factors, of which the low winter temperature is only one. More important is the short duration of the growing season, with snow-free surface, and the moisture balance. On the one hand, the total amount of precipitation in the polar regions is very low, comparable to desert regions, and precipitation falls as snow. On the other hand, the bottom remains permanently frozen except for a thin top layer that thaws in summer so that the defrost water cannot sink away. Deep-rooted plants thus have a double handicap: rooting can only take place in the thawed top layer, which is then under water in most places with soil development, so that no oxygen can reach the roots. On sloping terrain, the soil and weathering layer has been scraped away by the ice, often revealing bare rock, not a good or stable root soil for larger plants. In intermediate situations where the water can recede but broken and crumbled soil is still present, the desert drought will make itself felt with the suction effect of the permanently frozen ice on liquid water. Many other factors can also play a role: the strong and drying wind, generally unfavorable ecosystem conditions that hinder pollination and recycling of nutrients, etc.

The tree line, which is also felt vertically and occurs in mountains, will be based on the same factors: too short a growing season due to low temperature, slow metabolism, frozen soil and snow cover, too low humidity, too little soil to root and recycle nutrients .