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Monitoring the iconic quiver tree as an indicator of ecosystem integrity

By Dr Helga van der Merwe1 (SAEON) and Conrad Geldenhuys (DENC)
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A quiver tree in flower (Picture: Conrad Geldenhuys)

The Common Quiver tree2 (Aloidendron dichotomum, previously Aloe dichotoma) is a widespread and iconic succulent tree species distributed across the arid and semi-arid winter and summer rainfall regions of Namibia and South Africa.

The species is found on numerous inselbergs in the Nama and Succulent Karoo biomes and has been suggested as a species that would be suitable for monitoring climate change impacts in areas where complete biological inventories are lacking.

Over the past 20 years, numerous studies have highlighted large-scale mortality for quiver trees at various localities in Namibia and South Africa. Local mortality in many of these populations has been attributed to factors such as animal damage, disease, wind-throw, theft and climate change.

Recently, it has been found that mortality and recruitment patterns reflect prevailing differences in the summer rainfall zone (Nama Karoo Biome) and winter rainfall zone (Succulent Karoo Biome) climates.

Monitoring protocol

A scientific article recently published in the South African Journal of Botany proposes a monitoring protocol for quiver tree populations as indicators of ecosystem integrity. The article was a collaborative effort between SAEON and the Northern Cape Department of Environment and Nature Conservation (DENC).

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An example of leaf scale on a quiver tree (Picture: Helga van der Merwe)

Monitoring in a quiver tree population (Picture: Helga van der Merwe)

The proposed monitoring protocol combines several methodologies used over the years by various institutions in order to standardise field data collection procedures. This proposed protocol was tested by collecting data for 12 quiver tree populations situated throughout the Nama and Succulent Karoo biomes of the Northern Cape. The data were analysed in order to illustrate the application of the data collected for long-term monitoring purposes.

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Baboon-inflicted tree damage (Picture: Conrad Geldenhuys)

The baseline data collected for the Northern Cape populations indicated that there is a wide variation in population demographic characteristics between the different populations. The study highlighted that the choice of population surveyed is of critical importance and that a wide range of populations in close proximity to one another and at different sites situated far apart have to be surveyed in order to produce an unbiased assessment of populations.

In general, South African A. dichotomum populations appear to be in good health and are recruiting. 

Comprehensive data set

The results of the proposed standardised methodology allow for an improved understanding of quiver tree demography. Over time additional populations will be monitored. This, together with the continued collection of data at existing sites, will provide a comprehensive data set allowing for not only the more accurate interpretation of the dynamics of these populations but also for aiding in the understanding, management and conservation of these populations.

1 Dr Helga van der Merwe is a SAEON Research Career Advancement Fellow.

2 The quiver tree gets its English common name from the San people’s practice of hollowing out the tubular branches of Aloidendron dichotomum to form quivers for their arrows (PlantZAfrica.com).

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