Dry Miombo Woodland Research Articles

Carbon storage in a dry Miombo woodland area in Tanzania

A persistent need exists, both politically and academically, for knowledge on carbon storage and sequestration potentials of forest ecosystems and their contributions to the global carbon balance. This study assessed carbon stocks of a 6 065 ha dry Miombo woodland site in Iringa Rural district, Tanzania. The carbon stock for each of the six pools: trees, shrubs, deadwood, herbaceous vegetation (grass and herbs), surface litter and soil were estimated. Assuming a 50% carbon share of biomass, the results showed that the aboveground carbon stocks were: 24.71 Mg C ha−1 for trees; 10.88 Mg C ha−1 for shrubs; 0.54 Mg C ha−1 for deadwood; 0.58 Mg C ha−1 for herbaceous vegetation; and 2.47 Mg C ha−1 for surface litter. Soil carbon was estimated separately for two depth ranges: 0–15 cm and 15–30 cm below the surface. These soil strata contributed 21.24 Mg C ha−1 and 8.22 Mg C ha−1 respectively. The total carbon (C) stock of the six pools was 68.64 Mg C ha−1. Considering the vast area covered by dry Miombo woodlands in Tanzania and in 10 other countries in south-eastern Africa, the carbon stock of these ecosystems is clearly tremendous, underscoring the importance of conserving them.

A new Angolan species from the Triaspis hypericoides complex (Malpighiaceae) based on macromorphology and palynology

Triaspis hypericoides currently comprises three accepted subspecies endemic to woodland savannas of southern continental Africa. After an unusual population belonging to this complex was collected in southwestern Angola, we re‐evaluated the classification of this group. This study was based on the analysis of type specimens, protologues, field photographs and herbarium sheets of all subspecies within T. hypericoides. We reinstate T. canescens, T. hypericoides and T. nelsonii based on differences in leaf, flower and fruit traits. Additionally, the Angolan population is proposed here as a new species of Triapis endemic to the dry miombo woodlands of Bicuar National Park, Huíla Province, southwestern Angola. Triaspis bicuarensis sp. nov. is easily distinguished from T. nelsonii by its alternate (versus opposite) leaves, ovate to widely ovate (versus widely ovate to very widely ovate) leaf laminas, rounded (versus sagittate) leaf base, flat (versus undulate) margins, sparsely white‐tomentose (versus glabrous to glabrescent) abaxial leaf surface, petiole 4–8 mm long (versus 2–3 mm long), compound inflorescences in thyrses (versus corymbs) and bracteoles inserted below apex of peduncles (versus in the middle of peduncles). An identification key to all species belonging to this species complex is presented alongside a complete morphological description for the new species, photographic plates and comments on the distribution, ecology, palynology and taxonomy. These results underline the need for continued revisions of the taxonomy of the neglected African genera of Malpighiaceae.

Imprint of selective logging on seed regeneration of Brachystegia spiciformis Benth. in dry miombo woodland in Zimbabwe

Woodlands that have been affected by selective logging are ubiquitous in southern Africa but how this harvesting practice impacts seed regeneration of dominant tree species remains unclear. To address this deficiency, the study assessed the effects of different selective logging intensities on post-dispersal seed predation and seedling recruitment of Brachystegia spiciformis Benth. in Ruzawi Forest, Zimbabwe. Data were collected in blocks that were subjected to selective logging intensities of 0 % (00 L, unlogged), 5 % (05 L), 10 % (10 L), and 15 % (15 L). Ten years after logging, three transects (200×30 m) were laid in each block and five main plots (20×20 m) were laid in each transect, making a total of 60 plots. Low-intensity selective logging of 5 % did not cause any significant changes to seed predation levels, densities of true seedlings, seedling sprouts and root suckers, and soil nutrient concentrations. Seed predation was significantly greater in 10 L and 15 L sites than in unlogged and 05 L sites. As logging intensity increased, the relative importance of sexual reproduction decreased and that of vegetative reproduction increased. Canopy cover and seedling density decreased as logging intensity increased but a reverse trend was observed with grass biomass and root sucker density. Grass biomass was negatively correlated with the density of true seedlings and seedling sprouts. Soil organic C, N, P, and K were all significantly reduced in 10 L and 15 L sites than in unlogged and 5 L sites. Our results suggest that unless selective logging is practiced at very low harvest intensities, it can create opportunities for the growth of dense grass swards, potentially promoting rodent seed predation and limiting seedling recruitment. This study recommends effective use of low-impact selective logging practices as well the adoption of woodland management policies that prioritize sustainable utilization practices.

