Global warming reduces the carrying capacity of the tallest angiosperm species (Eucalyptus regnans)

Rising temperatures and increased drought intensity are driving accelerated tree mortality rates worldwide. We investigated how these changes affect the carrying capacity of mountain ash forests (Eucalyptus regnans), the world's tallest flowering plant and one of the most carbon-dense forests on Earth (450-819 tonnes carbon per hectare).

Using data from a large network of silvicultural experiments collected between 1947 and 2000 in southeastern Australia, we quantified temporal trends in mortality rates and carrying capacity, and their relationships to spatiotemporal climate variations. We analyzed how maximum stand density changes with tree size (self-thinning line) across different climatic conditions and over time, disentangling spatial variation among sites from temporal variation within sites.

Our results show forests growing in the warmest and driest conditions (highest vapour pressure deficit) had the lowest carrying capacity. This capacity further decreased with rising temperatures. Each one-degree Celsius increase in mean annual temperature was associated with a 9% reduction in carrying capacity. Based on these relationships, a projected three-degree Celsius increase by 2080 (CSIRO RCP8.5 scenario) could reduce tree density and carbon stocks by 24%, equivalent to losing 240,000 hectares of mature mountain ash forests or releasing 108 million tonnes of carbon.

Trees that died were 0.62 times the size of living trees (i.e., they were suppressed), with no detectable effect of climate on this ratio. These findings demonstrate that reduced carrying capacity could undermine carbon sequestration and global forest restoration efforts, particularly in seasonally dry regions where warming accelerates water limitations. We discuss implications for incorporating changing carrying capacity into forest management and carbon accounting.