Dr. Ryan W. McEwan

Callery pear (Pyrus calleryana) was an extremely popular ornamental tree in the United States that was planted in many neighborhoods. It has since become a highly problematic invasive tree and is presently banned from sale in some states. The story of this transformation is outlined in this article in The Conversation:

https://theconversation.com/once-the-callery-pear-tree-was-landscapers-favorite-now-states-are-banning-this-invasive-species-and-urging-homeowners-to-cut-it-down-198724

Understanding the ecology of Callery pear invasion has been a goal in the McEwan Lab for the past several years. In the lab we are exploring (a) the profile of traits that enable this species to become invasive, (b) the characteristics of ecosystems that make them more (or less) susceptible to invasion, and (c) the effects this species has on ecosystems . We are increasingly studying methods to control Callery pear invasion and restore ecosystems that have been invaded.

Here are some things we have discovered in our research so far:

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Callery pear has an extended leaf duration and is frost resistant

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In temperate regions, like Ohio, most woody plants drop their leaves in the fall and go into a semi-dormancy stage in the winter before sprouting leaves again in the spring. The length of time during the year when the leaves are active is important for the plant from the perspective of gaining energy. We noticed that Callery pear seemed to have a longer leaf duration than many native trees and wanted to examine this more closely. In this study, we monitored Callery pear and two co-occurring native trees in grasslands near Dayton, OH.  We found that Callery pear had a much earlier leaf out in the spring than either cottonwood (Populus deltoides) or sycamore (Platanus americana). 

In the graph above, the grey points with the stars are above the other colored points from mid-March through late May–> This indicates more advanced leaf development for Callery pear than the other plants in the study during this time period. There was a hard frost event during our study and we found that Callery pear suffered virtually no damage while both native species lost nearly all their leaves. We found that Callery pear also held its leaves nearly a month longer in fall.  The increased opportunity for growth could be an important component of the invasion biology of Callery pear.

You can read the entire open access paper at this LINK.

Here is the citation for this work:

Maloney, M.E., A. Hay, E.B. Borth and R.W. McEwan. 2022. Leaf phenology and freeze tolerance of the invasive tree Pyrus calleryana (Roseaceae) and potential native competitors. Journal of the Torrey Botanical Society 149: 273-279. Open Access: https://doi.org/10.3159/TORREY-D-22-00008.1

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Callery pear invasion into grasslands is facilitated by adjacent forest

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Ecological invasion into grasslands is a process that may be influenced by landscape features. The invasive plant must disperse into the habitat, and for Callery pear that dispersal mechanism is generally though to be through animal dispersal by birds. We were interested in how landscape features might influence the invasion process for Callery Pear. We sampled restored grassland sites with varying levels of invasion of Callery pear and measured the distance from each plot to various landscape features. The most important factor that we discovered in dictating initial invasion of Callery pear was distant to a forest edge.

In the graph above, each of the dots is a plot, how high the points are indicates the level of Callery pear cover in the plot and how far to the right each point is indicates how far away that plot was from a forest edge. We saw that after approximately 75 meters from the forest edge there were not plots containing Callery pear. This indicates an “edge – to – interior” pattern of the invasion process. This may be related to bird roosting sites in the forest edge. Hypothetically, the birds eat the Callery pear fruits then land at the edge of the field in trees and deposit the fruits while roosting. A lot more work is needed to verify this pattern and assess the mechanism(s) responsible. If you have observations of animals eating Callery pear fruits, please share them with us at this link.

You can read the dissertation chapter from which this article was created here (Chapter 2): LINK

The citation for this work is here:

Woods, M.J., G. Dietsch and R.W. McEwan. 2022. Callery pear invasion in prairie restorations is predicted by proximity to forest edge, not species richness. Biological Invasions 24: 3555–3564. LINK.

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Callery pear suppresses seed germination of some native grassland species

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In central Ohio were our lab is located, we have noted many dense stands of Callery pear in open fields. We were curious as to how those trees established dominance so quickly. One way many invasive species, including Amur honeysuckle, are able to establish dominance is through suppression of potential competitors through chemical inhibition of seed germination. To test this idea, we conducted a series of laboratory assays in which we applied leaf exudates to native grass and forb seeds and monitored germination.  We found strong evidence of allelopathic inhibition of native seeds when exposed to P. calleryana exudates.

In the graph above, look for the black circles and compare those to the open squares- where you see the black circles above the squares that is a point in time when the germination of the native plant is being inhibited by the exudates from leaves of Callery pear. We did this kind of experiment with grasses and with forbs and saw compelling results that suggest that Callery pear may be limiting competition by suppressing germination of potential competitors. Further work is need to verify this effect in field conditions and isolate the responsible compounds. 

