In Maputaland, which has been classified a Biodiversity Hotspot and Centre of Plant Endemism, natural land (original) (raw)
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Open Journal of Forestry, 2014
In Africa, information on time required for plants to develop from seed to mean size and maximum size is scarce. There is also a lack of information regarding accurate relationships between stem diameter, height and canopy dimensions. This type of information is however becoming a real necessity to allow the accurate measurement of carbon stocks and carbon stocks change to qualify for the UNFCCC's REDD+ mechanism. We evaluated these parameters for 22 Sand Forest and woodland tree species of South Africa's Maputaland. Results indicated that it takes approximately 66 and 35 years for current dynamics of Sand Forests and woodlands to occur respectively. Based on diameter and growth rates, larger trees of the forests can be older than 600 years, while large woodland trees can reach ages of 900 years. Our evaluation of linear relationships between stem diameter, height and canopy dimensions showed extremely robust results allowing the use of stem diameter to calculate height and canopy dimensions. The results are of interest for carbon related investigations and reconstructing stands dynamics.
Growth rate of selected woody species in northern Maputaland, KwaZulu–Natal, South Africa
South African Journal of Botany, 2008
Annual stem circumference growth rates of 23 woody species of the Sand Forest and woodland in Maputaland are presented for the first time. The rare Sand Forest, has been identified as an endemic, diverse vegetation type that is under threat from land transformation and human utilisation outside conservation areas. The growth rate of the selected woody species was evaluated over a 71-month survey that encompassed climatic extremes, oscillating from heavy rainfall to drought in a short period of time. The mean annual stem circumference growth rate among the investigated species varied thirteen fold, from a low of 2.04 mm/yr for Brachylaena huillensis to 26.28 mm/yr for Garcinia livingstonei. When the data of all species were considered, there was a significant positive relationship between stem circumference and growth rate, but no significant relationship between wood density and growth rate. The present results suggest that there is no significant difference in terms of mean annual growth rate between the Sand Forest and woodland vegetation types. In general, woodland species showed larger fluctuations in growth rate and the temporal pattern of Sand Forest species seemed to lag behind that of the woodland species. These trends can in part be ascribed to the woodland species all being deciduous, whereas the Sand Forest suite included both deciduous and evergreen species. Annual growth rates measured from dry season to dry season were not related to the seasonal rainfall pattern, but appeared to be highly dependent on the water availability at the time of the enumeration. The larger size classes reacted sooner to decreased water availability and a reduction in growth of larger trees was accompanied by an increase in growth in the smaller size classes.
Forest Ecosystems, 2016
Background: Tropical dry forests cover less than 13 % of the world's tropical forests and their area and biodiversity are declining. In southern Africa, the major threat is increasing population pressure, while drought caused by climate change is a potential threat in the drier transition zones to shrub land. Monitoring climate change impacts in these transition zones is difficult as there is inadequate information on forest composition to allow disentanglement from other environmental drivers. Methods: This study combined historical and modern forest inventories covering an area of 21,000 km 2 in a transition zone in Namibia and Angola to distinguish late succession tree communities, to understand their dependence on site factors, and to detect trends in the forest composition over the last 40 years. Results: The woodlands were dominated by six tree species that represented 84 % of the total basal area and can be referred to as Baikiaea-Pterocarpus woodlands. A boosted regression tree analysis revealed that late succession tree communities are primarily determined by climate and topography. The Schinziophyton rautanenii and Baikiaea plurijuga communities are common on slightly inclined dune or valley slopes and had the highest basal area (5.5-6.2 m 2 ha −1). The Burkea africana-Guibourtia coleosperma and Pterocarpus angolensis-Dialium englerianum communities are typical for the sandy plateaux and have a higher proportion of smaller stems caused by a higher fire frequency. A decrease in overall basal area or a trend of increasing domination by the more drought and cold resilient B. africana community was not confirmed by the historical data, but there were significant decreases in basal area for Ochna pulchra and the valuable fruit tree D. englerianum. Conclusions: The slope communities are more sheltered from fire, frost and drought but are more susceptible to human expansion. The community with the important timber tree P. angolensis can best withstand high fire frequency but shows signs of a higher vulnerability to climate change. Conservation and climate adaptation strategies should include protection of the slope communities through refuges. Follow-up studies are needed on short term dynamics, especially near the edges of the transition zone towards shrub land.
