Tree Growth and Water-Use Efficiency at the Himalayan Fir Treeline and lower altitudes: Roles of Climate Warming and CO₂ Fertilization

Abstract. Alpine forests are increasingly exposed to rising temperatures and intensified drought, potentially pushing species beyond their tolerance limits. However, the extent to which rising atmospheric CO₂ (Cₐ) can mitigate these stressors by enhancing tree intrinsic water-use efficiency (iWUE) remains unclear. We investigated the growth and physiological responses of Himalayan fir (Abies spectabilis) using basal area increment (BAI) and δ¹³C data to track ecophysiological processes over recent decades along an elevational gradient in warming and drying sites on the Tibetan Plateau. Significant growth increases were observed at all elevations in wet regions, while negative growth trends were noted at lower elevations in dry regions. Climate–growth correlation analysis revealed that growth is primarily constrained by growing season temperatures and spring moisture availability. Tree iWUE increased over time at all elevations, with a stronger increase in wet regions. Tree growth at lower elevations in dry stands was negatively related to iWUE, whereas BAI in wet regions was positively associated with iWUE. Leaf intercellular CO₂ (Cᵢ) increased proportionally to Cₐ after 1965. Structural equation modeling indicated that temperature was a key driver of BAI and iWUE at all elevations in wet regions, while temperature had negative effects on BAI at lower elevations in dry regions. These results suggest that elevated Cₐ and temperature can stimulate tree growth in high-elevation forests in wet regions, but the positive effects do not compensate for the negative impacts of reduced water availability at lower elevations in dry regions. Warming-induced drought stress may thus emerge as a more significant driver of growth compared to increasing Cₐ levels in comparable alpine forests. Our findings provide critical insights for refining assumptions about CO₂ fertilization and climate change effects in ecophysiological models.