Pyrus calleryana_Gary A. Monroe
Photo by Gary A. Monroe

Pyrus calleryana Risk Assessment

Common names: Callery pear

Pyrus calleryana -- California

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Evaluation Summary
Summary: 
General Evaluation Information
Date of Evaluation: 
August 9, 2016
Evaluation Time (hrs): 
2 Hours
Evaluation Status: 
Completed
Plant Information
Plant Material: 
If the plant is a cultivar, and if the cultivar's behavior differs from its parent's (behavior), explain how: 
Regional Information
Region Name: 
Climate Matching Map
These maps were built using a toolkit created in collaboration between GreenInfo Network, PlantRight, Cal-IPC, and Agricultural Sustainability Institute at UC Davis.
Climate Matching Maps PDF: 
Invasive History and Climate Matching
1. Has the species (or cultivar or variety, if applicable; applies to subsequent "species" questions) become naturalized where it is not native?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
This species has naturalized in many states in the Eastern US (USDA PLANTS), as well as Europe, South Africa and Australia (GBIF).
Reference(s): 
2. Is the species (or cultivar or variety) noted as being naturalized in the US or world in a similar climate?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
Answer / Justification: 
The following naturalized occurrences match California's climate according to the climate matching map: Kwazulu-Natal and the Northwest Province in Southern Africa, as well as some occurrences in New South Wales and South Australia. As well, areas in the Eastern US that match California's climate and have occurrences: southern Appalachian mountains. Calfora lists occurrences in 4 counties (Butte, Sacramento, Solano, Yolo).
Reference(s): 
3. Is the species (or cultivar or variety) noted as being invasive in the U.S. or world?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
High
Answer / Justification: 
Invasive problem in several areas in the US.
Reference(s): 
4. Is the species (or cultivar or variety) noted as being invasive in the US or world in a similar climate?
Yes or No: 
Yes
Points: 
3
Confidence Level: 
Low
Answer / Justification: 
Species is listed as invasive in areas that do not match Calfornia's climate: Ohio (Culley et al), Illinois (Invasive.org), Alabama, Mississippi, (Invasive.org/EDR maps). It is listed as invasive in Tennessee in Invasive.org, and checking the USDA PLANTS county distribution it is naturalized (presumably invasive) in eastern Tennessee which areas match California's climate (USDA PLANTS).
Reference(s): 
5. Are other species of the same genus (or closely related genera) invasive in a similar climate?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Pyrus communis and Pyrus betulifolia are listed in the Invasive Plant Atlas of the US, but neither appear to be significant invasive species.
Reference(s): 
6. Is the species (or cultivar or variety) found predominately in a climate matching the region of concern?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Overall, no. The native range in southeast China and surroundings does not match California's climate. As well, most occurrences in Europe, Egypt and many of the occurrences in Australia and the eastern US do not match California's climate. Vincent 2005: "It has already escaped in portions of USDA zones 8/9 to zone 5, and probably will not escape beyond these areas, given its tolerances as expressed by Dirr (1990). The seeds may also require a period of cold temperatures (328–368 F; Dirr 1990), this may limit its escape in warmer areas of the country."
Impact on Native Plants and Animals
7. Does this plant displace native plants and dominate (overtop or smother) the plant community in areas where it has established?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
Yes, indicated to crowd out native plants, form thickets.
Reference(s): 
8. Is the plant noted as promoting fire and/or changing fire regimes?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Nothing noted in any invasive summary page or literature sources concerning promotion of fire.
Reference(s): 
9. Is the plant a health risk to humans or animals/fish? Has the species been noted as impacting grazing systems?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
The fruit is edible, and there was nothing specifically listed as a health risk from any source.
Reference(s): 
10. Does the plant produce impenetrable thickets, blocking or slowing movement of animals, livestock, or humans?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
Dense, thorny thickets noted.
Reference(s): 
Reproductive Strategies
11. Does this species (or cultivar or variety) reproduce and spread vegetatively?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
Noted to reproduce vegetatively and by seed (Swearingen). Literature lists suckering.
Reference(s): 
12. If naturally detached fragments from this plant are capable of producing new plants, is this a common method of reproduction for the plant?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Although suckering is indicated, it is unclear whether fragments actually detach and form new individuals.
