Drivers and their influences on variation of aboveground carbon removals in miombo woodlands of mainland Tanzania


 Background

Removals caused by both natural and anthropogenic drivers such as logging and fire causes substantial carbon emissions. Better insights into drivers and their variations of aboveground carbon removals is therefore needed. We assessed the drivers of aboveground carbon (AGC) removals and quantified the dynamics of removals-induced carbon emissions due to drivers using the National Forest Resources Assessment and Monitoring (NAFORMA) data sets in R software. Miombo woodlands which is the largest forest formations covering about 93% of forest land in mainland Tanzania was the case study.
Results

Drivers of AGC removals in miombo woodlands of mainland Tanzania in order of importance were; timber, fire, shifting cultivation, charcoal, natural death, firewood collection, poles, grazing by wildlife animals, carvings, grazing by domestic animals, and mining. The average AGC removals by drivers range from 0.0–1.273tCha− 1year− 1.
Conclusions

Increased mitigation efforts in addressing removals by timber, fires, shifting cultivation, charcoal and natural death would be effective in addressing forest degradation in the REDD + process in Tanzania. Since NAFORMA provides national picture on drivers and their variation on AGC removals, site-specific studies need to be conducted to bring information that would be used for local forest management. This kind of study need to be conducted in other vegetation types like Montane and Mangrove forest in Tanzania.


Abstract Background
Removals caused by both natural and anthropogenic drivers such as logging and fire causes substantial carbon emissions. Better insights into drivers and their variations of aboveground carbon removals is therefore needed. We assessed the drivers of aboveground carbon (AGC) removals and quantified the dynamics of removals-induced carbon emissions due to drivers using the National Forest Resources Assessment and Monitoring (NAFORMA) data sets in R software. Miombo woodlands which is the largest forest formations covering about 93% of forest land in mainland Tanzania was the case study.

Results
Drivers of AGC removals in miombo woodlands of mainland Tanzania in order of importance were; timber, fire, shifting cultivation, charcoal, natural death, firewood collection, poles, grazing by wildlife animals, carvings, grazing by domestic animals, and mining. The average AGC removals by drivers range from 0.0-1.273tCha − 1 year − 1 .

Conclusions
Increased mitigation efforts in addressing removals by timber, fires, shifting cultivation, charcoal and natural death would be effective in addressing forest degradation in the REDD + process in Tanzania.
Since NAFORMA provides national picture on drivers and their variation on AGC removals, site-specific studies need to be conducted to bring information that would be used for local forest management.
This kind of study need to be conducted in other vegetation types like Montane and Mangrove forest in Tanzania.

