In a study published in New Phytologist on March 10, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and Northeast Forestry University revealed that trees adopt distinct strategies to balance water transport, sugar distribution, and storage depending on their climate. This study is the first to systematically compare the functional ratios of mature, canopy-level woody plants.

The researchers used canopy cranes to collect standardized samples from four typical Chinese forests (tropical, subtropical, temperate) in Xishuangbanna, Ailao Mountain, Lijiang, and Changbai Mountain, covering both latitudinal and altitudinal gradients. They analyzed the allocation patterns of functional ratios (e.g., xylem, phloem, and ray parenchyma to leaf area ratios) in 55 canopy woody tree species under different climatic conditions and their environmental driving factors.

Their results showed that trees in cold, high-altitude, and high-latitude regions invest more in xylem and phloem per unit leaf area than trees in warm, humid tropical forests. This suggests that in cold climates, trees prioritize storage capacity and the ability to repair freeze-thaw cycle damage over high-capacity transport systems.

The ratio of ray parenchyma to leaf area also increased significantly at higher altitudes, reflecting a greater need for storage under light limitation and freeze-thaw stress. Interestingly, although functional ratios varied considerably across different climate zones, plants with different growth forms exhibited convergent strategies within the same forest environment. This convergence indicates that environmental filtering is the dominant force shaping branch-level allocation strategies.

The researchers also found that the xylem-to-phloem functional ratio is closely linked to plant height, wood density, and hydraulic conductivity, showing that these allocation strategies are tightly connected to overall physiological function.

"Our findings demonstrate that functional ratios diverge across climates but converge within similar environments, suggesting that environmental filtering drives storage-transport balances," said CHEN Yajun of XTBG.

By clarifying how canopy trees balance survival and growth resources across climate zones, these findings provide valuable scientific guidance for forest management and conservation in the context of global climate change.

Canopy view of the four typical Chinese forests. (Image by XTBG)