Natural Communities & Habitat

Wetland Woodland Wildland (available for here) by Liz Thompson, Eric Sorenson, and Bob Zaino, is the definitive guide to Vermont’s natural communities. I highly encourage you to buy the book and start leafing through its pages to familiarize yourself with the content. The authors define a natural community as “an interacting assemblage of organisms, their physical environment, and the natural processes that affect them.” Now you can debate all day and night about whether or not this is a valid framework for understanding landscapes, but philosophical arguments aside, this is one of the most useful tools for makings sense of the patterns of where various trees and shrubs species tend to congregate in Vermont hills and valleys. Plus the state and conservationists use the concept to assess ecological value of various habitats (Natural Community Inventory). ​​

Fundamental to the natural community concept is that plants overlap in what their limiting factors to survival are. That is, a plant might be limited in its ability to germinate and successfully reproduce by how well drained the soil is, the mean average temperature, the type of bedrock, available sunlight, the presence of repeated fire, etc. And not all factors are equally limiting; thus a white pine might prefer sandier areas to silty ones, but they can and do grow in less well drained soils in areas that were previously grazed by cattle. Since most plants can grow in a larger range of habitats than they actually do – see UVM’s campus map of trees for a demonstration of this – it is important to know that the focus is on their ability to successfully compete with other species. And again, it is the combination of a plant’s limiting factors that determine the range of places where they can and can’t compete with other plants, and it is the pattern of certain organisms to grow and live in overlapping habitats time and again that we call a natural community.

Shared limiting factors leads to overlapping distributions
We can expand on the original definition to state that a community of is the result of similar adaptations of different species of plants to deal with both different abiotic factors (soil, hydrology, climate, topography, bedrock type) and disturbance regimes (both human – agriculture, logging, etc. – and non-human – fire, flooding, wind storms, beavers, ice storms, etc.). We can call these natural communities when enough time has elapsed to iron out the strong destabilizing influence on vegetative patterns of major and non-cyclical disturbances (seasonal flooding would be a cyclical disturbance that results in a stable vegetative community). I hope this isn’t over complicating the issue. I want you to be able to see that a key part of identification and understanding the world is knowing context. If you see a Swamp White Oak growing in an area, you should keep your eyes peeled for Green Ash. Similarly, knowing that you just identified a Black Willow makes it more likely that you’re looking at the bark of a Silver Maple than a Sugar Maple.

Let’s continue with some examples of how this concept helps us build expectations about the natural world. Say you’re hiking up Camel’s Hump on the Burrows Trail. As you move away from the parking lot, the abiotic factors steadily change, often in a related way. The higher up you go, the more essential it is for plants to have adaptations for dealing with lower temperatures, increased cloud cover, more snow and rain, thinner soils, and so on. A plant can’t do everything well, so those that thrive and outcompete at the parking lot – sugar maples, American beech, yellow birch, white ash – have a harder time competing towards the summit where you see mountain ash, paper birch, and black spruce.

Or imagine that you’re paddling the Winooski River near Half Moon Cove in Colchester. You stop for a lunch break and as you move away from the wide, lazy river’s edge, you notice that while the trees stay roughly the same, the soils shift from sandy and well-drained in the first floodplain terrace to siltier and less and saturated in the next floodplain terrace. You also notice that you’ve moved away from a dense tangle of tall ostrich ferns to a lower expanse of a sensitive ferns. Whether on the summit or along the river, as these conditions change, the plants that are capable of competing well in these locations changes too.

​These changes in vegetation is not surprising when we are aware of the associated changes in disturbances and abiotic conditions. We can begin to build up expectations about what plants will live in certain environments (again, given enough time for patterns of succession to iron out initial conditions that favor primary succession species like paper birch, white pine, and staghorn sumac). These expectations serve us well when we want to go out and see an example of, say, a sycamore. Our next step could be to look up habitat preferences for Sycamores using the Sylvics Manual. Knowing its preferred habitat, we turn to a soil map, surficial geology map, or bedrock map to triangulate conditions that would be home to a sycamore. We could then hop on a bike, pedal to the trail head and field test if our hypothesis was correct.

Wetland Woodland Wildland in many ways is a shortcut here. It has triangulated these conditions for us and provides detailed descriptions of our state’s 80+ natural communities, providing a short list of places our sycamore might grow (each natural community description includes an incomplete but useful list of the plants you are likely to find in that area). Understanding what plants grow in different natural communities can greatly improve our ability to weed through an overwhelming number of possible species of trees and shrubs and narrow it down to a short list of highly likely suspects. It’s similar to the mnemonic for opposite branched trees MAD Capped Bucking Horse that allows us to winnow our list of 75+ trees and shrubs to a much more manageable number.

​Each example of a natural community is unique from other examples of the same community and when out in the field you may find an odd ball species not listed in the book growing there, one that you would not expect to see in that natural community type, or find that a major suspect is missing entirely from the patch. Again, these are patterns based on expectations, and while mostly accurate, nature never fails to surprise us.

​So much of Vermont’s landscape has been heavily modified over the past two centuries that a patch of land may not even be a natural community. Given enough time without human or non-human disturbance (roughly 200 years) the patch of land will return to a natural community type. ​​