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Invasive shrubs are becoming increasingly common in eastern forests. These shrubs are top competitors for native shrubs - they can break bud earlier in the spring and hold onto leaves longer in the fall. This phenomenon is called Extended Leaf Phenology (ELP), and allows these early-leafing invaders to take advantage of the greater amount of light reaching the forest floor in early spring.
ELP of these shrubs can create shading on the forest floor at times when native herbs, tree seedlings, insects, reptiles and more depend on that greater sunlight.
Shady Invaders is a project created by researchers at Penn State University to explore the timing of leaves on invasive and native shrubs. The goal of the project is to start to quantify ELP on a regional scale so that we can understand how or if increased shading is actually impacting deciduous forest ecosystems.
We are seeking observers in the eastern U.S. to document changes in the growth of invasive and native shrubs. Each list is ordered with the easiest plants to identify at the top and the most difficult at the bottom. However, not all of these species will be found in every forest. See some species identification tips
We are particularly interested in observations on the following invasive species:
- Japanese barberry (Berberis thunbergii)
- Amur honeysuckle (Lonicera maackii)
- multiflora rose (Rosa multiflora)
- burningbush (Euonymus alatus)
- privet (Ligustrum sp.)
- Morrow's honeysuckle (Lonicera morrowii)*
- Tatarian honeysuckle (Lonicera tatarica)*
And the following native species:
- spicebush (Lindera benzoin)
- mapleleaf viburnum (Viburnum acerifolium)
- flowering dogwood (Cornus florida)
- gray dogwood (Cornus racemosa)
- black haw (Viburnum prunifolium)
- alternateleaf dogwood (Cornus alternifolia)
- southern arrowwood (Viburnum dentatum)
- hobble-bush (Viburnum lantanoides)
*Note that these two species hybridize commonly and form a separate species, Lonicera x bella.
How to participate
1. Select one (or more) of the species to track from the list above. Choosing at least one native species and one invasive is ideal, but you can select just one or the other if you would like. Observations of these species made under a deciduous forest canopy (with trees that shed their leaves annually) are the most useful for this campaign.
2. Join Nature's Notebook. If you haven't already, create a Nature's Notebook account. See our specifics of observing if you need more details on getting started.
3. Sign up to receive our Shady Invaders campaign messaging (in the right sidebar of this page - you may need to scroll back up to see it). You will receive messages approximately every 4-6 weeks during the growing season, providing early results, encouragement, observation tips, interesting links, and campaign-specific opportunities. Don't miss out!
4. Take observations. We invite you to track leaf out in your plants ideally 2-4 times a week, in the spring and autumn. We are especially interested in the following leaf phenophases, though you are welcome to report on flowering and fruiting as well.
5. Report your observations. As you collect data during the season, log in to your Nature's Notebook account and enter the observation data you recorded. You can also use our smartphone apps to submit your observations!
Want to do more? Observations of overstory trees are a great complement to the Shady Invaders campaign. Find out if one of the trees in your yard is on the Nature's Notebook species list.
In 2017, we surpassed our 2016 totals for both number of observers and number of shrubs observed. IN 2017, you tracked an average of 6.7 shrubs per person.
What did we learn from your observations? Invasive shrubs broke buds several weeks earlier than native shrubs in both 2016 (23.4 days earlier) and 2017 (24 days earlier). For both invasive and native shrubs, breaking leaf buds were reported earlier in 2017 than in 2016, reflecting the very earlier spring that occurred across much of the Eastern US this year.
When we look at the fall phenology of invasive and native shrubs, we see a different story. In 2016, invasive shrubs had colored leaves several weeks later, on average, than native shrubs. This is what we would expect with species that have extended leaf phenology, keeping their leaves on longer to take advantage of the sunlight reaching the forest floor after deciduous overstory trees have shed their leaves.
In 2017, however, there is no significant difference between colored leaves of invasive and native shrubs. Colored leaves appeared on both invasive and native shrubs at about the same time and several weeks earlier in 2017 than in 2016.
What would cause this difference between years? You might have noticed that fall was late in 2017, with many of the overstory deciduous trees keeping their leaves weeks longer than normal. When this happens, an invasive shrub's advantage of keeping its leaves is removed, since more light isn't reaching the forest floor and helping the shrub grow.
It's possible that invasive species are more responsive to overstory shading than native shrubs. Or, there could be another environmental variable that we don't yet know about that was different between 2016 and 2017.
What makes a species invasive?
Non-native species are those that are found outside of their historic range, generally as a result of human activities. Invasive species are those non-native species that thrive outside of their historical range with a demonstrated detriment to the invaded ecosystem, economics or human health. Not all species that are non-native become invasive. In fact, very few individuals survive outside of their native range, because the conditions are different from what they have adapted to over many generations.
While invasive species are generally the unintentional product of increased global transportation, many invasive plants continue to be intentionally introduced. Invasive shrubs are an increasingly prevalent component of eastern forests. (Shrubs are generally defined as woody plants with multiple stems arising at or near the ground and are generally shorter and have smaller stem diameters than trees.) Studies estimate that 82% of the 235 invasive woody species in the United States, and 62% of invasive woody species globally were introduced intentionally for horticultural purposes.