Stable isotopes in tree rings record physiological trends in Larix gmelinii after fires

Fire is an important regulator of ecosystem dynamics in boreal forests, and in particular has a complicated association with growth and physiological processes of fire-tolerant tree species. Stable isotope ratios in tree rings are used extensively in eco-physiological studies for evaluating the impact of past environmental (e.g., drought and air pollution) factors on tree growth and physiological processes. Yet, such studies based on carbon (delta C-13) and oxygen (delta O-18) isotope ratios in tree rings are rarely conducted on fire effect, and are especially not well explored for fire-tolerant trees. In this study, we investigated variations in basal area increment and isotopes of Larix gmelinii (Rupr.) Rupr. before and after three moderate fires (different fire years) at three sites across the Great Xing'an Mountains, Northeastern China. We found that the radial growth of L. gmelinii trees has significantly declined after the fires across study sites. Following the fires, a simultaneous increase in delta C-13 and delta O-18 has strengthened the link between the two isotopes. Further, fires have significantly enhanced the C-13-derived intrinsic water-use efficiency (iWUE) and largely altered the relationships between delta C-13, delta O-18, iWUE and climate (temperature and precipitation). A dual-isotope conceptual model revealed that an initial co-increase in delta C-13 and delta O-18 in the fire year can be mainly attributed to a reduction in stomatal conductance with a constant photosynthetic rate. However, this physiological response would shift to different patterns over post-fire time between sites, which might be partly related to spring temperature. This study is beneficial to better understand, from a physiological perspective, how fire-tolerant tree species adapt to a fire-prone environment. It should also be remembered that the limitation of model assumptions and constraints may challenge model applicability and further inferred physiological response.

Automated summary

Fire plays a crucial role in regulating boreal forest ecosystems, especially in the growth and physiological processes of fire-tolerant trees. However, studies on the effects of fires on tree isotopes, specifically carbon and oxygen, are scarce, especially for fire-tolerant species. This study examined the changes in growth and isotopes of Larix gmelinii before and after three moderate fires in Northeastern China. The results showed that the fires significantly reduced radial growth and increased both carbon and oxygen isotopes. Fires also enhanced the intrinsic water-use efficiency derived from carbon isotopes and altered the relationships between isotopes, water-use efficiency, and climate factors. A conceptual model revealed the initial response of stomatal conductance and photosynthesis, which varied over time and were influenced by spring temperature. This study provides valuable insights into the physiological adaptations of fire-tolerant tree species in fire-prone environments, while acknowledging the limitations and constraints of the model.