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  • Writer's pictureORC collaborator

Long-term vegetation monitoring infrastructure at Ongava

The SECO project aims to increase our understanding of carbon fluxes across dry tropical vegetation. Globally, savannas, woodlands and dry forests form the dominant land cover in the tropics, occupying more land area than rainforests. Dry tropical vegetation harbours a unique biodiversity, and is home to more than a billion people, many of whom derive their livelihoods from these ecosystems. Dry tropical ecosystems are in flux, being both a source of carbon emissions due to deforestation and land degradation, and also a carbon sink due to enhanced growth of trees and woody ecosystem expansion. Both of these processes however, are poorly understood. We currently lack clarity on both the magnitude and the underlying mechanisms driving these carbon fluxes. This results in large uncertainty in our estimates of how dry tropical vegetation contributes to the global carbon budget, which is key to predicting climate change and its impacts.

SECO is pioneering the expansion of a systematic network of vegetation monitoring sites across the global dry tropics, to bring us closer to understanding how dry tropical vegetation is changing, and its effects on the global carbon cycle.

Each SECO monitoring site consists of multiple permanent sample plots, within which the growth, mortality, and composition of woody vegetation (trees and shrubs) can be measured repeatedly over many years.

Measuring trees in a mopane-Terminalia-Commiphora woodland (left) and A numbered tree tag attached to a tree stem (right)

At Ongava Game Reserve SECO has collaborated with Ongava Research Centre (ORC) and the Namibian University of Science and Technology (NUST) to establish 11 of these permanent sample plots. Each plot is 100x100 m square, located within a particular vegetation type, e.g. woodland, parkland, shrubland etc. Within each plot, every woody stem >5 cm diameter is recorded, its diameter measured, species and mortality information recorded. Each measured woody stems is marked with a numbered metal tag nailed into the stem so the same stems can be identified in future years to see how much they have grown, and which stems have died. For more information on the plot measurements, see the field guidedeveloped by SEOSAW, the Socio-Ecological Observatory for Southern African Woodlands.

SECO first started looking to establish plots in northern Namibia when it became apparent that existing plots were drastically under-sampling the more arid end of seasonally dry tropical woodland vegetation in southern Africa. The permanent plots in Ongava are in mopane woodlands, a major woodland type that is underrepresented in long-term monitoring and very understudied.

The climate space of dry tropical vegetation in southern Africa taken from the green shaded area of the map below, with Mean Annual Temperature (MAT) on the X axis, and Mean Annual Precipitation (MAP) on the Y axis. Points show existing permanent plots in the region (red), and the new plots at Ongava (yellow).

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The extent dry tropical vegetation in southern Africa (green), showing existing permanent plots in the region (red), the new plots at Ongava (yellow), and the extent of mopane woodlands in Angola and Namibia (blue).

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Now that we have measured the trees on each of these plots at Ongava, we can use those measurements to estimate woody biomass, i.e. how much carbon is stored in that particular patch of woodland. In the future, these biomass estimates could be combined with remote sensing technology, particularly radar, to estimate woody biomass stocks across Ongava game Reserve. In conjunction with other plots located across the region, we can also start to build regional biomass maps.

Our aim is to return to these plots in two years to conduct a second census on the trees. In the meantime further surveys of shrubs, small tree stems, and ground layer vegetation will be conducted to develop a full vegetation profile of the plots. With continued funding and support, we hope these censuses can continue every 3-5 years thereafter. The value of such plots only increases as more data is collected over time. From this we will build a picture of how biomass stocks and vegetation composition is changing, both locally within the Reserve, and also globally across the dry tropics.

It’s important to mention that although in the first instance these vegetation monitoring plots will be used by the SECO project for their carbon dynamics research, the plots will have a life beyond this project. Permanent sample plots are ideal to act as a general infrastructure for future research, both for vegetation and other parts of the ecosystem where a deeper understanding of the vegetation might enrich a particular research question. Already we are brainstorming ideas on the interactions between soil microbial populations and tree species composition, the role of herbivores in shaping woodland structure, and many other exciting possibilities.

For Ongava Game Reserve, we hope these plots can act as part of a broader programme of data collection to monitor the natural landscape, the effects of management actions, and the ramifications this may have on the vegetation which plays host to such a diverse array of charismatic animal species.

Authors: John L. Godlee & Vera De Cauwer

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