Ontogenetic shifts in biomass allocation and xylem structure of the world's highest-occurring plants: balancing growth, storage, and resilience in the extreme Himalayan subnival zone.

Abstract

Understanding how plants allocate biomass to different organs and tissue types is essential for revealing their adaptive strategies across life stages and environments. This study investigates ontogenetic shifts in biomass allocation in Ladakiella klimesii, a long-lived alpine forb in the Himalayan subnival zone at 5900 m, to understand how plants adapt to extreme environments. Biomass distribution to roots, stems, and leaves, plus xylem tissue composition were measured in 205 individuals to determine how plant size and age influence resource prioritization across development stages. Root collar cross-sections were examined to determine plant age, annual growth increments, and tissue fractions. Smaller plants prioritized roots for nutrient uptake, while larger plants allocated more biomass to parenchyma for storage and metabolic activities. Lignified tissues decreased with increasing size, reflecting reduced structural requirements, while vessel fraction and radial growth were higher in younger plants to support water transport. Age modulated these patterns independently, with younger plants focusing on establishing structures and older plants emphasizing storage tissues for resilience. Ladakiella klimesii adapts to the extreme subnival zone through narrow xylem vessels to prevent freezing-induced embolism, the absence of fibres to minimize freezing risks, and high leaf mass fractions to optimize photosynthesis during short growing seasons. Its simplified xylem structure, dominated by parenchyma and single-lignified vessel rows, reflects thermal constraints and functional efficiency. These findings highlight the importance of integrating plant size and age in ecological studies and underscore this species' specialized strategies to thrive in a challenging subnival environment.