Above a certain altitude on the sides of mountains, trees case to thrive because the climate is too extreme. The air is too cold, the winds are too strong and and in many cases there isn't enough water to sustain life. Trees are constantly losing water through their leaves by evaporation, so any interruptions in the water supply can cause them to dry out in harsh environments. In the mountains, ground water freezes more often, so it isn't always possible for the roots to deliver sufficient water to the rest of the tree. The lack of water causes pressure within the tree to fall to a point where the water column rupters and fatal air bubbles form. When temperatures are extremely low it is even possible for the water inside the tree to freeze.
The trees most frequently seen at high altitudes on mountain sides are conifers, because hey have be better protection against excessive evaporation. The timber line-the altitude at which trees cease to grow-varies according to climate and latitude. It typically lies somewhere between 600 m (as it is in Sweden) and 4000 m (as it is in Nepal). The timber line in the Central Alps of Europe lies at about 1900 m.
Trees cannot survive in the harsh climate found at high altitudes.