Trees in agricultural landscapes maintain soil organic carbon following miombo woodland conversion to shifting cultivation

The main driver of deforestation and degradation of miombo woodland is shifting cultivation (SC). SC is the main farm system and livelihood of the people in miombo woodlands. Assessing the current body of knowledge, the following apparent gaps were identified: (i) researches analysing the response of soil-C to woodland (miombo) conversion to SC are contradictory; (ii) studies reporting all the stages of agriculture-induced ecological succession (woodland, shifting cultivation, fallow vegetation, and reconversion to shifting cultivation or to woodland) are lacking. The response of soil-C to (1) dry miombo woodland (DM) conversion to SC, (2) SC conversion to fallow vegetation, (3) reconversion of fallow vegetation to SC, (4) the length of the cropping and fallowing periods, and (5) land-use intensification, was studied. The study was carried out in a landscape mosaic consisting of treeless SC, tree-based SC, grass fallow (treeless SC-fallow), and woody fallow (tree-based SC-fallow). DM conversion to treeless SC resulted in depleted soil-C by up to 72%; the depletion increased with the increasing length of the cultivation period. The fallow period (grass fallow) recovered part of the soil-C lost during the cultivation period: grass fallow lands were approximately threefold larger in soil-C stocks than treeless SC lands. Soil-C increased with increasing grass fallow age. No significant changes in soil-C were observed when DM was converted to tree-based SC, or when tree-based SC lands were left under fallow (woody fallow); consequently, soil-C did not vary with the increasing lengths of cultivation and fallow periods. Woody fallows were up to 45% higher in soil-C stocks than grass fallows. Reconversion of grass fallow to treeless SC resulted in soil-C loss; no changes were observed when woody fallows were reconverted to tree-based SC. Land-use intensification by increasing the frequency of SC cycles did not alter soil-C in either treeless or tree-based SC. It was concluded that, under the study area conditions, the response of soil-C to agriculture-induced land-use changes depends mainly on the presence or absence of the arboreal component. Maintaining or introducing the arboreal component is recommended to avoid or mitigate soil-C loss following DM conversion to SC.

Marula (Sclerocarya birrea subsp. caffra, Anacardiaceae) thrives under climate change in sub‐Saharan Africa

AbstractClimate change threatens the persistence of some plant species, especially those highly adapted to their microclimates. The aim of the study was to determine the current suitable habitat of marula (Sclerocarya birrea subsp. caffra) and to forecast the habitat under different climate change scenarios. Predictions were made in Maxent using 490 occurrence records, four representative concentration pathways (RCPs) and three general circulation models (GCMs). Variables used were altitude, soil characteristics, monthly precipitation, monthly minimum and maximum temperatures and 19 bioclimatic variables. The current suitable habitat of marula largely occurs in mopane and dry miombo woodlands of southern Africa, including Madagascar. Permutation importance test showed that precipitation in November (30.1%), temperature seasonality (22.1%) and mean temperature of wettest quarter (14.3%) contributed significantly to the model of the current suitable habitat. When forecasted, the suitable habitat expanded under all GCMs and RCPs. On average in 2050, the range expanded by 26.0% (RCP2.6), 33.8% (RCP4.5), 24.8% (RCP6.0) and 42.3% (RCP8.5), while in 2070, it expanded by 30.0% (RCP2.6), 39.9% (RCP4.5), 33.3% (RCP6.0) and 45.1% (RCP8.5). The subspecies is not directly threatened by climate change, but indirect threats remain uncertain.

