Rare Species Guide

  Synonyms

Crataegus tomentosa

  Basis for Listing

Crataegus calpodendron (late hawthorn) is endemic to forested portions of eastern and central United States and southern Ontario (Phipps 2014). There is no indication it is rare across its entire range; however, available evidence leads to the conclusion it is rare in Minnesota.

Determining the conservation and management needs of C. calpodendron in Minnesota is complicated by a lack of historical information. An analysis of herbarium specimens collected by field botanists over time usually reveals patterns or trends, but not in this case. Authentic herbarium specimens of C. calpodendron from Minnesota are very scarce. Although this absence of specimen data is some indication of rarity, it is widely known that thorny stems and complicated taxonomy discourage some field botanists from making collections of hawthorns. 

Early written reports of the species from Minnesota (Clements et al. 1912) are even less helpful. They were largely based on information we now know to be incorrect, leaving us with little historical context with which to evaluate the current status of this species.

Based on what we have learned from modern observations and specimen collections (Smith 2008), it appears that C. calpodendron exists in Minnesota as small and scattered populations surviving in fragmented habitat remnants. These remnants occur primarily in the southeast portion of the state (Eastern Broadleaf Forest Province), where recent increases in human populations and development activities are often in conflict with conservation goals.  For these reasons, C. calpodendron was listed as special concern in 2013.

  Description

Crataegus calpodendron is typically a large shrub, or occasionally a small tree, with ascending branches, slender twigs, and numerous compound thorns on the lower portion of the trunk. The leaves are elliptical to broadly elliptical or rhombic, 5-10 cm (2-4 in.) long, and 3.3-6.5 cm (1.3-2.6 in.) wide; the base is acutely to obtusely angled or occasionally somewhat rounded. There are 15-45 white flowers in each inflorescence, and the sepals are glandular-serrate or glandular-laciniate. There are 15-20 stamens per flower, and the anthers are pink or red. The fruit is an orange-red broadly ellipsoidal pome, 6-9 mm (~1/4 in.) long. For certain identification, look for the combination of 15-20 stamens, reddish anthers, relatively large pointed leaves, and petioles that are densely hairy on all sides.

  Habitat

Crataegus calpodendron is strongly associated with mesic-hardwood forests in the southeastern portion of the state, where it is typically found in mesic loamy soil, with Ulmus spp. (elms), Tilia americana (basswood), Fraxinus spp. (ashes), or Acer saccharum (sugar maple). It does occur in alluvial soil in river valleys, though usually not on active floodplains, and not in fine silts or coarse sands. It also seems to be absent from historically fire-prone landscapes such as prairies, savannas, oak ecosystems, and pine forests.

  Biology / Life History

Crataegus calpodendron was given the common name of “late hawthorn” because it is the last hawthorn in Minnesota to flower each year, usually during the second or third week in June, which is at least a week later than any other hawthorn. It also has the smallest fruit of any Minnesota hawthorn, only about 6-9 mm (~1/4 in.) in diameter. Edge-grown plants often have multiple stems and may reach a height of about 5.5 m (18 ft.). When suppressed in a forest understory, it usually has only a single stem, 2-3 m (7-10 ft.) tall. Suppressed specimens often persist for long periods and may flower frequently, if not every year, making this one of the most shade tolerant of the hawthorns. Seedlings, however, are rarely found in deep shade, indicating that openings or edges are required for reproduction. Pollination is presumably accomplished by flying insects, though few details have been reported in the literature. The flower has no specialized structures, which would indicate a limited pollinator relationship, leading to the conclusion that a variety of bees, or bee-sized insects, likely serve as pollinators. Dispersal of the seeds is likely facilitated by birds, which eat the fruit and spread the seeds in their droppings (Smith 2008).

  Conservation / Management

Crataegus calpodendron presents some unusual conservation and management challenges. This species is clearly associated with remnants of the large expanse of mesic hardwood forest that once covered the southeast portion of the state. Yet C. calpodendron does not reproduce well, if at all, under a dense canopy of Acer saccharum (sugar maple), which is the tree that typically dominates these habitats. Since conservation of these habitat remnants typically favors sites with a uniform and unbroken canopy of sugar maple, the specific habitat for late hawthorn is often (unintentionally) excluded. Larger tracts that have a significant amount of topographical relief and a greater amount of micro-habitat included within the forest would provide much better conservation value for C. calpodendron. Within this type of site, no specific management would be needed to maintain a population of C. calpodendron; it would be able to find the small edges or gaps needed to serve as a seedling nursery. Any attempt to artificially create such gaps through commercial logging activity would likely fail because natural regeneration of the canopy trees would begin immediately and would provide too much competition for seedlings of C. calpodendron; they would simply grow too slowly to keep up with the saplings and root suckers of canopy trees.

  Best Time to Search

The best time to search for C. calpodendron is when flowers are present, which is typically between June 10 and June 20. Secondarily, plants can also be identified when ripe fruit are present, typically from September 14 to September 22.

  Authors/Revisions

Welby Smith (MNDNR), 2018

(Note: all content ©MNDNR)

  References and Additional Information

Clements, F. E., C. O. Rosendahl, and F. K. Butters. 1912. Minnesota trees and shrubs: an illustrated manual of the native and cultivated woody plants of the state. Volume IX. University of Minnesota, Minneapolis.

Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 394 pp.

Minnesota Department of Natural Resources. 2006. Tomorrow's habitat for the wild and rare: An action plan for Minnesota wildlife, comprehensive wildlife conservation strategy. Division of Ecological Services, Minnesota Department of Natural Resources. 297 pp. + appendices.

Phipps, J. B. 2014. Crataegus. Pages 491-643 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 9. Oxford University Press, New York, New York.

Smith, P. G., and J. B. Phipps. 1987. Studies in Crataegus (Rosaceae, Maloideae). XVI. Taxonomy of C. series Rotundifoliae in Ontario. Canadian Journal of Botany 65(12)2646-2667.

Smith, P. G., and J. B. Phipps. 1988. Studies in Crataegus (Rosaceae, Maloideae). XIX. Breeding behavior in Ontario Crataegus series Rotundifoliae. Canadian Journal of Botany 66(10)1914-1923.

Smith, W. R. 2008. Trees and shrubs of Minnesota. University of Minnesota Press, Minneapolis. 703 pp.