Urban context and neighbouring lands: how reforestation could have a role in the implementation of Sustainable Energy and Climate Action Plans

Reforestation is the appropriate natural tool to mitigate the climate change. The authors illustrate how to manage cuts and plantations of trees making profit from unused lands, to reach different carbon capture goals. Unused lands, moreover, are frequently in the neighbouring of Municipalities which often are politically responsible of these territories. Their destination as carbon sink inside the implementation of the Sustainable Energy and Climate Action Plans (SECAPs) is particularly suitable, participating in a synergic way to reduce the CO2 municipal emissions by 55 % in 2030 with respect to the 1990 datum. In the framework of a SECAP, this level is mandatory and participates to the common huge effort to decarbonize energy needs. After having modelled some intrinsic aspects related to the dynamics of the carbon capture due to the growth of trees, the research demonstrated how, with a proper management of cuts and plantations, a new concept of mutualism between city and territory can be designed, recognizing the crucial role of neighbouring unused lands alongside those direct actions usually implemented to reduce the carbon intensity of a city. Introduction Urban areas are responsible for almost 80% of total GHG [1], so recognising the central role of Municipalities, the importance of the sustainable energy action plans (SEAPs) and subsequent sustainable energy and climate action plans (SECAPs) [2]. To reduce the majority of GHG emissions, interventions should be used in urban context and the most important one appears to be the reforestation [3],[4]. According to the Paris Agreement about the safeguard limit of 450 ppm of CO2 in the atmosphere, forests are the green lung of the world and play an important role in climate mitigation limiting the rise of mean temperature below 2°C [5]. A decisive help could be given by the sequestration of the CO2 from the atmosphere; the least expensive way is to store it inside the wood of the forest, according to a suitable management of them. This storage will offer more time to develop new low carbon technologies, renewable energy generation and also awareness and lifestyles based on a more responsible behaviour [6]. It is important to remark that this carbon storage will be only temporary because all the carbon absorbed by the atmosphere during growing  Corresponding author: simona.abbate@graduate.univaq.it © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). E3S Web of Conferences 312, 10002 (2021) https://doi.org/10.1051/e3sconf/202131210002 76° Italian National Congress ATI

Introduction Urban areas are responsible for almost 80% of total GHG [1], so recognising the central role of Municipalities, the importance of the sustainable energy action plans (SEAPs) and subsequent sustainable energy and climate action plans (SECAPs) [2]. To reduce the majority of GHG emissions, interventions should be used in urban context and the most important one appears to be the reforestation [3], [4]. According to the Paris Agreement about the safeguard limit of 450 ppm of CO2 in the atmosphere, forests are the green lung of the world and play an important role in climate mitigation limiting the rise of mean temperature below 2°C [5]. A decisive help could be given by the sequestration of the CO2 from the atmosphere; the least expensive way is to store it inside the wood of the forest, according to a suitable management of them. This storage will offer more time to develop new low carbon technologies, renewable energy generation and also awareness and lifestyles based on a more responsible behaviour [6]. It is important to remark that this carbon storage will be only temporary because all the carbon absorbed by the atmosphere during growing surface of land, in terms of plantings, cuts and spatial partitions, using a genetic algorithm. There are two different targets: insure the maximum carbon uptake in the minor time (emergency scenario) and guarantees a fairly constant rate of carbon uptake (constant-rate scenario).

Assumptions
To evaluate the capacity of a forest to uptake carbon and reduce the carbon emissions of the area a model has been developed to describe the performance over time of the forest. To calculate the carbon uptake the IPCC approach [16] has been adopted, evaluating the overall biomass growth and using the specific carbon fraction coefficient. Firstly, the Verhulst approach has been used to represent trees growth [17], which are consequently modelled by a logistic curve [18], [19]. Furthermore, forest management in terms of rotation crops has been modelled following two different goals: maximize carbon uptake and obtain a constant uptake rate. Knowing the national and international scenarios and goals, it has been possible evaluate the relevance of reforestation.

