Influence of bark beetle infestation on stem diameter dynamics

Bark beetle epidemics have increased globally due to drought and a warming climate, impairing forest ecosystem services by inducing severe forest disturbances. Bark beetle infestation monitoring is urgently needed for pre-emptive forest management. However, we have a lack of thorough understanding of the physiological responses that occur on trees after a bark beetle infestation. More specifically, we need insights into how quickly trees decline post-infestation and how the rate of tree decline relates to the bark beetle brood development. To increase understanding of trees’ physiological responses to bark beetle infestation, we monitored stem diameter variations at 15-minute intervals across 31 healthy and 26 infested mature Norway spruces affected by the European spruce bark beetle (Ips typographus L.) in Southern Finland. We observed that the infestation led to reduced or negligible growth, followed by a sudden decline in stem diameter. Using dendrometer data, we also established a point of ceased stem expansion for trees affected by bark beetle infestation, serving as a proxy for the tree's physiological response. This allowed us to explore the duration between the infestation date and the onset of tree decline. We observed considerable temporal variation in the physiological response of trees to the bark beetle infestation. The timing of the point of ceased stem expansion ranged from 15 to 81 days after the onset of the simulated infestation date, with a mean of 46.8 days. Only 58 % of the trees showed a point of ceased stem expansion before the bark beetle brood development was completed. This suggests that trees may take longer to respond to the infestation than it takes for the bark beetle brood to develop. Considerable temporal variation in tree response to the bark beetle infestation was observed, which calls for more research to understand the temporal dynamics of the tree's response to bark beetle infestations.