The Rare Nature Of Tropical Rainforests
Understanding the unique features of equatorial forests to grasp their global significance
[Photo: Courtesy L. Lee Grismer]
My tree education started early on while growing up in the Great Plains of America among the conifers - white spruce and Ponderosa pine - of the Black Hills of South Dakota. In the second grade, as the class representative, I helped plant a spruce sapling near the school entrance. After moving southward to Missouri and Texas, I often camped out among a variety of central hardwoods, such as Hickory-Oak and Elm-Ash-Cottonwood forests.
On graduation day from forestry school in the Loblolly-Shortleaf pine forests of East Texas, I affixed a pine cone, saved from the Black Hills, on my cap tassel. This represented my personal tree growth from elementary school to college. All of these typical temperate forest associations were the precursor to something much grandeur in my future.
Temperate forests are notable for their grouping of dominant species: Beech-Birch-Maple, Hemlock-Sitka Spruce or Pine forests. The key point is that these tree communities are more simple in structure (overstory and understory) and contain far less tree species diversity than their tropical forest counterparts.
In contrast, equatorial rainforests are otherworldly. They display a chaotic assembly of plant life that overwhelms sensory input. Rainforests surround your visual space and encroach upon your sense of structure to distort reality. There are no dominant tree groupings upon which to get a grip; there’s just a lot of singular species obfuscation.
The tropical rainforests of the Amazon and Southeast Asia have always attracted adventurers and researchers, especially those from temperate climes, to try and explain this grand difference.
Everyone knows Charles Darwin, but it is the co-founder of the theory of natural selection, Alfred R. Wallace, that provides a better description of species distribution. Taken from his Tropical Nature And Other Essays, published in 1878:
If the traveller notices a particular species and wishes to find more like it, he may often turn his eyes in vain in every direction. Trees of varied forms, dimensions and colours are all around him, but he rarely sees any of them repeated. Time after time he goes to a tree which looks like the one he seeks, but a closer examination shows it to be distinct. He may at length, perhaps, meet with a second specimen a half a mile off, or he may fail altogether, till on another occasion he stumbles on one by accident.
The old Victorian naturalists had a way with words - usually too damn many to sort through. Let’s update the description with my favorite quote about the rainforest.
As biologists, Adrian Forsyth and Ken Miyata spent years in the tropical forests of Central and South America making astute observations. In Tropical Nature, published in 1984, they broke down the complexity of describing the key feature of a rainforest:
It is one of the ironies of tropical rain forest that common species are rare and rare species are common.
Brevity is clarity. This single sentence shines a bright light on the rare nature of rainforest ecology. It makes sense out of everything written by Wallace or Darwin.
Now that you understand the simple elegance of rainforest plant diversity, let’s throw some hard statistics into the equation. In the 1980s in Malaysia, scientists studied the floristic composition of a 50-hectare (equivalent to 123 acres or 70 soccer fields) plot located in the lowland ecosystem of Pasoh Forest Reserve. According to the researchers, this single plot area comprised of a total of 338,924 individual trees and 818 different tree species, representing 295 genera and 81 families.
Taking the diversity of tropical forests as compared to temperate zone forests, try to envision the difference and complexities of forest management practices (clearcutting vs. selective cutting). In terms of biological diversity, envision the loss of species due to deforestation, loss of nature reserves and habitat intrusion. And that’s without considering animal species.
Wallace collected species in the field and sold them to European buyers to support his work. This is one account of his attempt to summarize the main features of tropical vegetation:
The primeval forests of the equatorial zone are grand and overwhelming by their vastness, and by the display of a force of development and vigour of growth rarely or never witnessed in temperate climates. Among their best distinguishing features are the variety of forms and species which everywhere meet and grow side by side, and the extent to which parasites, epiphytes, and creepers fill up every available station with peculiar modes of life.
Competition for sunlight, water and nutrients is fierce in the jungle. This makes it more difficult for common species to survive and allows for rare species to thrive. Here’s how Wallace interpreted the dilemma:
… the never-ceasing struggle for existence between the various species in the same area has resulted in a nice balance of organic forces, which gives the advantage, now to one, now to another, species, and prevents any one type of vegetation from monopolising territory to the exclusion of the rest.
Temperate forests exude grandeur on a linear plane, over a landscape terrain. It reveals the strength and beauty of dominant species communities by stretching them out far and wide. In the rainforest, the linear plane is vertical. The horizontal plane is too broken by species diversity to expose its overall grandeur. Every species is trying to reach or capture the sun or find a separate niche for survival.
This vertical structure gives rise to the prevalence of rainforest descriptive terms such as “cathedrals” and “temples” and the exalted sense of majesty and magnitude. During his long voyage of discovery, Charles Darwin encountered equatorial forests and gave high praise:
Among the scenes which are deeply impressed on my mind, none exceed in sublimity the primeval [tropical] forests, ... temples filled with the varied productions of the God of Nature. No one can stand in these solitudes unmoved, and not feel that there is more in man than the mere breath of his body.
As humans, we’d be better off to understand and apply the dynamics of diversity within the rainforest as a means for species survival and well being. And realize that our grandeur lies in being rare too.
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- Rick Scobi (@rickscobi)