Epiphytes, which grow non-parasitically on other plants, are thought to experience minimal variation in internal nitrogen isotopic composition (δ¹⁵N) due to extreme nitrogen scarcity in the canopy. However, it remains unclear how these canopy-dwelling plants adapt to severe nitrogen limitation in their unique habitat, due to the lack of empirical evidence.

In a study published in Journal of Ecology, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and Lanzhou University demonstrated for the first time that the leaf-root nitrogen isotope is smaller in vascular epiphytes compared to soil-rooted plants, but still has ecological significance, and that the degree of nitrogen isotope fractionation between leaves and roots is consistent within the same group, but has significant differences among species.

Researchers investigated nitrogen isotope fractionation in vascular epiphytes from the tropical forests of Xishuangbanna, Southwest China. Through a meticulous sampling effort involving single-rope climbing, free climbing, and pole pruners, they collected leaf and root samples from 24 epiphyte species. They calculated the difference in δ¹⁵N between leaves and roots and conducted statistical analyses to identify key drivers behind the observed patterns.

The results demonstrated that epiphytes exhibited a smaller but noteworthy leaf-root δ¹⁵N fractionation compared to soil-rooted plants. The leaf-root δ¹⁵N fractionation of epiphytes was species-specific and independent of phylogeny, and it varied across habitat scales and taxonomic categories, showing a strong association with the availability of external nitrogen sources or their acquisition efficiencies.

Besides, researchers found that fractionation values showed a marked increase in correlation with leaf and root nitrogen as well as their ratios. This finding suggested that the internal nitrogen transport and assimilation in epiphytes were strongly influenced by the availability of nitrogen. Also, water deficit was found to significantly affect negative δ¹⁵N fractionation.

“Our study demonstrates a significant degree of species specificity in the adaptation of epiphytes to microenvironments and nitrogen cycling. It highlights the need to account for species specificity when investigating plant nutritional and physiological characteristics,” said LI Su from XTBG.