Basic density and CO2 sequestration in seven species from the Commemorative Arboretum of 500 Years of Brazil in Alberto Löfgren State Park, São Paulo

Luís Alberto Bucci ______________________________________________________________________________________ Abstract: Carbon sequestration involves the removal of CO2 from the atmosphere, aiming to reduce the greenhouse effect. Wood basic density is a direct part of this process. We selected five trees in each of the following species to determine wood basic density and quantify carbon, both sequestered and fixed, for 10 years: Alchornea sidifolia, Ceiba speciosa, Gallesia integrifolia, Guazuma ulmifolia, Inga marginata, Maclura tinctoria, and Prunus cerasoides. The study was conducted in the Commemorative Arboretum of 500 Years of Brazil in Alberto Löfgren State Park, São Paulo. Using maximum moisture content capture of CO, we studied variations of wood density and CO2, both fixed and sequestered, by an individual tree in the same species and between species. Values of fixed and sequestered CO2 showed variations among species with a high dependence on wood density such that trees with higher amounts of CO2, both fixed and abducted, were also trees that showed the highest increment both in height and diameter (DBH). Based on these metrics, G. ulmifolia, I. marginata, M. tinctoria, and P. cerasoides showed the most potential to sequester carbon. Our calculations showed that planting these four species would result in the sequestration of around 30 tons of carbon per hectare.


Introduction
The incidence of pollutants emitted into the air has risen steadily in the last few decades, and all countries are contributors. Carbon dioxide (CO 2 ) gas, one of the compounds released into the atmosphere by man, is produced in all parts of the planet, mainly by burning petroleum-derived fuels and by producing cement (75% of total emissions). However, land use processes, especially deforestation and burning, are responsible for a large part of the remaining 25% ( C&T Brasil, 2006).
CO 2 is one of the greenhouse gases by the absorption of thermal infrared light. Carbon becomes available to living beings through plants by photosynthesis, and because carbon is stored, it is often called fixed carbon. Biological decomposition is one of the ways in which this process is reversed, releasing CO 2 into the atmosphere. Anthropic activities have generated a considerable increase in CO 2 concentration in the air. In view of the increase in CO 2 emissions, which have intensified the greenhouse effect, likely accounting for a significant increase in temperature on the planet, governments have debated ways to minimize these emissions without harming economic growth. Some initiatives are based on reducing emissions, acquiring carbon credits, implementing projects based on clean technologies in other countries, and planting more trees in order to absorb CO 2 emissions. The Clean Development Mechanism (CDM) was created at the Kyoto Conference as an instrument by which developed countries could invest in projects to promote carbon sequestration and thus reduce CO 2 emissions (Barreto et al., 2009).
The concept of carbon sequestration was enshrined in the 1997 Kyoto Conference with the purpose of containing and reversing CO 2 accumulation in the atmosphere and, hence, reduce the greenhouse effect. The conservation of carbon stocks in soils, forests and other vegetation types; preservation of native forests; implantation of forests and agroforestry systems and recovery of degraded areas are some methods that can contribute to the reduction of CO 2 concentration in the atmosphere (Environment Brazil, 2010).
The mass of carbon contained in trees is governed by the volume and density of the wood (Phillips et al., 2019). This means that wood density can be used to quantify the amount of carbon in trees. In addition, results of carbon sequestration can be quantified by estimating plant biomass above and below ground, calculating the carbon stored in wood products and the amount of CO 2 absorbed in the photosynthesis process. The study of energy balance and carbon cycle in the atmosphere depends on the quantification of plant biomass in each of the following components of vegetation: aerial part, roots, and layers decomposed on the soil (Ambiente Brasil, 2010).
The sequestration of forest carbon is a viable alternative to mitigate the rise in global temperature. Vegetables, using their photosynthetic capacity, fix atmospheric CO 2 , biosynthesizing in the form of carbohydrates and, finally, depositing on the cell wall (Renner, 2004).
According to Baird (2002), CO 2 can be removed from the atmosphere through the selection of plants grown especially for this purpose. Fast growth correlates with faster CO 2 absorption as a result of vigorous tree growth in the tropics. Indeed, one hectare of this type of forest can sequester much more carbon than one hectare of temperate forest (Renner, 2004).
Referring to the Climate Convention, Yu (2004) stated that carbon sequestration is a palliative and not a permanent measure, although it can bring secondary ecological benefits. Variation in the cost of CO 2 absorption is associated with several factors, such as regional differences in climate, existing variations in soil quality, differences in management, time for cutting and technologies used.
Brazil has the largest plant biodiversity on the planet with more than 55 thousand species of superior plants and about 10 thousand species of bryophytes, fungi and algae, a total equivalent to almost 25% of all existing species of plants (Brazil RU, 2010). However, of all these species, it is not known which species best contribute to CO 2 sequestration.
Considering the importance of studies on CO 2 sequestration of tree species, we selected seven 10-year-old species, six native and one exotic, for our study. Our goal was to determine the effect of basic density on the amount of carbon sequestered these trees planted in an arboretum.

