A Forest with Latitude, diversity in the Garden Route

It is recorded that Amazonian Indians have the capability to distinguish between 64 shades of green, a necessary ability when the ambient tone of their environment is made up by this colour. By comparison, the average westerner can distinguish a mere 16 shades of green. Living in this gem of the Garden Route we have the luxury of no less than 139 indigenous species of trees gracing our forests and coastal vegetation. This is a staggering 130 more trees that there are in the entire country of Canada, a country that can boast a mere 9 species of trees, three of which are deciduous while the rest are conifers. But why this disparity in the number of trees between a region and a country? The basic answer is : Disease and it’s dispersal across the range of latitudes. In the artic and temperate climates in higher latitudes, the severe cold of winter provides a sanitizing action which kills off terminal pathogens on an annual cycle during the harsh freezing conditions. Without pathogen populations building up, a species of tree can grow in dense populations that are free of the threat of an epidemic destroying them. By contrast, in the sub tropics and the tropics, pathogens are able to survive winter and build up to epidemic proportions. In the event that trees where to grow in dense stands of mono specific populations, a diseased tree could infect the entire population, killing off a large section of the forest. The average spacing for a healthy population of a species of trees is approximately 500m between individuals. This means that for two mature trees of the same species to be separated by this distance, there is plenty of space between them for other tree species, all with the same spatial requirements. Spaced at 5m between mature trees of different species, this would calculate to a potential of 100 species of trees to maintain a healthy forest. With this requirement of speciation, the fun begins. Each species now has to compete for resources, water and sunlight, and space, yet all the time not expanding beyond the sanitary boundary of 500m. Within this context, the variation of life stories erupts. Up the Touw River there are three sections along the Giant Kingfisher Trail where Yellowwoods, both the Outeniqua and the Real Yellowwoods, grow in lines, a memory of an ancient past event. The youngest line is between 40 and 65years old, one that I would love to witness in about 500 years time when they are in their prime. Yet it was not till the autumn of 2009 that I figured out what had caused the trees to grow in a line, the precision of which hinted at either primate or human intervention. But it was simpler – this was the high water mark of various past floods that deposited seeds in a line to germinate, a natural memorial of a series of floods spanning more than 300 years. Indeed, it is on the oldest of these tree lines that another story unfolds. Entwined around members of the oldest line, which I estimate to be between 270 and 310 years old, are a number of figs which will in the future strangle and kill the Yellowwoods. Penetrating the Podocarpus armoury of auto peeling bark to dispel all epiphytes that could introduce a pathogen, five or more trees have been invaded by one of the most intriguing stories in the floral kingdom, namely one of the strangling figs. Fig trees have about 1600 species distributed around the world, each of them requiring a specific wasp to fertilize their flowers. The specific female wasp will enter the fig to lay eggs and while she does so, she carries pollen amongst flowers in the fig and pollinates them. When she exits the fig and enters a second fig, again she pollinates the flowers inside while she is laying eggs. But it does not stop there. If the wasps lay too many eggs, the wasp population increases and the larva destroy the figs and reduce the number of seeds available for dispersal. This is where the tree steps in – as the population of wasps increases, it raises the temperature of the fruit by up to 6º C. Inside the fig is an endemic population of intestinal nematodes, host specific for the wasp larva. As the fig temperature increases, the nematode population increases and destroys the wasp population. This results in reducing the number of fruit destroyed and subsequently increasing the amount of seed available for dispersal. But, the tree can’t survive without the wasps to pollinate the fruit, so as the wasp population declines, the tree lowers the fruit temperature and reduces the nematode population. Again the wasp population increases. The final step for the tree is to disperse it’s seeds. Here the tree employs birds and bats. While we easily see birds, it is infact the bats that are the primary seed dispersal agents of figs. Bats, unlike most birds that feed from fig trees, pick the fruit and fly to a safe location to eat. To ensure that their seed is carried off, the tree produces the first crop of fruit that is 30% larger than the rest, thereby enticing the bats to pick their fruit and carry it away to a new location. For these and other examples of the diversity in our forests visit www.gardenroutetrail.co.za for information on our selection of guided walks.

 

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