Abstract: Aims With the intensification of climate change, shrub expansion or encroachment in high-altitudes and high-latitude has become increasingly significant. However, the patterns and driving mechanisms of changes in plant defensive chemicals such as total phenols and condensed tannins during such shrub expansion processes in alpine regions remain poorly understood. Methods Taking the alpine shrub-meadow transition zone or ecotone in western Sichuan on the eastern Qingzang Plateau as the study area, sample plots were established along a shrub expansion gradient including control (0% coverage), light expansion (<30% coverage), light-moderate expansion (30%–45% coverage), moderate-heavy expansion (45%–60% coverage), and heavy expansion (>60% coverage). The impact of shrub expansion on plant defensive chemical compounds and their relationships with environmental factors such as elevation, soil pH, and moisture content were analyzed by a space-for-time substitution approach. Important findings The results showed that: (1) Shrub expansion significantly increased the density, height, volume, and weighted density of shrubs in the alpine shrub-meadow transition zone of western Sichuan; (2) Compared to herbaceous plants and Sorbus rufopilosa, Rhododendron lapponicum exhibited higher levels of phenolic compounds during the shrub expansion process; (3) Changes in species composition at the community level had a significantly greater impact on the variation in plant phenolic compounds than intraspecific variation resulting from phenotypic plasticity among individuals; (4) Concentrations of phenolic compounds in both plant leaves and roots showed significant correlations with their carbon to nitrogen ratios, while soil factors had a relatively minimal effect, supporting the Carbon-nutrient Balance Hypothesis (CNBH). In summary, this study sheds light on the positive impact of shrub expansion on plant defense compounds and provides critically theoretical underpinnings for ecosystem management and plant conservation in the context of global change.

Key words: shrub encroachment, phenolic compounds, intraspecific variability effect, carbon-to-nitrogen ratio, carbon-nutrient balance hypothesis