Spatial variation, Ithilien and the Old Forest

A strength of Tolkien was that he put so much effort into creating an amazingly detailed secondary world. He did this by adding as much realism, often ecological, as he could to the mix. Despite the presence of dragons, trolls and hobbits there is an overwhelming feeling that Middle-earth is a place that we could walk about and feel like we knew. (Indeed, living in New Zealand I actually can walk around parts of Middle-earth and feel at home!).

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“Looking ahead they could see only tree-trunks of innumerable sizes and shapes”. Spatial heterogeneity in the Old Forest.

One example of Tolkien’s aim to make things as familiar as possible was to make the environments that he described as realistic as possible.  Take this example from the hobbits tramping through the Old Forest, just outside the borders of The Shire.

Looking ahead they could see only tree-trunks of innumerable sizes and shapes: straight or bent, twisted leaning, squat or slender, smooth or gnarled and branched; and all the stems were green or grey with moss and slimy, shaggy growths.”

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“as they went forward it seemed that the trees became taller”

We get a sense of the sheer variety of the forms of life that we would encounter if we walked in such a wood. Not only do we imagine the variation in species present but we picture how they differ as we look around. We are not in a plantation where the same few species radiate out in all directions around us. Species also differ spatially. Further on Tolkien writes;

to the left of their path the land seemed to be drier and more open, climbing up to slopes where the trees were thinner, and pines and firs replaced the oaks and ashes and other strange and nameless species of the denser wood.

and;

The ground was rising steadily, and as they went forward it seemed that the trees became taller, darker and thicker.

Tolkien paints a compelling image for the reader because he acknowledges that things change, even over small spatial scales. Walk ten paces and you will be next to a different type of tree of bush. It matters if you are moving down-hill, away from water or onto different soils. Tolkien emphasises this in many of his descriptions of habitats. For example, when Sam, Frodo and Gollum reach Ithilien;

A little way back from the lake they found a deep brown bed of last year’s fern. Beyond it was a thicket of dark-leaved bay-trees climbing up a steep bank that was crowned with old cedars.

As ecologists we are alway interested in variation and particularly in spatial variation. Surprisingly, we often do not incorporate spatial variation into our models when we seek to explain ecological processes. Often spatial variation is too complicated to deal with. We also have issues with how we go about measuring spatial variation, especially when comparing the distribution of, say, two species, and most methods have serious problems (usually because they assume no spatial variation).

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“Rising like a bald head” Tree-line at Isengard (or Paradise, near Glenorchy, as we Kiwi know it as).

Clearly, we know that spatial variation is important. We seldom go more than a few steps in a forest without encountering it. At a larger scale, and common in New Zealand, are the changes that we find as we approach and move through the tree-line, a phenomenon that Tolkien was also familiar with;

The path stopped climbing, and became for a while nearly level. The dark trees drew aside, and ahead they could see the path going almost straight forward. Before them, but some distance off, there stood a green hill-top, treeless, rising like a bald head out of the encircling wood.

If spatial variation is so important then what can we do to measure it? Hannah Buckley and Brad Case (Lincoln University) and several colleagues have produced, perhaps appropriately, a trilogy of papers that discuss a method that can be used to directly measure spatial complexity. This method is called codispersion analysis (codispersion is a fancy way of saying that different species and/or traits are found in the same place at the same time). Codispersion analysis can visualise and measure the co-occurrance of different species.

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The hobbits realized that the world is not a uniform environment everywhere you step.

In papers in Ecology, New Phytologist and Chance, Buckley and Case outline how codispersion data is collected and analysed and provide testing through simulations and examples. They looked at spatial variation in a woodland near Harvard (USA), near St Louis (USA), in Puerto Rico, the Pyrenees and on the Tibetan Plateau. In all cases they were able to detect significant signals generated from gradients within the sample sites. They were also able to measure other features of the system to examine what was driving these distributions.

Using this new approach as a tool we can begin to appreciate the science behind the distribution of species in habitats, much as those described by Tolkien, once again in Ithilien;

their day’s march drew on and afternoon waned, the forest opened out, and the trees became larger and more scattered. Great ilexes of huge girth stood dark and solemn wide glades with here and there among them hoary ash-trees, and giant oaks just putting out their brown-green buds. Among them lay long launds of green grass dappled with celandine and anemones, white and blue

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