Georgofili World

Newsletter of the Georgofili Academy

Urban trees and carbon dioxide sequestration

It is now well known that trees sequester and store CO2 by fixing carbon in permanent forms of biomass. The amount of gas exchange between a tree and the atmosphere changes depending on the age and health status of the tree itself, but the overall net balance of a patch of vegetation in equilibrium with the surrounding environment can be considered stable in time. This balance, however, is altered by man through some factors such as the increase of fossil fuel emissions and the relationship between the crop and the utilization of biomass. In this regard, the peri-urban forests, city parks and gardens, serving as CO2 sinks, play a vital role in combating the rising levels of atmospheric carbon dioxide.
For these reasons, the managers of urban green areas are considering whether projects of planting trees in urban areas can be financed through the carbon market (Carbon Trading is a market based mechanism for helping mitigate the increase of CO2 in the atmosphere.  Carbon trading markets are developed that bring buyers and sellers of carbon credits together with standardized rules of trade), especially since it is now a market internationally accredited and somewhat preferred by the buyers (Poudyal et al. 2011). The biggest concern about the projects on planting trees in urban areas and the question we should answer is whether these projects are cost-efficient investments.
A better understanding of how the variables predominantly affect the efficiency of these programs could help in understanding if we can intervene with the management decisions to improve the project or if uncontrollable variables such as climate, play a major role in determining the potential for integration of such projects in the carbon credits markets.
For urban green managers it is also important to know how to create potentially new and more efficient projects in terms of cost; even if the projects will not reach the market, these studies are of great interest to government agencies that voluntarily seek to minimize emissions of the entire community by also making a budget of carbon allowances produced and emitted.
From the “biological” point of view the quantity sequestered and then stored depends, as said, by the growth rate and mortality of trees, which in turn depend on the species, age, structure and the degree of plants health.
Young trees sequester and store CO2 rapidly for several decades, before the annual increase of CO2 decreases, while, for example, the so-called "old growth forests", i.e. the forests of "old" growth or virgin forests can release a quantity of CO2 resulting from the decomposition of dead biomass, equal to the quantity fixed with the new growth; also plants subjected to various stresses such as arid and dry seasons (or the most commonly urban stresses, such as soil compaction) may lose the normal ability to fix CO2 by closing the stomata to avoid dehydration.
The plantations in rural areas, due to their greater density, accumulate a quantity of CO2 per unit area which is approximately double (4.8 t/ha) compared to those in urban areas, but the individual tree growth is greater in urban areas as each plant has, usually, a larger surface and volume for growth (the data present in literature indicate that CO2 sequestration in healthy urban trees can be 4-5 times higher than their counterparts in the forest).
The accumulation can vary from 4 to 16 kg/year for small trees (8-15 cm), slow growing, up to about 360 kg/year for larger, fast growing  trees and is linked to their maximum rate of increase. However, although fast-growing trees initially accumulate more CO2 than others, this advantage may be lost if death occurs at a younger age. 
The mortality rate for street trees and those in residential areas is unfortunately very high, ranging between 10-30% for the first five years and then from 0.5 to 3% for each following year. A possible solution to minimize losses is to select suitable species to the plant site; species not suited for a specific site, can be easily stressed, showing slower growth (if not dieback) and therefore be inefficient for the purpose of CO2 sequestration.
For a typical tree in the forest the amount of CO2 sequestered is, on average, stored for 51% in the trunk, branches 30%, and 3% in leaves. The thick roots (diameter> 2mm) accumulate about 15-20% of total carbon, while in the fine roots there is a quantity of carbon that is comparable to that of the leaves.
The amount of CO2 stored in the urban forest depends on several variables such as the density of coverage that already exists, the pattern and the density of planting. For example in the city of Sacramento (California) with a high planting density, the CO2 stored has been calculated around 172 t/ha, while the amount in the less densely planted Oakland (California) drops down to 40 t/ha
The release of CO2 determined by the life processes and by tree maintenance is compensated by the quantity sequestered in the woody biomass and the amount of emissions avoided through the presence of trees that affect, for example the heating and cooling of buildings for the reduction of the Urban Heat Island effect (UHI), and positively influence the stormwater management. Then, in the evaluation of a program, the net reduction in CO2 is simply the difference between the reductions in CO2 emissions and the same, in metric tons (t):

Net balance = CO2 (CO2 Sequestred CO2 emissions avoided) - CO2 released (eq.1)

Prof. Francesco Ferrini –