15 Landscape Structure and Composition

In art, they are paintings of places, usually from a distance, a mosaic of forms interwoven to portray a recognizable place familiar and pleasing to the eye. That is the function of the painting. Ecological landscapes also can be viewed as a mosaic. They have structure, as portrayed by the pattern of the patches comprising the landscape. They have composition, as portrayed by the types of patches on the landscape. They have function, as defined by the resources of concern—in our case, patches of habitat for a species or collection of species. Forested landscapes, as habitat for vertebrates, are patterns of habitat patches distributed over forest-dominated areas, with each patch representing a set of resources used or needed by the species. The pattern of habitat patches across a landscape differs from one species to another. We, as humans, tend to view forested landscapes as collections of forest stands, a mosaic of units of vegetation that is relatively uniform within stands, but heterogeneous among them. We do that because, to us, those stands represent different resources, such as economic value, places to hike, or different aesthetics. For some other vertebrates, the stands that we see also represent patches of different values, habitat values. But not for all species. Take the shrubassociated Swainson’s thrush that is widely distributed across North America. What type of mosaic of patches would a Swainson’s thrush see? The thrushes’ search image (what they seek) for a territory, likely includes patches of shrubs with varying quality for foraging on insects in the leaf litter and foliage, as well as nest sites low in the shrubs. The overstory vegetation seems to have little, if any, influence on what Swainson’s thrushes see as habitat patches, except through the effect it has on shrub development. Old-growth, stand establishment, or understory reinitiation conditions are not really as important to this species as the shrub cover present in the stand. If we were to develop a very simple map of habitat with white patches being habitat and black being unusable, then we would map patches of shrubs of a certain cover and height for Swainson’s thrushes, and these patches might be quite unrelated to tree species’ composition or structure.

Mapping habitat for northern flying squirrels would focus more on snags, tree species, size and canopy cover, and not so much on shrubs. And, to make things even more complicated, consider habitat for cottontail rabbits that feed in grassy areas but use dense shrubs as cover. They live on the edge between these two types of vegetation, and the acceptable habitat (white) on our map would be a linear strip extending perhaps 30 m into the grassy area and 20 m into the shrubs. Everything else would be black. Three species, three landscape patterns, all in the same forest. Managing stands in this forest could influence the patterns of habitat for each of these species, depending on the types of vegetation and other features provided in each stand.

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