Fig5. Annual oxygen production (Kg/m2) based on the GWR model in
Isfahan city
Estimating the total amount of carbon sequestration,
CO2 absorption, and Oxygen production
The results obtained from the spatial distribution map showed that the
biomass of all trees within the city sequestrated about 7704.22 tons of
carbon. This amount of carbon absorbs a total of 28274.502 tons of
carbon dioxide. Accordingly, 20570.16 tons of oxygen is produced by all
trees across the city per year.
Discussion
Regarding the calculation of above-ground and below-ground biomass,
previous studies used allometric equations using biophysical parameters
like DBH, height, and wood density (Aboal et al., 2005, Basuki et al.,
2009, Bond-Lamberty et al., 2002, Cai et al., 2013, Djomo et al., 2010,
Henry et al., 2010, Joosten et al., 2004, Segura and Kanninen, 2005).
Similarly, in this study, to calculate the biomass in an individual
unit, we used the allometric equations that were developed by
Ponce-Hernandez and colleagues in 2004.
In terms of calculating carbon sequestration, in this study, we used a
photosynthesis equation to estimate carbon storage in the biomass. While
previous studies used a constant to convert the biomass into carbon
storage. In different species, it varies between 44.4 to 55.7 percent.
Generally, in most of the studies, an average of 50 percent of the
weight of the dried biomass is considered as a constant to convert
biomass into carbon storage (Elias and Potvin, 2003, Singh et al., 2011,
Zhang et al., 2009, Zhu et al., 2010).
Applying different processes to extract the trees’ canopy, we used
different spectral variables, included band analysis, vegetation index,
and texture analysis. The vegetation indices included the Excess Green
Plant Index (ExG), the Excess Red Plant Index (ExR), and the difference
between these indices (ΔExGR) (Meyer and Neto, 2008). Amongst all the
variables the ΔExGR index showed a significant relationship with carbon
sequestration.
Regarding carbon sequestration and CO2 absorption by
green areas in the urban ecosystem, the results of this study are in
line with previous studies that emphasized the role of the green area to
absorb CO2 in cities (Dwivedi et al., 2009, Groffman et
al., 2006, Nowak et al., 2013, Qing-Biao et al., 2009, Raciti et al.,
2014, Tor-ngern and Leksungnoen, 2020, Townsend‐Small and Czimczik,
2010, Velasco et al., 2016, Zirkle et al., 2012; Schlesinger and
Lichter, 2001).
The green infrastructure of Isfahan city with a high diversity of tree
species can provide climate regulation services. Addressing the monetary
valuation can highlight the importance of the carbon sequestration
service. Simply, previous studies have shown that the cost of separating
carbon dioxide (CO2) from major point sources such as
fossil fuel power plants and transporting to a storage site, and
ultimately storing in an underground natural reservoir cost about 100 to
$ 300 per ton of carbon (Bui et al., 2018, EASAC, 2019, Rubin and De
Coninck, 2005). The results showed that the trees in Isfahan store
28274.502 tons of carbon in their biomass per year. If the average cost
of carbon sequestration is assumed $ 200 per ton, then the annual value
of carbon sequestration by trees will be $ 5654,900.
In addition, the results of the study confirm that the GWR method
contributes to high accuracy in modeling a spatially heterogeneous
pattern (i.e., carbon sequestration distribution pattern) within the
city (in this research R2 was 0.915). Because the GWR
method provides a separate regression equation for each observation
rather than calibrating only a single regression equation for the whole
statement (Fotheringham et al., 2001) . The result of this study
is congruent with findings from other studies, arguing that the GWR
method possesses a better potential to address the spatial distribution
of parameters like primary production, land surface temperature, and
fire density (Li et al., 2017, Oliveira et al., 2014, Wong and Lee,
2005).
Moreover, the results of this study indicated that determining the drip
line radius (approximate radius of the canopy) plays an important role
in matching the ground data of each tree and the spectral data of the
satellite image. Because the surface of tree canopies in an urban area
usually is not homogeneous and uniform in comparison with the canopies
in a forest. Then, using plots in sampling to measure the variables is
not recommended. Also, the drip line (approximate radius of the canopy)
is calculated based on the trunk diameter for each single tree. So, the
drip line can be more appropriate than the plot in establishing a
regression relationship between the ground-collected information of
every sample tree and image data related to the canopy of each sample
tree.
Acknowledging the limitations in this study, in order to calculate the
annual carbon sequestration, we used the annual diameter growth rate and
the height growth rate which was obtained by previous research (i.e.,
these parameters are not the same in all trees, so it leads to error in
carbon sequestration calculation). In addition, parameters like wood
density can be different not only among species but also among trees of
the same species (Domec and Gartner, 2002). In this study, we divided
the trees into two categories: coniferous and broad leave. Therefore,
this generalisation with using average densities brings about errors in
allometric formulas. Weighing the biomass in the field may solve these
kinds of issues and subsequently may contribute to high accuracy. But
field measurement is considered a costly method and is just applicable
in small areas. Besides, in this study, due to the high cost of
worldview image, we just access red, green, and blue bands. We did not
have a near-infrared band which may assess the green area better than
other bands.
The results of this research can be implemented by urban land-use
planners and decision -makers, because there is a growing need to
integrate urban ecosystem service concept (i.e., carbon sequestration)
into impact assessment, urban planning processes towards sustainability,
livability, and resilience (Cortinovis and Geneletti, 2018,
Gómez-Baggethun et al., 2013, Haase et al., 2014). In this way, for
instance, the distribution map of CO2 absorption can
help the planners to better understand which neighborhood needs to be
planted to mitigate CO2 concentration.
Conclusion
Increasing concern with the climate change has led to the research which
focusing on the green areas impacts in mitigating CO2concentration. However, the potential effect of urban forests on air
quality and climate change mitigation remains an object of debate,
mainly due to a lack of reliable data. In this research we proposed a
modeling to contribute to estimating carbon sequestration and its
spatial distribution within Isfahan city. Therefore, we developed a GWR
model in which calculated carbon sequestration was the dependent
variable, while the vegetation index of ΔExGR was regarded as the
independent variable.
In general, it can be concluded that integrating high-resolution data
with allometric calculations can make a contribution to analyze carbon
sequestration and its spatial distribution efficiently and economically.
As a recommendation for future research, this research can be coupled
with a study that analyze carbon emission and addresses its spatial
variation within the city. The combination of these two approaches can
give an insight, for instance, to recognize which sites need more
planting strategy or whether there is a balance between
CO2 emission and carbon sequestration in different
places of the city.