Dwarf hackberry (Celtis tenuifolia) COSEWIC assessment and status report: chapter 6
Biology
General
Dwarf hackberry is a drought-tolerant small tree or shrub occurring in open woodlands, including alvars and near-shore sand deposits. It is wind-pollinated, bird- dispersed and a food plant for a number of host-specific and often rare insects.
Reproduction
This species is monoecious with unisexual flowers, solitary or in clusters of two or three. The flowers, appearing in late May to early June, are wind-pollinated. The fruit, a drupe with a single seed and sweet mesocarp, matures in late summer and is primarily bird-dispersed, but mammals, also, are known to consume the fruit of Celtis (Cypher and Cypher, 1999). Self-compatibility was confirmed on one isolated individual in the author’s garden in Guelph. Elais (1970) suggests that some species of Celtis may be apomictic but no confirmation of this is known. No other form of asexual reproduction was observed. Three of the populations occur with common hackberry, and forms that appear to be intermediates exist, which suggests the possibility of hybridization between these two native species.
Survival
Individual trees could be expected to survive for several decades. Dunster (1992) aged 16 of the largest trees at Northville (part of the Grand Bend population) and Point Pelee; she found ages between 40 and 64 years (adjusted to the height of coring). Trees in dynamic locations are likely more short-lived; along the west shore of Point Pelee and Fish Point the loss of trees has been documented following changing lake levels and severe storms. Dunster (1992) found a high level of infestation of bark beetles and an annual mortality associated with the beetles of 10% at Point Pelee, during two years of observation. She also found very low survival for 1-3 year old seedlings.
Physiology
This perennial woody species occurs primarily below the southern limits of glaciation, with the outlying populations in the Great Lakes region being the exception. One would expect that the individuals in these populations might be limited by the occurrence of localized moderate microclimates. However, in cultivation, the species thrives much further to the north, with known specimens in Ottawa and Guelph. In Guelph, seedlings readily establish from a single parent tree in nearby uncultivated areas. Thus, it is not restricted even at early life stages from far cooler climates than in its natural range. Most of the sites where it occurs are dry and calcareous, and thus with a high pH. Exceptions include a few sites on sand dunes in the Grand Bend area where the soils are slightly acidic (Dunster, 1992), although Wagner (1974) found pH readings of 8+ at this site. The adaptations of this species allow it to maintain a relatively high rate of photosynthesis during prolonged droughts even though it exhibits reduced stomatal conductance (Kubiske and Adams, 1993).
Dispersal
Dwarf hackberry is wind pollinated. Its seeds are likely most effectively dispersed by fruit eating birds (who consume the whole fruit and defecate the seed later), though mammals are no doubt responsible for some local dispersal. Dunster (1992, Figure 2) maps the occurrence of the Great Lakes disjunct populations and the Mississippi flyway (i.e., of migrating birds), showing the high coincidence between the two. Raccoons (Procyon lotor) have been documented as ingesting Celtis occidentalis fruit, but germination rates of seeds were significantly lower that non-ingested controls (Cypher and Cypher, 1999); however, it was recognized that the benefits of dispersal may outweigh a reduction in germination rate.
Nutrition and Interspecific Interactions
This species thrives on dry and often calcareous sites. The local site conditions may help maintain open wooded habitats, reducing the development of heavy shading that would likely cause it to decline.
A number of butterflies, gall insects, a scale, twig borers and bark beetles use this species as their host plant. Many have a minor impact but some, especially the bark beetles (Bright et al., 1994), are associated with a significant level of mortality (Dunster, 1992); one species of gastropod was observed voraciously browsing on seedlings (Dunster, 1992).
Adaptability
Dwarf hackberry tolerates dynamic conditions with periodic disturbance as well as drought and poor calcareous stony or sandy soils. It can be cultivated easily and transplanting cultivated plants for re-introductions should not be a significant challenge, if such action is considered a viable recovery strategy.
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