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Solving the Case of the “Shady Invaders”

Posted: December 14, 2017

If you spend time outside in Pennsylvania, you’ve probably seen shrubs creeping into eastern deciduous forest stands. You may have also noticed that these invasive shrubs, such as honeysuckles, Japanese barberry, and burning bush tend to be green much longer than native shrubs in the understory.
The left panel represents the forest during the growing season. Extended leaf phenology (ELP) becomes apparent at the extreme ends of the growing season (early spring and late fall) when most other species have lost their foliage, as depicted on the right

The left panel represents the forest during the growing season. Extended leaf phenology (ELP) becomes apparent at the extreme ends of the growing season (early spring and late fall) when most other species have lost their foliage, as depicted on the right

Picture a bird flitting about in search of insects or fruit. When danger approaches it can swiftly retreat and hide. In contrast, plants are generally rooted in place. Beyond making them comparatively easier to study than animals (much to the chagrin of my colleagues who are currently wading through coldwater streams to relocate radio-tagged fish), it also makes plant survival and success fascinating. Because plants are spatially static, they are dynamic through time. Plants can optimize their fixed situation through the timing of important life history events. For example, young leaves tend to be more delicious and less defended than older leaves. Some plants have been found to break buds and leaf out much earlier than when their insect herbivores emerge, giving leaves time to grow and develop defenses. Yet the benefits of early leaf production have to be balanced with the risks of a late frost and leaf damage. Recurring or cyclic life history events such as breaking leaf buds, flowering, fruiting, fall color change, and senescence are called phenophases. The study of how phenophase timing varies across a landscape and through years is called phenology. You can think of phenology as how tuned a species is to their surroundings. As such, phenology is one of the most important drivers of the success of a species. In fact, species that become highly successful in a new range (those we call invasive species) frequently exhibit novel phenology compared to the communities that they invade. For example, invasive grasses in western rangelands green up earlier than native species, taking up space before natives can.

If you spend time outside in Pennsylvania, you’ve probably seen shrubs creeping into eastern deciduous forest stands. You may have also noticed that these invasive shrubs, such as honeysuckles (Lonicera maackii, L. morrowwii, L. tartarica), Japanese barberry (Berberis thunbergii), and burning bush (Euonymus alatus) tend to be green much longer than native shrubs in the understory. This earlier bud burst and/or later leaf color change, compared to native species, is called extended leaf phenology.                                   

Why does extended leaf phenology of the shrub layer matter in eastern deciduous forests? To start, there is more light available in the understory of deciduous forests in the early spring and late fall when the overstory canopy is leafless. Over the course of the growing season, this has been shown to provide significantly more food (through photosynthesis) for some invasive shrubs, when compared to natives. Beyond this direct benefit to the invasive shrubs, their extended leaf phenology has the potential to impact ecosystems through novel shading. The majority of the plant diversity in an eastern deciduous forest occurs in the herbaceous layer, and many native herbaceous species have extended leaf phenology compared to the overstory canopy. In fact, spring ephemerals are a group of taxonomically unrelated herbaceous plants that are adapted to green up, flower, and begin to die-back while more light is available prior to overstory canopy closure in the spring. Additionally, many tree seedlings and saplings have extended leaf phenology compared to mature individuals of the same species in the overstory - an adaptation helping young trees to survive in their parents’ shade. So it’s not surprising that the presence of some invasive shrubs has been associated with decreased native herbaceous diversity and decreased tree regeneration. However, novel shading from invasive shrubs can also impact temperature, decreasing daily maximum temperatures which could affect insect and herptofaunal (reptile and amphibian) emergence. Additionally, the novel shading produced by Amur honeysuckle (L. maackii) has been found to reduce the pollination of herbaceous species. It turns out that many pollinators don't like the cooler, darker conditions created under a honeysuckle canopy. Another study found that small mammals (e.g. mice, chipmunks) use honeysuckle as habitat and preferentially browse natives in the understory. Ecologists call this “apparent competition.” In other words, it appears as if honeysuckle is a strong competitor, better at taking advantage of light and soil resources, when in actuality its success may be the result of indirect mechanisms.

While some invasive shrubs remain highly understudied, the research that has occurred is spatially limited. A single researcher or research group can only look at so many species and so many places. However, the cues that a plant might use to trigger phenophases such as budburst, flowering, and leaf color change frequently vary across species. Furthermore, the cues themselves, including day-length, temperature and precipitation, vary through space and between years. Is the difference in leaf phenology between a given native and invasive shrub in central Pennsylvania comparable to the difference seen in North Carolina or Missouri? How do these differences vary for other native and invasive shrubs? How widely applicable is research on the impacts of extended leaf phenology across species and space? To answer these questions, I need help, from all across the eastern US. So, I teamed with the USA National Phenology Network to create a citizen science campaign called Shady Invaders that began in the spring of 2016. We have grown to over 200 sites in eastern deciduous forest stands this year, and are continuing to recruit volunteers to help gather data to answer the questions above. If you go on a weekly walk in the woods, or have a woodlot with at least a partially deciduous canopy nearby, your help would be greatly appreciated. To participate, sign up for an account with Nature’s Notebook. This is where you will enter your site and plant information, print data sheets, and log your data. We have some ID tips online, too! Observations of ‘no’ for phenophases are just as important as observations of ‘yes’ in order to catch an accurate beginning and end for each phenophase for each individual you follow.

What have we found so far? As a group, 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 occurred earlier in 2017 than in 2016, reflecting the earlier warming across much of the eastern U.S. this spring. When we look at the fall phenology of invasive and native shrubs, the pattern is less consistent. 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. So far in 2017, however, there is no significant difference between colored leaves of invasive and native shrubs. We need more observations to tease apart the different environmental cues that plants are responding to. With the help of citizen scientists, I hope to also tease apart the environmental cues and species-specific patterns across eastern deciduous forests in order to understand the consistency of the impacts of invasive shrubs, and to help guide management efforts.

Want to participate or have questions? Contact me.

Contact Information

Erynn Maynard
  • PhD Student, Ecology