Nature’s Notebook

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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

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

Results from the Shady Invaders Campaign

The map below shows the sites where observers reported data on invasive and native shrubs. While the campaign focused on the eastern U.S., we had a few sites reporting on these species in the west, including one site in Canada. Many sites reported data in all three years of the campaign (indicated by dark green on the map).

Shady Invaders map of sites with years reporting 2016-2018

The year with the highest number of observers was 2017, with 417 observers, followed by 2018 with 388 observers reporting. All 878 observers submitted a total of 375,635 observations over the course of the campaign.

Shady Invaders campaign participation 2016-18










Your data captured 1,605 onsets of breaking leaf buds, and 1,585 end dates for leaves.

Shady Invaders number of onsets captured 2016-18











Researcher Erynn Maynard analyzed your data to answer the question of whether invasive shrubs have Extended Leaf Phenology, and what environmental variables contribute to the difference.  

She found that invasive shrubs leaf out earlier than native shrubs, but this effect diminishes at northern latitudes. This is likely because at the most northern latitudes, cold spring temperatures end up limiting both native and invasive plants. Invasive shrubs are better able to take advantage of warmer spring temperatures that occur at lower latitudes. Given predictions of warmer spring temperatures across the East due to climate change, we could see an increased advantage of invasive species across latitudes in the future.

Shady invaders onset of invasive and native shrubs by latitude

Figure: Relationships and 95% credible intervals (shaded regions around lines) showing fit between onset date and latitude. The model's R squared is 0.74, meaning that 74% of the variation in the data is explained by the model.


To look at the differences between species, Erynn estimated the onset of leaves by species for the mean latitude of the dataset.The species follow the pattern of invasive species leafing out earlier than native shrubs, with the exception of burningbush. Why is this species different? Burningbush has green branches, which may mean that this species doesn't need leaves to take photosynthetic advantage of extra sun in the spring.   

Shady Invaders invasive and native shrubs species specific onsets

Figure: Modeled onset of leaves by species for the mean latitude of the dataset, 40 degrees N. Mean onset for invasive and native shrubs is also shown on the right.


Erynn also looked at which environmental variables have the most impact on shrubs, and found that both native and invasive shrubs have an earlier onset of leaves with warmer average spring temperatures. Invasive shrubs also respond to warmer bud chilling days, greater spring precipitation, and higher elevation.  

Shady Invaders environmental variables influencing onset

Figure: Environmental variables influencing whether leaves appear earlier or later on  native and invasive shrubs.

While data on fall phenology are still coming in, Erynn has done some preliminary summaries of your data on end of the leaves phenophase, fall color, and falling leaves. Invasive shrubs have later average dates for all three phenophases, showing they hold onto their leaves longer than native shrubs. 

Shady Invaders fall phenology of invasives and natives

Figure: Average end of leaves, onset of fall color, and onset of falling leaves for native (n = 270) and invasive shrubs (n = 55), 2016-2018.


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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