Wednesday, May 27, 2020

Ecological Succession

In 1859, Henry David Thoreau wrote an essay called "The Succession of Forest Trees" in which he described succession in an oak-pine forest. "It has long been known to observers that squirrels bury nuts in the ground, but I am not aware that any one has thus accounted for the regular succession of forests." His essay was more about replacement than succession as that term had come to be used in modern ecological studies. He first observed the obvious – that many tree seeds are designed to be transported by wind, also that the small seeds of wild cherry, wild grape and various berries can be consumed by birds and other animals and defecated elsewhere. (We know that Oriental bittersweet berries are consumed by robins and spread that way.)

Wreath of Oriental bittersweet vines, with berries. In the
spring, returning robins perched on it to eat the berries.
Thoreau also described the less obvious, how, for example, oak seedlings might emerge from the soil after a pine tree forest was cut. Some botanists of his era took the position that acorns could lay dormant in the soil for decades, even centuries, but when the forest trees above them were cut, be viable, triggered by sunlight. Thoreau observed, rather, that even in a mature pine forest, there were oak seedlings amongst the underbrush the result of squirrels burying acorns or else dropped by blue jays and other birds. The same dispersion seen for beech, hickory, chestnut and other nuts.

Ecological succession is the process of change by all species (plants, animals, bacteria) of a community over time. During the first half of the twentieth century, succession theory was dominated by the idea that for a given location there was a convergence toward a climax community regardless of the starting conditions, that, for example, that moist-soil lowlands in eastern Massachusetts would always end up as a forest dominated by sugar maple and beech trees regardless of whether the starting conditions were abandoned farmland, fire, flood or hurricane. Similarly, a drier, hilly terrain would always end up being pine/oak. In this school of thinking, a climax forest was a stable, interrelated community with a near-constant total biomass – trees dying being replaced by the same species.

Current theory allows for more complexity and chance. In these models the finding of certain species being found together is because terrain and climate are beneficial to each species individually without ‘community’ interaction. In both models, prolifically reproducing and fast-growing species will populate a disturbed area first, followed by shade-tolerant, slower growing but more competitively successful species. American beech is an example of an extremely shade-tolerant tree that can abide in the understructure for years until a break occurs in the canopy.    

One point that has become clearer is that it is not just about plants. The local extermination of beaver changed terrain. The return of same created wetlands and flooded ex-forests. The local extermination of deer allowed for lush undergrowth and greater survival of tree seedlings. The present-day surfeit of deer in New England – now at a population higher than before the European colonists arrived – is denuding all the undergrowth. Trees that were part of the North American mosaic got diseases. American chestnut trees are long gone, native dogwood, ash and hemlock are struggling. Invasive species challenge the status quo. Climate change is affecting the entire biome.         

Dandelion: The common dandelion (Taraxacum officinale) was introduced to
North America be English colonists in the early 1600s as a medicinal plant.
In Maynard and Stow, some of the best evidence for ecological succession is abandoned farmland. Some of this was seized by the U.S. Army via eminent domain during World War II, subsequently allows to go to forest, now the Wildlife Refuge. The trees are all around 70-80 years old. There is not a lot of dead wood on the ground. Same for the Summer Hill forest. The woods traversed by the Assabet River Walk, once pasture, are older. There, there are many downed trees in varying states of decay.   

The greenspace bordering the Assabet River Rail Trail was cleared during construction. Dandelions, an exemplar of windblown propagation, are common, as are other early-growth perennials. Tree seedlings are present. Without maintenance of borders, our trail could become a green-flanked, green-roofed tunnel. An excellent report “Rail Trail Maintenance and Operation: Ensuring the Future of Your Trail – A Survey of 100 Rail Trails” suggests that trails need roughly $1,500 per mile per year in maintenance and operating costs. Maintenance tends to be a combined effort of municipal budgeting and volunteer organizations.

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