Reference(s): 
13. Does the species (or cultivar or variety) commonly produce viable seed?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Swearingen: "Different genotypes growing near each other (e.g., within about 300 ft.) can cross-pollinate and produce fruit with viable seed. Also, cultivars are often grafted onto seed-grown rootstocks with varying genotypes; if the plant produces shoots from the rootstock (which it often does), then these shoots and the graft can pollinate one another. "
Reference(s): 
14. Does this plant produce copious viable seeds each year (> 1000)?
Yes or No: 
No
Points: 
0
Confidence Level: 
Low
Answer / Justification: 
This species is self-incompatible (unable to set seed from pollen from the same individual) and it was also wrongly assumed to be functionally sterile, however, seed set is noted when grown in proximity to other cultivars that are genetically different, as in Vincent 2005, where he states that this species will hybridize and produce seed set at a "very low rate," but notes Nesom (2000) as documenting a "higher rate of seed set" even when grown in isolation. Nesom, incidentally, noted an average of 1.6 seeds per fruit for the 30 fruits produced (although only one tree was studied). "Increased" seed production is also noted in Hardimann and Culley (2010), "Overall, these data indicate that invasive parents are capable of producing more seeds than cultivated parents...", however, they only report biomass and viability of seeds produced, not the number. Culley and Hardiman (2007) support the report of low seed set per fruit: "Individual flowers are protandrous... consist of five sepals, five petals, two sets of 10 anthers each that differentially dehisce, and two to five carpels (Cuizhi and Spongberg 2003) with two ovules per locule. This produces a maximum seed number of 10, although the actual number is usually between 2 and 6." It is unclear what the fruit set actually is, variously noted as "rare" but now noted to be "abundant" in invasive populations.
Reference(s): 
15. Is there significant germination (>25%) of seeds the next growing season, with no requirement of an infrequent environmental condition for seeds to germinate (i.e. fire) or long dormancy period?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Answer / Justification: 
There is some indication of dormancy, but this would be broken by appropriate climate conditions where the species can occur, not infrequent conditions. Germination rates were found to be high.
Reference(s): 
16. Does this plant produce viable seed within the first three years (for an herbaceous species) to five years (for a woody species) after germination?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Can set seed at 3 years of age.
Reference(s): 
17. Does this plant continuously produce seed for >3 months each year or does seed production occur more than once a year?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
Culley and Hardiman (2007) give detailed documentation of reproduction: "Fruits take several months to develop and remain on the tree until they mature in early to late autumn (August to October).
Reference(s): 
Dispersal
18. Are the plant’s propagules frequently dispersed long distance (>100 m) by mammals or birds or via domestic animals?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Attractive, sugary fruit, listed as bird dispersed in several sources.
Reference(s): 
19. Are the plant’s propagules frequently dispersed long distance (>100 m) by wind or water?
Yes or No: 
No
Points: 
0
Confidence Level: 
Low
Answer / Justification: 
Unlikely that this would be dispersed by wind or water given the heavy fruit.
Reference(s): 
20. Are the plant’s propagules frequently dispersed via contaminated seed (agriculture or wildflower packets), equipment, vehicles, boats or clothing/shoes?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Seeds are dispersed from ornamental trees via birds. Seeds are not likely accidentally introduced by the vectors above.
Reference(s): 
Evaluation Notes

Several sources note that this invasive type is a result of hybridization of several horticultural types. Most invasive species websites treat this as one type, under Pyrus calleryana. Culley et al. (2001): Ultimately, this study indicates that increased spread of P. calleryana has been initiated by introduction of multiple cultivar types and subsequent widespread planting and is not the result of an inherent fi tness advantage of hybrid progeny"

Mid-Atlantic States Invasive Site: https://www.nps.gov/plants/alien/pubs/midatlantic/pyca.htm

Invasive.org: http://www.invasiveplantatlas.org/subject.html?sub=10957

GISD: http://www.iucngisd.org/gisd/speciesname/Pyrus+calleryana

Reviewed by Gina Darin.

Total PRE Score

  • < 13 : accept (low risk of invasiveness)
  • 13 - 15 : evaluate further
  • > 15 : reject (high risk of invasiveness)

PRE Score: 
16
Number of questions answered: 
20
Screener Confidence (%): 
57.0
PRE Content Access and Privacy
Evaluation visibility: 
Public - accessible to all site users

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