A Brief Summation of the Discussion regarding Urban Carbon Sequestration

On this preceding Thursday we discussed the problem of carbon sequestration by urban forests.  The general consensus seemed to be that while this is in theory a valuable service that trees can provide in an urban environment, there are simply not enough urban trees to really make much of a dent in global CO2 levels.  Beyond that, it's likely that the contribution that urban trees make towards sequestering carbon are mitigated in part by the effort put into growing, transporting, planting, maintaining and removing them.  Data on this is as yet relatively thin.  

Further, some work has found that the allometric equations used to calculate the biomass (and hence stored carbon) of urban trees tend towards being a poor fit , though this effect reduced as the number of trees included in the calculation increased.

Often the methods used to quantify the urban forest are strongly associated with a traditional forestry approach: random plots are selected within a patch of forested land, these are surveyed, and these results are then extrapolated to the forest as a whole.  This typifies the approach used by Nowak in the paper we discussed. Unfortunately, this method is perhaps not well suited for use in urban environments, where the individual parcel is probably the best unit of analysis.  Parcels owners may produce wildly variable forest structure through differences in values and what they consider to be desirable outcomes.  Large scale remote sensing methods are likely too coarse to capture this variability, and it strikes us as unwise to take data generated from remnant, park, or regrowth forests in urban areas and apply the results to single family residential properties, even if the degree of canopy cover appears to be similar.

We understand the appeal of attempting to import a forestry approach to the urban environment.    It is much more costly in time and resources to negotiate for access to land that is privately owned, and in doing so one may have to wrestle with heterogeneity of urban ecosystems much more than with other methods.  However, until this more fine grained analysis is performed (and hopefully combined with attempts to understand what motivates and characterizes the land management practices that shape the urban environment), much of this work will remain of questionable validity.

Beyond this, we feel that there is often a somewhat myopic focus on carbon sequestration by urban trees.  Urban trees are not an unmitigated good.  They can contribute to photochemical smog through the release of biogenic hydrocarbons.  They can pose a direct threat to life and property if not well managed in storm prone areas, and can produce a significant cost on cities when struck by disease en masse (as evidence first by Dutch Elm disease, and now the Emerald Ash Borer), and can require quite large volumes of water to maintain.

This is not to say that urban trees do not produce a raft of benefits that are indeed very valuable, in directly maintaining the integrity of urban ecosystems, providing economic benefits to cities through aesthetic value, water management, and urban heat island abatement, and potentially acting as a locus around which communities can organize and build social capital.  Far from it.  We simply feel that the conversation that surrounds urban trees is often unrealistic and overly focused on carbon storage and sequestration, and does not truly contribute much to the discussions that city planners, citizens, and scientists need to be having about the urban forest.


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