How does elevation affect the performance of chainsaws

Typically, it is recommended for the best performance that you have the carburettor adjusted by an authorised service centre when you change elevation significantly. The unit will run rich in the thinner atmosphere at higher elevation. You may or may not notice a loss of power, but this will not cause any long term problems. You will notice excess smoke and possibly some clogging of the spark arrestor screen.

Your unit will experience a loss of 1.5% of its power for every 1000 feet (approx. 305 m) of elevation above sea level at a given temperature and humidity.
You should have your equipment readjusted every 2500–3000 feet (approx. 760–920 m) of elevation change.
The authorised servicing dealers that are in the area you will be working in will be familiar with how the carburettor needs to be adjusted and should be able to assist you.