You can read the entire open access paper at this Link.

Here is the citation for this work:

Woods, M.J., D. Schaeffer, J.T Bauer and R.W. McEwan. 2023. Pyrus calleryana exudates reduce germination of native grassland species, suggesting the potential for allelopathic effects during ecological invasion. PeerJ 11:e15189. Open Access: https://doi.org/10.7717/peerj.15189

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Callery pear invasion influences soil biology

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Invasive plants have strong potential to alter ecosystem function in areas where they invade. These effects can include altering soil nutrients and microbial activity. We were interested in testing for potential effects of Callery pear on soil nutrients and microbial activity. We used soil enzyme activity as a way to measure the soil microbial community. We analyzed soil under and away from Callery pear stems in two conditions: (a) “treated” stems, which had been previously cut by land managers as part of control efforts and (b) “untreated” stems that had never been cut.  We found strong evidence that cutting the stems activates biological changes in the root system that alters soil chemistry.  We analyzed soil enzyme activity as a way to assess overall soil microbial ecology and found significant differences among treated and untreated stems.  Future studies aimed at understanding how invasion & control activities alter soil ecology are needed.

You can read the dissertation chapter from which this article was created here (Chapter 3): LINK

Here is the citation for this work:

Woods, M.J., G.K. Attea and R.W. McEwan. 2021. Resprouting of the woody plant Pyrus calleryana influences soil ecology during invasion of grasslands in the American Midwest. Applied Soil Ecology 166: 103989. LINK.

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Callery pear sprouts aggressively and can survive both fire and ice!

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In a series of field experiments we clipped Callery pear stems in the field then applied treatments to control sprouting.

We applied fire to small plots containing Callery pear stumps:

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We also applied liquid nitrogen to Callery pear stumps to freeze the stumps:

We applied these treatments to both stems that had been repeatedly mown before the start of the experiment (we called this “trees-sprouting”), and also to trees that had never been cut (we called this “trees-intact”).

Above you can see a graph from the paper, from the trees-sprouting part of the study. The height of the line in the middle of the boxes represent the number of sprouts that came back after the Callery pears where cut and then treated with one of the treatments. “Cut” are stems that were only cut and not treated. “Fire” were cut first and then burned with prescribed fire. “Freeze” were cut first and then frozen with liquid nitrogen. “Herb” were cut and sprayed with herbicide. “Neg” is a “negative control”- those stems were not cut at all. The point of the graph is that only herbicide actually reduced the number of stems (box below the dotted line). The number of sprouts actually increased in all the other treatments. What is really interesting in this graph is that the number of sprouts from Callery pear stumps that were burned (red box) is actually increased relative to the negative control (white box), suggesting that fire increased the number of sprouts! Fire is a common practice in maintaining grasslands in the American Midwest, so this result is very concerning.

You can read the entire open access paper at this LINK.

Here is the citation for this work:

Maloney, M.E., E.B. Borth, G. Dietch, M.C. Lloyd and R.W. McEwan. 2023. A trial of fire and ice: assessment of control techniques for Pyrus calleryana stems during grassland restoration in southwestern Ohio, USA. Ecological Restoration 41: 25-33. Open Access: https://muse.jhu.edu/pub/19/article/883911

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A way out of the pear predicament?

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Callery pear has a profile of advantageous traits including early phenology, allelopathy, and aggressive sprout response that make it an extremely effective invader.  This species is influencing grassland ecosystems in the Midwest and “natural” control measures are ineffective.  Herbicides are required to manage this plant; however, once you apply herbicide, the site is likely to be re-invaded. Ongoing work in the lab focuses on how to advance native species success in the presence of this invasive tree – we are currently running experiments to see if we can “hold the niche” against re-invasion.

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McEwan Lab Publications focused on the biology of Callery Pear invasion:

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Woods, M.J., D. Schaeffer, J.T Bauer and R.W. McEwan. 2023. Pyrus calleryana exudates reduce germination of native grassland species, suggesting the potential for allelopathic effects during ecological invasion. PeerJ 11:e15189. Open Access: https://doi.org/10.7717/peerj.15189

Maloney, M.E., E.B. Borth, G. Dietch, M.C. Lloyd and R.W. McEwan. 2023. A trial of fire and ice: assessment of control techniques for Pyrus calleryana stems during grassland restoration in southwestern Ohio, USA. Ecological Restoration 41: 25-33. Open Access: https://muse.jhu.edu/pub/19/article/883911

Maloney, M.E., A. Hay, E.B. Borth and R.W. McEwan. 2022. Leaf phenology and freeze tolerance of the invasive tree Pyrus calleryana (Roseaceae) and potential native competitors. Journal of the Torrey Botanical Society 149: 273-279. Open Access: https://doi.org/10.3159/TORREY-D-22-00008.1

Woods, M.J., G. Dietsch and R.W. McEwan. 2022. Callery pear invasion in prairie restorations is predicted by proximity to forest edge, not species richness. Biological Invasions 24: 3555–3564. LINK.

