The environment of the northern taiga to tundra transition is highly sensitive to climate fluctuations. In this study from northeastern European Russia, stable carbon and oxygen isotope ratios (¹³C, ¹⁸O) in α-cellulose of Sphagnum fuscum stems subsampled from hummocks and peat plateau profiles have been used as climate proxies. The entire isotope time series, dated by lead (²¹⁰Pb), caesium (¹³⁷Cs) and AMS-radiocarbon (¹⁴C) dating, spans the past 2500 years. Plant macrofossil analyses were used as an aid in single species selection, but are also helpful in identifying past surface moisture conditions. The most significant relationships were found between the recent ¹³C record and summer (July–August) temperatures (R² = 0.58, p < 0.01), and the recent ¹⁸O record and winter (October–May) precipitation anomalies in the tundra region (R² = 0.36, p < 0.01). The study demonstrates that stable isotopes preserved in northern peat deposits are useful indicators for summer temperature and winter precipitation at decadal to millennial timescales.