| Scientific Name | Aloe pearsonii Schönland | Higher Classification | Monocotyledons | Family | ASPHODELACEAE | Common Names | Pearson's Aloe (e), Richtersveld Aloe (e) |
National Status | Status and Criteria | Critically Endangered A3ce+4ace | Assessment Date | 2022/05/23 | Assessor(s) | D. Raimondo, P.C.V. Van Wyk, C. Eastment, N. Jurgens, S. Loots, C.J. Geldenhuys, W. Foden, M.T. Hoffman, P. Jacobs, E. Swart, K.C. Kelly, H. Bezuidenhout & D. Guo | Justification | This arid adapted aloe occurs in the Gariep Centre of Plant Endemism, the richest centre of succulent plant endemism in the world (Van Wyk and Smith 2001). It has a limited distribution in the northern Richtersveld and southern Namibia. Until recently it occurred as locally common subpopulations and even dominated whole plant communities. It has however suffered severe and rapid population decline since 2015 due to extreme drought conditions with both the length of the dry spell experienced and the severity of heat exceeding those of historically recorded droughts. Within the Richtersveld portion of its range there has been additional decline to the population caused by livestock and wild fauna grazing. Niche based climate models using plausible future CO2 emissions scenarios indicate that there will be a reduction of between 81% (RCP 2.6) and 100% (RCP 8.5) of its suitable climate envelope by the time period 2061-2080. Combining the observed mass dieback of individuals witnessed over the past five years with the projected future shifts of suitable climate envelopes, a decline of over 80% of the population is inferred within both a shifting time frame which starts in 1980 and ends in 2080 as well as in the hundred year period projecting forward from 2021 to 2121. This long lived species has a generation length in excess of 100 year and it therefore qualifies for listing as Critically Endangered under criterion A. |
Distribution | Endemism | Not endemic to South Africa | Provincial distribution | Northern Cape | Range | This species is highly localised, occurring in the north west corner of South Africa and the southern parts of Namibia in the Lüderitz district. It has a narrow distribution that is limited to the area between Kuboes in the South African Richtersveld National Park and the mining town of Rosh Pinah in southern Namibia. Its distribution falls within and contributes towards the Gariep Centre of Plant Endemism (Van Wyk and Smith 2001). |
Habitat and Ecology | Major system | Terrestrial | Major habitats | Upper Annisvlakte Succulent Shrubland, Central Richtersveld Mountain Shrubland, Richtersveld Sheet Wash Desert, Noms Mountain Desert | Description | The species is found in shallow sandy or stony soils mainly of granite, sandstone and shale origins (van Wyk and Smith 2014). Subpopulations occur predominantly on steep (15° to 20°) rocky slopes between 300 and 1150 m.a.s.l. in hot, xeric habitat (Carter et al. 2011; Jankowitz 1977). It does not deal with frost and cold well as death or terminal damage has been recorded once temperatures reach -5°C to -7°C (Cousins and Witkowski 2012). This and the fact that CAM is the most likely photosynthesis pathway utilised (e.g. Grey et. al. 2019 results on Aloidendron dichotomum; Cousins and Witkowski 2012) allows the species to persist during extreme summer temperatures of >50°C.
Despite extreme heat tolerance, the species relies on winter rainfall events for recruitment and mist for moisture. As a result, populations are found predominantly on south-west facing rocky slopes due to the availability of mist and rainfall coming from south-westerly low pressure systems in winter (Jankowitz 1975; Rothmann 2004). This species is able to survive periods of lack of moisture and has been recorded to persist through the droughts of the 1980s (BIOTA/SASSCAL observation system) and the droughts in the 1940s (Reynolds 1950) however it appears to occupy a niche at the edge of its ecological tolerance since mortality linked to the extended 5 year droughts that took place between 2015 and 2020 has been observed. This species is therefore sensitive to increases in dry spell duration and mean and maximum temperatures.
Flowering predominantly in summer, the nectar provides a scarce source of nutrients and moisture for a variety of avian and insect species during dry summer months. Seeds are wind dispersed and broken stem pieces are able to root in ideal conditions. The average age of mature individuals is suspected to be very old as 20 year monitoring in an exclusion plot by the BIOTA/SASSCAL Project showed no change in size or biomass of individuals. Aloe pearsonii is believed to be one of the slowest growing of all aloes, eventually growing to two metres tall after several hundred years (Walker and Vanden Bon 2018). Germination of seeds at the Richtersveld Park Sendelingsdrif nursery show that even seedlings grow extremely slowly reaching only 0.5 cm after 3 years.
