Topography- and age-driven growth response of Himalayan birch to climate in Trans-Himalayan Manang valley, Nepal

Abstract

This study aims to evaluate how climate variability influences both the long-term radial growth dynamics and regeneration patterns of Himalayan birch (Betula utilis) across contrasting slope aspects in the Trans-Himalayan region of central Nepal. We investigated radial growth responses and regeneration status of B. utilis using tree-ring width and stand regeneration consensus. We produced tree ring-width chronologies of B. utilis; 304 years (1720–2023) from the north facing moist slope of Milarepa, and 58 years (1966–2023) from the south facing dry slope of Ngawal. Results indicate that warming temperatures in moist Himalayan forests negatively affect the growth of B. utilis, particularly through increasing minimum (night-time) temperatures. Radial growth at Ngawal is negatively affected by higher late- and early-growing season temperatures but benefits from winter precipitation, whereas growth is more strongly limited by both minimum and mean temperatures across seasons at Milarepa. The declining basal area increment and ring-width indices from the late twentieth century, especially in the moist forest, further indicated higher growth sensitivity. Also, growth sensitivity to climate strengthened from young stands toward old-growth forests, suggesting that stand development stage modulates climate–growth relationships. Further, B. utilis shows stable regeneration overall, with stronger recruitment at Milarepa, but more limited regeneration at Ngawal due to environmental and anthropogenic constraints. These findings highlight that warming in moist Himalayan forests may suppress birch growth and increase vulnerability of late-successional forests to climate change.