Effects of conspecifics and rocks on seedling recruitment of Brachystegia spiciformis Benth.in a dry miombo woodland in southern Africa

Natural objects that buffer climate extremes and potentially function as regeneration niches are ubiquitous in dry miombo woodlands, yet few studies have explored their effects on early stages of plant recruitment. We used Brachystegia spiciformis to investigate the influence of conspecifics and rocks on early seedling recruitment in Cecil Kop Nature Reserve in Zimbabwe. B. spiciformis-dominated woodland was stratified into canopy and gap sites which were further divided into rock and rock-free microsites. In each microsite, we laid 3 blocks of 100 × 100 m and sow seeds of B. spiciformis in five 1 m2 plots close to rocks under canopies (CR), close to rocks in gaps (GR), rock-free in gaps (GF) and rock-free under canopies (CF), making a total of 60 plots. We laid additional 1 m2 plots to quantify the presence of naturally occurring seedlings at 1–12 m distances from boles of 15 trees in four cardinal directions. We found that photosynthetic photon flux density (PPFD), soil and air temperatures were significantly lower in CR, CF and GR than in GF while soil moisture displayed a reverse trend. Although seedling emergence was above 65% in all microsites, more than 60% of the seedlings died because of drought-related mortality which was increased in GF compared with other microsites. Survival probabilty of seedlings was highest in CF and CR, intermediate in GR and lowest in GF. Seedling biomass was increased in GR than under canopies despite higher concentrations of soil organic C, N and P in the latter. The density of naturally occurring seedlings was highest at the parent location than away from it, contradicting the Janzen-Connell recruitment pattern. We conclude that in dry miombo, conservation efforts may be well-served by focusing on the role of nurse objects in the plant recruitment process.

Optimising Sentinel-2 MSI spatial resolution for estimating foliar nitrogen concentration in miombo woodlands

ABSTRACT The landscape-level estimation of nitrogen will constitute a significant input in global change models. High resolution broadband satellite technology is a promising solution to the limitation of spatial coverage. This study sought to identify an optimal spatial resolution for estimating foliar nitrogen in dry miombo woodlands. The contrast texture measure, calculated at increasing window size on Sentinel-2 imagery, was related to field measured nitrogen via random forest regression models. Our results show that the optimum size for estimating nitrogen is 50m. This result is significant in understanding the scale at which dynamics in foliar nitrogen concentration can be remotely measured.

Dry and wet miombo woodlands of south-central Africa respond differently to climate change.

It is important to understand how species distributions will shift under climate change. While much focus has been on species tracking temperature changes in the northern hemisphere, changing precipitation patterns in tropical regions have received less attention. The aim of the study was to estimate the current distribution of wet and dry miombo woodlands of sub-Saharan Africa and to predict their distributions under different climate change scenarios. A maximum entropy method (Maxent) was used to estimate the distributions and for projections. Occurrence records of dominant tree species in each woodland were used for modeling, together with altitude, soil characteristics, and climate variables as the environmental variables. Modeling was done under all four representative concentration pathways (RCPs) and three general circulation models. Three dominant tree species were used in models of dry miombo while seven were used for wet miombo. Models estimated dry miombo to cover almost the entire known distribution of miombo woodlands while wet miombo were estimated to predominate in parts of Angola, southern Democratic Republic of Congo, Malawi, Tanzania, Zambia, and Zimbabwe. Future climate scenarios predict a drier climate in sub-Saharan Africa, and as a result, the range of dry miombo will expand. Dry miombo were predicted to expand by up to 17.3% in 2050 and 22.7% in 2070. In contrast, wet miombo were predicted to contract by up to - 28.6% in 2050 and - 41.6% in 2070. A warming climate is conducive for the proliferation of dry miombo tree species but unfavorable for wet miombo tree species.

