The question we address in our paper is whether continued warming, drought, and wildfires in the Western U.S. have the potential to transition current forested lands permanently into shrublands or grasslands.  It has long been suggested that this is possible in certain locations.  For example, some studies question the long-term viability of the lodgepole pine forests in Yellowstone, however, whether they transition to grassland or to a different type of forest is still uncertain [Clark et al., 2017; Westerling et al., 2011].  It is clear that many areas of the West have experienced increasing drought intensity as a result of climate warming, as well as increased fire frequency and burn severity, and this trend is projected to increase in the future with most models. Some projections have shown that over 50% of future landscapes in the western United States will have climates incompatible with the vegetation communities that now occur on those landscapes [Rehfeldt et al., 2006].  We think it unlikely that warming and drought alone will cause such a shift, but if these lead to more wildfires, then that has the possibility of resetting the stage, so to speak, to allow grasses or shrubs to dominate if the changed climatic state favors those species.  We also address the question of how such a shift, or even just the new climatic state, will affect carbon dynamics and the resultant terrestrial carbon sink – i.e. will the land become a greater carbon source to the atmosphere or not?

We have conducted a modeling study using the Terrestrial Ecosystem Model [Felzer et al., 2011], a biogeochemical model of carbon, nitrogen, and water cycling.  We use two of the common future emissions scenarios, the Representative Concentration Pathway (RCP) 4.5 and 8.5 scenarios.  RCP4.5 is a moderate scenario in which emissions peak in 2040 and then decline, whereas RCP8.5 is a high-end scenario in which emissions continue to increase throughout the 21^st century.  Climate model output is from 20 global climate model runs for these two scenarios as downscaled and bias corrected by the Multivariate Adapted Constructed Analogs (MACA) [Abatzoglou and Brown, 2012].  In addition to changing climate, CO₂ levels increase accordingly with each scenario, and that generally has a positive effect on vegetation through the fertilization effect.  However, if warming exceeds any increases in precipitation, then drought conditions can occur and lead to wildfires.

In our study we employ a physical fire module based on one used by the National Center for Atmospheric Research (NCAR) Community Land Model (CLM), in which fire occurrence is based on fuel availability, ignition, combustibility, fire spread, and fire suppression, factors based on biomass, atmospheric humidity, soil moisture, wind speed, and population density [Lawrence et al., 2018; Li et al., 2012].  Existing forests are only allowed to shift if a replacement-level fire occurs first.  At that point, we assume the grid has a new starting point for regrowth, and we assess the optimality of forests, shrubs, and grasses by the end of the century.  We employ two criteria:  first is a bio-climatic limitation for each biome, to eliminate those biomes that fall outside of their theoretical limits in terms of temperature, solar radiation, and moisture conditions, based on the BIOME4 model [Kaplan et al., 2003; Prentice et al., 1992].  Secondly we employ a Net Primary Productivity (NPP) “bakeoff”, in which the biome with highest NPP is considered the most likely to survive and thrive.  Only if a grid passes the bioclimatic limitations and has an optimal NPP in a majority of the last 30 years of the 21^st century is it considered the likely biome to survive.

Results (Figure 1) show that 40% of grids originally dominated by trees during the 1984-2014 period will transition to shrubs (7%) and grasses (32%) by the end of the century under RCP4.5.  In contrast, under RCP8.5, 58% will transition to shrubs (18%) or grasses (40%).  Shifts predominantly occur in grids projected to experience the greatest temperature rise and decreased soil moisture, associated with severe, stand-replacing fires.  Geographically, the most pronounced shifts from forests to grasslands are observed in the Northern Rockies and Puget Trough, while transitions from forests to shrublands are predominant in the Southwest, consistent with historical trends (e.g. [Guiterman et al., 2022]).  Many of the forests in Yellowstone and the intermountain region are projected to persist. Additionally, our model indicates an upslope shift in the mean elevation of boreal and temperate coniferous forests under both climate scenarios. This trend indicates biome’s adaptive response to warmer and drier conditions.

In terms of carbon, the vegetation shifts lead to an overall decrease in NPP, vegetation and soil carbon by the end of the century.  This decline is primarily attributed to fires and unfavorable climatic conditions, which have enabled grasses and shrubs to expand at the expense of trees. In some locations where forests persist, the NPP actually increases due to more favorable moisture conditions.  However, Net Ecosystem Productivity (NEP) tells a different story, and is projected to increase.  While NPP is a measure of plant growth and carbon accumulation in the vegetation, NEP accounts for the soils and standing dead material as well, and includes decomposition.  The increase in NEP in both scenarios is primarily due to more grasses and shrubs, suggesting their ability to thrive in warmer, drier climates and expand into areas previously dominated by trees. However, if we account for carbon lost from fires themselves (often termed Net Biome Productivity), then there is a net carbon loss for both scenarios, of -60 GgC for RCP4.5 and -82 GgC for RCP8.5 scenario. In both scenarios, temperate coniferous forests contribute the most to the loss. 

The transition from trees to shrubs and grasses, predominantly in the northern states and Southwest region, marks a critical alteration in the ecosystem's capacity to store carbon, as forests are known for their higher carbon storage capacity compared to shrub or grass-dominated landscapes.  The elevation and range shifts suggest an adaptive response of ecosystems to the changing climate, with forests persisting in cooler and wetter regions. However, this pattern also indicates a potential narrowing of suitable habitats for trees, which are crucial for high-carbon storage.  As forests provide many critical ecosystem services, we suggest proactive forest conservation strategies, such as fuel reduction treatments, prescribed burning, fire breaks, and planting climate-adapted trees, to mitigate wildfire impacts and adapt to changing conditions.  Post-fire reforestation can also enhance recovery, biodiversity, and carbon storage. The focus should shift to maintaining these ecosystems in areas already transitioning to grassland or shrubland.  Instead of restoring the original forest, which may not be possible due to climate-vegetation mismatch, the focus should shift to maintaining these ecosystems.

 References

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