Background
Managing the carbon stocks of the land use sector is currently a key focus for climate change mitigation in developing countries [1,2,3]. In terrestrial ecosystems, forests and woodlands play a major role for the mitigation and adaptation to climate change via carbon storage [4,2]. After oceans, forests are the world's largest storehouses of carbon and they provide ecosystem services that are important to human wellbeing [5]. Tropical forests alone store a quarter of a trillion tons of carbon in above and below ground biomass [6]. Notwithstanding their contribution to the climate change mitigation, Tanzania's forests face enormous challenges including deforestation and forest degradation [7].
Deforestation and forest degradation are amongst the major anthropogenic sources of greenhouse gas emissions (GHG), contributing about 17 per cent globally [8]. Of the total emissions, degradation is responsible for at least one-fifth in the Brazilian Amazon [9], two-thirds in Indonesian forests [10], and almost half in African tropical forests [11]. Forest degradation also leads to forest fragmentation and can contribute to deforestation [12]. While deforestation refers to a permanent or long-term conversion of forest to non-forest land [13,14], forest degradation is the changes within the forest that negatively affect the structure or function of the stand and/or site, and thereby lower the capacity to supply products and/or services [15,16].
The changes within the forests involves removals of trees and hence contributing to carbon emissions. The drivers of carbon removals are multifaceted and cannot be reduced to a few variables; rather they operate at different levels and scales in the human-environment linkage [17]. These drivers are divided into two broad categories: proximate and underlying causes. Proximate causes are typically human activities operating at the local level. They include shifting cultivation and cattle ranching, wood extraction through logging or charcoal production, and infrastructural development such as transportation, markets and settlements. On the other hand, underlying causes do not directly cause removals but influence the proximate causes. This category includes a complexity of economic issues, policies and institutions, technological factors, socio-cultural, and demographic factors [11,17,18].
Aboveground biomass (AGB) is not static, but rather spatially and temporally highly variable, particularly in the tropics with the same factor likely having different results [19,20,21]. This makes its quantification challenging. It is generally assumed that about half of AGB consists of carbon in different vegetation types including miombo woodlands in Tanzania. Miombo woodlands are the largest vegetation types in Tanzania covering about 93% of the forest area of 48.1 million ha [22]. As in other tropical forest landscape, complex matrices of low to high AGC removal densities can be expected in entire miombo woodlands in Tanzania and its management categories due to varying drivers. Additionally, which drivers contribute more to the variations of AGC removals in the entire miombo woodlands and its management categories is to a large extent unknown. This has been due to lack of appropriate assessment mechanism. Nevertheless, the Mainland Tanzania National Forest Inventory (NFI) data source which is commonly referred as NAFORMA, have recently become available based on country REDD + readiness activities that allow assessment of AGC removals and their amount of AGC emissions in miombo woodlands [3]. The objective of the present study was to identify the drivers of AGC removals and assess which of the identified drivers contribute more to the variation of AGC in miombo woodlands of Tanzania mainland. Specifically the study sought to: (1) identify drivers of AGC removals (2) quantify the amount of AGC removals by each driver and, (3) Ranking the identified drivers in order of their contribution to the variations of AGC.
Understanding the drivers of AGC removals and their amount of AGC removed is fundamental for better design of REDD + strategy. In some cases, REDD + incentives would be channeled directly to affect drivers. Moreover, a better understanding of drivers of AGC removals are required as part of developing mitigation interventions at sub-national levels to ensure improved land-use change. This kind of understanding is also crucial for subsequent development of management plans in order to tackle each driver in response to the amount of AGC emissions caused.

Drivers and their corresponding number of stems and aboveground carbon removals
We identified eleven drivers for tree cutting and these were, forest fires, firewood collection, grazing by both wildlife, domesticated animals, carving, poles, shifting cultivation, timber, and mining activities (Table 1). Table 1 also shows the contribution of the drivers in terms of the number of stems and AGC removals per hectare per year for miombo woodlands in Tanzania. Higher number of stems/ha/year were removed by shifting cultivation, followed by charcoal, natural death, firewood collection and poles. In terms of biomass however, we observed higher AGC removals by timber followed by fire, shifting cultivation, charcoal and natural death ( Table 1).

Drivers and their variations on the number of stems and AGC removals
The contribution of the drivers with regards to number of stems and carbon removals were further expressed on the basis of different management categories and subcategories of miombo woodlands.
Considering Tanzania Forest Services Agency (TFS) administrative zones, large number of stems were removed by charcoal followed by firewood collection and shifting cultivation whereas grazing was the least in the central zones (Table 2). Onn the other hand, charcoal removed more AGC followed by firewood collection, natural death and shifting cultivation while grazing had the least removals. In the other zones, the drivers seem to change leading positions between charcoal production, timber and fire ( Table 2).
Considering vegetation types, natural death, timber production and shifting cultivation appear to be leading causes of removals interchangeably in the closed woodlands, open woodlands and Woodlands with scattered cropland for both number of stems and AGB (Table 3). Grazing, mining and carvings are among the least contributors to removals in the three vegetation types. Regarding ownership types, higher number of stem removals were observed due to natural death followed by fire, poles and timber in the central government land (Table 4). While the least number of stems removals per hectare per year was observed due to carving followed by grazing domestic and charcoal. The highest AGC was removed as timber followed by natural death, fire and charcoal. Carvings, grazing domestic and shifting cultivation accounted for the least AGC removals in this ownership types ( Table 4). The contribution of the drivers of removal in terms of number of stems and AGC removals appear to be changing leading positions in other ownership types i.e. general land, local government land, private land and village land (Table 4). Table 5 indicates drivers and the variations of number of stem and carbon removals in the different land use types. Regarding protection forestland, the highest number of stems removed were due to natural death followed by poles, firewood collection and timber (Table 5). In terms of AGC, the highest AGC were removed as timber followed by charcoal, natural death, poles and fire. Grazing by domestic animals, carvings and grazing by wild animals accounted for the least AGC removals in protection forest. In other land use types such as production forest, grazing land, shifting cultivation, water bodies or swamps and wildlife reserves, drivers of removals appear to be changing leading positions in terms of the number of stems and AGC.

