Krummholz at the Forefront of Treeline in the Himalaya: The Biotic Lock Decoupling Treeline Shift due to Climate Warming

Globally, alpine treelines are undergoing a spatially heterogeneous and frequently inconsistent and unpredicted altitudinal range expansion in response to accelerated climate warming. Previous findings in the Himalaya region reveal a significant decoupling between climatic control and treeline shift, suggesting that non-climatic factors are hindering the expected upward migration. We implemented a hierarchical sampling approach across krummholz and non-krummholz transects in Rolwalling and Langtang region extending from 3900 to 4300 m altitude. Soil was analysed for SOC, TN, microbial biomass to evaluate nutrient limitations and microbial stoichiometry. Allelochemical profiling was conducted for the analysis of secondary metabolites in leaf and root tissues of Rhododendron spp. Dendroecological climate sensitivity analysis was done through tree-rings study for drought response of the krummholz-forming R. campanulatum against the subalpine treeline species Abies spectabilis.Our results reveal that Krummholz soils exhibit significantly higher acidity and elevated allelochemical concentrations profiling such as carboxylic acids, fatty acids, phenolics, and terpenoids as potential inhibitory metabolites in Rhododendron tissues. Krummholz site maintained a significantly higher soil C:N ratio (25:1) and an exceptionally low mean microbial quotient (qMIC = 0.17%), reflecting nitrogen immobilization and stagnant nutrient turnover. The lower dwarf shrub heath zone exhibited the highest mean MBC 1,170.8 µg g⁻¹ soil and MBN 111 µg g⁻¹ soil, while lower krummholz had the highest MBP mean 299.6 µg g⁻¹ soil. Furthermore, dendrochronological analysis showed that A. spectabilis is significantly more sensitive to drought severity than the resilient R. campanulatum. These findings suggest a 'Biotic Lock' mechanism: R. campanulatum not only modifies an edaphic niche through soil interference and nutrient dynamics but also exhibits greater physiological relaxation under climatic stress. This study identifies the krummholz forest as a critical biotic frontier that is inhibiting subalpine forest advance through complex edaphic interactions, allelopathic constraints, and higher resilience to moisture stress.