Combining tree-ring width and carbon isotope data to investigate stem carbon allocation in an evergreen coniferous species

• The iWUE increased with aggravation of drought in semi-dry area • The impact of drought on BAI fluctuates by months and it weakens and shortens with time • The iWUE's response to drought is about 2 months later then BAI • The growth of conifer species under different drought degrees shares the same pattern at the monthly and yearly scales • Drought in semi-drought area causes legacy effect for 1–3 years and the second growth rate decreased in the third year Carbon allocation in tree stems critically affects how trees respond to climate variability and extreme weather. However, it is difficult to directly measure nonstructural carbohydrates in tree stems, and monitoring them over time is expensive and usually lasts only a few years at most. Hence, we utilized a method substituting nonstructural carbohydrates with the basal area increment (BAI) and intrinsic water-use efficiency (iWUE) to study tree stem carbon allocation. The results indicated that the values of the RAW (BAI-based tree-ring width index) and iWUEres (iWUE residual serial) in the current year could reveal information about early growing season climatic conditions. Thus, coniferous species might utilize carbon produced by photosynthesis during the early growing season to support tree growth. Wet years caused the RAW in the next year to significantly increase, and the RAW in drought years showed a significant correlation with the iWUEres in the previous year, while the RAW in the next year showed little correlation with the iWUEres in the current year. These results suggested that favourable climatic conditions could stimulate the accumulation of carbon, which promoted radial growth in the following year. In addition, drought in semiarid areas has a 1–3-year legacy effect on tree growth, suppressing the growth rate below the normal rate. The decline in tree growth was most noticeable in the year after drought, suggesting that inter-annual allocation of carbon reserves is a long-term process, yet trees prioritize using the newest carbon source for growth during adjacent years. This study broadens the understanding of nonstructural carbohydrates and their supply in evergreen coniferous species and has important implications for forest climate–carbon cycle models under changing climates.