Sampling
Alberto Löfgren State Park (PEAL, Portuguese) is located in the northern area of São Paulo City, São Paulo State, Brazil (23°47′S, 46°38′W, elevation 814 m). Climate is Cwa in the Köppen classification, mesothermal and humid with rainy summers and dry winters (Rossi et al., 2009). Ombrophilous Dense Forest is the main vegetation formation of PEAL; anthropic areas are also found with arborets of one or more species (Arzolla et al., 2009). The trees investigated in this study are located in one of these arborets, denominated Commemorative Arboretum of 500 Years of Brazil. In 2000, 24 native species and other exotic species were planted at the site with 4 x 5 m spacing, occupying an area of one hectare (Bonucci et al., 2007).

CO 2 sequestration
After collecting the samples, the following procedures were performed by tree and species, according to Bucci et al. (2010): Calculation of trunk volume or bole volume as where = trunk volume (in m 3 ), DBH = diameter at breast height, and H = tree height saturated volume.
Calculation of branch and root volume depends on such factors as species, age, and location. In this case, we estimated 25% (1.25) over the trunk volume and calculated the total volume as We calculated weight (W, in kg) as Basic density is the relationship between absolutely dry mass and saturated volume of wood, as described in the previous item. Then, to calculate fixed carbon, we applied a factor of 0.5 to tree weight, considering that 50% of the wood consists of carbon with the remainder constituted mainly by water and nutrients. We calculated absorbed CO 2 by multiplying the fixed carbon content by 3.67, as obtained from CO 2 / C, or a ratio of 44/12.

Data analyses
We undertook a parametric analysis of variance (one-way analysis of variance-ANOVA). When a significant difference was o served, Tukey's test was used to identify pairs of significantly different means between species.

