Dendrosenecio

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Giant groundsels
Senecio kilimanjari.jpg
Dendrosenecio kilimanjari
Scientific classification OOjs UI icon edit-ltr.svg
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Eudicots
Clade: Asterids
Order: Asterales
Family: Asteraceae
Subfamily: Asteroideae
Tribe: Senecioneae
Genus: Dendrosenecio
(Hauman ex Hedberg) B. Nord. (1978)
Type species
Dendrosenecio johnstonii
Synonyms [1] [2]
  • Senecio L. subg. Dendrosenecio(L. Hauman ex Hedberg)

Dendrosenecio is a genus of flowering plants in the sunflower family. [3] [4] It is a segregate of Senecio , [1] in which it formed the subgenus Dendrosenecio. [1] Its members, the giant groundsels, are native to the higher altitude zones of ten mountain groups in equatorial East Africa, [5] where they form a conspicuous element of the flora.

Contents

Description

They have a giant rosette habit, with a terminal leaf rosette at the apex of a stout woody stem. When they bloom, the flowers form a large terminal inflorescence. Concomitantly, two to four lateral branches are normally initiated. As a result, old plants have the appearance of candelabras the size of telephone poles, each branch with a terminal rosette. [5]

Species

Dendrosenecio varies geographically between mountain ranges, and altitudinally on a single mountain. There has been disagreement among botanists as to which populations of Dendrosenecio warrant recognition as species, and which should be relegated to the status of subspecies or variety. The following list, taken from Knox & Palmer [5] , will be used for articles about this genus.

Distribution

Groundsels of several species are found throughout the world as common roadside weeds, but nowhere except in the highlands of Africa do they exhibit such large tree forms.

Theodore Roosevelt 1914 [6]

The giant groundsels are found in the alpine zone of the mountains of equatorial East Africa - Mount Kilimanjaro and Mount Meru in Tanzania, Mount Kenya, the Aberdare Range, and Cherangani Hills in Kenya, Mount Elgon on the Uganda/Kenya border, the Rwenzori Mountains on the Uganda/Democratic Republic of Congo (DRC) border, the Virunga Mountains on the borders of Rwanda, Uganda and the DRC, and Mitumba Mountains (Mount Kahuzi and Mount Muhi) in the east of the DRC.

With the exception of D. eric-rosenii, which occurs on several of the mountains of the Albertine Rift (Rwenzori, Virunga and Mitumba Mountains), and D. battiscombei and D. keniodendron, which are shared by Mount Kenya and the Aberdare Range, the species are individually confined to a single range. In several of the ranges different species, or subspecies, are found at different heights.

Distribution chart

(after Knox & Palmer [5] )
rangeKilimanjaroMeruKenyaAberdaresCheranganiElgonRuwenzoriVirungaMitumba
higher altitudeD. kilimanjari ssp. cottoniiD. keniodendronD. keniodendronD. elgonensis ssp. barbatipes
intermediate altitudeD. kilimanjari ssp. kilimanjariD. meruensisD. keniensisD. brassiciformisD. cherangiensis ssp. daleiD. elgonensis ssp. elgonensisD. adnivalis (two subspecies)D. eric-rosenii ssp. alticola
lower altitudeD. johnstoniiD. battiscombeiD. battiscombeiD. cherangiensis ssp. cherangiensisD. eric-rosenii ssp. eric-roseniiD. eric-rosenii ssp. eric-roseniiD. eric-rosenii ssp. eric-rosenii

Evolution and adaptation

The mountains of central and eastern Africa are an almost ideal model system for studying speciation and adaptation in plants. The mountains rise far above the surrounding plains and plateaus, [7] tall enough to reach above the tree line [8] forming "islands in the sky" or isolated habitats. [7] These predominantly volcanic peaks further simplify the model by their age and arrangement around the Lake Victoria basin and proximity to the equator. [7]

The species found on Mount Kenya are by far the best model for altitudinal variation. Dendrosenecio keniodendron is the species which grows at the highest of altitudes, Dendrosenecio keniensis is found at the lower altitudes of the range where the species grows and Dendrosenecio battiscombei grows at the same altitudes as D. keniensis but in the wetter environments. The other mountains which are not tall enough to have a "big one at the top" have the two, one species for the drier land and one for the damper environments or just one because the environment is not so extreme. This simplification works extremely well as an introduction to the giant groundsel of East Africa with one exception, Kilimanjaro who has the one species that lives at the top and only one species that lives below; subspecies and varieties living in the moister environments.

