The Ecologic Status And Environment Of Aloe polyphylia In Lesotho
CACTUS & SUCCULENT JOURNAL (U.S.), Vol. 52, 1980
ALAN C. BEVERLY
Gilbert Westacott Reynolds, perhaps the greatest student of the genus Aloe, wrote of Aloe polyphylla in 1934 (9), “Growing in a locality of such difficult access, and large specimens being too heavy to move, plant lovers can rejoice in the knowledge that there need be little fear of this remarkable and unique aloe ever being eradicated.” And in 1969 (10), “Few have seen A. polyphylla in its natural habitat, and it appears to be the least known, one of the rarest, and one of the most unique of all the Aloes.” In this paper I intend to demonstrate that his prognostication was false, and the latter assertion true.
The sad truth is that in several populations poaching has been heavy enough to eliminate all Aloe polyphylla from the site. In the 1960-1970 period illicit traffic occurred to such an extent that at least 11 populations are now extinct. In 34 other sites I studied, habitat deterioration triggered by overgrazing is threatening the future existence of the population.
Of the 324 species in the genus throughout Africa, the Arabian peninsula and related islands, A. polyphylla is unique both ecologically and morphologically. No other spiral phyllotaxy is so striking. No other species, except A. percrassa Tod. in northern Ethiopia at 2,500m (8,000') has adapted to such severe seasonal extremes of temperature and moisture. The nearest phylogenetic relative to A. polyphylla is A. pratensis Bak. which is found mainly at 1,927m (6,300') on basalt slopes of 'the eastern facing Natal Drakensberg (11).
Other aloe communities in Lesotho are Aloe ferox Mill. which dominates slopes of the Cave Sandstone formation in the southwestern Qu,thing district near the Senqu River. Aloe saporuzria (Ait.) Haw. covers some stream banks on the Red Bed formation in the same area, but neither of these are found above 1,835m (6,000') .
DISTRIBUTION-PAST AND PRESENT
Aloe polyphylla is found along the southern Thaba Putsoa Range and extending east into the Central Range. Several sites in the higher elevations of the Drakensberg Range are noted, where the major river in southern Africa is born, the Senqu (Orange) (2, 3). I found no positive information concerning Aloe polyphylla in the northern Maluti Range, but the extreme NE corner of Lesotho has an extinct site. This creates a discontinuity on the map and presents a problem, since there are many habitats there which could support Aloe polyphylla. It seems fairly certain that Aloe polyphylla has always been endemic to Lesotho only. There are no confirmed records of natural populations outside the present political boundary.
Palynologic stratigraphy of Aliwal North in the northeastern Cape and Florisbad in the Orange Free State have yielded much insight into recent environmental changes in southern Africa. In the interior (26°-30° SLat.) colder periods with heavier winter precipitation farther north than today alternated with warmer and drier periods. These changes broadly correlate with glacial and interglacial periods in the higher latitudes of the northern hemisphere. There is some "periglacial" phenomena evidence in Lesotho's Drakensberg above 2,600m (8,500') (4). Beginning at 29,00025,000 years B.P. (before present) and terminating 17,000-16,000 years B.P. temperatures were at least 6°C lower and precipitation double that of today (5). The ecotone between the drier Karoo, with its steppe vegetation, and the grasslands of the Orange Free State has fluctuated in a NE-SW manner in response to climatic changes in the Quaternary (5). After about 10,000 years B.P. (Early Holocene) increasingly xeric conditions began to prevail and continue today. These events are certain to have broadly contributed to and shaped the evolution of Aloe polyphylla and to its presently highly restricted distribution. It is probable that the phylogenetic precursor of Aloe polyphylla found suitable habitats to the northeast of Lesotho along the Drakensberg escarpment at lower elevations during a colder, wetter period of the Quaternary but since the Early Holocene has been increasingly confined to the higher elevations in Lesotho. I suggest that both A. polyphylla and A. pratensis have a common ancestor and that they are young species, only becoming distinct during the last 10,000 years.
I located 36 extant populations throughout Lesotho and estimate the number of plants to be 2,500. From twelve more reliably reported sites I estimated 500 plants, but it would not be unreasonable to assign the number 3,500 for the entire country-wide population. I believe I only found a large fraction of the total. The populations are small, with 72% of the confirmed sites having less than 50 plants (figure 2). In only a few sites was the area greater than one hectare (2.47 acre). The ten largest populations were found to contain 74% of the 2,500 observed (2).
A. polyphylla grows on very steep north facing basalt slopes with little or no soil cover. The aggressive root system firmly anchors the plant in crevices and fractures in the basalt. The altitudinal range, 7,500'-8,500', encompasses the majority of the 36 sites studied. Above the A. polyphylla site there is usually a thick grassland or bog which regulates water flow for the aloes below. In summer a continuous flow of water bathes the slope. The thick grass sod holds a reservoir of water (like a sponge) which is slowly released, also delivering nutrients. It is this factor which allows A. polyphylla to occupy high and steep positions on basalt walls with little soil cover. Most grasses and shrubs require more soil merely to anchor themselves. Thus the native grasses Themeda triandra Forsk. and Festuca caprina Nees. play a critical role in the habitat of A. polyphylla.
