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.
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.
