Increased tree mortality rates have been observed worldwide in connection to climate warming-related processes, such as drought, heat, fire, and insect pest outbreaks. An understanding of the drivers of tree mortality during the Anthropocene is urgently needed to estimate forest vulnerability in a warmer climate. In this study, we assessed the drivers of tree mortality in an urban recreational boreal forest area in Helsinki, Finland, of approximately 830 ha, where increased tree mortality rates have been recently observed. A time series of aerial images was used to quantify tree mortality over the area to detect dead trees from 2005 to 2021 at seven timestamps. In total, 6008 dead trees were observed from the aerial images collected during the monitoring period. Forest environmental and climatic variables were used to explore the tree mortality drivers for individual trees and tree communities using logistic regression and correlation analysis. Our results showed that droughtrelated variables, i.e., the standardised precipitation evapotranspiration index and the Palmer drought severity index, were linked with increased tree mortality rates. We found that the stand-level basal area predicted tree mortality risk and was linked to site type; smaller basal area stands were located on rocky dry soils, resulting in a greater probability of tree mortality. We also observed that trees at high elevations or on steep slopes showed a greater mortality risk. Our results can increase the understanding of tree mortality in urban areas and help the planning of built and green areas in a changing climate.

In recent decades, increases in severe drought, heat extremes, and pest burden have contributed to increased global tree mortality. These risks are expected to be exacerbated under projected climate change. So far, observations of tree mortality are mainly based on manual field surveys with limited spatial coverage. The lack of accurate tree mortality data over large areas has limited the development and applications of tree mortality models. However, a combination of high-resolution remote sensing data, such as aerial imagery and automated imagery analysis, may provide a solution to this problem. In this study, we analysed the dynamics and drivers of forest canopy mortality in 117 366 ha of boreal forest in Southeast Finland, between 2017 and 2023. For this purpose, we first developed a fully convolutional semantic segmentation model to automatically segment forest canopy mortality from aerial imagery in 2017, 2020, and 2023 with a spatial resolution of 0.5 m. Secondly, we trained the model using a dataset consisting of 32555 canopy mortality segments manually delineated from aerial imagery from various geographic regions in Finland. The trained model showed high accuracy in detecting forest canopy mortality (with an F1 score of 0.86-0.93) when tested using an independent test set. To estimate standing deadwood volume, we combined the observed yearly forest canopy mortality with open forest resource information based on extensive field campaigns and airborne laser scanning. In our study area, forest canopy mortality increased from 23.4 ha (0.02 % of the study area) to 207.8 ha (0.18 %) between 2017 and 2023. Consequently, standing deadwood volume was estimated to increase from 5192 m3 (0.04 m3/ha) to 52800 m3 (0.45 m3/ha) during the study period. Both the volume of standing deadwood and the extent of forest canopy mortality increased exponentially. The majority of the forest canopy mortality occurred in Norway sprucedominated forests (64.1-77.3 %) on relatively fertile soils (81.6-84.7 %) while 20-25 % of the forest canopy mortality occurred in Scots pine-dominated forests. The average age of stands where mortality was observed was between 60 and 70 years old (2017 = 69.7 years and 2023 = 62.6 years), indicating that mature forests were more susceptible to mortality than younger stands. Our findings highlight an exponential increase in forest canopy mortality over a relatively short time span (6 years). The increasing risk of tree mortality in boreal forests underlines the urgent need for large-scale and spatially accurate monitoring to keep up to date with fast-paced changes in boreal forest mortality. As climate change increases drought, extreme heat and bark beetle outbreaks, consistent canopy mortality mapping is essential for implementing timely risk management measures in forestry.

Interest in growing mixed species stands has increased in recent years, and the cultivation of mixed stands is promoted in silvicultural guidelines and forest certification systems. Mixtures of tree species have often increased biomass production, but the effect of species mixture on productivity has been small in studies conducted in the Nordic countries. However, several previous studies are based on temporary plots selected according to predefined criteria, usually removing possible effects of damage affecting forest growth. Sample-based forest inventories describe the actual development of forest resources and provide an opportunity to analyse the factors affecting forest growth. Using the data of the Finnish National Forest Inventory (NFI), we analysed the species mixture-productivity relationship that take place in real environmental and management conditions. We analysed pure and mixed species stands dominated by spruce and pine in southern Finland and compared their volume increment, growing stock, and damage, and present results separately for age classes 21-40, 41-60, and > 60 years. Mixed stands were more common in the 1920 s because of historical forms of land use, but tree species composition in the current forests is quite different, and the stands are conifer-dominated. Slight and moderate broadleaved species mixture did not notably affect the volume growth of coniferous stands on average, while it varied somewhat from case to case. Instead, the growth of pine- and spruce-dominated stands was related to the amount of growing stock, site fertility, and stand age, regardless of the level of broadleaved species mixture. In addition, the results showed an increasing proportion of damaged stands with an increasing proportion of broadleaved trees. Our results at regional level confirm the results of previous Nordic studies conducted at stand level that slight and moderate mixtures of broadleaved trees in coniferous stands have no or only a low reducing effect on wood production.