**3.2 Carbon fertilization and tropical forestry NPP**

Net Primary Productivity (NPP) refers to the balance between carbon gain through photosynthesis (gross primary productivity, GPP) and losses through autotrophic respiration (Ra).14 Practically, it is not possible to precisely measure forest NPP in terms of this difference. At the ecosystem scale, NPP is measured over a long period such as a year. As per Clark et al. [18], NPP comprises new biomass produced by plants, soluble organic compounds that diffuse or are secreted into the environment such as root or phytoplankton exudation15, carbon transfers to microbes through symbiotic association with roots as found in mycorrhizae and nitrogen-fixing bacteria, and the volatile emissions that are lost from leaves to the atmosphere. However, most field measurements of NPP consider only 'new plant biomass produced' and, therefore, probably underestimate the true NPP by at least 30%. For our practical understanding, we can say 'NPP is the net carbon gain by plants'. NPP is an important

<sup>13</sup> Information on forests biomass and GHG emission for project-based and national assessments still rely on making extrapolations from scant field inventories through either allometric models or application of remote sensing data at spatial scales that mask variability across geographies and local details—the scale of most anthropogenic activities. In tropical forest landscapes, the majority of anthropogenic actions and land use changes occur at a smallholder scales range in spatial extent between 0 and 2 hectares, this is without undermining the increasing tendencies of large-scale plantation establishment above the aforementioned range. <sup>14</sup> Practically, it is not possible to measure precise forest NPP in terms of this difference. At the ecosystem

scale, NPP is measured over a long time-interval such as a year.

<sup>15</sup> Carbon transfers to microbes through symbiotic association with roots as found in mycorrhizae and nitrogen-fixing bacteria, and the volatile emissions that are lost from leaves to the atmosphere. However, most of the field measurements of NPP considers only 'new plant biomass produced' and therefore probably underestimate the true NPP by at least 30%.

#### *CDR and Tropical Forestry: Fighting Climate Change One Cubic Meter a Time DOI: http://dx.doi.org/10.5772/intechopen.109670*

parameter in many forestry models that are used to assess the future mitigation potential of the sector. A forest management project can exploit NPP for carbon sequestration in forests and biomass production for climate change mitigation [19].

The increasing in human-induced CO2 emissions indirectly implies that forest worldwide will grow faster and reduce the amount of atmospheric CO2 which stays airborne—an effect known carbon fertilization, which is high in the tropics. There has been increasing carbon sink on land since the 1980s; living woody plants were responsible for more than 80% of the sources and sinks on land16,17 [20]. Globally, vegetation is locking away more carbon as atmospheric CO2 levels rise. Plants are growing faster, fueled by a more CO2-fertile atmosphere. Carbon stocks (i.e., standing plant production) are related to, but do not refer to the same thing as, NPP (i.e., rate of growth of plant production). Increased CO2 concentrations reduce photorespiration, which translates into greater plant productivity—NPP.18 Giving plants more CO2 increased net primary productivity by 24% on average.19 Factorial simulations with multiple global ecosystem models suggest that CO2 fertilization effects explain 70% of the observed greening trend.20 CO2 fertilization effects explain most of the greening trends in the tropics. Results show a considerable increase in net primary production (NPP) over the last century, mainly due to the CO2 fertilization effect.

Pastures uptake 5–50 tCO2-eq/ha/year of atmospheric CO2 and also hold Nitrogen, which is turned by each animal into something like 0,5 tCO2-eq/year of carbon-based products—protein.21 The associated methane emissions is a result of the balance between the atmospheric CO2 removed by pastures and what gets process through animals' digestive system. The more animals are grazing, the more atmospheric CO2 is turned into protein and other products—including fertilizers [21], resulting on a process of removing the gas and returning into Society as useful goods.

In identifying potential plant species with higher NPP capabilities for carbon sequestration, Vithal and Nadagoudar [19] found that Bamboo has the highest NPP(17.523).22 Secondary vegetation like Lianas, palms, bamboo and other non-tree life forms have been omitted from a number of Amazonian biomass studies, which often fail to report what biomass components are included23 [13]. For Brazil as a whole, an average aboveground carbon stock of 120 tCO2e/ha in savanna woodlands

<sup>16</sup> with soil, leaf litter, and decaying organic matter making up the rest. But they also saw that vegetation retained a far smaller fraction of the carbon than the scientists originally thought.

