Impacted radial growth of birch in the East-Ural Radioactive Trace Zone: Effects of ionizing radiation or of competition, weather conditions, and phytophages?

Tree ring analysis offers a historical record of environmental conditions spanning decades to millennia. We studied the relationship between the annual radial growth of silver birch (Betula pendula Roth) and the radioactive contamination levels of forests impacted by the Kyshtym accident (1957), a major nuclear incident. The overall effect of the radiation event from the Kyshtym accident appears to be inconsistent (increased and decreased growth) for the first two decades following the incident. Given the multitude of variables involved, it is challenging to establish a singular causal relationship. Initially, the combination of irradiation, drought, and spongy moth (Lymantria dispar L.) outbreaks significantly reduced radial tree growth. By 1960, short-lived radionucleotides had decayed significantly, resulting in a 100–1000 fold decrease in γ radiation exposure at the most contaminated sites, while radiosensitive pine (Pinus sylvestris L.) died in mixed pine–birch forests. From 1960–1967, birch trees at sites with dead pines and moderate contamination experienced the greatest radial growth from reduced competition, but smaller radial growth at heavily contaminated sites, recovering in 1973–1978 despite a major drought. From 1979–2019, birch tree radial growth in the East-Ural Radioactive Trace (EURT) and control sites remained stable.