Die-back and death of mountain birch have been observed in northern Japan. We try to know the reason why birch is declining via monthly monitoring the gas exchange of both declining and healthy trees and using open-top chambers for seedlings. Study sites were located at the somma of Lake Mashu. We have been monitoring atmospheric ozone [O₃]. We selected each 4 trees of around 20 years old of both declining (less than 50% leaves) and healthy ones. Six OTCs were used with each 3 charcoal filtered and non-filtered to access the effects of [O₃] on plant growth. Net photosynthesis (Pn) and other parameters were measured. [O₃] reached to 80-100ppb during April to June and was around 50ppb during summer. Soil is originated from immature volcanic ash with shortage in phosphorous. Net photosynthetic rate of declining trees were lower at CO₂ and/or light saturation than that of healthy ones with smaller leaves and lower chlorophyll content. With OTCs, seedlings at non-filtered had smaller allocation of photosynthates to roots and low infection of ectomycorrhiza. [O₃] effects on trees may be a cause of tree declining due to unique leaf phenology of mountain birch.