Tree growth model description
Data about growth of trees have been taken from the National Forest Inventory and Forest Reservoirs (INFC) [20]. In this paper, data about Abruzzo Region have been considered, mediating the national value on the regional trend in case of missing regional data. The chosen database allows to achieve a highly detailed level, defined as Tier 3 by IPCC guideline. In this work the most common species have been chosen (Scots Pine, Oak and Beech), representing a starting point for future studies in which the best native species, respecting biodiversity, should be analysed. Table 1 shows the raw data provided by INFC database: both tree density and increment of merchantable volume have been successively used as input data for the growth model proposed. The eq. (1) represents the analytical expression of logistic growth, [4]: where AGB(t) is the tree biomass expressed in t/ha, whereas Winf, W0, and K are logistic parameters, specific for each species, that represent respectively the limit value of AGB(t), the biomass (in kg or kg/ha) at year zero, and the initial growth rate. Table 2 summarizes values of logistic parameters, estimated starting from real measured data and employing fitting methods.  Figure 1, referred to scots pine, shows differences between real and logistic growth curve in a range of 100 years. It is clear how the tree growth model matches closely the real growth rate, reaching a maximum difference of 30 % that is completely acceptable considering the complexity of the real AGB process. Subsequently, tree growth model has been completed calculating trend of growth rate during years. It is expressed in terms of CAI (Current Annual Increment) and MAI (Mean Annual Increment), both expressed in t/ (ha  year). The CAI is the differential form of the logistic curve, calculated through eq. (2), instead the MAI is the mean value of growth rate.
being W0, Winf and K the same parameters described for Eq. (1).  Finally, trees carbon uptake in absolute terms (tC/ha) and annual rate (tC/(ha  yr)) have been quantified following the IPCC approach [16], by multiplying respectively the values of ABG and CAI by the Carbon Fraction (CF) that represent the portion of carbon inside the biomass (   Table 3 also summarizes the results carried out by trees growth model in terms of carbon uptake during 100 years period. The values obtained are in line with those reported in literature: many authors [4], [22] suggest values for biomass in the range 0.5-2 tC/(ha  yr).

Case study description
The aim of this scientific research has been to identify a model of management of unused land in Avezzano's area to achieve two basic targets: maximum carbon uptake in a given period of time (emergency scenario) or constant uptake rate during time (constant-rate scenario). The optimization has been carried out through a genetic algorithm developed in an Excel-VBA environment, able to apply the stochastic methodology proposed by Holland [23], [24] and built following the mechanism of biological evolution [25]. The genetic algorithm allows to generate solutions optimizing the parameters by iterations and selection based on Darwin's theory of evolution [26]. Two scenarios have been investigated for the optimization: emergency and constant-rate scenarios. The first applies when the need is to maximize the uptake in a minor time, this solution is more important when the need for reduction of emissions is immediate for the health of the local population and to respect the international target. After the specified period, the trend in per capita emissions is expected to decline and the contribution of reforestation will be not necessary. The second one has been called constant-rate because it permits to always adsorb the same quantity of CO2 in a long-time interval, this solution is essential to absorb emissions in sectors that are difficult to decarbonise and that will continue to produce emissions for years to come.
The model and results could be used in an urban contest to reduce the emissions and permit to respect the internationals goals. The logic of this work has been based on considering not only the city, but the neighbouring lands too, a sort of mutualism between production area and its surrounding territories. The production and economic development have a close link with emissions [27], [28]; neighbouring lands benefit from the city's economic development and will be expected to make a significant contribution to the reduction of emissions.
The study has involved 9 municipalities in the Marsica's area, 1 production centre (Avezzano) and 8 neighbouring municipalities. The Marsica is an agricultural area, the cleared area of Fucino's Lake is all cultivated, but 14% of the agricultural area is unused (4917 ha). Hectares are abandoned, and in this area some trees could be replanted for the mitigation of the emissions. Application of emergency scenarios has been evaluated in 2 situations: Pniec and Europe. The constant-rate scenario has been applied to simulate the absorption of long-term emissions for sectors of difficult decarbonisation.
The Avezzano's trend of emissions, from 1990, increased until 2005 with a value of 195.61 ktCO2eq and gradually declined thereafter to 158.5 ktCO2eq [14]. This reduction (-19%) was possible thanks to the political decisions and to the energetic actions implemented in the framework of SEAPs [29], Figure 3. From these values it has been possible to derive the future trend, based on two regulatory requirements: Pniec and Europe. The first one transposes European legislation and produces some national strategies to respect the international goals. The Italian's Pniec is not updated to the last Green New Deal goals, but it is very significant because it formalizes some limits on emissions and gives some economic, politic, and technological strategies to decarbonise the economy and production system. In the case of Europe, all the ambitious environmental goals have been accepted, even if not formalized yet. In both situations the emergency scenario has been applied.