Use of local and global maps of forest canopy height and aboveground biomass to enhance local estimates of biomass in miombo woodlands in Tanzania

Field surveys are often a primary source of aboveground biomass (AGB) data, but plot-based estimates of parameters related to AGB are often not sufficiently precise, particularly not in tropical countries. Remotely sensed data may complement field data and thus help to increase the precision of estimates and circumvent some of the problems with missing sample observations in inaccessible areas. Here, we report the results of a study conducted in a 15,867 km² area in the dry miombo woodlands of Tanzania, to quantify the contribution of existing canopy height and biomass maps to improving the precision of canopy height and AGB estimates locally. A local and a global height map and three global biomass maps, and a probability sample of 513 inventory plots were subject to analysis. Model-assisted sampling estimators were used to estimate mean height and AGB across the study area using the original maps and then with the maps calibrated with local inventory plots. Large systematic map errors – positive or negative – were found for all the maps, with systematic errors as great as 60–70 %. The maps contributed nothing or even negatively to the precision of mean height and mean AGB estimates. However, after being calibrated locally, the maps contributed substantially to increasing the precision of both mean height and mean AGB estimates, with relative efficiencies (variance of the field-based estimates relative to the variance of the map-assisted estimates) of 1.3–2.7 for the overall estimates. The study, although focused on a relatively small area of dry tropical forests, illustrates the potential strengths and weaknesses of existing global forest height and biomass maps based on remotely sensed data and universal prediction models. Our results suggest that the use of regional or local inventory data for calibration can substantially increase the precision of map-based estimates and their applications in assessing forest carbon stocks for emission reduction programs and policy and financial decisions.

Assessing spatial patterns of forest degradation in dry Miombo woodland in Southern Tanzania

Miombo woodlands support diverse biodiversity and livelihoods of millions of people in Africa. Although these woodlands contain less carbon than humid forests, they are important for having large c...

Remote estimation of nitrogen is more accurate at the start of the growing season when compared with end of the growing season in miombo woodlands

The growing need to include foliar biochemical measurements in global change models has triggered the landscape level application of broadband satellites in nitrogen (N) estimation. In addition, the determination of seasonal variations in the accuracy of estimating N from remote sensing platforms is critical for understanding the reliability of N data used as an input into these models. However, seasonal differences in the accuracies of landscape level remote sensing models in dry deciduous landscapes such as the savanna woodlands remain poorly understood. Our study was carried out in the dry miombo woodland, one of the most expansive deciduous woodland in Southern Africa. A bootstrapped random forest model in the R environment was used to estimate foliar N from sentinel-2 broadband satellite platform. The relationship between foliar N and spectral reflectance was determined at two key phenological stages; the start and end of the growing season. Our results showed that the start of the growing season model (RMSE = 0.42) estimated N concentration better than the end of the growing season model (RMSE = 0.49). The difference between the two statistics are significantly (p < 0.05) different. In addition, maps of the standard deviation of predictions also indicate that the start of growing season model is more consistent (lower standard deviation overall) in its predictions, when compared with the end of the growing season model. We conclude that compared to the end of the growing period, the start of the growing season is a better phenological stage for remotely estimating N in miombo woodlands. This finding is significant in providing higher accuracy maps of N concentration for input into global change models.

Management implications of tree growth patterns in miombo woodlands of Zambia

Brachystegia-Julbernardia (miombo) woodlands of east and southern Africa are divided into dry (<1000 mm annual rainfall) and wet (>1000 mm). These woodlands first became established about 14,500 years ago. Since the migration of the Bantu people 5000 years ago, miombo woodlands have experienced widespread degradation and deforestation. There is little long-term data on tree growth and forest productivity for miombo woodlands and yet such data are required for the proper management of these woodlands. This study used tree growth data from 35 temporary plots in regrowth of 5–49 years old, and six permanent sample plots (PSPs) in old growth forest, which were monitored over a 28-year period (1990–2018). These data were used to investigate tree diameter growth and develop models of forest dynamics in wet and dry miombo woodlands. The study found that variations in tree diameter and stand basal area increments are strongly driven by stand age, tree size and density and that growth rates are higher in wet than dry miombo. Overall miombo woodland trees grew slowly at a rate of 0.34–0.50 cm yr−1 in re-growth areas. Most of the species in uneven-aged old-growth stands in dry miombo grew by 0.15 cm yr−1 with no significant annual differences although variations among conspecifics were large. Models predicted tree diameter increments of 0.10–0.24 cm yr−1 for stands of 100 years old. Transitions from one 5-cm diameter class to the next was estimated to take over 20 years and significant shifts in stand tree diameter distribution curves require over 25 years because of the low annual diameter increments. Stand responses to degradation varied from site to site and were driven by the number of saplings transiting into small diameter size classes. On the temporary regrowth plots basal areas averaged 7.1 m2 ha−1 and 9.9 m2 ha−1 in dry and wet re-growth miombo, respectively, with positive increments estimated at 0.40 m2 ha−1 yr−1 in dry miombo and 0.53 m2 ha−1 yr−1 in wet miombo. By contrast, average basal area increment on old-growth PSPs was negative, at −0.26 m2 ha−1 yr−1 over the 1990–2018 period, due to cutting and fire damage. Such drivers of forest degradation need to be controlled, especially in old-growth miombo, to maintain forest health and prevent decline in biomass.