<sup>17</sup> Globally, vegetation is locking away more carbon as atmospheric CO2 levels rise. Plants are growing faster, fueled by a more CO2-fertile atmosphere. Carbon *stocks* (i.e., standing plant production) are not the same thing as NPP (i.e., rate of growth of plant production).

<sup>18</sup> although warmer temperatures counteract this effect by increasing photorespiration somewhat

<sup>19</sup> Terrestrial Ecosystem Model (TEM) predicts that doubled CO2 will increase 16.3% of the global NPP. Under real conditions on the large scale where water and nutrient availability are also important factors influencing plant growth, experiments show increases under unstressed conditions.

<sup>20</sup> followed by nitrogen deposition (9%), climate change (8%) and land cover change (LCC) (4%).

<sup>21</sup> while returning fertilizers and gases to the environment.

<sup>22</sup> followed by rubber (15.970), oil-palm (14.500), *Samanea* and *Erythrina* (13.350), coconut (12.150), *cassia* (10.350), *eucalyptus* (10.009), alnus (10.000), *sesbania* (9.433), *prunus* (9.000), *leucaena* (8.739), *acacia* (9.000) and *casuarinas* (7.550).

<sup>23</sup> Standardization for non-tree components, together with trees <10 cm DBH, removes almost all of the difference between aboveground live biomass.

classified as "forestland"24, and 45 tCO2-eq/ha in those classified as "shrublands" (65.6% of the area), giving a weighted average of 75 tCO2-eq/ha25 [13].

Net primary productivity (NPP) of a closed-canopy26 forest stand was assessed for three years in a free-air CO2-enrichment (FACE) experiment. NPP increased 21% in stands exposed to elevated CO2, and there was no loss of response over time. Wood increment rate cumulated significantly during the first year of exposure, but subsequently return to its initial value, reducing the potential of the forest stand to sequester additional C in response to atmospheric CO2 enrichment27 [22]. Currently, there is limited pool of knowledge regarding the long-term impacts of CO2 enrichment in tropical rainforests [22]. Young trees and other small plants responded well to higher CO2, but it remains undetermined how more mature trees would react. Brazilian Amazonian trees are dying faster than they are growing. On land, reports suggest a decline in the tropical forest CO2 sink, increased plant mortality and decreased plant productivity. Under low Nitrogen conditions28, plants will have difficulties to transform elevated CO2 into production29 [5].

Standing undisturbed tropical forest sites over the last 50 years lost total volume of trees to secondary invasive vegetation, making them naturally net emitters of CO2. As regards above-ground live biomass and carbon flux, the world's remaining intact tropical forests have been reported to be largely out-ofequilibrium [23, 24]. Following the current increasing trend of forest degradation over deforestation [25, 26] and dwindling resilience to changing climates and rainfall patterns [27, 28], it is anticipated in the next 50 years that tropical forest sites are to yet another part of its volume stocks to the increasing competition from secondary vegetation. With this trend, large protected areas at isolated areas in the Amazon region that are retained in unmanaged conditions should hold less biomass volume yearly than their managed and plantation counterparts. The CO2 fertilization up-take is much faster by secondary vegetation, and old forests trees are losing their competitiveness every year, without harvesting and silvicultural treatments.

Today30, spatial biomass analyses31 show major differences between all of the resulting biomass maps, including those with largely overlapping ground-based

<sup>24</sup> 34.4% of the total savanna woodland area.

<sup>25</sup> Conversion from the original text in Mgha-1 using 1:1 ratio for m, and 3,67 factors for C-CO2.

<sup>26</sup> Liquidambar styraciflua (sweetgum).

<sup>27</sup> Most of the extra C was allocated to production of leaves and fine roots. These pools turn over more rapidly than wood.

<sup>28</sup> CO2 may not much affect plant productivity because of lack of Nitrogen in the soil. Plant acclimatization and water availability.

<sup>29</sup> Moreover, in the long term, elevated CO2 condition may cause the accumulation of carbohydrates in the plant tissues which may reduce the photosynthetic rates or decrease photosynthetic response to elevated CO2.

<sup>30</sup> Usually, continuous Forest Inventory data, with a proportion of 0.1% (for the effective area) of sampling, is used to determined standing stocks volumes from which biomass estimates are made. A number of fixed size plots of 10 meters wide by 250 meters long, used for monitoring tree increment and mortality. <sup>31</sup> Using space-borne LiDAR (Light Detection and Ranging) from the US National Aeronautics and Space Agency (NASA) Geoscience Laser Altimeter System (GLAS) on the Cloud and Land Elevation Satellite (ICESat), together with optical data from MODIS imagery and radar data from the Global Quick Scatter meter (OSCAT).