Woods, M.J., G.K. Attea and R.W. McEwan. 2021. Resprouting of the woody plant Pyrus calleryana influences soil ecology during invasion of grasslands in the American Midwest. Applied Soil Ecology 166: 103989. LINK.

McEwan Lab Callery Pear Data sets:

Exit seminar portion of Peter’s successful Thesis Defense!

Recording of the Exit Seminar for Grace, which preceded her successful defense!

Dr. Ryan McEwan

Giving a presentation in front of a group of people is a normal part of professional life.  These presentations are nearly always accompanied by slides projected on to a screen.  As you grow as a professional, you want to make sure that you don’t have a computer glitch meltdown right before your talk is supposed to start.  “Uhhh sorry, the computer is not working” is not something you want to be saying.  Strategizing against these fails is a part of professional development and is a basic expectation of graduate students.

Here are some ideas on how to avoid this unfortunate situation.

Work from correct assumptions:

1. You cannot assume that the projector/podium computer will have a cable to connect to your own laptop.  “Every podium has an HDMI cable right?” – -> WRONG!  You must plan for the eventuality that you will need to transfer the talk from your computer onto the podium computer.

2. You cannot assume you will have access to a reliable network (eg, wifi).  You probably will, but it is not guaranteed.  This also means that you cannot rely on “emailing yourself the file” or pulling it off of a your drive over a network.  There might not be a network, or you might have one that is unreliable, and start to meltdown if you try to pull your presentation file onto the podium computer.  

3. You must assume that the projector/podium computer is running Windows, and potentially an older version. 

Tips for resiliency against computer freakout:

1. Develop your talk on Powerpoint, or at least download it into Powerpoint after it is developed (for eg, on Google Slides).  Powerpoint is still the standard platform for presentations and does not require a network, or Chrome, to run properly. Google slides might look great on Chrome, but it might freak out on other platforms and **the computer at the podium might not have Chrome** but, it almost certainly has powerpoint! Do you want to be standing in front of a crowd of people trying to download and install Chrome, when the wifi is sketch? No. You don’t.

2. Make your Powerpoint file as simple as possible.  I personally do not use animations (you can use “insert duplicate slides” to accomplish much the same thing and it’s a lot more stable), and I do not embed videos.  I also try to use “snips” instead of importing images in some cases to try and make the file size smaller.  If you really want videos and snazzy animations, you really need to focus on the following tips to make sure it is going to actually work on the computer you have to present from! Its really a bummer to have someone start the talk, click on an animation, and the system freaks out and goes “blue screen of death.” If you want to have videos embedded…then make absolutely sure you have an HDMI cable and bring your own machine.  

3. Download the talk to the computer and run it from there. Your are almost certain to have a glitchy meltdown if you try to just open your talk from your email and run it without downloading, especially given that your wifi connection should be viewed as unstable! Download to the desktop of the podium computer (or your own), then opent it and give the talk from there. This is much more stable than if you run the talk from a network (eg, via Google Slides), or a flash drive.

4. Work with your host ahead of time to have the talk already loaded.  Go to the podium room 20-30 minutes ahead of time and set things up. Or, if that is not possible, send the talk to the host using a drive link or otherwise transfer it.  Ask them to please download it onto the computer desktop prior to the talk. If the talk is waiting there on the desktop of the computer you are presenting from, then you are much safer.

5. Always have your talk on a “flash drive.”  Although old technology, flash drives are extremely reliable.  I actually bring my talk on 2 flash drives every time I give an invited talk, even if I have a good idea that I can use my laptop or access a network.

6. Consider a PDF format.  In Powerpoint or Google slides you can save (or print) your slides to PDF and then run the presentation from an Adobe platform using “full screen mode.”  If you are really worried about connectivity and computer speed, this is a good option. For example, if you are giving the talk in a field station, a PDF format could be a great option. The PDF version eliminates all slide formatting issues, creates a much smaller file, and practically all computers have at least Adobe reader.