Nature’s Notebook

Connecting People with Nature to Benefit Our Changing Planet

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Shady Invaders

Project background

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. 

See what we learned from this campaign in 2017

Learn more about what defines an invasive species

Join us!

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:

And the following native species:

*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.

Phenophase Definition


(Click to enlarge)

Breaking leaf buds One or more breaking leaf buds are visible on the plant. A leaf bud is considered "breaking" once a green leaf tip is visible at the end of the bud, but before the first leaf from the bud has unfolded to expose the leaf stalk (petiole) or leaf base. spicebush_breaking leaf buds and flowering
Leaves One or more live, unfolded leaves are visible on the plant. A leaf is considered "unfolded" once its entire length has emerged from the breaking bud so that the leaf stalk (petiole) or leaf base is visible at its point of attachment to the stem. Do not include fully dried or dead leaves. Japanese barberry_Berberis thunbergii_leaves
Increasing leaf size A majority of leaves on the plant have not yet reached their full size and are still growing larger. Do not include new leaves that continue to emerge at the ends of elongating stems throughout the growing season. Morrows and Tartarian honeysuckle_increasing leaf size
Colored leaves One or more leaves (including any that have recently fallen from the plant) have turned to their late-season colors. Do not include fully dried or dead leaves that remain on the plant. mapleleaf viburnum_Viburnum acerifolium_colored leaves
Falling leaves One or more leaves are falling or have recently fallen from the plant. Photo: Ellen G Denny, alternateleaf dogwood_falling leaves

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

2017 Results from the Shady Invaders Campaign

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.

Number of observers in Shady Invaders 2016-17

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. 

Shady Invaders breaking leaf buds 2016-17

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. 

Shady invaders colored leaves comparison 2016-17

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.

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