Results and discussion
We observed variations in basic density relative to both fixed and sequestered CO 2 among seven studied species. Basic density was higher in G. ulmifolia, I. marginata, M. tinctoria, and P. cerasoides (Figure 1a). Fixed and sequestered CO 2 values oscillated, but we observed higher values in G. ulmifolia and I. marginata, but lower values in C. speciosa (Figure 1b). Table 2 shows total values of fixed and sequestered CO 2 per hectare (ha) of each species according to tree number. Guazuma ulmifolia and I. marginata showed higher carbon sequestration capacity (Table 2 and Figure 1b), followed by M. tinctoria and P. cerasoides. Ceiba speciosa was the species with the lowest potential. According to the results in an area of one hectare with these species and number of trees of each species, it is possible to obtain about 101 kg of fixed CO 2 and 2606 kg of sequestered CO 2 .
Wood basic density expresses the amount of woody mass that constitutes a given volume and is, therefore, an important parameter in wood quality evaluation for several uses. In general, results in the present study are consistent with those in the literature, showing that 10-year-old trees, such as those in our study, have lower density than older trees, as expected. , adult trees in natural environment, age unknown). Trees normally show a standard behavior from base-top direction of tree with higher densities at the base, reduced in DBH (diameter at breast height 1.30m), and increased or reduced until the canopy (Boschetti et. al., 2020). In many studies to determine basic density, samples were taken at DBH level. This type of sampling is normally undertaken by the ease and convenience; however, according to the literature, a decrease in DBH occurs for some species, and this can skew the results of the average value of this property for the entire tree (Boschetti et al., 2020). No consensus has been reached on the best sampling positions to estimate the average density of trees, using either destructive or non-destructive techniques (Couto et al., 2012). The trees that showed highest increment values in both height and diameter also showed lower densities, confirming the belief of many authors that the greater the growth in diameter, the lower the basic density. However, for Eucalyptus species, no direct relationship has been found between growth in diameter and basic density (Souza et. al., 1986). Wood basic density can be altered according to species characteristics, as well as external influences, such as environmental variations and silvicultural interventions. The causes of changes in wood density may also be indirect, as through the modification of other characteristics of wood, to the detriment of species characteristics, or the influence of the environment in which trees develop (Latorraca, 2000). For example, when comparing wood density of 10-year-old G. integrifolia in the present study with 25-year-old trees cited by Andrade el al. (2009), we found a slightly lower density in our investigation, confirming that basic density is influenced by planting age. Moreover, both fixed and sequestered CO 2 values were found to be highly dependent on wood basic density, i.e., the higher the density, the higher the fixed and sequestered CO 2 values will be. This is an indication that basic wood density must also be considered when selecting species for reforestation when the aim is fixing and sequestering carbon. Normally, only growth is taken into account in these types of studies. Bellote et al. (2006) collaborated to evaluate the polynomial equations obtained for annual increase in carbon and mathematical models developed to estimate carbon accumulation as a function of DBH of Pinus taeda trees. They found and easily proved that the amount of carbon accumulated for trees with the same diameter is different when different locations are compared. Similarly, for average annual increase in tree height and diameter, it was observed that the trees of the same age, but in different sites, grow to a different diameter, resulting in greater increases in volume, biomass and, consequently, carbon accumulation.
When an area of one hectare is planted with trees of the same species, total carbon sequestered is estimated to be around 24 tons per hectare. Thus, 12-year-old Hevea sp. sequesters, on average, 62.10 tons of carbon per hectare, or 30%, in its living biomass (Fernandes et al., 2007).
The management of wooded areas in urban environments tends to decrease the levels of CO 2 in the atmosphere, helping to improve health and minimize global warming. However, few studies in the literature have reported on this topic in relation to urban green areas in Brazil (Rocha et al., 2017). Thus, although the data from our study is derived from trees in an arboretum, we have helped to fill this gap as Alberto Löfgren State Park is an urban park, and our data summarize carbon sequestration in this urban environment.
In the present report, we look at the increase of CO 2 levels combined with the accumulation of carbon in the tissues of seven tree species growing in an urban setting. This is in line with an increased demand for studies reporting on the dynamics of natural forests that also aim to quantify the carbon stored during succession. Therefore, studies, such as ours, serve as a basis for the management and conservation of these native forest remnants. This goal is intensified by the need to assess the potential to sequester and accumulate carbon of each species individually in relation to the greenhouse effect (Figueiredo et al., 2015). Torres et. al. (2017) report that native species that most contributed to carbon storage in forest fragments should be used in forest restoration programs that focus on mitigating climate change. Trees with DBH ranging between 5-25 cm already contribute to carbon storage, demonstrating their importance for forest fragments. Additionally, selection of native Brazilian species, to the detriment of exotic trees, is essential to avoid altering the environment for the other components of flora and fauna.

Conclusion
The species studied showed variation both within and between species. In some trees, the increment values in both height and diameter showed lower values of basic density. The fixed and sequestered CO 2 values are highly dependent on basic density. Guazuma ulmifolia, Inga marginata, Maclura tinctoria, and Prunus cerasoides had the best potential to sequester carbon. We confirmed reports in the literature that basic density is influenced by planting age. In this study, we also confirmed the value of understanding the potential of native species to sequester CO 2 as a key factor in urban afforestation plantations in cities.

Species
Nº of trees