*CDR and Tropical Forestry: Fighting Climate Change One Cubic Meter a Time DOI: http://dx.doi.org/10.5772/intechopen.109670*

#### **Figure 1.**

*Tropical forests standing stocks BAU scenario over 400 years (Illustrative proxy by authors).*

datasets.32 The way forward will require using remote sensing data together with ground-based measurements, with progress needed in both areas [13]. From RADAM Brasil studies, the highest dense tropical forest at the Amazon region used to hold average between 420 and 480 m3 /ha, toping 520–580 m3 /ha of tree biomass [29]. Nowadays, average biomass of standing stocks range from 248.92 ± 61.78 t/ha, passing by 293.19 ± 27.74 t/ha, and reaching up to 356 ± 47 t/ha [30, 31], based on measurements for trees ≥10 cm DBH (diameter at breast height: diameter at 1.3 m above the ground or above any buttresses) with a 12% correction for small trees [13]; roughly, making it 270 to 320 m3 /ha and topping 310 to 400 m3 /ha33, circa of 25 to 35% less volume than 50 years earlier, as stated at RADAM Brasil early studies.

As illustrated in **Figure 1**, following the current BAU (Business As Usual) scenario and considering a 400 years' time frame, tropical forests are going to become less and less tree covered as the atmospheric CO2 levels rises and if no management interventions are implemented (the data is illustrative, a proxy from the previous findings and this section highlighting increasing secondary vegetation in natural unmanaged tropical forests).

The graph in **Figure 1** illustrates that secondary vegetation gains competitiveness over trees as the atmospheric CO2 becomes more and more available, reducing the overall carbon stock of forest stands. The process is ongoing and tends to speed up with the increase of CO2 and reduction of tree cover, which favors even more secondary vegetation growth. The associated gains in productivity of secondary vegetation can be can be compared to those from Croplands. The Brazilian agricultural sector, for instance, has portrayed continuous productivity increase over the last 30–40 years [18], showcasing the positive effect of atmospheric CO2 enrichment on plant NPP. Meanwhile, the ability of tropical forests trees to absorb massive amounts of carbon has waned [32].

SFM + HWP = CDR.

Degradation is translated as: "change between forest classes (i.e. from "close" to "open") which negatively affects the site and, in particular, reduces its productivity

<sup>32</sup> Expanding the network of ground-based inventories is essential.

<sup>33</sup> Assuming1:1 ratio from biomass to m,

capacity.34 The Intergovernmental Panel on Climate Change—IPCC2006 guidelines for GHG inventories from different sectors includes accounting procedures for Dead Wood—DW and Harvested Wood Products—HWP.35 Thus, wood used for project activities such as fencing of boundaries, furniture, construction, energy and others must be accounted as DW when determining the carbon sequestration and storage in forest areas, including from those without a formal Sustainable Forest Management Plan—SFMP. For areas holding SFMP, the rule is the same regarding DW, and besides this logs, timber, firewood and others imports and exports are also to be accounted for as HWP for the balance of forest carbon areas, carbon sequestration and carbon storage [33]. At harvesting, a large portion of aerial biomass carbon is transferred to HWP (Harvested Wood Products) and will be available at one of the forest product categories. Forest areas biomass volume is used as starting point for HWP carbon estimates, applying specific conversion factors for each log destination. Estimates related to wood products baseline are available under the format of volumes delivered to industrial plants or in terms of their outputs, comprising industrial logs or primary HWP (boards, planks, panels or paper). Carbon availability at those HWP over the years is then estimate allocating other parameters which indicate carbon amount 'in use' and destined to landfills. Thus, HWP Carbon estimates, including recycling, rely largely on data availability.36

### **3.3 Improved forest management in the tropics**

Tropical forests are accountable for about 35% of global net primary productivity (NPP).37 The CO2 fertilization effect that increases CO2 concentrations in leaves enhances plants' capacity in fixing carbon through photosynthesis has been considered as a primary mechanism that maintains and enhances tropical forest productivity [34].

