| Scientific Name | Aloidendron dichotomum (Masson) Klopper & Gideon.F.Sm. | Higher Classification | Monocotyledons | Family | ASPHODELACEAE | Synonyms | Aloe dichotoma Masson, Aloe dichotoma Masson var. montana (Schinz) A.Berger, Aloe montana Schinz, Aloe ramosa Haw., Rhipidodendron dichotomum (Masson) Willd., Rhipidodendrum dichotomum (Masson) Willd. | Common Names | Kokerboom (a), Quiver Tree (e) |
National Status | Status and Criteria | Vulnerable A4ace | Assessment Date | 2022/04/08 | Assessor(s) | W. Foden, D. Raimondo, C. Eastment, K.-A Grey, C.J. Geldenhuys, P.C.V. Van Wyk, N. Jurgens, M.T. Hoffman, E. Swart, G. Midgley & P. Jacobs | Justification | This species is a conspicuous, long-lived, succulent tree with a broad distribution range that extends through the arid Namib and Succulent Karoo regions of Southern Africa (Burke 2004; van der Merwe and Geldenhuys 2017). It has an extent of occurrence (EOO) of 149 707 km². Individuals typically grow in discrete subpopulations of 100-10,000+ trees, straddling summer and winter rainfall zones, varied topography and geology and a range of habitat types. The species is slow-growing (generation length 100 years) and highly adapted to the regions with high temperatures and very low rainfall.
Because dead trees decay slowly in the species' arid habitats it is possible to estimate a comparative measure of mortality for subpopulations (Foden et al. 2007; Hoffman et al. 2010). Similarly, juvenile's very slow growth rates allow estimation of relative recruitment success. Ongoing population monitoring in 40 subpopulations across the species' range over the last 18 years, projected to a 100-year moving window, reveal a projected overall population decline of at least 26% by 2102 based on the conservative assumption of a linear rate of population decline.
Climate change species distribution models predict losses of suitable habitat of between 33% and 68% by 2070, with spatial patterns of loss largely corresponding to mortality and recruitment trends observed in situ. The species is projected to shift in a south-easterly direction, with the range in 2070 projected to be ~191km east of its current range, with suitable habitat area in the far northern and western regions being lost by 2070 (Brodie et al. 2021). Since it has not shifted its range polewards yet, despite increasing habitat suitability in these areas, it seems unlikely range gains will keep pace with losses occurring in areas of decreasing suitability (Brodie et al. 2021). Based on recent coupled paleo-range and future range predictions, the species would have to migrate at a rate 15 times faster than its modelled range shift after the Last Glacial Maximum when the species would have retreated into climate refugia (Brodie et al. 2021). Such a rate is extremely unlikely for such a slow-growing species. As a result, we expect the overall suitability of the species' occupied habitat to continue to decline at an escalating rather than a linear rate. We therefore project a population reduction of over 30% within 100 years and this species thus qualifies for listing as Vulnerable under criterion A. |
Distribution | Endemism | Not endemic to South Africa | Provincial distribution | Northern Cape, Western Cape | Range | With its range spanning approximately 11 degrees of latitude between 21 and 32 degrees south (Foden et al. 2007), this species has the widest distribution of any taxon within the Aloidendron genus. Its northernmost subpopulation occurs at high elevation on Brandberg massif, Namibia, and its southernmost in the Hantam mountains in the Northern Cape Province of South Africa. Its western range edge approximately follows the Atlantic coast, to a distance of ~10-50km inland, with subpopulations occurring in the Swakopmond and Luderitz districts of Namibia. The eastern subpopulations extend into the summer rainfall region of Prieska, South Africa. |
Habitat and Ecology | Major system | Terrestrial | Major habitats | Hantam Karoo, Umdaus Mountains Succulent Shrubland, Rooiberg Quartz Vygieveld, Central Richtersveld Mountain Shrubland, Namaqualand Shale Shrubland, Namaqualand Klipkoppe Shrubland, Northern Knersvlakte Vygieveld, Bushmanland Arid Grassland, Blouputs Karroid Thornveld, Lower Gariep Broken Veld, Kahams Mountain Desert, Eastern Gariep Rocky Desert, Upper Gariep Alluvial Vegetation | Description | This species grows in both winter and summer rainfall climates, predominantly within the biomes of Nama and Succulent Karoo. Plants typically grow in dense groups on mountainous sites (rocky or sandy) with aspects largely north-facing in southern populations and south-facing in northern populations. Average rainfall ranges from approximately 5-350 mm per annum throughout the range. It is believed that coastal plants, which grow in areas of negligible rainfall, are able to utilise the frequent ocean fog, possibly by channeling water down their trunks to their matt of adventitious roots.