Gridded Adaptive Speciation Studies
Each mountain has a vertical gradient of precipitation and temperature fluctuations. [9] Mount Kilimanjaro at 5,895 metres (19,341 ft), Mount Kenya at 5,199 metres (17,057 ft) and Ruwenzori at 5,109 metres (16,762 ft) are the three tallest mountains in Africa; each tall enough to support altitude based layers of vegetative zones. [10] Each mountain providing its own vertically placed array of isolated habitats. [7]
Located from 50 kilometres (31 mi) to 1,000 kilometres (620 mi) around the equator, the environmental fluctuations occur as daily events [7] of warm days and cold nights and are consistent throughout the year [9] or as Hedberg described this unique situation: "summer every day, winter every night". [11] In addition to the simplified environmental variables, these mountains are easily described for biogeographic analysis as their age and arrangement around the Lake Victoria basin make it easy to disentangle the effects of time and position. [7]
Vegetation zones
Simplified grid system. Oldest mountain on the left. Flora of East Tropical Africa Peaks Grid.svg
Simplified grid system. Oldest mountain on the left.
In the altitudes between 3400 meters (11,000 feet) and 4500 meters (15,000 feet) some of the most extreme examples of adaptations can be found, which include:
  • Massive leaf rosettes in which leaf development occurs in a large "apical bud"
  • Water storage in the pith of the stem
  • Insulation of the stem by retaining withered and dead foliage
  • Secretion and impoundment of ice-nucleating polysaccharide fluids (a natural anti-freeze)
  • Nyctinastic leaf movement (the leaves close when it gets cold) [7]
At altitudes below 3400 meters (12,000 feet)the daily temperature fluctuations are less extreme, the average daily temperature steadily increases, and the growth forms and ecology of the Dendroseneico reflect the increased influence of biotic factors (such as competition for light) over abiotic factors (such as nightly frost). [7]
Dendrosenecio keniodendron on Mount Kenya. MtKenyaSenecio.jpg
Dendrosenecio keniodendron on Mount Kenya.
3400-3800 meters (11,000-12,000 feet)
Given the name Afro-alpine region by Hauman in 1955. [13] There is a sharp boundary at 3400 meters (3000 meters on the North side) that separates the forest from the lower alpine zone, [8] the environment is a moorland (low growing vegetation on acidic soils) and it is here that the Dendrosenecio start to grow among the mountain tussocks and sedges. [14]
Dendrosenecio keniensis grows in this region on Mount Kenya. A variety or subspecies of Dendrosenecio johnstonii live within this altitude range on all three of the tallest mountains.
3800-4500 meters (12,000-15,000 ft)
The upper moorlands; this is where most of the D. brassica make their homes on all three of the mountains, living with tough dwarf shrubs.
4300-5000 meters (14,000-16,000 ft)
Dendrosenecio woodlands, where each mountain has its own special variety. Dendrosenecio keniensis on Mount Kenya, Dendrosenecio kilimanjari on Mount Kilimanjaro and other species each on their own mountain.
4500 meters-peak (15,000 ft)
Populations of Dendrosenecio start to dwindle. Mount Kenya has the least vegetation in its upper parts due to its freezing temperatures.
Dispersal and establishment
Dispersal and establishment -- descending altitude, descending time
Kilimanjaro
Dendrosenecio kilimanjari
D. johnstonii
Aberdare Range
D. battiscombei
D. brassiciformis
Mount Meru (Tanzania)
D. meruensis
Mount Kenya
D. keniodendron
D. keniensis
D. battiscombei
Cherangani Hills
D. cheranganiensis
Mount Elgon
D. elgonensis
Aberdare Range
D. keniodendron
Virunga Mountains
D. erici-rosenii
Mitumba Mountains
D. erici-rosenii
Ruwenzori Mountains
D. erici-rosenii
D. adnivalis
Biogeographic interpretation of the molecular phylogeny suggests that in the most recent one million years, the first giant senecios established themselves at higher elevations of Mount Kilimanjaro and became the species D. kilimanjari . As they moved down that mountain, adapting to live in the different environment at the lower altitudes of Mount Kilimanjaro, they became a new species, D. johnstonii . Some seeds found a way to Mount Meru and established themselves as the species D. meruensis , others found a way to get from Mount Kilimanjaro to the Aberdare Range and established themselves as D. battiscombei . D. battiscombei migrated into the wet alpine habitat on the Aberdares resulted in the formation of the species D. brassiciformis . Dispersal from the Aberdares to Mount Kenya established a second isolated population of D. battiscombei. Altitudinal speciation on Mount Kenya resulted in the formation of D. keniodendron and the "dwarf" D. keniensis . Dispersal from Mount Kenya back to the Aberdares established a second insular population of D. keniodendron. Dispersal from the Aberdares to the Cherangani Hills established two subspecies of D. cheranganiensis : D. cheranganiensis subsp. cheranganiensis and altitudinal (sub)speciation into the web alpine habitat resulted in D. cheranganiensis subsp. dalei. Dispersal from the Aberdares to Mount Elgon established D. elgonensis which is a point where several subspecies diverge and disperse: from Mount Elgon to the Virunga Mountains established D. erici-rosenii ; from Mount Elgon to Mount Kahuzi (Mitumba Mountains) established a second population of D. erici-rosenii and dispersal from the Virunga Mountains to the Ruwenzori Mountains established a third population. [7]

