Shifting potential tree species distributions from the Last Glacial Maximum to the Mid-Holocene in North America, with a correlation assessment

Pollen reconstructions of tree genera in North America since glaciation are available, but species distributions predicted for paleoclimate based on tree inventories may inform knowledge gaps. Here I examined the distributions of 25 species or species groups from 20 000 years ago (ka) to 5 ka to give potential paleoecological ranges of boreal and temperate tree species. I also assessed the effects of correlated climate variables on species distribution models for current and past climate, which were modeled with the non-linear random forests classifier. Climate change alone was enough to create unique, species-specific paths that did not run directly north. At 20 ka, black spruce (Picea mariana) occurred as far north as the ice sheet and most boreal species generally may have extended as far south as 32 degrees N, or the northern southeastern United States. Temperate eastern species may have extended as far north as 39 degrees N in low-elevation locations and temperate eastern species displayed both continuous and clustered distributions across the southeastern United States. Rate of movement was 5.5 km per century between 20 and 14 ka, 13.3 km per century between 14 and 10 ka, and 3.25 km per century between 10 and 5 ka. Species retreated southward between 7 and 5 ka. Regarding correlation, models with all variables had greater accuracy than models with the two most important variables, which had greater accuracy than models with two variables of intermediate importance, demonstrating both reduced accuracy with omission of relevant variables and isolation of important variables that improve accuracy in models with correlated climate variables. Models from different climates identified the same two most important variables and ranked the remaining variables similarly, revealing robustness in the models over time. Distribution models agreed with pollen reconstructions regarding timing and rate of change, while generating detailed species-specific information about movement trajectories and velocities, latitudinal extents, and distribution continuities.

Automated summary

This study analyzed pollen reconstructions of tree genera in North America since glaciation and predicted the paleoecological ranges of boreal and temperate tree species from 20,000 years ago to 5,000 years ago. The study found that climate change was the main factor driving the distribution changes of tree species and identified key climate variables that influenced species distribution. The movement speed of species gradually decreased over the past 10,000 years and retreated southward between 7,000 and 5,000 years ago.