Species and soil wetness shape tree growth responses to warming and drought in boreal old-growth forests

Abstract Boreal forests mitigate climate change by sequestering atmospheric carbon. Recent declines in the boreal carbon sink and tree growth suggest that this mitigating influence is weakening, yet the underlying drivers remain unresolved. Increasing drought frequency is one likely driver. Yet most studies have focused on managed forests, overlooking the significant but shrinking areas of old-growth forests, which recent evidence suggests exhibit greater drought resistance. We analysed tree rings from Norway spruce, Scots pine, and Birch across soil-moisture gradients in 12 Swedish old-growth forests to evaluate temporal trends and climatic drivers of tree stem growth during 2012–2021. Contrary to previous studies, we found no overall recent forest growth decline, only a species-specific decline for Norway spruce in the warmest region. Following the extreme 2018 drought, Norway spruce showed pronounced growth reductions that peaked one year after the event (a lagged response) and persisted for at least three years (a legacy effect) in warm, dry areas. In contrast, Scots pine and Birch were less affected. Warming and drought (i.e. periods when evaporative demand exceeds water availability) jointly affected growth, particularly in warmer climates, but with differences among species in timing and magnitude. Tree growth in old-growth forest appeared relatively resistant to warming and drought, especially in wetter locations, although Norway spruce appears increasingly vulnerable in already warm regions. We further found that to avoid misleading interpretations of climatic effects, statistical models had to account for both the diminishing legacy effect (by including an autocorrelation structure), and the lagged drought response (by including a lagged hydroclimatic variable). Together, our findings emphasise the critical role of tree species composition and landscape-level hydrological conditions in shaping climate sensitivity of tree growth, and suggest that old-growth forests may sustain C sequestration into long-lived pools under climate change and thereby be valuable models for climate-adapted forestry.