Recovery Strategy for the Dwarf Hackberry in Canada [Proposed] 2011: Species Information

Dwarf Hackberry is a small, deciduous¹ tree or shrub-tree with light grey bark and upright stiffly- divaricate² branches and twigs. It typically reproduces sexually. From wind-pollinated, hermaphroditic³ flowers, it produces small, round, orange-brown fruit with a single seed (Farrar 1995, Ambrose 2003, Waldron 2003).

1 Deciduous trees shed their leaves each year.

2 Divaricate branches and twigs spread apart at a wide angle.

3 Hermaphroditic flowers have both male and female sexes borne on a single flower.

Key characteristics of the distribution of Dwarf Hackberry in Canada are:

• The species reaches the northern limits of its distribution in southern Ontario, over 1 000 km north of the geographical centre of its range (Figure 1) (Dunster 1992).
• Six isolated populations, all extant, have been identified: Pelee Island, Point Pelee National Park, Lambton County, and three in Hastings County (Point Anne Alvar⁴, Stirling Slope Complex Area of Natural and Scientific Interest [ANSI], and the Salmon River Alvar [ANSI] [previously known as the Lonsdale population]) (Figure 2 and Table 1).
• The species' distribution in Canada is believed to be naturally fragmented, with intervening areas of seemingly suitable habitat (e.g. Caradoc, Bothwell, and Norfolk sand plains) apparently unoccupied.
• Distribution patterns coincide with two harsh, but naturally occurring substrate types that are limited in their availability (the dry sands of dynamic shorelines, more stable inland dunes, and kame ridge tops and dry limestone alvar/broken bedrock sites) as well as major migratory bird flyways in Ontario (Dunster 1992).
• The overall Canadian extent of occurrence (approximately 26 587 km²) exceeds the threshold of 20 000 km² for the Threatened category under criteria B of COSEWIC, while the area of occupancy (about 18.5 km²) is well below the threshold of 500 km² for the Endangered category.

aka = also known as, [ANSI] = Area of Natural and Scientific Interest, CA = Conservation Authority, NP = National Park, PNR = Provincial Nature Reserve

Key characteristics of the sizes and trends of Dwarf Hackberry populations in Canada are:

• The estimate of 893 mature individuals reported in Canada (Ambrose 2003) is now believed to be an underestimate. New information is available as follows:
• A detailed 2007/8 survey of the Lambton County Heritage Forest, L-Lake Management Area, and Thedford (now Ausable Cut) Conservation Area in the Port Franks/Northville (Lambton County) area documented an estimated 7 074 mature trees plus 1 518 saplings and seedlings for a total of 8 592 Dwarf Hackberry plants. Based on time of year, time constraints, and survey methodology, this is still believed to underestimate population size (Mills and Craig 2008). The current Lambton County population size is now estimated at 13 083 trees of all age classes (Mills and Craig 2008, MacKenzie pers. comm. 2010, Ausable Bayfield Conservation Authority unpub. data, Nature Conservancy of Canada unpub. data), but again is likely underestimated.
• More detailed surveys stretching from Grand Bend south to Kettle Point in Lambton County, including a resurvey of the above three sites, are expected to locate a much larger population, including new sites, than has been documented to date (Boyd pers. comm. 2009, Craig pers. comm. 2009, Wilkes pers. comm. 2009).
• Approximate sizes of the other five populations are: Pelee Island (12 trees – Ontario Ministry of Natural Resources [OMNR] unpub. data), mainland Point Pelee National Park (47 trees – Jalava et al. 2008), Point Anne Alvar (10 trees – Ambrose 2003, Natural Heritage Information Centre [NHIC] unpub. data), Stirling Slope Complex [ANSI] (54 trees – Ambrose 2003, Parks Canada Agency unpub. data), and Salmon River Alvar [ANSI] (Lonsdale - 5 trees – Ambrose 2003). However, surveys of some private lands at the Point Anne Alvar site have not been undertaken. Further field work is required to evaluate other areas along the ancient Lake Iroquois shoreline in Hastings County where suitable Dwarf Hackberry habitat may exist.