The human appropriation of net primary production (HANPP) provides a useful measure of human intervention into the biosphere. The productive capacity of land is appropriated by harvesting or burning biomass and by converting natural ecosystems to managed lands. HANPP has still risen from 6.9 Gt of carbon per y in 1910 to 14.8 GtC/y in 2005, i.e., from 13 to 25% of the net primary production of

<sup>37</sup> And store about 72% of global forest biomass carbon (C).

<sup>34</sup> Deforestation means: "changing on land use with reduction of tree crow cover below 10% by hectare" while resulting in land degradation afterwards according to IPCC.

<sup>35</sup> Within IPCC2006 Dead Wood (DW) is classified as all kinds of branches, leaves, roots, dead trees and other types of biomass not included as litter or soil. Harvested Wood Products (HWP) are all wood material leaving project activities boundaries—other materials remaining within boundaries are to be accounted as DW.

<sup>36</sup> Estimates of forest products contribution, in terms of carbon, use generic variables, including (i) domestic HWP and imports (tCO2-eq/year); (ii) annual variation of HWP produced domestically, including annual variations on exported HWP (tCO2e/year); (iii) annual imports of all kinds of wood and paper (tCO2e/year); (iv) annual exports of all kinds of wood and paper (tCO2e/year); and (v) annual HWP (tCO2e/year). The level of lost on solid products and paper, in a given year, are specified towards the use of a lost constant (k), which by convenience is expressed in terms of half-life in services, in years. Half-life in service describes the number of year necessary for half of the material to change environment, which can be, for example, from a home to landfill, within that sector where it remains stored. Solid wood and paper production, imports and exports are converted from m or tons into tCO2-eq. For annual estimates calculation the method uses yield data (Consumption = Domestic Production + Imports—Exports).

*CDR and Tropical Forestry: Fighting Climate Change One Cubic Meter a Time DOI: http://dx.doi.org/10.5772/intechopen.109670*

#### **Figure 2.**

*Tropical forests standing stocks IFM scenario over 400 years (Illustrative proxy by authors).*

potential vegetation. Biomass harvested per capita and year has slightly declined despite growth in consumption because of a higher conversion efficiency of primary biomass to products.38 The rise in efficiency is overwhelmingly due to increased crop yields. HANPP might only grow to 27–29% by 2050, but providing large amounts of bioenergy could increase global HANPP to 44%. This result calls for strategies that foster continuous and increasing land-use efficiency.

Harvesting and consumption of tropical timber products stimulates management opportunities of increased productivity, reverting the degradation process due to increase of secondary invasive species volume, and generate profits. With a profitable forestry activity in place, there is incentive to practice forestry and reduce conversion to other land uses. Therefore, tropical timber is a value added CDR that can reduce forest degradation and conversion of forests to other land uses, while increasing CO2 removals. Advanced silvicultural techniques can be applied to improve productivity [6], taking advantage of the CO2 fertilization. As illustrated in **Figure 2**, following Improved Forest Management (IFM) contemporary silviculture techniques, the scenario considering a 400 years' time frame shows tropical forests recovering tree volume against the competing secondary vegetation.

Silvicultural practices—planning; individuals' selection; seed collection, genetic improvement; seedling development; fertilization; maintenance; weed, insects and diseases control; harvesting—are applied to reduce the presence of secondary vegetation and introduce CO2 enriched environment with adapted trees` varieties. This will result in increasing yields and therefore reducing HANPP while supplying society with more industrial, energy wood and other Non-Timber Forest Products (NTFP). The positive effect of IFM techniques are widely known globally, and these have promoted the cultivation of native tree species all over the world [6].

Brazil holds the largest stock of hardwoods in the planet. Some of these tropical hardwoods have characteristics that make them therapeutic, comfortable, charming as well as immune to fungi and insect attacks. Brazilian tropical hardwoods, just as softwoods, possess a diversity of qualities that are hardly reachable by tree species in other parts of the world. These unique qualities are competitive advantages that can be used to enhance Amazon biodiversity cultivation and tropical timber consumption

<sup>38</sup> And decline in reliance on bioenergy.

role. With a growing and promoted timber consumption, rural landholders have markets available to justify necessary investments on Brazilian native tropical timber species cultivation, with the use of IFM39 [33].

