Computer modeling is an increasingly popular tool used in ecological studies. The rapidly changing climate is putting pressure on scientists to predict the trajectory of the change and the impacts those changes will have on the environment. However, a computer model is only as predictive as the assumptions used to build it. In other words, “garbage in, garbage out.”
That sets the stage for a study published in 2018 that predicted that “grassland may be a more reliable carbon sink than forests in California.”(1) The study was quickly adopted by native plant advocates as a weapon in their battle to destroy non-native trees in favor of grassland they prefer. (2) They prefer grassland because it was the pre-settlement coastal landscape. They don’t acknowledge that burning by Native Americans and grazing by native ungulates were the primary reasons why grassland did not succeed to shrubs and forests prior to settlement. Pre-settlement grassland was as much a human creation as any modern landscape.
Most carbon storage is below ground, in roots and soil. That is true of both grassland and forests. If the forest burns, the carbon it has stored in soil remains, just as the below ground carbon sink of grassland remains.
The study (1) that claims grassland may be a more reliable carbon sink than forests reaches its erroneous conclusion by comparing below ground carbon storage in grassland with above ground carbon storage in forests. It’s a classic case of inappropriately comparing apples with oranges to the disadvantage of forests. It seemed such an unlikely comparison that I asked the study’s authors to confirm they had compared below ground carbon storage in grassland with above ground carbon storage in forests. They confirmed that they did, indeed, make that inappropriate comparison.
The study also bolsters its mistaken conclusions by erroneously claiming that forests are more likely to burn than grasses:
“The fire resistance for grasses is 0.5 while that of trees range from 0.1−0.3, making grasses more resistant to wildfires than trees, which is roughly consistent with field-observations since in the event of a wildfire, when compared to trees, a smaller fraction of the biomass of grass is damaged.” (1)
However, the study cited as the source of this statement (3) says exactly the opposite:
“The fraction of individuals killed depends upon the prescribed PFT fire resistance, which represents the PFT survivorship during a fire (see Table 1). In the fire model, grasses and litter are fully consumed.” (3)
|Table 1 PFT parameter values for fire resistance|
|PFT||Fire Resistance (%)|
|Tropical broad-leaved evergreen||12.0|
|Tropical broad-leaved raingreen||50.0|
|Temperate needle-leaved evergreen||12.0|
|Temperate broad-leaved evergreen||50.0|
|Temperate broad-leaved summergreen||12.0|
|Boreal needle-leaved evergreen||12.0|
Table 1 is consistent with this statement in the abstract of the cited study: “Estimated litter moisture is the main driver of day‐to‐day fire probability.” (3) Forests retain more moisture in the soil and leaf litter because of the shade provided by the tree canopy. I wrote to the study author again, asking “where is the source of your statement that grasses are more fire resistant than trees?” He did not reply.
If a study doesn’t seem to make sense, or it contradicts other sources of information, it is worthwhile to look under the hood. What is driving the model? Is it fueled by hot air? Is it serving an activist agenda? Are cited studies accurately quoted?
Some truth emerges from the model’s black box
Despite the erroneous assumptions of the computer model used by this study, there is some truth in the conclusions it reaches. Vegetation type conversions are occurring now and they will continue as the climate continues to change because when the climate changes, the vegetation changes. We are presently witnessing the transition of native conifers at high altitudes to lower altitude hardwood trees. Although these changes will occur gradually and there will be many intermediary transitions, the fact is that grassland is more likely to survive than forests in a warmer, drier climate in the long run.
The Guardian has published a comprehensive report about the loss of forests all over the world. In the Rocky Mountains, one-third of places where trees burned 20 years ago are now occupied by shrubs and flowers. About 15% of forests in the Rocky Mountains are not expected to grow back if killed by fire because the climate is no longer suitable for them. About half of existing forests in Alberta, Canada are expected to vanish by 2100. The “megadrought” in south-western US is expected to convert 30% of forests to shrubland or another type of ecosystem.
In the short run, the loss of forests can be mitigated by reforestation with tree species that are better adapted to a warmer, drier climate. The study (1) acknowledges the potential for mitigation to preserve forest ecosystems: “Factors such as species traits, biodiversity, rapid evolution, and human management intervention could alter our model-based findings from the projections provided here. Consequently, our results indicate the potential direction of change as opposed to predictions that consider the full ensemble of ecological, physiological and management factors that can alter pathways and responses of ecosystems to climate change.”
From the standpoint of carbon storage, it is not good news that grassland is likely to inherit hot, dry lands previously occupied by forests. Forests and wetlands store more carbon than grasslands, as the above chart in a USDA publication about carbon storage shows. Sustaining below ground carbon sinks will depend on carbon sequestration by above-ground plants and trees. Because above-ground carbon sequestration is primarily dependent upon the biomass, forests will always do a better job than grassland in the long run. In the short-run, grassland will grow back more quickly than forests, but it will never achieve comparable biomass.
Forests are presently absorbing about one-quarter of all human carbon emissions annually. Forests make a significant contribution to reducing carbon emissions, but planting trees is not a panacea as long we continue to burn fossil fuels to generate energy. The loss of carbon-sequestering capabilities of forests will exacerbate climate change in the long-run. It’s one of many dreaded feedback loops that are reaching tipping points: the impacts of climate change are destroying the mechanisms that mitigate climate change.
The study (1) acknowledges that by the end of the 21st Century, under current climate conditions (warming limited to 0.3⁰ – 1.7⁰ Centigrade) forests will have removed 5 times more net carbon (carbon storage minus carbon loss) per hectare from the atmosphere than grassland in California. See Table 1 in the study (1). Thus, the study agrees that forests store more carbon than grassland.
From the standpoint of wildlife, it is not good news that grassland is likely to replace forests in a warmer climate. The insects, birds, and animals that live in the forest will lose their habitat. Forests are home to over 80% of terrestrial species. We will lose our shade in a warming climate and our windbreak.
Not an argument for destroying forests
This study (1) is unfortunately being used by the native plant movement to advocate for the preemptive destruction of healthy urban forests that are not more likely than native forests to burn in wildfires. Virtually all wildfires in California occur in native vegetation. There is no advantage to destroying healthy forests that are expected to live for another 100-200 years. We don’t amputate our limbs to avoid breaking them. Nor should we destroy our forests before they die.
(1)“Grasslands may be more reliable carbon sinks than forests in California,” Pawlok Dass, Benjamin Z Houlton, Yingping Wang and David Warlind, 10 July 2018, Environmental Research Letters, Volume 13, Number 7
(2) “Importance of Grasslands for Carbon Storage,” Yerba Buena Chapter of California Native Plant Council, Quarterly Newsletter, March 2021, page 6.
(3) “The role of fire disturbance for global vegetation dynamics: coupling fire into a dynamic global vegetation model,” Thonicke K, Venevsky S, Sitch S and Cramer W 2001, Glob. Ecol. Biogeogr.10 661–77