• Overall population trends are considered relatively stable, despite previous concerns raised (Ambrose 2003):
• Although it requires verification, the Point Pelee National Park mainland Dwarf Hackberry population is believed to be experiencing a decline (Ambrose 2003, Jalava et al. 2008).
• Only a few individuals appear to reach a maximum age of 40 to 65 years (Dunster 1992), with much mortality of juveniles and some continuing losses of reproductive individuals due to natural and human disturbances (Ambrose 2003).
• Although the Pelee Island Red Cedar (Juniperus virginiana) Savanna Dwarf Hackberry site has been lost, with the disappearance of the single, mature individual that was located there, no extirpations of any of the six known populations have occurred. The Pelee Island Dwarf Hackberry population persists at Fish Point.
• While it is possible that some Lambton County population gains noted since 2003 may be attributed to a decline in White-tailed Deer (Odocoileus virginianus) abundance, lack of other predators, and/or changing climatic conditions that favour increased recruitment and population expansion, the substantial increase in population size does not likely represent population expansion, but rather is the result of more rigorous surveys locating trees that were already present.
• Range wide, two of six populations have less than ten mature individuals, making them susceptible to extirpation from natural or human disturbances. However, because Dwarf Hackberry is apomictic or self-fertile (Whittemore and Townsend 2007), it is possible for these populations to persist in very low numbers for many decades, as has been noted in Hastings County (Ambrose 2003).

4 In the Great Lakes basin, "alvar" refers to naturally open areas with shallow soils over relatively flat, limestone bedrock, with trees absent or at least not forming a continuous canopy (Reschke et al. 1999, Brownell and Riley 2000).

In Canada, Dwarf Hackberry occurs in open habitats on dry sand or limestone-based substrates. In sandy sites, it occurs in open habitats near the dynamic, early successional shores of Lake Erie (Point Pelee and Fish Point [Pelee Island]), in open woods further inland on the ridges and south facing slopes of the more aged and stabilized dunes adjacent to Lake Huron's shoreline (Grand Bend to Kettle Point, Lambton County), and on kame ridge top prairies and savannas above the Trent River (Stirling Slope Complex [ANSI]). In limestone-based sites (alvars), Dwarf Hackberry occurred in open woods, maintained by extreme droughty conditions, in the interior of Pelee Island (Red Cedar Savanna), and continues to occur at the Point Anne Alvar and the Salmon River Alvar [ANSI] (Lonsdale) in Hastings County (Ambrose 2003). Other disturbances, such as trails, help, in some cases, to maintain suitable open habitat. The sandy soils where Dwarf Hackberry occurs tend to have a large calcareous component and a pH above 7. At the most inland site (Stirling Slope Complex [ANSI]), soils are dry to wet-mesic sand and gravel loams, with a slightly acid to neutral pH (NHIC 2010d).

Several plant communities in which Dwarf Hackberry occurs are considered rare to extremely rare (e.g. shrub and treed sand dunes, oak savannas, and Red Cedar treed alvars), and have limited distribution in southern Ontario (Ambrose 2003). Oak savannas are known to require occasional fires to ensure maintenance of the ecosystem. Although the fire ecology of Dwarf Hackberry in Canada is unknown, research from the United States indicates that prescribed burns in Dwarf Hackberry habitat resulted in the emergence and persistence of the species where it was previously absent (Taft 2003). Sand dune habitats require the active disturbance mechanisms of wind, wave, and ice action to maintain them in the early successional stages favoured by Dwarf Hackberry. Location (dune ridges) and aspect (south facing slopes), along with wind throw, fire, insect infestations, and/or disease may help to maintain suitable habitat on older, more stabilized dunes. Alvar habitats are prone to extremes of temperature and moisture that make them inhospitable for many species, but suitable, in at least some locations, for Dwarf Hackberry.

