Don't they harm the environment?

Timber for frame manufacture is mainly obtained from Scandinavia, USA and Britain where proper forestry management ensures that more trees are planted than are felled.

In addition, growing trees absorb carbon dioxide (CO₂) and convert it to oxygen, but mature trees absorb less CO₂ and produce little oxygen so by harvesting mature trees and replanting with young specimens a constant cycle of CO₂ absorption and oxygen production is ensured.