Parallel evolution

The communities of giant Dendrosenecio and giant lobelias found on these African mountains are an exceptional example of parallel or convergent evolution and repeated convergent evolution between these two groups; providing evidence that the unusual features of these plants are an evolutionary response to a challenging habitat and an environment which can be easily described for biogeographic analysis. [7]

Cytological uniformity

Little variation was found in molecular phylogeny among the 40 recorded giant senecio collections (40 accessions), yet as a group they differ significantly from Cineraria deltoidea , the closest known relative. [5] The gametophytic chromosome number (is the number of chromosomes in each cell) for the giant Dendrosenecio is n = 50, and for the giant lobelias. Specifically Lobeliaceae, Lobelia subgenus Tupa section Rhynchopetalum it is n = 14. Only five of the 11 species of giant senecio and three of the 21 species of giant lobelia from eastern Africa remain uncounted. Although both groups are polyploid, Dendrosenecio is presumed to be decaploid (ten sets; 10x) and the Lobelia more certainly tetraploid (four sets; 4x), their adaptive radiations involved no further change in chromosome number. The cytological uniformity within each group, while providing circumstantial evidence that they descended from a single ancestor and simplifying interpretations of cladistic analyses, provides neither positive nor negative support for a possible role of polyploidy in evolving the giant-rosette growth-form. [15]

Related Research Articles

<span class="mw-page-title-main">Aberdare Range</span> Mountain range in Kenya

The Aberdare Range is a 160 km (99 mi) long mountain range of upland, north of Kenya's capital Nairobi with an average elevation of 3,500 metres (11,480 ft). It straddles across the counties of Nyandarua, Nyeri, Muranga, Kiambu and Laikipia. The mountain range is located in west central Kenya, northeast of Naivasha and Gilgil and lies just south of the Equator. The mountain range is called Nyandarua among the Agikuyu people in whose territory this forest and mountain range is located. The name Nyandarua comes from the Kikuyu word rwandarua meaning a drying hide, due to the distinctive fold of its silhouette.

<span class="mw-page-title-main">Mount Elgon National Park</span> National park in Uganda and Kenya

Mount Elgon National Park is a national park 140 kilometres (87 mi) northeast of Lake Victoria. The park covers an area of 1,279 square kilometres (494 sq mi) and is bisected by the border of Kenya and Uganda. The Ugandan part of the park covers 1,110 km2 (430 sq mi) while the Kenyan part covers 169 km2 (65 sq mi). The Kenyan part of the park was gazetted in 1968, the Ugandan part in 1992.

<i>Dendrosenecio keniodendron</i> Species of flowering plant

Dendrosenecio keniodendron or giant groundsel is a species of the genus Dendrosenecio of the large family Asteraceae and is one of the several species of giant groundsels endemic to the high altitudes of the Afrotropics, including Dendrosenecio johnstonii (Senecio battiscombei) occurring on Mount Kilimanjaro, Mount Kenya, and the Aberdare Mountains, Dendrosenecio keniensis occurring the lower alpine zone of Mount Kenya and D. keniodendron occurring in higher and drier sites on Mount Kenya. The giant rosette plants, sometimes 6 metres (20 ft) tall, often grow in even-sized stands, with different understory communities under different-aged stands.