The species' flowering time varies between subpopulations, but spans a few months in winter. Pollinators include short-billed generalist nectarivores (e.g. weavers, white-eyes and starlings), and sunbirds and honeybees are common pollinators and visitors (Van Jaarsveld 2011). Fruit contain hundreds of small, light, seeds that are wind-dispersed during spring. A small 'wing' allows each seed to be blown along the ground through the arid and frequently very windy landscape, theoretically enabling long-distance seed dispersal. Seeds can remain dormant for up to three years (Van Jaarsveld 2011, Giddy 1973). Seedlings typically require nurse plants (e.g. Stipagrostis brevifolia) or rocks for successful germination, presumably for shade and protection from herbivores. Young plants are highly susceptible to damage and mortality from livestock and wild animals (Geldenhuys 2019).
A. dichotomum is regarded as a keystone species. Flowers provide nectar to a wide range of birds and insects, and as typically the only trees in an otherwise sparse landscape, they are very often used as nest sites for a wide variety of bird species and a vantage point for raptors (Midgley et al. 1997, Powell 2005). Sociable weaver (Philetairus socius) nests are often found in larger individuals. Adult plants contain large water reserves and in periods of extended drought its trunk and branches are eaten by porcupines, antelope, baboons, hyrax and livestock, despite its extremely bitter taste. This occurs particularly in the seasonally variable summer rainfall region (Powell 2005; Jack et al. 2016).
Heat tolerance is remarkable in the species with summer temperatures exceeding 50° C in parts of its range (Cousins and Witkowski 2012). A. dichotomum performs crassulacean acid photosynthesis (CAM), a carbon fixation pathway adapted to arid conditions that allows plants to photosynthesize during the day, but only exchange gases at night (Grey 2019, Grey et al. under review). Due to limitations of mesophilic carbon storage, juveniles likely have a reduced ability to photosynthesize during cool winter nights. This physiological difference between age classes of the species is a possible limiting factor to the hypothesized (e.g. Foden et al. 2007) cooler leading edge expansion of the distribution as a response to climate change (Grey 2019).
While ongoing recruitment has been observed, the subpopulations rely on periods of suitable climate, presumably high rainfall, for the majority of recruitment events (Hoffman et al. 201, van Blerk 2013, Gallaher 2014, Jack et al. 2016). This, in combination with the fact that juveniles are less drought resistant, has resulted in a higher concentration of recruitment within the southern, winter rainfall region of the distribution where precipitation in the form of fog and rain is more reliable (Gallaher 2014, Jack et al. 2016). Biological interactions also play a role in limiting growth rate of populations. Long-term monitoring within permanent plots observed over a 40 year period from 1980 to 2020 at the Fish River Canyon by the BIOTA Biodiversity Observatories in Africa project (Jürgens et al. 2012) noted that successful recruitment is dependent on the presence of nurse plants, and the majority of juvenile plants are lost to weevil predation of the growth points, adult plants typically have colonies of ants living in their branches that protect the growing points from weevil damage.
Tree growing forms in this family usually present poor fire tolerance due to a lack of the insulating 'skirts' of dead leaves that are typically found in grassland and savanna Aloe species (Bond 1983). It is speculated that an absence of dormant bud banks on the stem prevents resprouting of the species, reducing ability to respond to disturbance events (Cousins and Witkowski 2012). This lack of resilience to fire is concerning given that projected future suitable habitats lie eastwards in savanna and grassland regions where fire is a regular feature in the landscape. |
Threats | | Anthropogenic climate change is the primary current and long-term threat to this species. Climate models for the likely emission scenarios where emissions stay at present day levels (RCP 2.6) (Hausfather and Peters, 2020) and worst case scenarios where emissions continue to increase during the 21st century (RCP 8.5) indicate that there will be a loss of suitable bioclimatic envelope of between 33% and 67% by 2070 for this species. Climate models also include new suitable habitat becoming available, an expansion of 84% of suitable habitat under RCP 2.6 emission scenario and 99% areas are projected under the RCP 8.5 scenario. However as mentioned above, only 5% of this predicted new range will likely be colonised.