Two basic scenarios
The model shown in the previous sections has been used to obtain the characteristic trends in two basic scenarios considered, the emergency one and the constant-rate one.

Emergency scenario:
In this case all the trees must be planted in the same year. Figure 4 shows the situation in a time interval of 50 (figure a) and 75 years (figure b). The annual carbon uptake rate (red line) increases till to the maximum value around 40 years. In the case of 50 years the total carbon uptake (blue area) shows a linear trend of growth; when the period is 75 years, the total uptake rate following a logistic curve, after 40 years the annual carbon uptake starts to decrease (red line) but the integral remains growing and reach its maximum value (blue area). The maximum annual uptake rate is the same for both periods considered (Table 4) instead the value of total uptake grows up from 91 tC to 134.2 tC for a single hectare.
The model has permitted to evaluate the situation after 100 years, this interval of time involves a cut and a rotation of crops. This situation lies outside the scope of this study because the emergency scenario is an immediate response to a problem of emissions.

Constant-rate scenario:
This scenario supposed that the uptake trend has been constant over a long period of time, but for this reason the first 20 years should be excluded in the process of optimization because they represent the inertial trend of the initial plantation phase, so the objective of the optimization has been to minimize the variance of the curve after 20 years. For a period of 150 years the optimal management of the single hectare provides a division of the surface in 4 equal parts. All trees are planted in the first year but, after 28 years, ¼ of hectare is cut and replanted (rotation 1); after 5 years (33 years from the beginning) another fourth of hectare is cut and replanted (rotation 2). This continues after 25 years (58 years from the first year) (rotation 3) and again after 16 years (74 years from the beginning) (rotation 4) ( Figure 5). All the plantations are left growing till to the same residual carbon uptake rate is reached (0.21 t C/yr), at this value the rotation is cut and replanted. The temporal interval of rotation 1 and 2 is less than 5 years, so for a better and easier management of the land a simplification factor has been introduced. Hence rotations 1 and 2 have been considered in the same year, assuming the time of cut and planting in the middle of the two time-lags; the hectare provides a division of the whole area in 4 parts yet, but the first and second fractions are cut and replanted after 30 years together, Figure 5. The results of this solution have been reported in Table 5, after 150 years the total uptake is 250.4 tC (blue area Figure 6 b ) and the value of the medium uptake (blue line Figure 6 a) is 1.82 tC/yr for 1 hectare.   Figure 6 Constant scenario, trend is related to the individual hectare. a) annual uptake, the red line represents the annual carbon uptake; b) total carbon uptake.

Case study "PNIEC" case:
In this case the Italian's scenarios of emission reported in Integrated National Energy and Climate Plan (PNIEC) [15] Table 6 distinguishing contributions from energy and other sources. Table 6 Trend of emissions in Italy, PNIEC [15] Considering the Avezzano's area, emissions until 2018 represent the real trend, calculated in the SEAPs monitoring process and reported in Figure 3, while from 2020 to 2040 the trend has been derived applying the same percentage of reduction present in PNIEC. The total trend is represented in Figure 7, the maximum value of emissions is 196 ktCO2eq in 2005 and the minimum value is 124 ktCO2eq reached in 2040 with a percentage of reduction of 37% referred to 2005 (Table 7).  The PNIEC case is illustrated in Figure 7. In this situation an operation of reforestation could be executed with the emergency scenario. The total agricultural unused land in Avezzano is 4917 ha: considering a reforestation of pine, the reduction of emission ranges from a value of 3% in 2021 to a value of 17% in 2040. As shown in Figure 8, the amount of avoided CO2eq emissions is in the range 0-21 kt CO2/yr. The cumulative emissions without reforestation in 2040 are 2740 ktCO2eq, instead with reforestation the total emissions at the end of the period of observation are 2514 ktCO2eq., The quantity of total CO2eq not emitted in 20 years is 227 kt CO2 Table 8.   141  134  129  124  -1483  2740  -PNIEC with  reforestation  141  128  119  102  -1412  2514  -CO2 not  emitted  0  7  10  21  -71 227 -