The effect of shortening fallow length on recovery of plant species richness, composition and growth in shifting cultivation landscapes of Kilosa District, Tanzania

There is insufficient knowledge on theeffects of shifting cultivation on vegetationchange, especially in situations of land useintensification. This study investigated theeffects of shortening fallow length onrecovery of small (Dbh < 5 cm) and big(Dbh ≥ 5 cm) plant species richness,composition and height growth in a dryMiombo woodland in East Central Tanzania.Vegetation sampling was conducted on 40(10 m x10 m) plots and 80 (5 m x 5 m) subplotsfrom adjacent sites under fallow for 3,5, 7 and 15 years and a control secondaryforest. Results showed that shorteningfallow length to not less than 7 years did nothave a significant negative effect onrecovery of plant species richness and heightgrowth for both small and big sized plantspecies. However, there was a remarkableshift in composition of dominant specieswhen fallow length was less than 15 years.Results showed that big size Miombowoodland defining species such asBrachystegia bussei, B. boehmii, and B.spiciformis were deprived in fallowsyounger than 15 years but dominant insecondary forest. Non-Miombo woodlanddefining species such as Cassia burtii andDombeya shumpangae were insteaddominant in younger fallows. These resultsimply that if the fallow length is shortenedto 3 years as required by Village Land Useplans in Kilosa District, the sustainability ofthe shifting cultivation systems will bethreatened, affecting ecosystems servicesoffered by landscapes under shiftingcultivation. It is therefore recommended thatfallow length be prolonged to at least 7 yearsand that communities are encouraged toretain some of the Miombo woodlanddefining species in the fields undercultivation.

Coppicing ability of dry miombo woodland species harvested for traditional charcoal production in Zambia: a win\u2013win strategy for sustaining rural livelihoods and recovering a woodland ecosystem

The miombo woodland is one of the most extensive woodlands in Africa, supporting livelihoods based on biomass fuel for millions of rural people. However, there are growing concerns about the sustainability of harvesting for biomass fuel (mainly charcoal). Thus, the aim of this study was to examine whether regeneration by coppice is a viable option for sustainably managing miombo woodlands for biomass fuel production. We tested the hypotheses that (1) species, stump diameter, stump height and time since cutting significantly affect the number of sprouts per cut stump (coppice density) and mean sprout height (shoot vigour) and (2) higher coppice density reduces shoot vigour due to competition among coppice shoots in a given stump. In an inventory in areas that were harvested for charcoal production by the local people, 369 stumps of 11 species were recorded with mean coppice stumps ranged from 6 to 84. The mean coppice density ranged from 5 to 8 shoots per stump while the mean height of coppice shoots ranged from 46 to 118 cm with marked interspecific variations. Stump size was significantly and positively correlated with coppice density for some of the species, but not with shoot vigour for the majority of the species. However, shoot vigour was significantly positively correlated to time since cutting of trees for nearly half of the species. Coppice density had a significant negative correlation with shoot vigour for two species, and a positive correlation for one species. In conclusion, the results provide evidence about the importance of coppice management as a win–win strategy for sustaining charcoal-based rural livelihoods and recovering the miombo woodland ecosystem.