For specific populations, further severe damage to all life stages including adult plants are caused by Kudus, with this damage being more significant during drought years. For example, monitoring of 100 plants in a permanent plot at Prieska in the Northern Cape South Africa, demonstrated that 34% of the population died from Kudu damage during the drought that took place between 2015 and 2018. Livestock have also been observed to damage subpopulations especially juveniles. Despite these observations, the overall influence of non-climatic variables such as herbivory on A. dichotomum for the 40 subpopulations that have undergone long-term monitoring was found to be small in comparison to the climatic influences (Kaleme 2003; Foden et al. 2007). However recent drought events and their negative influence on grazing/browsing availability for wildlife, in combination with high concentrations of livestock, have increased pressure on many subpopulations (van Wyk pers comms. 2021; Geldenhuys 2019).
Illegal collection is also a threat to this species, there is evidence that during the 1960s and 70s, truckloads of succulent seedlings were removed from the Richtersveld by plant collectors (Duncan et al. 2006). The illegal removal of wild Quiver Trees (A. dichotomum, A. pillansii and A. ramosissimum) from the Northern Cape is still a threat (Duncan et al. 2005, 2006; Powell 2005; Powell pers. comms. 2021).
The coincidence of high rainfall and wind speed events in the summer rainfall region puts the shallow rooting system of the species at risk. High rates of windthrow mortality have been found in large adults predominantly in the northern and eastern summer rainfall part of the distribution (Jack et al. 2016).
Much of the distribution of this species is also subject to mining industry expansion with associated direct loss of habitat and increased exposure to risks being a threat to the species (Foden et al. 2007; Duncan et al. 2005). |
Population | Subpopulations are scattered into discrete subpopulations, some with >100 kms between them, and range in size from <100 to 50,000+ individuals. Observations have been largely restricted to road-sides and since roads are sparse in this vast arid region, much of the range remains unsurveyed. A dichotomum occurs in both summer and winter rainfall areas, varying topographies and on multiple soil types and gradients (Foden et al., 2007). The reasons for the discontinuities between subpopulations are unknown.
Because dead trees decay very slowly in the speciesâ arid habitat, often remaining standing for at least a decade, it is possible to measure each subpopulationâs relative mortality. Similarly, individualsâ slow growth (they reach maximum height at approximately 80-120 years) and clear age stages to senescence, at least 200 years (Vogel, 1974) and possibly up to 350 years (Foden 2007), make subpopulation age structure and recruitment history apparent.
Ongoing long-term monitoring of 40 subpopulations from across the speciesâ range spans 18 years to date (2003 to 2021), with all surveyed in 2003, most again in 2008, and a subsample of sites resurveyed in 2018, 2020 or 2021 (Foden et al., unpublished). The most northerly subpopulations remain to be surveyed after 2008. Calculated using a 100-year âshifting windowâ from 2003-2102, based on the longest of the monitoring intervals measured since 2003, an overall population decline of 26.2% by 2102 was inferred, assuming linear population decline rates. Further, by 2102 sixteen individual subpopulations are projected to become extinct, nine to decrease by >30% and seven to increase by >30%. In eight subpopulations no juveniles were found at any point during the survey. Twenty-four subpopulations are projected to lose all juveniles by 2102, but six to increase juvenile percentage by >30%. With higher levels of mortality present in northerly populations once the northern subpopulations are resampled it is likely that the population decline projections to 2102 will exceed 30%.
A climate change explanation for A. dichotomumâs suspected decline is supported by three independent lines of evidence. Firstly, observed declines correspond to changes in the regionâs meteorological records (Foden et al. 2007, IPCC, 2021) which show marked increases in mean temperatures and decreases in water balance during the monitoring period. These may have led to surpassing of the speciesâ physiological thresholds in subpopulations in the warmest and driest parts of its range.
Secondly, based on observations (Foden 2002, Foden et al. 2007, Jack et al. 2016) and long-monitoring (van der Merwe and Geldenhuys 2017, Foden et al., unpublished), clear patterns of mortality and recruitment occur across latitude and elevation. The greater mortality in northern (i.e. equatorial, warm range-edge) and lower elevation subpopulations, and population age structure with a relatively high percentage of juveniles in southern populations is a characteristic signal of climate change impacts on a sedentary species (Foden et al. 2007).
