Spatial-temporal dynamics of savannah ecosystems (tree-grass interactions, grass quality/quantity, biodiversity) as a life support system to wildlife and livestock production in and around Kruger National Park
The Kruger National Park (KNP) and surrounding areas are located in the savannah Lowveld ecosystem of North-Eastern South Africa. This ecosystem is charaterised by a continuous grass layer mixed with forbs and sedges with variable cover of fine and broad-leaved trees and shrubs. The coexistence of the two life forms, trees and grasses in the KNP savannah has been attributed to the impacts of disturbance factors including fire and herbivory and to exclusive and differential access to limited water resources by grasses and trees. This causes greater within-life form competition than between-life form competition, thereby promoting tree-grass coexistence. The co-existence of trees and grasses in the savannah of the KNP has served as a life supporting system to some of the most charismatic wildlife in the world, including the presence of the Big five, that is the elephant, buffalo, rhinoceros, lion and leopard. Three ecosystem services are therefore identified as key to the KNP including ecotourism, grazing and browsing resources for wildlife and domesticated animals, and wood energy resources in the surrounding communities. The continuous provision of these ecosystem services depends on a number of essential biodiversity variables (EBVs) including rangeland condition (the quantity/quality of grass), tree-cover percentage and phenology. Therefore, the main focus of the storyline was to quantify the spatio-temporal dynamics of the above EBVs.
Maps of the following variables have been produced using remote sensing data:
(i) Seasonal variation of leaf nitrogen concentration (in percent).
Models for leaf nitrogen were developed using field-spectroscopy data and subsequently applied on Sentinel-2 images to map the distribution of leaf nitrogen in the KNP (Fig. 1).
Leaf nitrogen is a key component of rangeland quality and determines the feeding patterns of grazers and browsers within the broad landscape. The distribution of leaf nitrogen is not only impacted by the geological substrate of the landscape but also by climatic variables including rainfall and temperature. Models of mapping and monitoring leaf nitrogen from Sentinel-2 data are therefore essential in the context of climate change and for proper rangeland management.
(ii) Tree-cover percentage
The varying proportions of tree and herbaceous cover in the savannah biome of southern Africa determine their capacity to provide ecosystem services. The variability of grass and trees-cover is controlled by a number of environmental and anthropogenic factors. For example, increasing levels of anthropogenic CO2 are expected to favour tree growth at the expense of grasses. The consequence of this could be the expansion of woodlands into grass dominated systems, a phenomenon known as bush or woody encroachment. Human activities including deforestation, overgrazing, fencing-off of land for wildlife conservation, clearing of land for agriculture or farm abandonment are equally changing the dynamics of trees and grasses, or forest cover, in the region. Bush encroachment can suppress the productivity of herbaceous plant species and reduce land accessibility by wildlife. Therefore, continuous monitoring of tree-cover is be important for adaptive management to avoid the phenomenon of bush encroachment. Highly accurate maps of recent tree-cover percentage (Fig. 2) have been produced for the KNP and environs from Synthetic Aperture Radar data. Furthermore, a methodology for detecting bush encroachment has been developed from MODIS NDVI 250 m data.
(iii) Land surface phenology
The annual cycle of vegetation greening and senescence (aging) in the savannah plays an important role in the functioning of the biosphere, e.g. carbon and water cycles, migration of wildlife, spread of fire. This allows African savannahs to provide numerous ecosystem services of high social and economic importance, such as wildlife tourism, provision of food (e.g mopane worms, marula fruits, honey) and grazing land for livestock production. The periods corresponding to the start and end of the growing season, and indeed the length of the growing season (i.e. the phenological periods) determine the availability of the above ecosystem services both in space and time. Furthermore, understanding the relative contribution of grasses and trees to the various phenological periods can enhance our ability to manage the savannah for optimal benefits to people and wildlife. This has become even more critical in the advent of climate change and variability characterised by erratic rainfall patterns in Southern Africa. A methodology was established to map land surface phenological metrics in the savannah system of Southern Africa from MODIS imagery. The study on the response of land surface phenology on variation in tree cover during green-up and senescence periods in the semi-arid savannah of Southern Africa revealed that rainfall is the predominant factor that explains the inter-annual variability of the day corresponding to the start of the growing season for areas dominated by grasses (< 20% tree cover). For example, the start of the growing season was delayed in 2015 in the KNP when the region was affected by drought between October December 2015 (Fig.3).
On the other hand, tree cover is the predominant factor that explains the variability in the day corresponding to end of the growing season. In fact, the length of the growing season increases with increasing tree cover (Fig. 4).
The above EBVs shall be integrated into the KNP threshold of potential concern (TPC) management framework in order to enhance adaptive management of the protected area.
Ramoelo A., Cho M.A. (2018). Explaining leaf nitrogen distribution in a semi-arid environment predicted on Sentinel-2 imagery using a field spectroscopy derived model. Remote Sensing, 10, 269; DOI:10.3390/rs10020269. https://www.mdpi.com/2072-4292/10/2/269
Cho MA, Ramoelo A. (in review). Optimal dates for assessing tree cover change (2001-2018) in the semi-arid biomes of South Africa using MODIS NDVI time series. International Journal of Applied Earth Geoinformation and Earth Observation.
Cho M.A, Ramoelo A., Dziba L. (2017). Response of land surface phenology to variation in tree cover during green-up and senescence periods in the semi-arid savannah of southern Africa. Remote sensing, 9, 689: DOI:10.339/rs9070689. https://www.mdpi.com/2072-4292/9/7/689
Cho M.A, Ramoelo A., Dziba L. (2018). Understanding seasonal variations of vegetation greening and senescence in the Southern African savannahs. Ecopotential newsletter n.8. http://ecopotential-newsletter.igg.cnr.it/2018/04/understanding-seasonal-variations-of-vegetation-greening-and-senescence-in-the-southern-african-savannahs/
Cho MA. (Poster 2018). Assessing tree cover change in the dryland forests of southern Africa using tree cover maps derived from MODIS 205 m imagery (2001-2018). AGU 100 Advancing Earth and Space Science. Fall meeting, Washington, D.C., 10-14 Dec 2018.
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Last update: May, 2019