Sky Islands of East Africa
“Sky Island” is an evocative term for an element of geography, I have only recently been introduced to. But from my hikes in East Africa, I have been familiar with the idea for twenty years.
The concept refers to the concentration of up to five climate zones on a single isolated mountain, straddling the equator. It is possible to walk across a zone, or more accurately up or down, in from a couple to half a dozen hours.
Such dramatically different climates are usually the product of latitude and are hundreds or thousands of kilometres wide. In “sky islands” the separation is in 100s of metres of altitude.
On the two highest mountains of East Africa, Mt. Kilimanjaro and Mt. Kenya, one can walk from the “tropics” to the “Arctic”, through five zones, in less than 20 hours. This has a symbolic allure for hikers and mountaineers. There are three other East Africa mountains within a radius of about 400 km. where there are the four lower zones.
Both Kilimanjaro and Mt. Kenya are world heritage sites mainly because of the huge diversity of fauna in such a concentrated area. Because of the isolation, many of the plants may exist here and nowhere else, other suitable environments being too far away for the plants to migrate.
One of the most dramatic plant examples is the Gladiolus Watsonioides or MacKinder’s Gladiolus (sometimes “lilly”). Although apparently on Kilimanjaro, I have only seen it in two places, both on Mt. Kenya at approximately 3,900 m. altitude. This puts them at the margin of the third zone, moorland and heather and the fourth zone, alpine or high desert.
Especially remarkable about this plant is how tall and succulent it is. While there are some other flowers most live close to the ground or are quite dry. In addition, the large bright red blooms make it singularly spectacular.
Daily it experiences temperature swings from slightly below zero to as high as the mid-teens C.
There is an outcrop on the northwest side of Mt. Kenya where, so far, I have been able to count on finding it.
The most dominant and other world plants at the top of this zone are Giant Groundsels and Giant Robellia.
The lowest zone of a sky island is tropical lowlands at the base of the mountain from 800 to about 1800 m. in East Africa. There may be huge trees and if on the dry side of the mountain are often coniferous and on the wet side, deciduous.
Next on the way up the mountain is the tropical rainforest (1800 to 2800 m.) with a wide variety of plants and animals. The upper reaches of it may be bamboo forest. This area hosts the richest variety of plants.
The three most striking flowers I have seen in this zone are the Pjama Lilly, Gloriosa Lilly and the ‘Fireball Lilly’ or ‘African Blood’.
The trees here are smaller than lower down and are mostly gone above 3,000 metres altitude in the moorland and heather zone where robellial and groundsels are the most striking.
Above 4,000 m. grass, thistles and some close-to-the-ground flowers persist. On Mt. Kenya groundsels live comparatively high, but are usually shorter with more altitude.
No plants grow in the Arctic zone, even on the equator. It is characterized by intense sunlight, low levels of oxygen and sub-zero temperatures continually, out of the sun.
Local geography can effect local climate, especially rainfall, hence the extent of various plants within the altitude range.
I am only aware of the sky island phenomenon in East Africa, but I could imagine it near the equator in the Andes.