The lush grass attracts mixed herds of cattle, sheep and goats. The heavy cattle break the sod cover, sometimes sinking to their knees while the sheep and goats eat the grass down very closely. Grazing is excessive only relative to the capacity of the grassland for recovery. With a short growing season, varying from 36 months, productivity is very limited. While grazing is outstripping productivity, less palatable species are moving in. Colonizing species of Compositae from the dry western steppe of southern Africa steadily gain an advantage under the grazing pressure and have for- the last two hundred years (since white man first settled in southern Africa) been moving toward the east (1). Chrysocoma tenuifolia Berg. and Helichrysum aureonitens Sch. Bip. are now becoming dominant over much of Lesotho's montane veld.
This process is tantamount to desertification; an artificially induced decline in the biological productivity of a plant community, and a destabilization of essential relationships. Energy and water equilibria are disturbed, the new system holds less water at any given time and less solar energy is captured. This fragile ecosystem is easily physically abused, and is becoming dry and infertile. The hard and profuse, short thundershowers carry tons of topsoil down slope in a matter of minutes. The slope becomes very dry quite soon again after a rain when there is no water flow regulation from above. The irregularity introduced in space and time into the water flow through the A. polyphylla community stresses the adult plants, guarantees a rough time for any seedling, and drastically decreases the chance of a viable seed finding a place which affords germination and establishment. Figure 2 shows the relative age-class distribution of normal and disturbed populations.
The two largest populations had the only seedlings I observed, and the difference was plain enough. Water was flowing everywhere across the slope. The sheen of mucilaginous algae on the black basalt testifies to the continuity of water flow at these sites. There was no alien vegetation, no evidence of grazing and seedlings of Aloe polyphylla were scattered about the slope.
There are many large clumps of plants on the slope, with as many as 12 plants closely appressed together. This occurs when a lateral bud develops and produces a second rosette which becomes as large as the parent plant. The growth orientations are nearly always contrary. These new rosettes can be separated from the others when they form roots of their own. In the population as a whole the ratio of right to left hand spirals is close to one. It is probable that the spiral orientation is not genetically fixed but is a chance phyllotaxic phenomenon where right and left spirals occur with equal frequency. I noted one large speciment changing from left to right.
There are really only two seasons to consider, a wet warm (cool) summer and a cold dry winter. After a winter dormancy lasting from April through September mature plants start sending up inflorescence stalks (peduncles) the first week of October and continue flowering throughout December (remember the seasons are reversed in the southern hemisphere) .
The Malachite Sunbird, Nectarina famosa (L.) Roberts, visits the salmon-colored Howers and with its slightly recurved, thin, narrow bill can suck nectar from the base of the long tub~lar perianth. Reynolds (9) noted both "proterandrous" (anthers mature first) and "proterogynous" (pistil matures first) flowers, and incorrectly equated this condition with heterostylism (different lengths of styles in the flowers), which he believed to be, "so far unknown in the genus Aloe." Clearly more information is needed on pollination biology, with particular reference to the allegation that only cross-fertilized seed is viable.
Seed formation occurs in December and January. At this time female gnats lay eggs within the developing capsule, the larvae hatch, bore a small hole in the seed coat and utilize the endosperm food reserves (and the embryo) to metamorphose into adults. Up to 60% of the seed may be damaged in this way.
By the end of March the capsules have fully desiccated and the triangular seeds with small membranous wings have been blown across the slope. The seed must over winter and can only germinate in October or November if a successful seedling is to be frost tolerant by April. There is no dormancy mechanism; fresh seed will germinate without the winter cold treatment. After the equinox the daily range of temperature increases, and with the decrease in moisture the plants begin acclimatizing for the winter period.
It is a myth that these mountains receive substantial snowfall in winter. A strong high pressure cells builds up and prevents cold fronts from reaching very far north. Winter days are cold with bright sun and wind and low humidity. A reasonable estimate of the annual rainfall at an Aloe polyphylla site would be 50-60", 78% of which falls between September 1 and March 31.
With one assumption, I would like to explore the consequences of the processes and factors herein described on the future of A. polyphylla. Dr. A. Jacot-Guillarmod (7) postulated a cycle of diminishing returns beginning with poachers removing flowering size plants, and the decreased attraction of the populations for the Malachite Sunbird, without which pollination cannot occur. If this is true, then the remaining populations are effectively made into biologic islands and no gene exchange is accomplished between and within populations. Poaching will reduce the genetic diversity further. The failure of seed germination and establishment guarantees that genetic recombination can not contribute to the fitness of Aloe polyphylla in a rapidly deteriorating habitat. No new or different heterozygous plants are being created by these populations. If the lack of the pollinator is preventing cross-pollination and if seed is formed, by selfing, the percentage of homozygous Loci (genes) increases by 50% with each generation. This alone would reduce the ability of a population to respond to the environments' selection pressures, and amounts to a serious genetic drift. The only factor tending to stabilize populations is asexual reproduction. Eventually the large plants may become weakened and attacked by the rust fungus, Uromyces aloe (8). Many populations are now below the viable size necessary for continued self perpetuation and renewal into the future. As Figure 2 shows, with no seedlings produced the population is getting older and smaller very quickly.
If Lesotho cannot begin to effectively manage her grassland resource by controlling grazing, they will soon lose all opportunity to do so. The root of Lesotho's general ecologic crisis is in their flawed approach to resource allocation and management. They have created a commons, where no land is privately owned, and little or no responsibility is borne by the users. Chiefs give permission to use a plot for grazing or cultivation. The grazing ethic, "Locks each herdsman into a suicidal competition for the available range resource, compelling him to permit his herd to increase without limit, in a world of limited resources" (6). The threat to the entire country's watersheds, flora and fauna is grave, and we can realistically understand why Aloe polyphylla has joined the list of endangered species.
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