One of the potential benefits of increasing the number of urban and suburban trees in Rhode Island is the potential that trees have for long term storage of carbon. Unfortunately it is difficult to determine the exact amount of carbon that a given tree will be able to sequester over its lifespan. Variable weather conditions will cause great variations in growth. Using a model that is available through the Department of Energy (PDF or Excel) we attempted to make predictions for the amount of carbon the Rhode Island could potential sequester in its urban forests over the next fifty years. (All models are linked to a document containing full sized versions of the graphs and the associated data, or they are linkied to a full sized version of the graph. All graphs may be found in the above mentioned document.)

This model produces an estimate of the total potential amount of carbon sequestration because it does not look at the amount of carbon that is released from dead trees and branches. This would require that all dead material be salvaged. (For the purposes of these models when we say salvaged we mean that the material is either turned into lumber of some kind or it is placed in a landfill. We can consider trees placed in a landfill to be carbon that is still sequestered, and if the landfill is equipped to capture methane, than any carbon that does escape may be used.) For the most part the dead material from urban trees is either turned into mulch or burned, and the carbon that it contained is no longer sequestered. The linked page shows examples of the potential for carbon sequestration under several different planting schemes.

In order to produce more realistic models of the potential sequestration for the state, we factored a decay rate of a release of 10% of the sequestered carbon in a given year each year for the next ten years into the DOE model. This simply means that if there is 20 tons of carbon sequestered in year A then 2 tons of carbon will be released into the atmosphere each year for the next ten years. Models were made including this decay factor for scenarios in which 0% of dead material is salvaged, 20% is salvaged, and 50% is salvaged.

The models are based around several potential planting programs for the state. John Campanini (Rhode Island Tree Council) and Jennifer Cole Steele (Providence Neighborhood Planting Program) along with others have suggested that the city of Providence should plant 40,000 trees over the next four years. They have named this plan "40 in 4". The purpose of this plan is to increase the total canopy cover of the city to 25%. This plan will be very helpful in dealing with urban heat islands, but it will not produce a great deal of carbon sequestration for the state. We felt that this plan could be scaled up to the entire state. If one looks at the number 40,000 not on a spatial scale, but instead as a fraction relative to the population of Providence one can then scale it up to the entire state. The new number is then 250,000 trees.

We crafted several potential planting programs around this number and increased planting to make up for mortality in the plantings. The replacement planting numbers were based upon survivorship factors set in the DOE sequestration models that were used. In the plans 50% of the planting done in an individual year will be replaced 11 years after the planting. Starting on the 21st year 10% of the original planting will be replaced very ten years. These replacement plantings would be done whether or not the mortality of the original plantings was actually that high. It is meant to not only replace dying trees, but also to allow the urban forests of the state to continue to grow. The models that were then finally created are based upon an initial plan of planting 250,000 trees over 20 years, over 10 years, and over 5 years. The five year plan uses an alternative replacement plan, which consists of stepping down the number of trees planted every 5 years by 50% until it eventually oscillates annually between 500 and 1,000 trees. The "40 in 4" plan was also modeled as was a very ambitious but intriguing plan to plant 1,000,000 trees in the state of Rhode Island over the next 20 years with replacement. All of the models were carried out to the year 2055.

Several things are clear from these models. One is that the faster you plant larger numbers of trees the more potential you have for sequestering trees. Another is that even with a 0% salvage rate the amount of carbon that can be sequestered in these programs is significant to Rhode Island. This also suggests that the state should consider creating a market for building products from urban trees. The state might also wish to consider placing dead trees in landfills.