Thirdly, species distribution models for A. dichotomum project losses of between 33% and 68% of currently suitable habitat space by 2070 based on RCPs 2.6 (likely emission scenario) and 8.5 (worst case scenario) respectively (Hausfather and Peters 2020). Models predict habitat gains which range from 84% and 99% by 2070, also based on RCPs 2.6 and 8.5 respectively (see supporting information). The spatial patterns of change in habitat suitability approximately follow theory-based expectations for climate change, with losses more prevalent in warmer and drier, more northerly subpopulations as physiological tolerances of heat and water availability are crossed (Grey 2019). Modelled gains occur in more southerly range areas, which were historically too cold for the species, and areas of no change to 2070 remain in the south-east and along a mid-longitude mountainous âspineâ running north-south through much of the latitude range, under both RCPs. In order to predict if dispersal into modelled future suitable habitat is possible we considered paleo-range reconstruction which indicates that this species would have retreated poleward and westward to climate refugia during the Last Glacial Maximum (LGM) where conditions would have remained suitable for the species to persist (Brodie et al. 2021). During this time, models predict that suitable habitat space for the species would have declined by 69% relative to its current distribution (Brodie et al. 2021). Following this, the species would have expanded its geographic range to its current extent at a rate of ~0.4 km/decade (Brodie et al. 2021). Subsequent modelling of its future geographic range predict a range expansion of the species by 191 km eastwards, with suitable habitat space being lost in the most northerly and westerly extents of its current range (Brodie et al. 2021). This range shift would require the species to migrate at a rate of 6 km/decade, which is a rate 15 times faster than its historical migration rate (Brodie et al. 2021), and one that is unlikely to be possible for such a slow-growing species. Since the species has not extended its range to date, despite clear climate change impacts, and the fact that there is high pressure on juvenile recruitment from wildlife and livestock grazing, and ongoing habitat degradation, a zero dispersal scenario is assumed. Under likely emission scenarios the population is predicted to decline by 33% by 2070 and is suspected to exceed 35% by 2102.
| Population trend | Decreasing |
Conservation | | The species is listed in CITES Appendix II (CITES 2019), and present in 10 ex situ collections (BGCI 2020).
Portions of A. dichotomumâs distribution range occur within protected areas such as /Ai /Ais-Richtersveld Transfrontier Park, Namaqua National Park, Namib-Naukflut National Park and Tsau //Khaeb National Park, however, the extent to which these areas are able to mitigate the effects of climate change are not well understood.
Key research needs include determining rates of decay of tree carcasses which will allow more accurate estimation of mortality timing. Establishing growth rates, particularly of juveniles, is also needed. Both are possible through long-term monitoring, and appropriate methods are laid out by Van der Merwe and Geldenhuys (2017). Other needs include researching the conditions for successful germination and survival of young plants, but this relies on the availability of local climatic data, which are generally poor in this sparsely populated area. Grey et al. (2019) carried out laboratory experiments to establish cold tolerance thresholds for southern-most subpopulations. Expanding these to establish heat and moisture tolerance thresholds of the speciesâ and individual subpopulations is greatly needed, including for validating species distribution models.
To understand the speciesâ dispersal/migration potential, research into seed dispersal mechanisms and distances, as well as pollinator types and dispersal distances are needed. Recent work by Brodie et al. (2021) on paleo-historical range shifts relative to major climatic shifts helps to understand the speciesâ potential for climatic responses, but population genetics studies are needed to better determine speciesâ colonisation potential in the future.
Research on the scope and severity of non-climatic threats is highly recommended, including on the role of herbivore and insect damage at subpopulation and species scales. |
Notes | | The species is traded for horticultural purposes, but being easily and prolifically cultivated in nurseries, demand is largely met legitimately. However, despite being listed on CITES Appendix 2, plants continue to be removed from the wild, particularly in road-side populations. |
Assessment History |
Taxon assessed |
Status and Criteria |
Citation/Red List version | | Aloidendron dichotomum (Masson) Klopper & Gideon.F.Sm. | VU A3ce | 2014.1 | | Aloe dichotoma Masson | VU A3ce | Raimondo et al. (2009) | |
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Citation | | Foden, W., Raimondo, D., Eastment, C., Grey, K.-A, Geldenhuys, C.J., Van Wyk, P.C.V., Jurgens, N., Hoffman, M.T., Swart, E., Midgley, G. & Jacobs, P. 2022. Aloidendron dichotomum (Masson) Klopper & Gideon.F.Sm. National Assessment: Red List of South African Plants version . Accessed on 2025/04/23 |
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