As a moderately shade intolerant species, the availability of light becomes a limiting factor for the species as ecosystem disturbances naturally slow or are altered by humans. Forest openings and edges appear to be important for effective seedling germination and recruitment, as is some level of soil disturbance. Within Dwarf Hackberry habitat, fruits fall onto microsites that have the following optimum conditions necessary for seed germination and seedling survival:

• the site is protected from desiccating (drying) winds and burial by shifting sand;
• the soil contains humus, which retains moisture and provides nutrients;
• seeds are covered, but not buried, by leaf litter or plant debris, which reduces the chances of consumption by wildlife and provides protection from frost kill; and
• the site is positioned to receive the required amount of light and heat necessary to break dormancy and allow the seedlings to photosynthesize (Dunster 1992).

Primary long-distance seed dispersal is most likely carried out by fruit-eating birds, although the species responsible in Canadian populations are unknown. Germination is stimulated by weakening of the seed coat by acids in a bird's gut (Dunster 1992) and fruit-eating birds have been observed to disperse more seeds in tree fall gaps than the surrounding forest (Hoppes 1988). How the seeds are consumed and dispersed have implications for the distance of individual dispersal events, and thus the genetic mixing or isolation of sub-populations. Squirrels, mice and other small mammals may also play a secondary dispersal role, but they are more likely to consume or otherwise damage a higher proportion of the seeds that they come in contact with. Long-distance seed dispersal is therefore believed to be a limiting factor (Dunster 1992).

Since Dwarf Hackberry was designated as Threatened (COSEWIC 2003), additional threats have come to light, including the role of altered disturbance regimes in accelerating habitat succession, the grazing impacts of snails, inappropriate logging activities, development, certain recreational activities, and others (Section 4.2.9). Threats to the species were reassessed in 2009 at a recovery strategy writing workshop. Major threats are presented in order of priority in Table 2. The overall level of concern (high, medium, or low), extent (range wide or local), occurrence (current or anticipated), frequency (one-time, seasonal, continuous, recurrent, or unknown), severity, and causal certainty of the threats on Dwarf Hackberry populations are presented in the table for each.

? = Unknown; H = High; M = Medium; L =- Low

At many sites, a lack of natural disturbances (fire) or altered patterns, frequency, and severity of disturbance regimes (coastal processes at Point Pelee National Park and Fish Point [Pelee Island], wind throw and disease) threatens Dwarf Hackberry and its habitat. European settlement brought with it fire suppression. The resultant resource (water, nutrient, and light) competition from other plants (native, clonal, allelopathic⁵, exotic, and/or invasive tree, shrub, and plant species), has accelerated habitat successional processes and canopy closure within the open vegetation communities favoured by Dwarf Hackberry, altering habitat conditions and displacing Dwarf Hackberry. This is happening within the largest populations, located in Lambton County, Point Pelee National Park and Fish Point (Pelee Island), and is also suspected to be an issue at some Hastings County locations. Decreased flowering and germination rates are suspected.

Extensive shoreline protection and alteration has disrupted the natural coastal processes that shape the dynamic Point Pelee and Fish Point sand spits. The western shoreline of Point Pelee National Park was historically an accreting or growing shoreline. Between 2004 and 2006 however, it eroded an average of 5.5 m per year. The Colchester to Southeast Shoal Beach Nourishment Study (Baird 2010) determined that, without erosion mitigation measures and sand replenishment in the littoral cell, 126 hectares could be lost from the western shore within the next 50 years. Increased erosion means less land is available for Dwarf Hackberry germination and that trees are closer to the shore and more susceptible to the uprooting action of storm waves and ice.

