Computer models predict the future? Garbage in, garbage out

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.

Source: US EPA, 2018

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
PFTFire Resistance (%)
Tropical broad-leaved evergreen12.0
Tropical broad-leaved raingreen50.0
Temperate needle-leaved evergreen12.0
Temperate broad-leaved evergreen50.0
Temperate broad-leaved summergreen12.0
Boreal needle-leaved evergreen12.0
Boreal summergreen12.0
C3 grass100.0
C4 grass100.0

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

California’s Urban Greening Grant Program: An opportunity to speak for the trees

In September 2016, the State of California passed a law that allocated $1.2 billion to create a cap and trade program to reduce Greenhouse Gas (GHG) emissions.  The California Natural Resources (CNR) Agency was allocated $80 million to fund green infrastructure projects that reduce GHG emissions.  The CNR Agency is creating an Urban Greening Program to fund grants to cities, counties, and other entities such as non-profit organizations in URBAN settings.  75% of the funding must also be spent in economically disadvantaged communities.

These grants must reduce GHG emissions using at least one of these specific methods:

  1. Sequester and store carbon by planting trees
  2. Reduce building energy use from strategically planting trees to shade buildings
  3. Reduce commute, non-recreational and recreational vehicle miles travelled by constructing bicycle paths, bicycle lanes, or pedestrian facilities.

Clearly, planting trees is one of the primary objectives of this grant program.  That sounds like good news for the environment and everyone who lives in it until you read the draft program guidelines which are available HERE.

Unfortunately, as presently drafted, the grant program will NOT increase California’s urban tree canopies, because the program requires the planting of “primarily” native trees.   That requirement is explicitly stated several times in the draft guidelines, but there are also places in the draft where the reader might be misled to believe the requirement applies only to plants and not to trees.    Therefore, I asked that question of the CNR Agency staff and I watched the public hearing that was held in Sacramento on October 31st.  CNR Agency staff responded that the requirement that grant projects plant “primarily” native species applies to both plants and trees.

The good news is that the grant program guidelines are presently in draft form and the public has an opportunity to comment on them.  If you agree with me that we need our urban forest, you will join me in asking the CNR Agency to revise their grant program guidelines to remove restrictions against planting non-native trees.   Public comment must be submitted by December 5, 2016.  Send comments to:  Urban Greening Grant Program c/o The California Natural Resources Agency Attn: Bonds and Grants Unit 1416 Ninth Street, Suite 1311 Sacramento, CA 95814 Phone: (916) 653-2812, OR Email: Fax: (916) 653-8102

Here are a few of the reasons why limiting trees to native species will not increase tree canopies in urban areas in California:

Many places in California were virtually treeless prior to the arrival of Europeans.  Non-native trees were planted by early settlers in California because most of our native trees will not grow where non-native trees are capable of growing.  According to Matt Ritter’s California’s Guide to the Trees Among Us, only 6% of California’s urban trees are native to California:


Draft guidelines for the Urban Greening grants refers applicants to the California Native Plant Society for their plant palette (see page 24 of guidelines).  If applicants use this as the source of their plant palate, they will find few trees on those lists.  This is another way to understand that if you want trees in California, most of them must be non-native.

Most California native trees are not suitable as street trees because of their horticultural requirements and growth habits. 

  • The approved list of street trees for the City of San Francisco includes no trees native to San Francisco.  There are many opportunities to plant more trees in San Francisco because it has one of the smallest tree canopies in the country (12%).  The US Forest Service survey of San Francisco’s urban forest reported that 16% are eucalyptus, 8% are Monterey pine, and 4% are Monterey cypress.  None of these tree species is native to San Francisco.
  • The approved list of street trees for the City of Oakland includes 48 tree species of which only two are natives. Neither seem appropriate choices:  (1) toyon is a shrub, not a tree and the approved list says it will “need training to encourage an upright form.”  It is wishful thinking to believe that toyon can be successfully pruned into a street tree; (2) coast live oak is being killed by the millions by Sudden Oak Death and the US Forest Service predicts coast live oaks will be virtually gone in California by 2060.



