The tree, or rather its nutrient-conducting outer skin, the cambium, is sensitive to excessive heat (> 40 – 45° C). The cambium and proteins break down, the bark flakes off and cracks and stresses appear.
Some trees have very thick bark and can even survive small forest fires (e.g. cork trees), others are extremely sensitive to heat and even get a “bark burn” from too much sunlight (e.g. young beech trees).
If a fireplace is too close to a tree, the tree can be damaged, especially by the continuous hot air that rises. Therefore, stovepipes should also be placed in such a way that the hot air does not damage the tree and its branches.