Detection of subtle deforestation due to logging using satellite remote sensing in wet and dry savanna woodlands of Southern Africa

Tropical deforestation through logging activities poses a direct threat to biodiversity. However, the detection of logging has remained a challenge. Based on study sites in Zimbabwe and Zambia, we tested whether the Normalized Difference Vegetation Index (NDVI) and the Coefficient of Variation in NDVI (CVNDVI) derived from high and medium spatial resolution satellite data could be used to detect logging in dry and wet miombo woodlands. Separately, we integrated NDVI and CVNDVI in logistic regression to test whether each can be used to successfully predict logging in the study sites. We tested whether the spatial resolution of satellite data has an effect in detection of logging using NDVI and CVNDVI derived from Landsat 8 and Worldview-2. Based on the ROC curves, we concluded that remotely sensed data could provide an effective predictive tool for detecting logging. However, in wet miombo woodlands the predictive power of remotely sensed data is weak.

Interactions between Canopy Structure and Herbaceous Biomass along Environmental Gradients in Moist Forest and Dry Miombo Woodland of Tanzania.

We have limited understanding of how tropical canopy foliage varies along environmental gradients, and how this may in turn affect forest processes and functions. Here, we analyse the relationships between canopy leaf area index (LAI) and above ground herbaceous biomass (AGBH) along environmental gradients in a moist forest and miombo woodland in Tanzania. We recorded canopy structure and herbaceous biomass in 100 permanent vegetation plots (20 m × 40 m), stratified by elevation. We quantified tree species richness, evenness, Shannon diversity and predominant height as measures of structural variability, and disturbance (tree stumps), soil nutrients and elevation as indicators of environmental variability. Moist forest and miombo woodland differed substantially with respect to nearly all variables tested. Both structural and environmental variables were found to affect LAI and AGBH, the latter being additionally dependent on LAI in moist forest but not in miombo, where other factors are limiting. Combining structural and environmental predictors yielded the most powerful models. In moist forest, they explained 76% and 25% of deviance in LAI and AGBH, respectively. In miombo woodland, they explained 82% and 45% of deviance in LAI and AGBH. In moist forest, LAI increased non-linearly with predominant height and linearly with tree richness, and decreased with soil nitrogen except under high disturbance. Miombo woodland LAI increased linearly with stem density, soil phosphorous and nitrogen, and decreased linearly with tree species evenness. AGBH in moist forest decreased with LAI at lower elevations whilst increasing slightly at higher elevations. AGBH in miombo woodland increased linearly with soil nitrogen and soil pH. Overall, moist forest plots had denser canopies and lower AGBH compared with miombo plots. Further field studies are encouraged, to disentangle the direct influence of LAI on AGBH from complex interrelationships between stand structure, environmental gradients and disturbance in African forests and woodlands.

Does the abundance of dominant trees affect diversity of a widespread tropical woodland ecosystem in Tanzania?

Abstract:Dominant woody species can determine the structure and composition of a plant community by affecting environmental conditions experienced by other species. We explored how dominant tree species affect the tree species richness, diversity, evenness and vertical structural heterogeneity of non-dominant species in wet and dry miombo woodlands of Tanzania. We sampled 146 plots from eight districts with miombo woodlands, covering a wide range of topographic and climatic conditions. We recorded 217 woody plant species belonging to 48 families and 122 genera. Regression analysis showed significant negative linear associations between tree species richness, relative species profile index of the non-dominant and the relative abundance of the dominant tree species (Brachystegia spiciformisandBrachystegia microphyllain wet, andBrachystegia spiciformisandJulbernardia globiflorain dry miombo woodlands). Shannon diversity and evenness had strong non-linear negative relationships with relative abundance of dominant tree species. A large number of small individual stems from dominant and non-dominant tree species suggesting good regeneration conditions, and intensive competition affecting survival. We suggest that dominant miombo tree species are suppressing the non-dominant miombo tree species, especially in areas with high recruitments, perhaps because of their important adaptive features (extensive root systems and ectomycorrhizal associations), which enhance their ability to access limited nutrients.

