# Direct Observation of the Developing Intra-Annual Density Fluctuation (IADF) for Scots Pine in Semiarid Siberian Belt Forest: External Stress Targets Cambium

## TL;DR

Scots pine trees in Siberia showed unusual wood patterns during a heatwave, indicating environmental stress mainly affects the cambium zone.

## Contribution

The study identifies the cambial zone as the primary target of environmental stress during tree-ring formation.

## Key findings

- An IADF in tree rings formed during a heatwave and drought in 2024.
- Environmental stress primarily affects the cambial zone, not later stages of wood formation.
- Normal wood cell production resumed after the heatwave subsided.

## Abstract

Long-term observations of the seasonal growth of Scots pine (Pinus sylvestris L.) tree rings in the arid conditions of the Khakass-Minusinsk Basin (southern Siberia) revealed that in 2024, trees had formed a tree ring with a typical intra-annual density fluctuation (IADF) in the transition wood. An analysis of the timing and causes of this wood structure anomaly was conducted using a combination of three approaches: (1) analyzing images of cross-sections of the forming tree ring throughout the season; (2) comparing the timing of anomalous cells’ differentiation with daily climate data; (3) comparing seasonal growth observations with calculated characteristics of the modeled growth rate and its derivatives: soil moisture and transpiration. We found that during the most severe heat wave and drought (from 22 June to 9 July), the last normal earlywood cells were yet expanding, IADF cells were being produced in the cambial zone, and the first of them began expansion, while normal cells began being produced again immediately after the subsiding of environmental stress. Apparently, low soil moisture and very high temperatures mainly impacted cells in the cambial zone, marking it as the primary target of external factors influencing tree-ring formation and structure, which is important for dendroclimatology and digital wood anatomy. This result is supported by both indirect and limited direct evidence from other sources.