Cucumber tree (Magnolia acuminata L.) recovery strategy: chapter 2

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BACKGROUND

OMNR Status: Endangered (regulated)

8.1      Description of the Species:

The Cucumber Tree is the only native Magnolia species in Ontario.  It is a forest species which can grow to a height of 30 m in its Ontario range.  The leaves are large, simple and without teeth.  They can be 10 to 24 cm in length and half that in width.  Large greenish-yellow flowers emerge in early summer and are pollinated by beetles and other insects.  The fruit matures in late summer and is composed of many red, fleshy pods, each containing one or two seeds.  The tree is named for the slight resemblance of the immature fruit to a cucumber. 

9.1      Global Range: 

Cucumber Tree is a minor forest tree species of eastern North America, with its central distribution from south-eastern New York to northern Georgia, with outlying populations occurring in southern Ontario to Florida  (including Indiana, Illinois, Missouri, Oklahoma and Louisiana) (see Figure 1).  

Figure 1.   Natural Distribution of Magnolia acuminata in North America (after Ambrose, 1987).

9.2       Canadian Range:

Cucumber Tree only occurs in two regions of southern Ontario: Town of Pelham in the Regional Municipality of Niagara and south and west of the Town of Simcoe in Norfolk County (see Figure 2). 

9.3       Percent of Global Distribution in Canada: 

It is unlikely that the Ontario populations represent more than a fraction of a percent of the entire distribution, since populations in their central range are not designated and thus assumed to be more abundant.  Isolated populations occur in peripheral areas, some with sufficient restriction to warrant listing by state authorities.

9.4        Distribution Trend : 

It is difficult to determine the rate of change in geographic distribution because of a lack of complete historical records and a recent intensified effort to search for Cucumber Trees.  Although the number of records has increased in recent years, this does not necessarily represent an expansion of the species range in Canada.  This increase is more likely a result of the intensified effort to study the species.  Of the 31 known locations for Cucumber Tree in Ontario, 8 have been lost since 1930; 4 were in Norfolk County and 4 were in Niagara Region. 

10.1      Global Range:  

This species is considered secure globally with a rank of G5, thus there are assumed to be numerous populations of this species in its central Appalachian range. While widespread in eastern North America, it is listed as S1 in three states (Indiana, Oklahoma and Florida), S2 in Ontario and secure in only two states (North Carolina and Pennsylvania: S5).   In all other states Cucumber Tree remains unranked (S?) or only as reported without rank (SR), according to the current NatureServe (2004) web site.  See Table 2.

Table2.  Natural Heritage Rankings for Magnolia acuminata (NatureServe, 2004)

G: global, N: national, S: sub-national

1: Critically Imperilled

2: Imperilled (i.e., extremely rare or especially vulnerable)

3: Vulnerable (i.e., rare and uncommon)

4: Apparently Secure (i.e., uncommon but not rare)

5: Secure (i.e., common, widespread, and abundant)

?: Unranked

R: Reported (i.e., reported but not ranked)

10.2       Canadian Range : 

There is some evidence that Magnolia acuminata has experienced decline in Canada, specifically Ontario, over the past century.  Historical records suggest Cucumber Tree was previously more abundant in both regions of Ontario than at present.  Some sites are confirmed to have been lost within the past 20-70 years (Ambrose & Aboud, 1983); (see section 19).   Over the past 20 years thorough and systematic searches have been made of Cucumber Tree populations by OMNR staff, consultants and scientists.  During this same period new populations have been discovered by forestry staff of OMNR and private landowners.  In the past two years more thorough field surveys have led to the discovery of one new population and additions of individual trees and saplings to known populations.  Recent mapping has confirmed the presence of 283 trees and saplings on 32 properties (or 13 endangered plant communities (EPC), since some share the same EPC).  While the loss of a few individual trees has been documented, there likely has not been a significant decline of this species since its designation, due to the increased awareness of its jeopardy and increased actions for its protection.

