Carbon stress causes preferential storage over growth in treeline trees

Abstract Non‐structural carbohydrates (NSCs) play an important functional role in determining plant survival as a backup carbon (C) source, while they also represent a large C sink being traded off against other C sinks when facing stress. However, information regarding how such trade‐offs in C allocation under C stress vary among species remains limited. Here, we experimentally induced C stress in juveniles of three tree species by exposing them to in situ artificial shading, and then we quantified the dynamics of whole‐tree C allocation patterns between storage and growth. For light‐demanding deciduous conifer larch and shade‐intolerant deciduous broad‐leaved birch, tree growth (represented by relative tree height growth (RHG) and relative basal area increment (RBAI)) was significantly reduced by shading, whereas for shade‐tolerant evergreen conifer fir, RHG and RBAI remained constant. Variations of whole‐tree NSC reserves were consistent for non‐treeline tree species (i.e., birch and fir), showing that shading remarkably reduced NSC storage. In contrast, shading‐induced NSC accumulation was found only in treeline tree species (i.e., larch). More importantly, C allocation to NSC storage was prioritized over structural growth only in larch under C stress, suggesting a strong trade‐off between survival and growth in treeline trees. Overall, our results discovered different patterns of C allocation in response to C stress across species with different plant functional types, further highlighting the highly conservative survival strategy of treeline tree species. Our study may contribute to an enhanced understanding of how forest biomes mitigate the negative impacts of stressful events in the context of global changes.