A bog ecotone

Go exploring in the higher latitudes of North America and you may find yourself in a forest that look like this one below. Small hardwood trees and an understorey dominated by royal ferns (Osmunda regalis). This ancient-looking scene hides a more interesting story. Here the ground is swampy and often spotted with small pools of water. Dig down and you will reach bedrock after a small depth – this place is a recent formation, with its soil forming after the retreat of glaciers after the last ice age at approximately ten thousand years ago.

But this fen is not the only thing to find here – walk a short distance further and the terrain changes. A drop in elevation of no more than a single meter, guiding the runoff and subsurface water into a wide expanse. This area is also shaped by the retreat of glaciers, but the pooling of water and different soil creates a sharp shift in the vegetation. A drop of one meter and the forest abruptly becomes a bog. Bogs have low pH, meaning acidic soils. Sphagnum moss grows well under these conditions, so the bog is dominated by a spongy carpet of these plants. As the plants die, their biomass is retained and preserved in the bog and eventually becomes a thick (tens of meters) layer of peat. This is an incredibly fast transition between a forest and bog landscape. These transitions are generally called ecotones. A few tree species make it into the bog (you can see a few stunted conifers) but they are very different species than can survive in the forest. Climate niches shift across an ecotone, so often the most interesting biology and highest diversity of species is found on the ecotone itself.

One species that does well in the sphagnum-dominated bog is Sarracenia purpurea, the pitcher plant. The low-nutrient conditions of the soil have resulted in the evolution of a species that can obtain mineral nutrients from insects, just as we see in my blog post on alpine fens.

A bog is a glorious place to explore, but a delicate one. Stepping on the soil easily crushes many delicate plants and compresses the sphagnum, limiting its role as an ecosystem engineerThis bog (in Orono, Maine) has a boardwalk built across parts of it to allow many people to safely enjoy its strange beauty.

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