What has less impact from an ecological point of view — camping below or above the tree line? US and Swiss organisations appear to contradict each other:
From the American lnt.org comes the advise (or rather order) to not camp above tree line:
Travel and Camp on Durable Surfaces: While visiting these areas stick to the trail or bare rock to help protect this beautiful environment. Camping is not permitted above tree line.
However, from the swiss alpine club comes the advice it is better to camp above tree line:
Aus ökologischer Sicht sind Standorte oberhalb der Waldgrenze meist unbedenklich.
Meaning from an ecological point of view, locations above the tree line are usually unproblematic. They advise to particularly avoid the forest near the tree line, as animals seek shelter there, but otherwise their reasoning appears to be: the higher up, the less life, the less humans are disturbing (plus the lowest elevations in the Alps are more often in use by humans).
It would seem that in alpine territory, durable unvegetated surfaces are more common the higher one goes, until reaching the territory of only snow and ice, where all surfaces are durable (ecologically speaking). On the other hand, a lush warm rainforest may recover much more quickly if I do camp on vegetated surfaces. From an ecological point of view, aiming to leave no trace, should I rather camp above or below the tree line?
The other advice from the Swiss organisation (bury human waste, carry out toilet paper, camp on durable surfaces, etc.) agrees with what I have read from American organisations.