<i>Dendrosenecio keniensis</i> Species of plant

Dendrosenecio keniensis is one of the giant groundsels endemic the higher altitudes of Mount Kenya. It is in the family Asteraceae and the genus Dendrosenecio. Dendrosenecio keniodendron occurs the upper alpine zone of Mount Kenya and D. keniensis in the wetter areas of the lower alpine or the moorlands.

<i>Dendrosenecio battiscombei</i> Species of flowering plant

Dendrosenecio battiscombei is one of the giant groundsels that lives on the slopes of Mount Kenya and the Aberdare Range. Like Dendrosenecio adnivalis on the Ruwenzori Mountains and the Virunga Mountains, Dendrosenecio battiscombei grows in the lower wetter areas of the Afro-Alpine zone.

<i>Lobelia deckenii</i> Species of plant in the family Campanulaceae

Lobelia deckenii is a species of flowering plant in the family Campanulaceae. It is a giant lobelia endemic to the mountains of Tanzania. It is listed as a threatened plant of the forests of Cherangani hills, Kenya. It grows in moist areas, such as valley bottoms and moorland, in contrast to Lobelia telekii which grows in a similar but drier habitat. These two species produce occasional hybrids. Lobelia deckenii plants usually produce multiple rosettes. Each rosette grows for several decades, produces a single large inflorescence and hundreds of thousands of seeds, then dies. Because individual plants have multiple rosettes, they survive to reproduce repeatedly, and plants with more rosettes flower more frequently. It is iteroparous.

<i>Lobelia telekii</i> Species of flowering plant

Lobelia telekii is a species of flowering plant in the family Campanulaceae, that is found only in the alpine zones of Mount Kenya, Mount Elgon, and the Aberdare Mountains of East Africa. It occurs at higher altitudes on well-drained sloped hillsides. It is a semelparous species, putting all its reproductive effort into producing single large inflorescence up to 3 metres (10 ft) tall, and then dying. Inflorescences of L. telekii also possesses a large pith-volume for internal water storage and marcescent foliage which could provide insulation. It secretes a polysaccharide into this reservoir, which may be useful for its survival in the cold climate. The plant is named after the Austro-Hungarian explorer, Count Sámuel Teleki.

<i>Afrosciadium kerstenii</i> Species of flowering plant

Afrosciadium kerstenii, synonym Peucedanum kerstenii, is a member of the carrot family, Apiaceae. It is native to east tropical Africa and the Democratic Republic of the Congo (Zaïre). It grows among the giant groundsels (Dendrosenecio) atop of the mountains of east Africa: Mount Kilimanjaro, Mount Kenya, Rwenzori Mountains and the Virunga Volcanoes.

Dendrosenecio meruensis is one of the East African giant groundsel, this one is endemic to the slopes of Mount Meru. Once they were considered to be of the genus Senecio but since then have been reclassified into their own genus Dendrosenecio.

Dendrosenecio cheranganiensis is one of the East African giant groundsel, this one endemic to the Cherangani Hills. Once it was a genus of Senecio but has recently been reclassified as a Dendrosenecio.

Dendrosenecio elgonensis is one of the giant groundsel of East Africa; this one is endemic to Mount Elgon. They used to be considered part of the genus Senecio but recently have been reclassified to their own genus, Dendrosenecio.

Dendrosenecio erici-rosenii one of the East African giant groundsel and this one can be found on the Rwenzori Mountains, Virunga Mountains and the Mitumba Mountains. It is a species of the genus Dendrosenecio and is also a collection of reclassified Senecio species.

<i>Dendrosenecio kilimanjari</i> Species of flowering plant

Dendrosenecio kilimanjari is a giant groundsel found on Mount Kilimanjaro in Africa, below 4,000 metres (13,000 ft).

Dendrosenecio johnstonii, formerly Senecio johnstonii, is a species of giant groundsel found in the middle altitudes of Mount Kilimanjaro in Africa. A recent botanical reclassification split off some species formerly in Senecio, putting the giant groundsels in the new genus Dendrosenecio. It also redefined the former species Senecio cottonii, as a subspecies of Dendrosenecio johnstonii. Both genera are in the family Asteraceae. The giant grounsels of the genus Dendrosenecio evolved, about a million years ago, from a Senecio that established itself on Mount Kilimanjaro, with those that survived adapting into Dendrosenecio kilimanjari. As it moved down the mountain, the adaptations necessary for the new environment created the new species, Dendrosenecio johnstonii. Various subspecies are found on other mountains.