Drivers of aboveground carbon and number of stems removals
Several drivers contributed number of stems and AGC removals in mainland Tanzania. These drivers included charcoal, wildfire, firewood collection, grazing by both wildlife and domesticated animals, carving, poles, shifting cultivation, timber, and mining activities. Since drivers of AGC removals are similar to drivers for forest degradation in the woodlands meant for both protective and production purposes, comparison across studies were based on studies conducted to determine forest degradation drivers. The result found in the present study is comparable to results found in miombo woodland in Masito forest in western Tanzania and Liwale district southern Tanzania [23,24,25].
These studies documented only six drivers responsible for forest degradation. Sites specific and the methodologies applied on these studies explains fewer documentation of drivers. On the other hand, [26] documented ten drivers for forest degradation in Philippines that agrees with results from the present study. The methodology employed, particularly on the sampling procedures could explain the similarity.
In terms of the contribution of drivers on the number of stems and AGC removals nationally, removals by shifting cultivation, natural death, poles and charcoal production account for the highest number of stem removals. The reason could be attributed by high demand of charcoal in the country for cooking energy in which small diameter trees are involved. Tanzania's annual consumption of charcoal is 1,658,000 tons [27]. About 85% of the total urban population depends on charcoal for household cooking and energy for small and medium enterprises [28]. Additionally, more than 40% of the tree removals can be attributed to charcoal use alone in Tanzania [29]. Higher removals by shifting cultivation is probably due to intensification of shifting cultivation in Tanzania. Shifting cultivation in Tanzania occupies 7.6% of the total country land area and 33% of area classified as woodlands in Tanzania [22]. Other scholars [30,31] asserted that shifting cultivation contribute more to forest degradation due rising demand for agricultural products, dietary changes, agricultural trade and adjustment. Firewood collection and poles on the other hand, rank third and fourth in taking large amount of stems in the woodlands. This is probably because; firewood is the main source of energy rural areas [32]. The same author noted that, lack of alternative and affordable sources of energy dependence of communities on forests. Construction purposes both in the rural and urban areas probably account for higher removals of trees as poles. Furthermore, climate change impacts like diseases eruptions and severe drought naturally kills trees. These effects have recently increased tremendously. Mining and grazing by domesticated animals appeared as the least drivers responsible for stems removals. This is because of the smallest area subjected into mining and carvings activities.
In terms of AGC removals, timber and fire accounts for the highest AGC removals. This may be explained by the large trees removals that comprises of the largest biomass. According to [33], large trees tend to account for a large proportion of the AGB in mature forests; often between 30 and 40% of the AGB can be found in trees with diameters greater than 70 cm. Elsewhere in miombo [34], found that most miombo had been heavily disturbed because of local benefits attached to them like dryseason fodder for large livestock populations, fuelwood for domestic use and rural industry and construction materials for farm structures and homes for millions. Higher AGC removals in miombo woodlands due to fire is because of its roles as the management tools. When fire is frequently and uncontrolled, it could kill trees and eventually cause carbon emissions.
Considering administrative zones, charcoal and timber account for higher AGC removals in the Eastern zone. Conversely, charcoal and firewood collection account for higher number of stem removals in this zone. This is due to the highest charcoal and timber consumption that may be linked to the closeness to Dar es Salaam city. Dar es Salaam, Tanzania's largest city, accounts for more than 50% of all charcoal consumed in the country [35]. Moreover, higher timber consumption in this zone could be attributed to high demand of timber for furniture and infrastructure development particularly houses in the Dar es Salaam city. Dar es Salaam is the primary destination of timber and timber products (including all round and sawn timber) and accounting for 87% of timber felled in southeast Tanzania [36]. Other important domestic markets of timber and wood products from the zone are Zanzibar, Mafia and Arusha [25]. Shifting cultivation and charcoal account for the largest number of stems and AGC removals in the northern zone probably due to intensification of shifting cultivation. In the lake zone, fire, firewood collection and pole account for the large stems and AGC removals probably due to heavily dependence trees for cooking energy and constructions purposes.
Furthermore, presence of dry litter that foster fire occurrence explains the removals due to fire in this zone. The regular fires in the miombo region can, if too frequent or intense, cause mortality of large and small trees and prevent regeneration [37]. Likewise, long-term plot-scale experiments had shown that under annual burning miombo woodland is converted to grassland [38,37], and that in the absence of fire, miombo starts to form closed canopy forest [37].
Regarding vegetation types, shifting cultivation, charcoal, timber poles, and firewood collection accounted for the highest AGC and number of stems removals in the woodland with scattered woodland. Shifting cultivation type of farming in the country is practiced by more than 70% of the population. Other scholar [25] found that shifting type of Agriculture is common and practiced for all annual crops grown in Tanzania