5 Allelopathic plants suppress or inhibit the growth of other plants through the release of chemical toxins.

Between 1989 and 1991, there was an epidemic infestation of the large reproducing Dwarf Hackberry trees in Point Pelee National Park. Tree mortality was documented at 10% per year for a two-year period, rising to 17% in the third year (Dunster 1992). Seven native species of bark beetles (Hickory Bark Beetle [Chramesus hicoriae], Beech Bark Beetle [Scolytus fagi], Hackberry Engraver Beetle [Scolytus muticus], Asian Ambrosia Beetle [Xyleborinus saxeseni], Black Stem Borer [Xylosandrus germanus], an ambrosia beetle [Hypothenemus eruditus], and Phloeotribus dentrifrons) were found. The Asian Ambrosia Beetle was also recorded from the Port Franks (Lambton County Heritage Forest [Lambton County]) area (Dunster 1992, Bright et al. 1994). Bark beetles are integral species in forest ecosystems and are attracted to trees by chemicals, called monoterpenes, produced in resin, and released from freshly damaged bark. Bark beetles are known to detect stress in plants and invade weakened trees. Adults burrow through the bark to lay eggs, while their larvae burrow and feed under tree bark. Natural predators of bark beetles include woodpeckers and parasitic insects. Given the impact of other bark beetles (16.3 million ha. of dead Lodgepole Pine [Pinus contorta] in British Columbia due to the Mountain Pine Beetle [Dendroctonus ponderosae] and millions of dead trees in southwestern Ontario and the Great Lakes states due to the Emerald Ash Borer [Agrilus planipennis]), this threat should not be taken lightly. The reasons for a stress event at Point Pelee National Park are unknown and could be attributed to many factors such as climate change, plant overcrowding, and loss of bark beetle predators. The extent to which these beetle species also utilize Common Hackberry (C. occidentalis), which occurs in habitat adjacent to Dwarf Hackberry, is also unknown. Bark beetle problems were not observed during the 2002 survey to update the COSEWIC report (Ambrose pers. comm. 2009). The presence and current level of impact at sites where these beetles were originally detected is unknown. Several of the bark beetle species are known to be expanding their ranges northwards from the United States (Dunster 1992), and warmer winter weather due to climate change could result in another northwards expansion toward the Lake Huron population.

Seedling grazing by the native Webbhelix [syn. Triodopsis] multilineata, a very rare to rare species tracked by the NHIC, was observed in epidemic proportions in 1989/90 at Point Pelee National Park and is considered a major threat to population demographics. In an experiment, 62% of 365 Dwarf Hackberry seedlings were chewed to the ground by three snails within 24 hours of having been located (Dunster 1992). Whether this snail species is present and having the same level of impact today is unknown.

High intensity grazing is thought to be related to the high density of a species in one place (Ridley 1930, quoted in Harper 1977). Since Dwarf Hackberry is not an abundant species at Point Pelee National Park, it may be a victim of snail grazing due both to its proximity to the abundant Common Hackberry and to the morphological similarity between the two species (Dunster 1992). It is suspected that grazing events may be episodic, potentially related to climate conditions that stress all hackberry species and lead to epidemic grazing (Dunster 1992). Consistent seedling grazing reduces both population numbers and the opportunities for seedlings to become reproductive adults that yield seed for local recruitment and long distance dispersal.

It is uncertain whether the seed bank contains sufficient seeds to perpetuate the population at Point Pelee National Park if continued snail grazing occurs. No snails were observed around Dwarf Hackberry in the Lambton County Heritage Forest (Lambton County) and it is unknown if snail grazing is an issue at any other site in Lambton County or within the Hastings County populations.

Active forest management can lead to habitat loss or creation. Uninformed, poorly planned and/or implemented forest management can be detrimental to the health and survival of Dwarf Hackberry populations. Negative effects include understory removal and the breaking, crushing, or bark abrasion of Dwarf Hackberry trees due to the felling and skidding of trees; smothering of seedlings by slash piles; soil erosion and compaction; reforestation with monoculture pine stands or other inappropriate species; and loss of habitat connectivity. Over time, this can result in a species' decreased ability to adapt to change and its potential extirpation from sites.

