# Change in C, N, and P Characteristics of Hypericum kouytchense Organs in Response to Altitude Gradients in Karst Regions of SW China

## TL;DR

This study examines how the nutrient content of a shrub species changes with altitude in China's karst regions, highlighting phosphorus as a key factor in adaptation.

## Contribution

The study provides new insights into nutrient allocation and adaptation strategies of Hypericum kouytchense shrubs along altitude gradients in karst ecosystems.

## Key findings

- Hypericum kouytchense organs showed higher nitrogen content compared to global and Chinese plant averages.
- Phosphorus content in plant organs followed a concave pattern with increasing altitude, influencing nutrient ratios.
- Phosphorus was identified as the key driver of nutrient cycling and plant-soil coupling in these shrubs.

## Abstract

The environmental heterogeneity caused by altitude can lead to trade-offs in nutrient utilization and allocation strategies among plant organs; however, there is still a lack of research on the nutrient variation in the “flower–leaf–branch–fine root–soil” systems of native shrubs along altitude gradients in China’s unique karst regions. Therefore, we analyzed the carbon (C), nitrogen (N), and phosphorus (P) contents and their ratios in flowers, leaves, branches, fine roots, and surface soil of Hypericum kouytchense shrubs across 2200–2700 m altitudinal range in southwestern China’s karst areas, where this species is widely distributed and grows well. The results show that H. kouytchense organs had higher N content than both global and Chinese plant averages. The order of C:N:P value across plant organs was branches > fine roots > flowers > leaves. Altitude significantly affected the nutrient dynamics in plant organs and soil. With increasing altitude, P content in plant organs exhibited a significant concave pattern, leading to unimodal trends in the C:P of plant organs, as well as the N:P of leaves and fine roots. Meanwhile, plant organs except branches displayed significant homeostasis coefficients in C:P and fine root P, indicating a shift in H. kouytchense’s P utilization strategy from acquisitive-type to conservative-type. Strong positive relationships between plant organs and soil P and available P revealed that P was the key driver of nutrient cycling in H. kouytchense shrubs, enhancing plant organ–soil coupling relationships. In conclusion, H. kouytchense demonstrates flexible adaptability, suggesting that future vegetation restoration and conservation management projects in karst ecosystems should consider the nutrient adaptation strategies of different species, paying particular attention to P utilization.