How many trees are there in the North American boreal forest?

Boreal forests, the largest terrestrial biome on Earth, are highly varied in local tree density. Despite previous attempts to estimate tree density in boreal forests, the accuracy of the estimation is unknown, leaving the question how many trees there are in boreal forests largely unanswered. Here, we compiled tree density data from 4367 plots in North American boreal forest and developed tree height-based generalized linear and machine learning models to address this question. We further produced the current boreal tree density map of North America, and projected tree density distribution in 2050 under the shared socioeconomic pathways (SSP) 126, 245 and 585 climate change scenarios. Our best-performed and cross-validated random forest model estimated a total of 277.2 (± 137.7 SD) billion trees in the North American boreal forest, 31.3% higher than the previously estimated 211.2 billion. Our projected tree density distributions in 2050 showed at least 11% increase in tree density in the region. This study improves our knowledge about boreal tree density and contributes to understanding the role of boreal forests in regulating forest ecosystem functions and informing adaptation and mitigation policy-making. The projected warming-induced increase in tree density suggests the potential of the North American boreal forest for carbon sequestration.