Habitat conversion through land development for cottage, rural residential, agricultural, and commercial purposes poses a moderate threat to Dwarf Hackberry populations. The Lambton County population south of Grand Bend, the largest in Canada, currently faces the greatest development pressures. The northeast corner of the Salmon River Alvar [ANSI] (Lonsdale), where Dwarf Hackberry is located, is also being surrounded by development. Habitat fragmentation leading to less suitable habitat between populations and sub-populations and the respective dwindling number of individuals may reduce opportunities for seed dispersal and gene exchange, leading to genetic bottlenecks and a reduction in population size. Over time, this can result in a decreased ability to adapt to change, which can lead to extirpation. Fragmentation likely also impacts habitat for seed dispersers, reducing optimal seed dispersal through loss of suitable habitat.

Sand and limestone removal has the potential to lead to habitat loss, fragmentation, and isolation in four of six populations. The Point Anne Alvar population in Hastings County occurs on an alvar, portions of which are within a privately owned, active, licensed, limestone quarry. The Port Franks Road (Lambton County) site is adjacent to a sand pit used for road maintenance. Expansion of an adjacent, active sand pit is considered a threat to the Stirling Slope Complex [ANSI] Dwarf Hackberry population, while the potential for future limestone quarrying to expand from a quarry to the south of the Salmon River Alvar [ANSI] (Lonsdale) may also be a concern.

Exotic and/or invasive plants and allelopathic tree species compete with Dwarf Hackberry for water, nutrient, and light resources. Invasive, exotic species of concern at Point Pelee National Park include Garlic Mustard (Alliaria petiolata), European/Common Buckthorn (Rhamnus cathartica), Norway Maple (Acer platanoides), White Mulberry (Morus alba), and White Sweet Clover (Melilotus alba). Nitrogen-fixing species, like the latter, improve soil conditions for other species that normally could not establish themselves in the nutrient depleted environments that Dwarf Hackberry can, and therefore increase competition and shading by other species. Garlic Mustard and White Mulberry are also present at Fish Point (Pelee Island), while the former is also a concern in the Lambton County population (Grand Bend to Kettle Point). European/ Common Buckthorn is of concern in the Port Franks/Northville (Lambton County) area and at the Point Anne Alvar (Brinker pers. comm. 2010).

As hackberries are themselves known to be allelopathic, the positive, negative, and synergistic (combined) impacts of allelopathic inhibition by other species on Dwarf Hackberry trees and plant communities are unknown. However, allelopathic species that may be of concern include Common Hackberry, Fragrant Sumac (Rhus aromatica), Red Cedar, and Common Juniper (Juniperus communis). Poison Ivy (Toxicodendron radicans), Drummond's Dogwood (Cornus drummondii), Fragrant Sumac, Common Juniper, and Witch Hazel (Hamamelis virginiana) can crowd out Dwarf Hackberry through their growth habits. In the Lambton County Heritage Forest (Lambton County), areas with dense populations of Witch Hazel have little or no Dwarf Hackberry and vice versa (Mills and Craig 2008). Low populations of both species were found growing amongst each other, which would eliminate competitive exclusion⁶ as a theory.

This relationship may be due to niche differentiation⁷, or some other ecological factor. Niche differentiation might be explained by spatial partitioning of resources. If Dwarf Hackberry has a general ecological niche on the entire slope of a dune, but its realized or actual niche is only the top portion of the south-facing slope, it may be because Witch Hazel, which is a better competitor and can survive in more shady conditions, but cannot survive on the top portion of the slope, has excluded Dwarf Hackberry from the lower portion of the slope.

Competitive exclusion relates to two species competing for the same resources that cannot stably coexist if other ecological factors are constant. One of the two competitors will always overcome the other, leading to either the extinction of the weaker competitor or an evolutionary or behavioural shift towards a different ecological niche.

6 Competitive exclusion relates to two species competing for the same resources that cannot stably coexist if other ecological factors are constant. One of the two competitors will always overcome the other, leading to either the extinction of the weaker competitor or an evolutionary or behavioural shift towards a different ecological niche.

