Giant Senecio Diversification in East African Tropical Mountains

For June I will be going through an article that further follows my love for mountains and their amazing ecology and vegetation.  In Equatorial Africa, there is a group of particularly unusual plants that grow within an altitude band from 2500 to 4600m on 10 mountains in 5 countries, the giant senecios (known as the genus Dendrosenecio).  These 5 countries are the Democratic Republic of Congo, Rwanda, Uganda, Kenya and Tanzania and are all homes to mountains that are taller than 3300m and fall within 4˚ of the equator.

Figure 1. Mount Kilimanjaro in the foreground and Mount Meru in the background.

What makes these senecios remarkable is their extraordinary size, with some growing up to 6m in height.  What makes this truly amazing is that their closest, non-montane relatives rarely grow larger than a metre tall.  The Dendrosenecio’s size is not their only amazing characteristic.  They have a host of morphological features that allow them to successfully inhabit their extreme habitat.  So, besides their giant rosette growth form, they have a large pith volume, marcescent foliage and nyctinasty. 

Figure 2.  The relative size of the giant senecios.

To translate: the large pith volume allows them to store large amounts of water; marcescent foliage means that they retain withered or dead foliage for insulation; and nyctinasty is the response of leaves to cold, meaning they close once temperatures are too cold for photosynthetic activity.  Furthermore, members of Dendrosenecio secrete polysaccharide fluids which act as a natural anti-freeze.  These features have evolved independently in the montane giant lobelias (also African) and the Andean genus Espeletia, which invokes that these features are adaptations to high altitude tropical environments.

All of these morphological features paint the picture of an extreme environment, which has been described by Karl Hedberg, a Swedish botanist, as “summer every day, winter every night”.  As all of these mountains are located within 1000km of the equator, the environmental fluctuations occur daily instead of seasonally, with every day being warm and every evening being cold with nightly frost.

Figure 3. A population of giant senecios on the third highest and most ascended peak of Mt. Kenya, Lenana.

The article, titled “Chloroplast DNA variation and the recent radiation of the giant senecios (Asteraceae) on the tall mountains of eastern Africa” (Knox & Palmer, 1995. PNAS Vol. 92. pp. 10349-10353) challenged the common dogma that the ancestors of the Dendrosenecio lived at low altitudes allowing them to migrate in the lowlands of the region and that the current distribution reflects parallel evolution on each mountain.  The authors joke that this train of thought is very anthropo-centric because humans are obliged to live at lower altitudes and climb mountains one at a time and therefore expect plants to do the same.  They invoke that long-distance dispersal is more likely than migration and that it is possible for colonists to have started near the top and evolved their way down a mountain.  To explore this hypothesis, they look at the family relationships (phylogeny) between 11 species within the Dendrosenecio by analysing chloroplast DNA. 

The authors found 60 mutations among the 11 species that were informative regarding the relationships between the species.  Their results indicate that the giant senecios are an isolated lineage relative to the greater Senecioneae clade, with few molecular similarities to their closest relatives.  Furthermore, they believe that it could be possible that other high-altitude tropical Senecioneae around the world may be more closely related.  They go on to compare the Dendrosenecio with other giant-rosette plants in the Asteraceae (the larger group that Senecioneae falls within) and found little difference and found that across the world, a consistent feature of these plants was the radical morphological divergence from their closest known relatives. 

The heart of this article is the discussion on the radiation of the Dendrosenecio.  Their results indicate that Dendrosenecio originated in Tanzania, in eastern Africa, at high-altitude on Mt. Kilimanjaro and that the subspecies D. kilimanjari subsp. kilimanjari and D. kilimanjari subsp. johnstonii evolved via a downward altitudinal radiation, with a long distance dispersal event giving rise to D. meruensis on Mt. Meru.  The recent ages of the mountains that the giant senecios inhabit indicate that the radiation of Dendrosenecio occurred within the last one million years.  The non-Tanzanian giant senecios can then be sub-divided into 3 clades: a Mt. Kenya/Aberdares sub-clade, a Western Rift Zone/Mt. Elgon sub-clade and the Cherangani Hills sub-clade. 

Figure 4. The phylogeny of Dendrosenecio species and their geographical locations.

In general, the molecular data indicates that the diversification of the giant senecios involved repeated altitudinal radiation – meaning once a mountain had been colonised, species would diversify within the altitudinal ranges available to them, moving mostly down, but also up the mountain slopes.  Importantly, there was no evidence found to suggest that they had colonised mountains from a lowland ancestor, further invoking the theory of many long-distance dispersal events.

The biogeographic component shows that, as one would expect, species on neighbouring mountains are more closely related than species on farther off mountains, with just one exception.  Interestingly, there was no evidence found of recent establishment of one species that another species currently inhabited, indicating that habitats need to be unoccupied for successful colonisation.

This article outlines the basic trends of diversification of the remarkable genus Dendrosenecio and how they came to populate the mountain tops of east Africa’s tropical peaks.  I truly hope that one day I’ll be able to visit these amazing plants.  I just hope that it’s on an all-expenses paid working field trip.