#### **3.4 Quantifying anthropogenic contributions to GHG flux**

Following the UNFCCC global estimates of anthropogenic net land-use emission, there is a discrepancy of about 4 Gt CO2 per year between aggregated national GHG inventories and the global models in IPCC assessment reports. According to Grassi et al. [35] a great proportion of the discrepancy (3.2 Gt CO2 pr–1) is due to differences in concepts implemented in estimating anthropogenic forest sinks—a representation of environmental changes and managed areas. Such differences between inventories and models of GHG emissions and sinks need to be addressed [36] to enhance monitoring and achieve collective progress towards the goal of the Paris Agreement on global temperature.40 Following global estimate of GHG fluxes from forest, uncertainties in global gross removals and net flux are mostly attributable to extremely high uncertainty in applying the removal factors from the IPCC Guidelines to old secondary forests41,42 [10]. Global models and maps of GHG emissions and fluxes are based on spatially clustered inventories that are translated to make predictions across geographies and forest types [37]. Although they provide important insights on global trends, such models may support misleading applications such as using some default values in IPCC recommendations [38] management actions and policy as they do not capture variability and uncertainties across geographies and generalize assumptions of carbon flux to unknown local and regional spaces [12]. Globally, forests store approximately 8.4 billion tCO2-eq and are capable of retaining some further billions; meanwhile, about 4.2 to 20 billion tCO2-eq are estimated to be stored within HWP "in use".43 The 3.4 billion m3 of yearly global harvested wood is equivalent to just 20% of total yields (some 17 billion m3 /year) [33]. A lot from what is harvested is used for direct and inefficient burning as fuel wood. Increasing the sustainable removal of senescing biomass from forests and harvesting yields would have a profound positive effect to fight global warming. With the use of extra 2 billion m3 /year, industrial woods will be possible to reduce between 14 and 31% of all cement and steel GHG emissions and between 12 to 19% of all fossil fuel consumption by the use of residues from industrial wood production chains for clean energy appliances. With the intensification of sustainable forest management, more CO2 is sequestered and stored

<sup>39</sup> Biodiversity banking regional strategies implementation and the use of contemporary industries (MDF, HDF etc.) value aggregation will increase social inclusion chances and, by that, project activity sustainability over time.

<sup>40</sup> By and large, inventory data is scarce or absent for tropical forests, and there is large variability in the methodology for and quality of existing data.

<sup>41</sup> To make estimates at large scales, inventory (activity) data vital in making extrapolation from information and models based on spatially continuous data collected from airborne or satellite-based remote sensing procedures.

<sup>42</sup> The absence of 'activity data' constitutes a key impediment to and source of error (over- or underestimation) in estimating GHG emissions and CDR in tropical forests.

<sup>43</sup> World wood production includes more than 1.5 billion m/year of industrial logs, accounting for something like 1.1 billion tCO2-eq/year, with 420 million m of sawed lumber and 220 million m on plywood and panels—representing some 20% of total in long life-spam forest products, which sequester and store close to 200 million tCO2-eq each year.

#### *CDR and Tropical Forestry: Fighting Climate Change One Cubic Meter a Time DOI: http://dx.doi.org/10.5772/intechopen.109670*

avoiding emissions from alternative materials and still producing renewable energy from harvesting residues. Besides, harvested volumes are renewed. Brazil has by far the largest global stock and growth of "hardwoods", which have the longest life-span between tree species, making them relevant suppliers of HWP storing carbon for many years.

The International Wood Culture Society (IWCS) is a non-profit organization formed by wood enthusiasts, dedicated to research, education and promotion of wood culture. IWCS advocates for a harmonious living between people and nature, explores the value of wood use from a cultural perspective and supplies a platform for studying wood culture, encouraging its practice and promotion.44 IWCS established March, 21st as Wood World Day, a data to disseminate the value wood aggregates to daily life [33]. Tropical forestry must be accompanied by similar public and private efforts towards trade and use of tropical hardwoods, creating the synergies that might help in removing huge amounts of atmospheric CO2 and returning to society noble wood products. The current stocks of billions of m3 of dying mature trees, ready to be harvested on Brazilian Amazon region alone, have the capacity to remove billions tCO2-eq from the atmosphere, just by turning them into timber and having new trees planted. Thus, cutting down trees do not necessarily implicate on GHG emissions, and neither is the change of land use directly linked to atmospheric CO2 generation. The use of wood could also generate millions of jobs and trillions of dollars in revenue over the next decades. Tropical forests hold capacity to regenerate after harvesting, and the magnitude and benefits that this capacity would mean is directly related to silvicultural practices, which will impact global GHG balance positively with broad use of tropical HWP.