Operationalizing measurement of forest degradation: Identification and quantification of charcoal production in tropical dry forests using very high resolution satellite imagery

Quantification of forest degradation in monitoring and reporting as well as in historic baselines is among the most challenging tasks in national REDD+ strategies. However, a recently introduced option is to base monitoring systems on subnational conditions such as prevalent degradation activities. In Tanzania, charcoal production is considered a major cause of forest degradation, but is challenging to quantify due to sub-canopy biomass loss, remote production sites and illegal trade. We studied two charcoal production sites in dry Miombo woodland representing open woodland conditions near human settlements and remote forest with nearly closed canopies. Supervised classification and adaptive thresholding were applied on a pansharpened QuickBird (QB) image to detect kiln burn marks (KBMs). Supervised classification showed reasonable detection accuracy in the remote forest site only, while adaptive thresholding was found acceptable at both locations. We used supervised classification and manual digitizing for KBM delineation and found acceptable delineation accuracy at both sites with RMSEs of 25–32% compared to ground measurements. Regression of charcoal production on KBM area delineated from QB resulted in R2s of 0.86–0.88 with cross-validation RMSE ranging from 2.22 to 2.29Mg charcoal per kiln. This study demonstrates, how locally calibrated remote sensing techniques may be used to identify and delineate charcoal production sites for estimation of charcoal production and associated extraction of woody biomass.

Spatial patterns of subsistence extraction of forest products – An indirect approach for estimation of forest degradation in dry forest

Successful implementation of a forest based climate change mitigation mechanism such as REDD + depends on robust and available methods for measurement and estimation of forest degradation. Currently available methods are for application in single-hit degradation incidents in high density humid forests. However, it has been suggested that gradual degradation, especially in dry forests, is more widespread and that methods are needed for measuring and estimating associated emissions. We assess the applicability of an indirect remote sensing approach for monitoring forest degradation: infrastructure and other indicators of human activities are mapped and used for spatial prediction of degradation activities. For proxy variables we tested distance to forest edge, distance to roads, and population pressure calculated as the sum of inhabitants per pixel in the Landscan 2010 population raster dataset multiplied by an inverse power distance decay function. Wood extraction incidents were counted in 160 plots in two dry forests in Tanzania with infrastructural entry from one side only. We analyzed the spatial pattern of forest degradation as a function of the chosen proxy variables using zero inflated count models which allows for an excess of zero counts. A jack-knife bootstrap using 10,000 runs was applied to optimize the population distance decay function. We found that the impact of forest degradation is highest near high population concentration, above 1000 individuals. Furthermore, distance to nearest forest edge or road was a significant proxy for estimation of the number of wood extraction incidents (p < 0.001), where degradation incidents decreased with increasing distance to forest edge or road. At 3000 m from the forest edge towards the forest core the probability of wood extraction is 20% and dropping. The population distance decay function was found to have a steep decline indicating a relative small impact on forest degradation. Further, and perhaps larger, studies are needed to be able to recommend a distance decay function for general application in Tanzania. However, the results are useful for understanding spatial patterns of wood harvesting as a function of distance to nearest forest edge or road in dry Miombo woodland areas with average population pressure at 1685 ± 101 persons within a radius of 4000 m from the wood extraction sites.

Floristic Composition, Structure, and Species Associations of Dry Miombo Woodland in Tanzania

For the majority of forest reserves in Tanzania, biodiversity is poorly documented. This study was conducted to assess species richness (woody species), diversity, and forest structure and to examine relationships between species occurrence and topographic and edaphic factors in the Gangalamtumba Village Land Forest Reserve, a dry Miombo woodland area in Tanzania. A total of 35 nested circular plots with radii of 5, 15, and 20 m were used to collect data on woody species and soil samples across the 6,065 ha community-managed forest reserve. Stumps were measured 20 cm above ground. A total of 88 species belonging to 29 families were identified. Generally forest structure parameters and diversity indices indicated the forest to be in a good condition and have high species richness and diversity. Vegetation analysis revealed four communities of which two were dominated by the family Caesalpiniaceae, indicating large variation of site conditions and possible disturbances in the study area. The high level of diversity of woody species and the high basal area and volume indicate that the forest is in good condition, but the effect of anthropogenic activities is evident and stresses the need for proper management to maintain or enhance the present species diversity.