Tuesday 27 May 2014

Parasites, Epiphytes and Symbiosis - Costa Rican Adventure Part 2

Before I went there I assumed that rainforest was rainforest; you know, big trees, dripping water, humidity, biting insects and wary animals. But rainforests are not just big forests. Lets start with the basics: 'primary' and 'secondary' rainforest is the most important distinction, and one that is vital to understanding why the destruction of the rain forests is so devastating to the global environment. Primary rainforest is 'real' rainforest, that's always been there, with the depth of complexity and biodiversity that distinguishes it from so many other natural environments. Secondary rainforest is forest that is being regrown after the land has previously been cleared. The Reserve at La Leona is a small area of primary forest. Much of the Corcovado National Park is primary forest, with some pockets of secondary forest, areas which was forcibly reclaimed by the government from local people who farmed there. These areas have been regrowing for about forty years now. The problem is that we have no idea how long it might take for secondary forest to return to the level of complexity of primary forest, if ever. Some scientists think that once the forest has been cut down the biodiversity that you have lost is gone forever.

Imagine what happens in this country when a piece of land is allowed to go wild, it will become quickly overrun with a few very dominant species; nettles, brambles, dandelions etc. The biodiversity that exists in the rainforest can only come about because of a very delicate balance between the strong and the weaker species, with different kinds of relationships existing between plants, and between plants and animals. Secondary forest is much more vulnerable because the subtleties take such a long time to emerge. This little tree stump shows, even to the untrained eye, the immense diversity of plants that will colonise every available inch of forest, but no one thing is able to dominate. Where Britain has a mere 32 native tree species (and the wiki page also states "There is no woodland in Britain that has not been profoundly affected by human intervention"), a square kilometre of rainforest may have in excess of 300 species

A sign on one of the walkways in the Selvatura Park pointed out that an ordinary medium size tree will support about 70 other plant species living on it. Mostly these plants are epiphytes, taking their water from the rain, or sending down roots as many of the long dangling vines do, and merely using the tree as a platform, not damaging it in any way.
This photo shows a strangler fig. It is a parasitic plant. It begins life high in the canopy as a little shoot, gradually sending out roots down the trunk of its host tree. Over the course of about twenty years it will gradually wrap the host tree, depriving it of water and nutrients from the soil and eventually killing it. This benefits the forest because it returns the tree's nutrients to the ground and provides a living environment for many insects and the hollow left by the decayed tree provides a home for many animals. But how good would it be if every tree in the forest was throttled by a strangler fig? This does not happen precisely because of the balance: the seed has to land in the right kind of spot, the shoot has to be not eaten by animals or insects, or not pulled out by passing monkeys. No humans have to go in and chop off the burgeoning strangler figs to protect the trees, nature does this all by herself.

This next photo is an acacia tree. It thrives in a symbiotic relationship with a species of ant which lives in its hollow thorns and feeds on the tree. In return the ants provide protection against herbivores and keep the ground around the tree clear of other plants that would compete for water and nutrients.

Intimate relationships like this exist between species and then the whole forest also exists in a cycle of growth and death. When a tree dies and falls it creates a light hole within the forest allowing a whole new layer of growth to happen in the understory. I noticed in several places along sloping areas and by streams that trees looked as if they were falling over, their huge roots hanging on like grim death, but Jim our guide said no, they grew like that. Light is at a premium in the dense forest so trees will grow in the direction of light however apparently precarious the position. 

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