Forests, 2016
Biomass and carbon stocks are key information criteria to understand the role of forests in regulating global climate. However, for a bio-rich continent like Africa, ground-based measurements for accurate estimation of carbon are scarce, and the variables affecting the forest carbon are not well understood. Here, we present the first biomass study conducted in South Africa Mistbelt forests. Using data from a non-destructive sampling of 59 trees of four species, we (1) evaluated the accuracy of multispecies aboveground biomass (AGB) models, using predictors such as diameter at breast height (DBH), total height (H) and wood density; (2) estimated the amount of biomass and carbon stored in the aboveground compartment of Mistbelt forests and (3) explored the variation of aboveground carbon (AGC) in relation to tree species diversity and structural variables. We found significant effects of species on wood density and AGB. Among the candidate models, the model that incorporated DBH and H as a compound variable (DBH 2ˆH) was the best fitting. AGB and AGC values were highly variable across all plots, with average values of 358.1 Mg¨ha´1 and 179.0 Mg¨C¨ha´1, respectively. Few species contributed 80% of AGC stock, probably as a result of selection effect. Stand basal area, basal area of the ten most important species and basal area of the largest trees were the most influencing variables. Tree species richness was also positively correlated with AGC, but the basal area of smaller trees was not. These results enable insights into the role of biodiversity in maintaining carbon storage and the possibilities for sustainable strategies for timber harvesting without risk of significant biomass decline.
A spatial and temporal analysis of Sand Forest tree assemblages in Maputaland, South Africa
South African Journal of Wildlife Research, 2008
Maputaland is a centre of plant endemism, within the Maputaland-Pondoland-Albany hotspot of biodiversity shared by Mozambique and South Africa. The Sand Forest vegetation is the most valuable vegetation type in this region due to the endemics it harbours, but it is currently under threat from growing animal population densities within conserved areas and from a growing human population outside. To improve our understanding of Sand Forest dynamics the present study investigated the woody plant assemblages in Tembe Elephant Park. A total of 59 plots were sampled and analysed using classification and ordination methods. The results indicate that Sand Forest in Tembe is a complex assemblage of at least three floristically and structurally distinct woody communities. An ordination of the combined results from the present study together with two other studies could link the floristics and structure of the Sand Forest to the level of disturbance by herbivores and humans. Large herbivores induce both structural and floristic changes in the Sand Forest of Tembe Elephant Park and a lack of disturbance in the nearby Tshanini Community Conservation Area leads to a state of equilibrium. The results therefore suggest that Sand Forest dynamics and complexity may be strongly linked to small-to medium-scale animal disturbance.
Accurate height-diameter allometry is crucial for the estimation of forest biomass and carbon stocks. Tree height measurements over a large range of diameters and species are urgently needed in the tropics, specifically in central Africa, for the development of locally derived height-diameter allometric equations and the conversion of forest inventory data into biomass estimates, and for the validation of remotely sensed canopy height that mostly rely on a few specific field sites. In this study, we aimed to identify the variation in height-diameter allometry of tropical trees between forest types and among species in central Africa, and we examined the consequences for biomass estimation. Height and diameter were measured for a total of 521 trees over a large range of diameters in two forest types in southern Cameroon, 10–240 cm in the evergreen forest and 11–182 cm in the semi-deciduous forest. A total of ten allometric models including asymptotic and non-asymptotic models were fitted to the height-diameter data. Measured tree diameters, grouped into 10 cm wide diameter classes up to 150, from commercial forest inventory data (0.5 ha plots, n = 2101 and n = 5152, respectively in the evergreen and in the semi-deciduous forests) were converted into biomass estimates using general allometric models with and without including our site-specific height-diameter allometry. Though debated in the literature, our results supported a saturation of tree height with tree diameter both at site and species level, with asymptotic models better depicting the height-diameter allometry. Height-diameter allometry significantly differed between forest types and these local height-diameter equations also differed from published equations. For a given diameter, trees tended to be taller in the semi-deciduous forest than in the evergreen forest, as already reported between moist and wet forests in pantropical studies. Similar trends were reported within species for the three species shared by both forest types, suggesting an environmental control of tree allometry. Because of the low performance of the bioclimatic stress variable to predict tree height and of the slight soil differences between the two forest types, the environmental determinants of height-diameter allometry remain to be explored. In addition to tree allometry variation, structural differences (basal area and density) were also identified between the two forest types using commercial forest inventory data at genus level, and both allometry and forest structure (taller trees and denser stands) contributed to the greater biomass per hectare of the semi-deciduous forest.
