Carbon storage in the Greater Yellowstone Ecosystem under future warming scenarios

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This article is part of the Spring 2021 issue of the Earth Science Matters Newsletter. 

Climate change has contrasting impacts on forests of the Rocky Mountains.  Rising temperatures can enhance growth of cool, high-elevation forests where growing seasons are short.  However, warming can also induce dry conditions that increase fire risks and drought stress, and lead to the loss of forests and the ecosystem carbon stocks they hold.  Therefore, understanding the long-term consequences of climate change for mountain forests requires research approaches that integrate multiple ecological processes such as tree establishment, growth, mortality, and wildfire, all of which are sensitive to climate change. 

USGS scientists studied how a warming climate interacts with forests and wildfire in The Greater Yellowstone Ecosystem (GYE).  The GYE, which spans parts of Wyoming, Idaho, and Montana includes extensive public lands.  Fire is the dominant disturbance in the GYE, a reality that was driven home by extensive wildfires in Yellowstone National Park during the summer of 1988.  Past research in the region separately projected both massive increases in area burned and increases in forest growth, rendering the long-term outlook for forests and forest carbon stocks in the region uncertain. 

USGS scientists recently used the LANDIS-II forest landscape model to untangle the conflicting impacts of climate change on forests in the GYE.  LANDIS-II simulates ecological processes by breaking a region into a grid of 250 × 250 meter pixels.  Tree establishment, growth, and death are simulated in each pixel, and the model tracks tree biomass and carbon stocks in wood and soils.  Simulated wildfires spread between pixels, killing trees and releasing carbon to the atmosphere.  Simulated tree growth and fire probabilities are based on past observations in the region and are sensitive to changing climate.  The research design included a control scenario under historical climatic conditions, and future scenarios using climate data from five general circulation models (GCMs) that bracket the range of projected climatic conditions through 2100.    

 

Map of Greater Yellowstone Ecosystem

Left – Map of the Greater Yellowstone Ecosystem (GYE), which includes parts of Wyoming, Idaho, and Montana. Green areas denote federal lands managed by the National Park Service, U.S. Fish and Wildlife Service, U.S. Forest Service, and Bureau of Land Management.  Yellow areas are held by Tribal governments, states, and private land holders.  Top right – Cumulative area burned simulated by the LANDIS-II model for 1988 – 2100 under historical climate conditions (black line) and climate projected by five general circulation models (GCMs; colored lines).  Bottom right – The difference in carbon held in live aboveground biomass between simulations with historical and GCM climate.

(Credit: Jeremy Havens, USGS. Public domain.)

 

LANDIS-II simulations indicated that the GYE can maintain forest growth and carbon uptake in a warming climate.  Carbon held in soils, live trees, and dead wood can delay losses of carbon at the regional scale, especially before 2050.  However, with warming greater than 3 °C in average annual temperature, simulated increases in area burned reduced live carbon stocks by 4 – 36% relative to the historical climate scenario.  Consequently, forest-carbon losses to wildfire ultimately exceeded warming-driven gains in forest productivity.  Whereas fires will likely be the main factor limiting forests during the coming decades, drought may increasingly limit seedling establishment, especially at low elevation.  Furthermore, larger future fires would mean longer distances to seed sources in unburned patches.  Therefore, both drought and seed availability may limit post-fire forest recovery, and forest regeneration may require management intervention.

The paper, “Increased burning in a warming climate reduces carbon uptake in the Greater Yellowstone Ecosystem despite productivity gains,” was published in the Journal of Ecology. It can be read here, https://doi.org/10.1111/1365-2745.13559.

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