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Invasive Plants Change Expected Pathways of Plant Succession

- by Charlotte Pyle, USDA Natural Resources Conservation Service

You probably have observed that in recent years there have been some new twists to the textbook model of plant succession. In the textbook model of old field succession, abandoned farm land is expected to revert to a mixture of grasses and herbaceous plants and from there, change to domination by shrubs and tree seedlings and ultimately to forest. Invasive shrubs are changing old-field succession. In addition, on logged sites, invasive plants are creating changes in the pathways to re-establishment and development of new forest cover.

In the past, people have used the word “invade” to describe what happens during plant succession when the seeds of native plants come into an open site and new plants get established. This “invasion” would lead to a more or less predictable type of vegetation depending on climate, site conditions, and surrounding seed sources.

Today, when people talk about invading plants, they are talking about non-native plants that have disruptive effects. The President’s Executive Order 13112 (of February 1999) defines an invasive species as a species that is (1) non-native (alien to the ecosystem under consideration) and (2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health.

Connecticut’s roadsides and fencerows are a good place to see invasive shrubs such as Multiflora Rose, Winged Euonymus, Autumn-Olive, Japanese Barberry, and several non-native shrub Honeysuckles. Other roadside invasives include Tree-of-Heaven, Asiatic Bittersweet, Japanese Knotweed, Phragmites, Spotted Knapweed, Cypress Spurge, and Garlic Mustard. On sunny roadsides and fence rows, invasive plants grow vigorously and produce good seed crops.
Although you may first spot invasive plants along a roadside, their seeds spread widely to farms, forests, wetlands, and additional roadsides. Seeds are spread by vehicle tires, mowing machines, roadside reclamation of eroded soil, contaminated hay (harboring invasive plant seeds), birds, mammals, moving water, and wind.

In many forests and agricultural lands where invasive plants are already present, certain controlling factors keep plant sizes small and numbers few. In closed canopy forests, shade slows the growth and seed production of invasive plants, but does not completely prevent their establishment. In pastures, grazing tends to keep invasive shrubs small in size, but their roots can become well-established over time. Grazing will not control herbaceous plants such as Cypress Spurge, Spotted Knapweed, and Narrowleaf Bittercress that are disliked by livestock. Frequent mowing holds invasive plants in check, but they may flourish in rocky patches or field corners missed by the mowing equipment.

Often when the controlling force of grazing or mowing is taken away, fields become overrun with invasive shrubs such as Multiflora Rose, Autumn-Olive, non-native shrub Honeysuckles, or Japanese Barberry, and Asiatic Bittersweet ( a woody vine). These plants shade the ground preventing the establishment of the native herbaceous plants and shrubs expected to crop up in old fields soon after farm use abandonment. A thick cover of invasive shrubs can hold the site in a shrub stage where species diversity is low.

There is a lack of data for what the long-term outcome of the changes in the expected pathway old-field succession will be across Connecticut. However, because invasive plants are increasingly prevalent in the landscape, it is increasingly likely that we will be seeing invasive plants disrupting old-field succession because it is increasingly likely there will be a seed source or established plants waiting to be released when farm land is abandoned or land is cleared.

In forests, the additional light to the forest floor created by logging often releases small Japanese Barberry plants to grow into large thickets. In addition, logging equipment and birds may bring in seeds of other invasive species that grow vigorously in the sunlight. In partial cuts, released Japanese Barberry is particularly a problem when it quickly expands to covers large areas of the forest floor making it difficult for tree seedlings to get established.

The over-population of deer in Connecticut also contributes to the invasive plant problem. The invasive plants that do well in Connecticut tend to be avoided by deer. It is more difficult for the expected pathway of forest succession to occur on open sites that have the combination of competition from invasive plants and excessive browsing by deer because fewer new native trees and shrubs can remain alive beyond the early seedling stage under these conditions.

Given the widespread presence of invasive plants in our landscape, a pro-active approach is needed. If you are letting an active field revert to old field succession (either for management as early-successional wildlife habitat or for long-term succession to forest), your plans should include invasive plant management up front. Plans for logging should take into account the existing locations of invasive plants and avoid moving equipment through them or opening them up without plans for subsequent invasive plant control.

For Further Information
The USDA Natural Resources Conservation Service (NRCS) Connecticut website includes printable materials, information on the Wildlife Habitat Incentives Program (WHIP), and links to a variety of invasive plant-related topics www.ct.nrcs.usda.gov/plants.html.

The Connecticut Invasive Plant Working Group website includes the 2003 lists of invasive plants that disrupt “minimally-managed” areas of native vegetation in Connecticut, a calendar of events, and species-specific information on invasive plant management www.hort.uconn.edu/CIPWG.

NOTE: Charlotte Pyle has complied an Invasive Fact Sheet which you can access at www.ct.nrcs.usda.gov/invas-factsheets.html

This article originally appeared in the June 2003 ECFLA/WDLT Newsletter.

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