A widely used land surface model may overestimate the future carbon storage capacity of forests by up to 30%, according to a study published on June 18 in Geophysical Research Letters by Cornell University researchers. This discrepancy is attributed to the model's failure to account for reduced tree growth under hotter, drier atmospheric conditions, even when photosynthesis continues.
Key Findings
- The study determined that a common land surface model may overstate future forest carbon storage by up to 30%.
- The model operates on the assumption that tree photosynthesis and growth are tightly coupled. Observational data from European forests indicates that hotter, drier air slows growth rates, even when photosynthesis is ongoing.
- Researchers used an eight-year dataset of growth records from Swiss forests, encompassing both broadleaf and coniferous species. They observed that atmospheric dryness, including factors beyond just rainfall, consistently reduced growth rates.
- Subsequent observations noted the same growth slowdown effect in North American forests.
- A statistical model used to project tree growth and carbon storage through 2069 was compared to simulations from the land surface model. The land model overestimated growth by a factor of two for broadleaf trees and a factor of three for conifers. The largest discrepancies were found in regions forecasted to become hotter and drier.
Background
- Land ecosystems currently absorb approximately 27% of carbon dioxide emissions from fossil fuel burning; oceans absorb about 25%, and the remainder accumulates in the atmosphere.
- A sustained slowdown in forest growth would reduce this land-based carbon sink, which recorded a sharp decline in 2023.
- Separate tree-ring studies have reported only minor effects of drought on tropical stem growth. The Cornell study focuses on European forests, identifying atmospheric dryness as the primary driver of the growth slowdown.
Statements
"The land models wrongly assume photosynthesis and growth move in step. So the tree may be photosynthesizing, but it’s not growing."
— Brendan Clark, postdoctoral researcher at Cornell University and lead author
"This research sharpened estimates of the extent of damage additional warming would cause to the land sink."
— Daniele Visioni, assistant professor at Cornell University and co-author
Implications and Ongoing Work
- Parts of Australia's tropical forests have already shifted from acting as a carbon sink to a carbon source, attributed to rising tree mortality surpassing new growth.
- Brendan Clark plans to write computer code that integrates the observed slower growth rates directly into land models, aiming to bridge a gap between ecological observations and climate modeling practices.