In the last post we discussed the four main drivers that determine the structure and composition of savannas such as the Kruger National Park. All savanna landscapes on the African continent can broadly be classified into two groups, irrespective of the interactions of the four drivers: Broad-leaved savannas
and Fine-leaved savannas
You will remember from the previous post that climate (and more specifically rainfall) has a major influence on savanna areas. This is especially true for areas at the extreme ends of the rainfall gradient. Very dry areas (close to 350 mm per annum) tend to develop into fine-leaved savannas. Similarly, wet savannas (above 700 mm per annum) normally fall within the broad-leaved group. The reason for this is the different physiological adaptations and ancestral origins of fine-leaved and broad-leaved trees.
trees probably originated in hot and dry climates
. Small leaflets may have evolved to prevent excess water loss through transpiration. Due to sufficient access to the sun in their dry habitats, they also did not need large leaves for sufficient photosynthesis. You will also find that many fine-leaved trees have thorns
for defense against herbivores. This has to do with the fact that drier environments are often associated with nutrient-rich soils.
(The reasons for this will be discussed in more detail in future posts).
In contrast, broad-leaved
trees probably evolved in wetter environments
where sufficient available water did not make it necessary to decrease leaf size significantly. Wet environments normally go hand-in-hand with lots of cloudy days. Logically, a larger leaf size would also provide a tree with a larger surface area for sufficient photosynthesis. Broad-leaved trees normally rely on chemical defenses
against herbivores. Once again, this type of defense is linked to the nutrient-poor soils
normally found in moist areas.
Please keep in mind that abovementioned description of the origins of broad- and fine-leaved savanna trees are over-simplified, as there are never just a few simple explanations for the way things work in nature. But let us not get bogged down in too much technical detail and rather continue with the topic.
Most of Kruger falls in a rainfall zone which lies in-between the dry and wet extremes mentioned above. Therefore the soil properties take on a more important role in determining the savanna group that will develop in an area. At average rainfall levels, a landscape can develop into either broad-leaved or fine-leaved savanna depending on the underlying soil characteristics. Savannas in Kruger are therefore divided into nutrient-rich
and nutrient-poor savannas
In general, nutrient-rich areas consist of clay soil made up of very small particles which tend to prevent water from filtering through to underlying soil levels. The deeper soil conditions are therefore similar to arid environments due to little moisture availability. Logically, clay soils therefore tend to produce fine-leaved trees.
Fine-leaved savanna near Satara, KNP
Nutrient-poor areas consist of sandy or shallow rocky soils with courser particles which allow water to filter through to deeper levels, resulting in higher moisture levels. The deeper soil conditions are therefore similar to the wet savannas and therefore produce more broad-leaved trees.
Broad-leaved savanna at Jones' Dam, KNP
Once again, this is an over-simplified description of the split between fine-leaved and broad-leaved landscapes in Kruger, but you get the idea
Of course, in nature there are always exceptions to the rule. Just when you think you know exactly which plants will grow where, along comes the Mopane to challenge your logic. In later posts we will discuss why the broad-leaved Mopane dominates areas where you should expect to find fine-leaved trees…
A last important concept worth mentioning in this post is the Catena
. All landscapes undergo weathering through the passage of time but in some landscapes the effect of the weathering is more pronounced. In areas underlain by very old rocks such as granite (like the western portion of Kruger) you find an undulating landscape with endless crests, slopes and valleys. The working of wind, rain and gravity over millions of years has resulted in rock minerals moving and leaching from the crests, down the slopes and into the valley bottoms. Therefore you normally find sandy, nutrient poor soils on crests and clayey, richer soils in the valleys. You guessed it: the result is broad-leaved trees
on the crests
and fine-leaved trees
in the valleys
. This sequence from crest, down the slopes to the valley bottom is called a Catena. All Kruger landscapes and indeed all savannas, exhibit the catena sequence. However, as mentioned earlier, the catenas in areas underlain by granites are more pronounced and more clearly visible.
Diagram of Catena in Kruger
The different ecological implications and workings of broad-leaved and fine-leaved savannas makes for very interesting reading. Therefore I will focus more on this when discussing specific Kruger landscapes in upcoming posts.
Your homework: Try to identify Catenas during your drives in Kruger. There are many good examples in the Park, for instance the H1-2 from Skukuza towards Tshokwane (the first section of road before the S36 turn-off). References:
Eckhardt, Holger & Scholes, Robert & Venter, Freek. The Kruger Experience: Ecology and Management of savanna heterogeneity, Chapter 5: The abiotic template and its associated vegetation pattern.
Washington DC-USA: Island Press, 2003. Print.
Hendry, O. Kruger Ecozone Map (drawing of Catena).
Johannesburg: Jacana Media, 2004. Print
Scholes, Robert and Walker, B.H. An African Savanna: Synthesis of the Nylsvley study.
. Cambridge-UK: Cambridge University Press, 2004. Print.