Recruitment appears to be episodic and extremely rare linked to past favourable climatic periods as most individuals within subpopulations appear to be the same age and all are mature old individuals. No seedlings have been seen despite ongoing searchers since 2013. Seed set is very low, as most seeds are parasitised by wasps. Seeds are viable only for short periods and seedbanks do not survive more than two years. Given its long-lived life history strategy it has a generation length that exceeds 100 years. |
Threats | | Anthropogenic climate change is the primary threat to this species, with the increases in dry spell duration, mean and maximum temperatures in the Richtersveld having caused mass die-off in formerly large subpopulations of this species during the extended drought that took place between 2015 and 2020. Abiotic changes linked to global change are expected to intensify into the future, with observed temperature change over the past 10 years already reaching those predicted for near future scenarios (2035) (van Wilgen et al. 2016). Models of future suitable climate envelope given the most plausible emissions scenarios, indicate loss of suitable habitat of between 81 and 100%. Natural dispersal to future suitable habitat is also extremely unlikely for this species, since it already occupies all the high altitude slopes available within its range and its inability to survive frost will prevent it moving east. Furthermore it has extremely low adaptive capacity as monitoring has showed that the vast majority of known individuals are mature old adults and seed set, recruitment and dispersal is not currently taking place.
In addition to the impacts of climate change, severe overstocking of livestock across the Richtersveld is causing ongoing degradation of this species' habitat. This is compounded by browsing species such as Baboon (Papio ursinus), Hyraxes (e.g. Procavia capensis) and occasionally Klipspringer (Oreotragus oreotragus). Overgrazing has reduced palatable species to such an extent that desperate livestock and indigenous fauna are now starting to browse this species, and many plants are dying as a result of the damage caused. The combination of overgrazing and climate-change driven droughts make it extremely unlikely that this long-lived, slow growing species will recover to its original population size. |
Population | Reports and observations of subpopulations conducted prior to 2010 indicate that this species occurred in dense stands within its restricted distribution (Jankowitz 1977; Carter et. al. 2011; Walker and Vanden Bon 2018). However, recent observations of the subpopulations at Koeboesberg, Helskloof, Pooitjiespram, Spitskop and Namusberg, indicate that the population has declined significantly over a short period of time due to drought related mortality.
The largest known subpopulation occurs at Helskloof within the Richtersveld National Park in South Africa where over a million plants were estimated to be extant in 2021. Monitoring data from the BIOTA/SASSCAL observation system (Jürgens et al. 2012) and South African National Parks shows that this subpopulation was stable between 1980 and 2014, however the extended drought between 2015 and 2020 has resulted in a 60% decline to this subpopulation. This decline is inferred from surveys conducted in January 2021 where the number of living versus dead plants were counted along six transects each of 625 m long. A total of 3 085 plants were counted, of which 1 220 plants were alive and 1 838 (60%) plants were dead. The remaining living specimens showed immense stress and loss of biomass (supporting information doc. 1). In order to confirm that none of the plants were dead prior to the extended drought, photographs taken at Helskloof between 1980 and 2017 were collated and the same sites rephotographed in 2021 (supporting information doc. 2). Matched photographs showed decline of between 72 and 100% of plants depending on the site. Old photographs provide evidence that no plants were dead prior to 2017. Two other smaller subpopulations known from South Africa, from Kodaspiek and Pooitjiespram while extant in 2014, have both been completely lost as a result of the extended drought. During monitoring no fruits were observed and no seedlings have been recorded since 2014, despite some rain falling in 2020.
In Namibia, observations of the subpopulation on Namusberg, show that it had declined by 50% between 2014 and 2019 and further declines are likely to have happened since then given the trend of ongoing mortality during 2020 witnessed at Helskloof. Nearby Namusberg, on the farm Spitskop, there has also been substantial dieback and only half of the original subpopulation is still alive, with those surviving occurring on sheltered south facing rocky slopes. Based on observed decline of subpopulations in South Africa and Namibia, we estimate that an overall population decline of 55-60% has taken place since 2014. Future declines to the population are also extremely likely since 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 81% and 100% for the time period 2061-2080.
| 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 Aloidendrendon dichotomum’s 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 species’ and individual subpopulations is greatly needed, including for validating species distribution models.
To understand the species’ 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 species’ potential for climatic responses, but population genetics studies are needed to better determine species’ 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 | | This is a well-known species but rarely encountered in cultivation, where it has a reputation for being difficult to grow well; this is attributed to the extremely harsh environment the species inhabits in Namaqualand (Walker and Vanden Bon 2018). |
Assessment History |
Taxon assessed |
Status and Criteria |
Citation/Red List version | | Aloe pearsonii Schönland | VU A2ac; B1ab(iii,v)+2ab(iii,v) | 2015.1 | | Aloe pearsonii Schönland | Least Concern | Raimondo et al. (2009) | | Aloe pearsonii Schönland | EN B1B2abce | Victor (2002) | | Aloe pearsonii Schönland | Vulnerable | Hilton-Taylor (1996) | |
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Citation | | Raimondo, D., Van Wyk, P.C.V., Eastment, C., Jurgens, N., Loots, S., Geldenhuys, C.J., Foden, W., Hoffman, M.T., Jacobs, P., Swart, E., Kelly, K.C., Bezuidenhout, H. & Guo, D. 2022. Aloe pearsonii Schönland. National Assessment: Red List of South African Plants version . Accessed on 2024/12/06 |
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