7 Niche differentiation is the process by which natural selection drives competing species into different patterns of resource use or different ecological niches, allowing two species to partition resources so that one species does not out-compete the other as dictated by the competitive exclusion principle. Coexistence is obtained through the differentiation of their realized or actual ecological niches. Resources may be partitioned though space or time.

A variety of recreational activities result in damage to Dwarf Hackberry trees of all age classes, soil disturbance, erosion, and/or the introduction of invasive and/or exotic plants into critical habitat. Unregulated recreational activities noted on private lands in the Port Franks area (Lambton County) include off-road vehicle use on steep dune slopes that may cause direct damage to Dwarf Hackberry as well as its habitat through erosion and dune blowouts. As Dwarf Hackberry seedlings often germinate in forest gaps, including trailside locations, off-road vehicle use, horseback riding, and hiking, even when confined to trails, can potentially lead to the trampling, crushing, and destruction of seedlings. Although many sites do not permit off-road vehicle use, such vehicles have been noted as a problem to varying degrees in the Lambton County Heritage Forest (Lambton County), Nature Conservancy of Canada's Van Valkenburg property (Lambton County), and Fish Point (Pelee Island) where trees occur next to a parking area on a road allowance.

Lake Erie (Point Pelee National Park and Pelee Island) and Lake Huron (Lambton County) populations occur within and adjacent to a national park and several provincial parks, nature reserves, county forests, and conservation areas. These areas receive high volumes of visitors during the summer months. Collection of firewood, even when not permitted, could be an issue where campfires and/or the use of rustic barbecues are permitted. Trail brushing and maintenance, typically a winter activity, can also have a direct impact on Dwarf Hackberry that grows alongside trails at several sites.

Other threats identified in 2009, but considered unsubstantiated, unavoidable, or currently of lesser importance to Dwarf Hackberry are:

• Unexploded ordinance removal and chemical contaminant clean-up – unavoidable, recurrent events that must be undertaken to ensure public safety at the former Camp Ipperwash Military Reserve (Lambton County) have a high likelihood of destroying some Dwarf Hackberry trees and habitat. Population level impacts are expected to be small. Application of specific measures identified in the Canadian Environmental Assessment Act assessment may mitigate the impacts.
• Chemicals - seasonal fertilizer and herbicide use (low causal certainty, severity, and level of concern) adjacent to the Lambton County Heritage Forest (Lambton County).
• Deer and domestic animal browse – Browsing of Dwarf Hackberry by White-tailed Deer (low causal certainty, severity, and overall concern) occurs range wide. Normally confined to the winter months, it becomes a continuous threat when deer populations are high. Browsing by cows is a current and continuous concern of medium causal certainty, low severity, and low overall level of concern along the south edge of the Lambton County Heritage Forest west of the Port Franks Road (Lambton County).
• Climate change – Climate change is widespread, current, and continuous. However, its potential effects on Dwarf Hackberry are unknown. Drier conditions could provide Dwarf Hackberry with a competitive advantage over Witch Hazel. However, if changes are too severe and quick for Dwarf Hackberry to adapt, extirpations may result. Seedlings in already harsh alvar environments may not be able to survive greater extremes of dryness and heat. Drought conditions are already thought to be responsible for making the species more susceptible to attack by detrimental species at Point Pelee National Park (Dunster 1992). In addition, severe drought could kill other dune-stabilizing vegetation, causing erosion, dune destabilization, and blowouts that would result in the loss of plants and habitat. In addition, climate change is expected to intensify storm events, increasing wind and wave driven shoreline erosion, while reducing the amount of ice cover and associated scour. On the other hand, if Dwarf Hackberry survives the new and changing environment, seeds could be dispersed to suitable, but previously unoccupied, habitats further north of its current range extent.
• Road construction – the construction of Outer Drive and Port Franks Road 3 likely had historic impacts on the Lambton County Dwarf Hackberry population. At the former Camp Ipperwash Military Reserve (Lambton County), temporary roads will be used during clean-up of the unexploded ordinance and contaminants. They will be located, to the extent possible, in areas that do not contain species at risk in order to avoid population-level effects.