Data Collection Sampling design
The data used for the assessment of drivers and their influence on variation of AGC removals presented in this paper were collected by NAFORMA [22]. Systematic double sampling for stratification with optimal allocation of individual plots in cluster was sampling design of the NAFORMA (Fig. 2).The design was chosen after sampling simulations to reduce uncertainty of estimates under given budget constraints. The detail of the planning of this design and other uncertainties are given in [42,22,2,3] Data acquisition All stumps with diameter ≥ 5 cm within the circular plot radius of 15 m were measured for diameter and height using calliper or measuring tape. In addition, age, name and end uses to which the removed trees were put into were identified. The details on how age of the stumps and end uses of the removed trees were decided are given in [43,3]. For the purpose of the present study, all plots that were surveyed for stumps measurement were extracted from NAFORMA database. A total 7 323 stumps from 16 803 plots were extracted.

Data analysis Analysis of drivers of aboveground carbon removals
To obtain the drivers of AGC removals, the identified trees with their corresponding drivers for their removals were listed. The drivers were sorted alphabetically in order to identify total number of drivers responsible for removals in miombo woodlands. Those drivers that were similar like removals due to firewood collection for domestic and industrial use were regarded as firewood collection.

Drivers and their influence on aboveground carbon removals
We included multiple drivers identified (11 drivers) in the analysis, so that the interrelationships between the drivers and AGB removed could be accounted. To define the influence of each driver on AGC removals, AGB removed per tree was estimated using allometric equation that estimates tree biomass from the remaining stump [7]. The estimated individual tree AGB removal in its corresponding driver was divided by age of the stump to get the rate of AGB removals per year. AGB removals per year per tree was summed up and expressed on per plot basis. Since each stratum had unique sampling intensity, it was necessary to calculate expansion factors (EF) for each respective stratum since simple mean of AGB would ignore the nature of the sampling design upon which the data were collected. The EF describes the area in which a sample plot represents in each stratum. The details on how the EF factor was calculated are shown in [2,3]

Ethics approval and consent to participate
Not applicable

Consent for publication
Not applicable

Funding
No financial support was provided regarding the preparation of this manuscript

Availability of data and materials
All authors declare that the datasets used in this study are available upon request from the Tanzania Forest Service, Ministry of Natural Resources and Tourism Tanzania.

Authors' contributions
BJM has been involved in designing the study, drafting the manuscript, data analysis and write up:  Cluster design (source: [22])