Plastic and adaptive response of carbon allocation to temperature change in alpine treeline trees

Understanding how tree individuals allocate nonstructural carbohydrate (NSC) between storage and growth is key to reveal the mechanism of alpine treeline formation as well as its response to climate warming. Yet, little is known about whether carbon allocation of treeline trees responds to temperature change by means of phenotypic plasticity and/or ecotypic adaptation. Here, we employed a reciprocal transplant experiment and a growth chamber warming treatment to explore the effects of temperature change on functional traits and carbon allocation between NSC storage and growth in treeline species Larix chinensis. In the reciprocal transplant experiment, we observed that the individuals grown at high elevation always displayed relatively lower specific leaf area (SLA) than those grown at lower elevations irrespective of species provenance, demonstrating the phenotypic plasticity of functional traits in relation to elevation. Higher growth rate and late-season NSC concentration in the individuals at their provenance elevation than at the elevations away from home, indicated that trees were locally adapted to their home environments in terms of NSC storage and growth. Additionally, individuals from high-elevation provenance showed considerably less phenotypic plasticity in carbon allocation (i.e., the ratio of NSC storage to growth) in response to temperature change compared to those from low-elevation provenance. In the chamber experiment, warming greatly accelerated shoot growth but not accompanied by a down-regulation in NSC storage in cold-edge trees, resulting in a lower ratio of NSC storage to growth. It was noteworthy that there was a time lag in variations of carbon allocation, especially in variations of NSC storage when cold-edge trees were grown in warmed climates. Our findings demonstrate that the responses of NSC storage and growth to temperature change might be a combination of short-term plastic and long-term adaptive reactions, and suggest a limited response of cold-edge populations in carbon allocation to climate warming.