Variable associations of annual biomass increment with age, latitude and germination year in four tree species in Sweden

Forests are pivotal for biodiversity, food webs, and human economies, and as carbon sinks their climate change mitigation potential is undisputed. However, whether trees continue to efficiently accumulate biomass at an increasing rate with age, how growth trajectories respond to climate change, and vary with latitude at species range margins is under debate. Here, we combine tree-ring data with biometric equation modeling to analyze how annual biomass increment varies according to tree age, species, latitude, and germination year. We generated 26,225 estimates of annual biomass increments for 136 individual trees, representing two evergreen gymnosperm conifer species (Picea abies (L.) H.Karst., and Pinus sylvestris L.) and two deciduous angiosperm species (Quercus spp. L. and Fagus sylvatica L.). Our dataset includes some old trees (range 150 - 405 years), and samples from locations spanning >10° latitude (55.0 - 66.3 °N) in Sweden. Annual biomass increments varied considerably between species, years and among individual trees within stands. On average, biomass increment remained positive throughout the lifespan across trees, species, and latitudes. Age-specific biomass increment was higher in the deciduous than in the evergreen species and declined with increasing latitude within species. For spruce and beech, biomass increment increased significantly with germination year, possibly reflecting faster growth in recent times in response to a warmer climate. The findings have implications for forestry practices aimed at productivity, biodiversity conservation, and climate change mitigation.