Climate change requires native plants and trees to change their ranges if they are to survive.  One of the indicators of the impact of climate change on our landscapes is that 70 million native trees have died in California because of drought, insect infestations, and disease.  The underlying cause of these factors is climate change.

  • 66 million native conifers have died in the Sierra Nevada in the past 4 years because of drought and native bark beetles that have spread because winters are no longer cold enough to keep their population in check.  Update:  A new survey of California’s trees now reports that 102 million trees are now dead.  That’s one-third of California’s trees.  62 million trees died in 2016 alone, which is an accelerating rate of death.  These trees are still standing and they pose an extreme fire hazard.  These are NATIVE TREES being killed by a combination of drought and NATIVE BARK BEETLES.  
  • 5 million native oaks have died since 1995 because of Sudden Oak Death. A study of SOD by University of Cambridge said in spring 2016 that the SOD epidemic is “unstoppable” and predicted that most oaks in California would eventually be killed by SOD. The Oak Mortality Task Force reported the results of its annual survey for 2016 recently.  They said that SOD infections increased greatly in 2016 and that infections that were dormant in 2015 are active again.  This resurgence of the pathogen causing SOD is caused by increased rain in 2016.
  • Scientists predict that redwood trees will “relocate from the coast of California to southern Oregon” in response to changes in the climate.

If you care about climate change, please join us in this effort to create a grant program that will expand our urban forests and reduce the greenhouse gas emissions that are causing climate change.  Restrictions against planting non-native trees must be removed from grant guidelines in order to increase our tree canopies in California’s urban environments. 

Update:  Final guidelines for California State Urban Greening grant applications were published on March 1, 2017, and are available HERE.  That program will distribute $76 million to cities that reduce greenhouse gas emissions by planting trees or reducing fossil fuels emissions.  The deadline for grant applications is May 1, 2017.  There will be a workshop for applicants at the Lake Temescal Beach House (6500 Broadway, Oakland) on March 27, 2017.

Final guidelines are improved from the draft guidelines.  Draft guidelines would have required applicants to plant only native trees.  The State agency received 62 public comments on the draft.  27 of those comments asked that the guidelines be revised to permit planting non-native trees as well as native trees.  One of the 27 comment letters was signed by 33 tree-advocacy non-profit organizations. 

Final guidelines reflect the public’s opposition to prohibiting the planting of non-native trees, which would have severely limited the number of trees that would survive.  Native trees have specific horticultural requirements that limit the places where they can be planted.

Final guidelines now say that only “invasive” trees cannot be planted by grant projectsIf the granting agency uses the classification of the California Invasive Plant Council to determine “invasiveness,” applicants would not be allowed to plant 15 specific tree species.  However, the California Invasive Plant Council is revising its inventory of “invasive” plants, so we don’t know if the number of “invasive” trees will be increased by that revision.

Update #2:  The California Invasive Plant Council has published the proposed revision to its list of “invasive” species.  There were about 200 plants on the existing list.  Now they propose to add another 99 species.  Ten of those species are added based on their current impacts in California.  One of the ten is a tree (glossy privet).  87 of the species are proposed for addition “based on risk of becoming invasive” in the future in California.  Twelve of the 89 potentially invasive plants are trees. 

There were 15 trees on the original list of “invasive” species.  That means that the revised list of “invasive” trees will now include a total of 28 trees that cannot be planted by Urban Greening projects that are applying for grant funds. 

The revised inventory of “invasive” plants was just published.  Public comments can be submitted on the proposed revisions by May 8.  The proposed revisions and how to make comments on the proposal are available HERE

Personally, I object to the introduction of a new category of 89 plants that are not presently having any “impact” according to Cal-IPC but are predicted to in the future.  These revisions will increase the inventory of “invasive” plants by 50%.  It represents a significant escalation of the crusade against non-native plants in the California. 

Nativist bias is not entirely absent from the revised guidelines for the Urban Greening program.  Applicants are required to explain why they plan to plant non-native trees.  However, applicants are also required to have a certified arborist or comparable horticultural expert certify that the plant list is appropriate to the planting location.  Hopefully, that will prevent the wasteful planting of native trees where they will not survive.