Shrub Encroachment Constrains Precipitation‐Driven Shifts in Herbaceous Biomass‐Diversity Relationships

Abstract

The relationship between herbaceous above ground biomass and species richness typically exhibits a unimodal pattern, shifting from positive under low‐precipitation conditions to negative under high‐precipitation conditions. While shrub encroachment generally reduces both herbaceous aboveground biomass and diversity, how shrubs mediate the herbaceous biomass‐richness relationship under altered precipitation regimes remains unclear. By synthesizing data from globally distributed precipitation manipulation experiments, we reveal contrasting herbaceous biomass‐diversity responses in grasslands and shrublands. In grasslands, reduced precipitation shifts the peak of the unimodal herbaceous biomass‐richness curve toward lower biomass, whereas increased precipitation shifts it toward greater biomass. In contrast, the peak of the unimodal relationship in shrub‐dominated systems shows no significant shifts across precipitation extremes. These global patterns are supported by a 5‐year field experiment in a desert steppe in Northern China, which further shows that precipitation change strongly affects species richness through soil water availability in grasslands, while exerting weaker effects through modifications of the fast‐slow leaf economics spectrum in shrublands. These findings demonstrate that shrub encroachment can constrain precipitation‐induced changes in herbaceous biomass‐diversity relationships, providing important insights into how woody plant expansion interacts with climate change to influence ecosystem structure and function.

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