International Journal of Applied Earth Observation and Geoinformation, 2019
The varying proportions of tree and herbaceous cover in the grassland and savanna biomes of Southern Africa determine their capacity to provide ecosystem services. The asynchronous phenologies e.g. annual NDVI profiles of grasses and trees in these semi-arid landscapes provide an opportunity to estimate percentage tree-cover by determining the period of maximum contrast between grasses and trees. First, a 16-day NDVI time series was generated from MODIS NDVI data, i.e. MOD13A2 16-day NDVI composite data. Secondly, percentage tree-cover data for 100 sample polygons (4 × 4) pixels for areas that have not undergone change in tree cover between 2001 and 2018 were derived using high resolution Google Earth imagery. Next, a time series consisting of the coefficients of determination (R 2) for the NDVI/tree-cover linear regression were computed for the 100 polygons. Lastly, a threshold R 2 > 0.5 was used to determine the optimal period of the year for mapping tree-cover. It emerged that the narrow period from Julian day 161-177 (June 10-26) was the most consistent period with R 2 > 0.5 in the region. 18 tree-cover maps (2001-2018) were generated using linear regression model coefficients derived from Julian day 161 for each year. Kendall correlation coefficient (tau) was used to determine areas of significant (p < 0.05 and p < 0.01) increasing or decreasing trend in tree-cover. Areas (polygons) that showed increasing tree-cover appeared to be more widespread in the trend map as compared to areas of decreasing tree-cover. An accuracy assessment of the map of increasing tree-cover was conducted using Google Earth high resolution images. Out of 330 and 200 mapped polygons verified using p < 0.05 and 0.01 thresholds, respectively, 180 (54% accuracy) and 132 (65% accuracy) showed evidence of tree recruitment. Farm abandonment appeared to have been the most important factor contributing to increasing tree-cover in the region.
Vegetation structure, dominance patterns and height growth in an Afromontane forest, Southern Africa
Journal of Forestry Research, 2018
Information on forest structure is fundamentally important to track successional vegetation dynamics for efficient forest management. This study reports on vegetation characteristics, dominance patterns and species height growth in a northern mistbelt forest type in South Africa. Common alpha-diversity indices (species richness and Shannon-Weiner diversity), structural vegetation parameters (tree density and basal area), and species importance value index were used. Size class distribution and height-diameter allometry were further examined for the overall stand and most important species. Stem densities (472.0 ± 43.5 and 605.3 ± 28.1 trees ha-1 for C 5 cm to \ 10 cm and C 10 cm dbh (diameter at breast height) classes, respectively) and basal area values (1.99 ± 0.19 and 48.07 ± 3.46 m 2 ha-1 , respectively) are comparable to other Afromontane forests in East Africa. The overall stand showed an inverted-J shaped distribution pattern which is a typical feature of stand size class distribution in most natural forests. Most ecologically important species also exhibited an inverted-J shaped distribution pattern, suggesting good regeneration and recruitment potential. There were significant differences in species on height, reflecting species-specific height growth patterns, possibly a result of intrinsic growth potential and competitive interactions. The present study suggests that conservation and management policies, including protection of surrounding land uses against fire, contribute to maintaining a successful recovery of these forests. However, it should be noted that these forests may be experiencing relatively slow dynamic flux as a result of the overmature state of some trees with several years under relatively strict protection.
Forests
While closed canopy forests have been an important focal point for land cover change monitoring and climate change mitigation, less consideration has been given to methods for large scale measurements of trees outside of forests. Trees outside of forests are an important but often overlooked natural resource throughout sub-Saharan Africa, providing benefits for livelihoods as well as climate change mitigation and adaptation. In this study, the development of an individual tree cover map using very high-resolution remote sensing and a comparison with a new automated machine learning mapping product revealed an important contribution of trees outside of forests to landscape tree cover and carbon stocks in a region where trees outside of forests are important components of livelihood systems. Here, we test and demonstrate the use of allometric scaling from remote sensing crown area to provide estimates of landscape-scale carbon stocks. Prominent biomass and carbon maps from global-scal...