<i>Dendrosenecio adnivalis</i> Species of flowering plant

Dendrosenecio adnivalis is one of the giant groundsels of the mountains of Eastern Africa. D. adnivalis grows on the Rwenzori Mountains and on the Virunga Mountains in Uganda and the Democratic Republic of Congo.

Dendrosenecio brassiciformis is one of the East African giant groundsel, this one is endemic to the slopes of Aberdare Range and bearing fruit but once, and dying after. Once considered to be of the genus Senecio but since have been reclassified into their own genus Dendrosenecio.

<span class="mw-page-title-main">Natural history of Mount Kenya</span>

The flora and fauna of Mount Kenya are diverse, due to the variation in altitude, rainfall, aspect and temperature. The mountain slopes can be divided into vegetation zones, with each zone having different dominant plant species. Although many plants on Mount Kenya have local names, here they are reported only with their English and scientific names.

<span class="mw-page-title-main">East African montane moorlands</span>

The East African montane moorlands is a montane grasslands and shrublands ecoregion which occupies several high mountain peaks in Kenya, South Sudan, Tanzania, and Uganda.

<span class="mw-page-title-main">Rwenzori–Virunga montane moorlands</span> Montane ecoregion in central Africa

The Ruwenzori-Virunga montane moorlands is a montane grasslands and shrublands ecoregion of the Rwenzori Mountains and Virunga Mountains in central Africa.

<i>Carex monostachya</i> Species of sedge

Carex monostachya is a species of sedge native to the mountains of East Africa.

References

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  3. Nordenstam, Rune Bertil. 1978. Opera Botanica 44: 40
  4. Tropicos, Dendrosenecio B. Nord.
  5. 1 2 3 4 5 Knox, ric B.; Jeffrey D. Palmer (October 24, 1995). "Chloroplast DNA variation and the recent radiation of the giant senecios (Asteraceae) on the tall mountains of eastern Africa". Proceedings of the National Academy of Sciences. National Academy of Sciences. 92 (22): 10349–10354. Bibcode:1995PNAS...9210349K. doi: 10.1073/pnas.92.22.10349 . PMC   40794 . PMID   7479782.
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  7. 1 2 3 4 5 6 7 8 9 10 11 Knox, Eric B. (2004). "Adaptive radiation of African montane plants". In Ulf Dieckmann; Michael Doebeli; Diethard Tautz; Johan A. J. Metz (eds.). Adaptive Speciation. Cambridge University Press. p. 476. ISBN   0-521-82842-2 . Retrieved 2008-03-29.
  8. 1 2 Bussmann, Fainer W. (June 2006). "Vegetation zonation and nomenclature of African Mountains - An overview". Lyonia. Retrieved 2008-04-27.
  9. 1 2 Weischet, Wolfgang; Endlicher, Wilfried (2000). Regionale Klimatologie Teil 2 Die Alte Welt: Europa - Afrika - Asien (Regional climatology, Part 2: The old world: Europe - Africa - Asia). p. 625. ISBN   978-3-443-07119-6.
  10. Hedberg, Olov (1955). "Vegetation belts of the East-African mountains". Proceedings of the Linnean Society of London (Botany). 165: 134–136. doi:10.1111/j.1095-8312.1955.tb00730.x.
  11. Hedberg, Karl Olov (1964). "Features of afroalpine plant ecology". Acta Phytogeographica Suecica. 49: 1–144. ISBN   91-7210-049-4 . Retrieved 2008-05-04.
  12. "Africa Ultra-Prominences: 84 Mountains with prominence of 1,500m (4,921 ft) or greater". Peaklist. Retrieved 2008-05-05.
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  14. Hedberg, Olov (1951). "Vegetation belts of the East-African mountains". Svensk Botanisk Tidskrift. 45: 141–196.
  15. Knox, Eric B.; Robert R. Kowal (July 1993). "Chromosome Numbers of the East African Giant Senecios and Giant Lobelias and their Evolutionary Significance" (PDF). American Journal of Botany . Botanical Society of America. 80 (7): 847–853. doi:10.2307/2445604. hdl: 2027.42/141794 . JSTOR   2445604.

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