High-latitude ecosystems are undergoing rapid climate warming, yet long-term ecological response remain poorly understood due to the scarcity of sustained monitoring records. We analyse a unique 40-year dataset from a subarctic mountain birch (Betula pubescens ssp. czerepanovii) ecosystem in northern Fennoscandia, integrating climate, plant reproduction, insect herbivory, phenology, and large herbivore performance. Mean annual temperature increased by 0.6 °C per decade since 1981. Despite strong warming signal, ecosystem dynamics were dominated by cyclic biotic interactions rather than linear effects of climate warming. Birch reproductive indicators (pollen accumulation rate and catkin production) and reindeer calving success exhibited 2–4-year cycles, while geometrid moth populations showed recurrent ∼10-year outbreak dynamics. While warming weakly correlates with birch reproduction, it is strongly associated with increased moth abundance, establishment of the previously temperature-limited winter moth, and reindeer calving success. Moth outbreaks, combined with reindeer grazing pressure, led to birch defoliation and delayed post-outbreak recovery lasting 6–8 years. Potential positive effect of climate warming on mountain birch reproduction in subarctic ecosystem is, therefore, largely counterbalanced by increased herbivory pressure. Birch flowering and moth larval emergence phenology remained tightly synchronized, with no detectable phenological mismatch under warming. This indicates substantial phenological plasticity, likely reflecting adaptation to historically high interannual climate variability. Our results demonstrate that climate impacts in subarctic ecosystems are best captured by multi-trophic biotic indicators reflecting trophic interactions, disturbance regimes, and species redistribution. We highlight the critical role of long-term monitoring for adaptive ecosystem management planning under continued climate change.