**3.2 CO2 fluxes**

Monthly averages of NEE and GPP are presented in **Figure 9**, where seasonal changes in NEE are shown to be more intense in the Caatinga and Pantanal sites than in the Amazon and Cerrado sites. The maximum monthly mean GPP values exceed

**Figure 9.** *Monthly variation of NEE (left) and GPP (right) at the analyzed sites.*

**Figure 10.** *Violin plot of air temperature at the analyzed sites.*

14.0 gC m<sup>2</sup> d<sup>1</sup> in the Cerrado and Amazon sites, while the lowest GPP values reach about 1.0 gC m<sup>2</sup> d<sup>1</sup> in the Caatinga site. The seasonality and magnitude of the CO2 exchange values found in the Amazon and Cerrado resemble each other, while the Caatinga and Pantanal patterns show very close values and variability. The maximum assimilation of the sites occurs in the month of February, when Pantanal and Cerrado show assimilations close to 4.0 gC m<sup>2</sup> d<sup>1</sup> , and the minimum is very close to 0.5 gC m<sup>2</sup> d<sup>1</sup> in the month of August at the Pantanal site, after the coldest and least rainy month of the site. Sites susceptible to seasonal flooding [21, 30] in the early months of the year (Cerrado and Pantanal) show increases in assimilation (more negative NEE) between January and February, while Amazon and Caatinga sites have marked increases in assimilation in the warmer months of the year. The density of NEE data (**Figure 10**) shows that most sites act as CO2 sinks, with higher variability at the Pantanal site and lower variability at the Caatinga site. The medians of GPP (**Figure 11**) are higher in the Amazon, Cerrado and Pantanal sites compared to Caatinga. However, the same does not occur for NEE, where the medians of the Caatinga site have higher values compared to the other sites. Despite this, the large density of data below 0 gC m<sup>2</sup> d<sup>1</sup> indicates that the Caatinga biome is much more

*The Relevance of Maintaining Standing Forests for Global Climate Balance: A Case Study… DOI: http://dx.doi.org/10.5772/intechopen.110533*

**Figure 11.** *Violin plot of air temperature at the analyzed sites.*

firmly established as a CO2 sink, despite its smaller vegetative size, than the Amazon site, for example, which have a bimodal NEE data density distribution pattern, showing a secondary maximum above 0 gC m<sup>2</sup> d<sup>1</sup> , indicating that the biome can act as a source in some seasons, probably controlled by CO2 emissions higher than assimilation due to the intensification of wildfires in the region, included within the arc of deforestation of the Amazon.

### **4. Discussion**

The results show that most of the sites act as CO2 sink, corroborating works in the literature [12, 23, 31, 32]. There are data of higher CO2 emissions (positive NEE) mainly at the Amazon site, where other work has shown the possibility of the environment acting as a moderate source [26, 33]. The overall magnitude of the carbon source/sink is generally highly sensitive to the choice of filter u∗ (to measure turbulence intensity), and even with all the corrections recommended by the literature, the pattern was reinforced by biometric measurements [16], which makes us look for alternative explanations for this unusual pattern, since the vast majority of forest sites in different areas of the same biome, point to a CO2 sink. Heyek et al. [33] suggested that continuous integrated responses to changes in meteorology, with increased humidity and decreased sunlight, rather than a temporary disturbance, were responsible for the high carbon source at the site. The authors also suggested that reduced photosynthesis, rather than increased respiration, contributed to the high NEE source in specific years at the site. This suggests that partial drought-induced damage to stillliving trees can adversely affect ecosystem-wide photosynthesis for several years, which is consistent with results from forest biometric studies at regional and global scales [34, 35]. Tian [36], analyzing series from 1980 to 1994, notes that the carbon balance of the Amazon forest can have great variability, sometimes positive and sometimes negative, depending on variables such as sunlight incidence, CO2 concentration in the atmosphere and rainfall volume. Gatti et al. [37] point out that regions of the Amazon forest, such as the site region of this study, are affected by environmental degradation and are leading the Amazon as a whole to emit more carbon than it can absorb. The authors point out that a secondary effect of deforestation has been

created: the indirect carbon emission caused by the impact of reduced rainfall on photosynthesis, corroborating what was pointed out [33]. Indirect emission happens because deforested regions have a greater loss of rainfall, especially in the dry season (August to October). With the drop in rainfall volume, the temperature rose 2°C in the northeast of the forest and 2.5°C in the southeast, and this "stress" affected photosynthesis, causing the trees to emit more CO2 than in normal situations to compensate for the imbalance, showing the importance of maintaining the forest, including for the maintenance of rainfall important for agriculture and cattle ranching in the region.

### **5. Conclusions**

The study showed an important meteorological control on the carbon cycle in the biomes studied, which infers that changes in surface cover will directly affect these variables and, consequently, the local carbon balance. Despite acting mostly as a CO2 sink, some environments already show worrying source data in certain periods, pointed out as a direct effect of the reduction of photosynthesis caused by land use changes. The preserved forest plays an important role in maintaining rainfall at a regional and global level, and its maintenance makes it possible, by the way, an important tool in combating global warming via carbon sequestration by trees, which requires commitment and public policies of environmental preservation and recovery of degraded areas.

### **Acknowledgements**

The authors are also thankful to the Coordination for the Improvement of Higher Education Personnel (CAPES) for the postdoctoral funding granted to KRM and to the National Council for Scientific and Technological Development (CNPq) for the research productivity grant of C.M.S.S (Process n° 303802/2017-0), the financial support of CNPq, through undergraduate research project (PIBIC-UFOPA for L. B. V and G. V. A) and the project NOWCDCB: National Observatory of Water and Carbon Dynamics in the Caatinga Biome (INCT-MCTI/CNPq/CAPES/FAPs 16/2014, grant: 465764/2014-2) and (MCTI/CNPq N° 28/2018, grant 420854/2018-5).