"EUROPE" case:
Applying all the strictest limits discussed, some not yet approved, referred to the GHG emissions, it has been possible to reconstruct the emissions trend in Avezzano. Unlike the previous case, in which the PNIEC considers technological development and economical trend, in this case a picture of emissions has been made to respect the international intentions without a concrete analysis of the real possibility of implementation. The new proposal [7] foresees 2 goals in 2030 and another in 2050. In the first year a reduction of 55% compared with 1990 levels will be obtained, in the second the achievement of zero emissions will be reached. This value applied in Avezzano returns the value reported in Table 9. Considering the total unutilised surface of 4917 ha and an intervention of reforestation with the emergency scenario, the carbon neutrality is achieved in 2041-2042, about ten years before the international target ( Figure 9). From 2042 the function of the forest in the study area is done, this must not become a pretext to slow the energy transition, because the forest continues to adsorb the GHG from the atmosphere and permits to reduce the total global emissions.
The annual CO2eq emissions could be reduced in the range 0-39 kt CO2eq /yr and the total CO2eq emissions pass from 1634 ktCO2eq in 2020 to 1101 ktCO2eq in 2050, a 32 % reduction (Table 10).

Constant-rate case:
This case study allows to simulate a scenario in which the EU CO2 emission target by 2050 is not achieved, due to the existence of production processes in which technological development has not allowed total decarbonisation. In particular, the annual CO2 emission related to the above-mentioned production processes have been quantified as the 15% of the total emission in 1990, while the remainder 85% follows a reduction rate in line with the EU target (European Trend). To quantify these considerations for Avezzano city, Table 11 summarizes the emission data (kt CO2) and shows a residual value amount to 18.93 ktCO2 eq. The CO2 emission trend of Avezzano from 1990 to 2170 is illustrated in Figure 10. Table 11 Trend of annual CO2 emissions in Avezzano. Data 1990-2018 are real data from SECAPs and monitoring [24] [14], instead data 2020-2170 reflect a "EUROPE" trend which considers residual emission related to production processes hardly to be decarbonised.  Figure 10 Annual emission trend of CO2 from 1990 to 2170, considering residual emission related to production processes hardly to decarbonised.
From Figure 10, for mitigating CO2 emissions in the scenario described above a carbon sink that ensures a constant uptake trend over a long period of time is necessary. This may be achieved by choosing the rotations strategy described in previous paragraphs and called "constant-rate scenario". In particular, taking into consideration the numerical information shown in Figure 5 and Figure 6, the effect of reforestation with a "constant-rate scenario" rotations management has been simulated starting from 2018. Furthermore, to ensure the reaching of zero emission target in 2050 an area of 4072.11 ha is required. In particular Figure  11 and Figure 12 show the effect of reforestation in terms of annual and cumulative CO2 emission.  The total CO2eq net emissions pass from 2057 ktCO2eq in 2018 to 1597 ktCO2eq in 2050. This 22 % reduction in 2020 ( Figure 12 and Table 12) tends to decrease slightly during the following years.

Conclusion
The work assessed the potential offered by reforestation as a carbon sink to reach the EU CO2 emission target in the Municipal context. In particular the carbon uptake dynamics has been highlighted, exploring the key role of the management of cuts and reforestation of trees with reference to lands not interested to agricultural needs (unused land). The paper has been introduced a new approach, considering not only the city but the neighbouring lands too: in fact, neighbouring territories benefit from the city's economic development and will be expected to give a significant contribution to the reduction of emissions. Different situations have been analysed. The effect of reforestation in Pniec and Europe cases is noticeable: planting all the unused land in the Marsica region the reduction of emissions, with respect to current levels, will be above 15% in 2040 around 28% in 2050. The "constant-rate scenario" represents an important instrument for political and scientific perspectives because it allows to create a constant sink; policy makers could decide which sectors decarbonise later and scientists have more time to study innovative technique to decarbonize the society. If 15% of the Avezzano's emissions in 1990 cannot be removed, to achieve the carbon neutrality in 2050 it is necessary to plant about 4070 ha in 2020.
In urban context, the reference made to unused lands in surrounding of Cities allows to plan a more strategic reforestation project. In fact, from an integrated point of view reforestation acts not only as carbon sink, but also as protection tools against environmental issues that will be more enhanced by climate change: soil erosion, landslides, extreme rainfall, landscape restoration, mitigation of high temperatures, reconstruction of biodiversity.