A report of a tree near Ipperwash, already cut down when reported by Fox and Soper (1952), was possibly erroneous.  It may have been confused with Tulip Tree (Liriodendron tulipifera) which is abundant in that area.  John Goldie, in his journal of 1819, states “in this part of the country [New York State] the inhabitants call it [Liriodendron] Cucumber Tree.”

Table 3 presents all available data on extant and extirpated populations of Magnolia acuminata in Canada that are believed to be naturally occurring.  It should be noted that for the purposes of this recovery strategy, populations are considered to be independent if separated by one kilometre or more, and that groupings of trees separated by less than one kilometre are considered sub-populations (NHIC, March 2001).  The sighting for Ipperwash in Lambton County will be assumed to be erroneous unless more supporting evidence can be found.  The absence of this species in adjacent Michigan and north-western Ohio (Voss 1985; Barnes & Wagner, 1981) leads to further questioning of this sighting.

10.3      Percent of Global Abundance in Canada:

The percent of global abundance in Canada is not known, however is estimated to be less than one percent .  

10.4      Population Trend: 

The rate of population change cannot be accurately measured due to incomplete records prior to 1978.  However, based on complete records since 1978, there has been a documented loss of 13 trees from 4 sites and regeneration noted at 8 sites.  This indicates that there has not likely been a significant decline of abundance in recent years.

Table3.  Estimated Abundance of Magnolia acuminata L.  in Canada 

 

11.   Biologically Limiting Factors:

Pollination:

Out-crossing promotes higher seed set (Ambrose & Kevan, 1990).  While this species is not fully self incompatible, the reduced seed set of individual trees reduces the reproductive potential of isolated, small populations.   Furthermore, the pollinators may further limit reproductive potential; only preliminary information is known about the beetles and other insect pollinators of this species (Thien, 1974; Ambrose & Kevan, 1990).   Better defining the major pollinators and understanding their foraging behaviour will help assess what impact forest management practices and habitat fragmentation may have on limiting out-crossing in this species. 

Seed dispersal :

Primary seed dispersal is most likely by birds.  The fruit has the typical characteristics of bird dispersal and van der Pijl (1969) gives Magnolia as an example of this syndrome.  However, it is not know what species of birds may be responsible for seed dispersal in Ontario populations.   How the seeds are consumed and dispersed have important implications for the distance of individual dispersal events and thus the genetic mixing or isolation of sub-populations.   However, frugivorous birds have been observed to disperse more seeds in treefall gaps than the surrounding forest (Hoppes, 1988) which is beneficial for shade intolerant species such as Cucumber Tree. Squirrels and small ground 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 have contact with.  While germination of some seeds with impervious seed coats can be improved with scarification, this is not the case with this species (Kock, 1998).

Shade intolerance:

Some opening of the forest canopy appears to be important for effective seedling recruitment.  Forest management that leaves small forest openings has been shown to be effective at promoting natural regeneration in southern Ontario (OMNR, 2000).  Currently a study is underway by the Long Point Region Conservation Authority (LPRCA) and the UofG, to compare regeneration under different forest canopy conditions, some of which have been opened by experimental forestry operations (Reader and de Gruchy, pers. comm, 2001).

Fragmentation of habitats:

Many populations are isolated due to habitat fragmentation, which likely reduces cross-pollination, range of seed dispersal and effective population size.  Fragmentation likely also impacts habitat for pollinators and seed dispersers thus reducing optimal pollination and seed dispersal.  Actual habitat loss appears to be a less significant threat, possibly because many of the habitats are in wet, headwater area woods that have been left when higher land was cleared for agriculture.

Low Connectivity and Small Population Size:

Less than optimal connectivity between sub-populations and their small population sizes may reduce opportunities for gene exchange.  Over time, this can result in a species’ decreased ability to adapt to change and possibly extirpation or extinction.  Several Cucumber sites have small populations that are isolated from others and two populations appear to be based on single parents. 

ForestManagement:

Poor forest management can be detrimental to the health and survival of Cucumber Tree populations.  The effects include indiscriminate cutting, bark abrasion to trees adjacent to skidder trails or felled trees, smothering of saplings by slash piles, soil erosion and soil compaction.  However, selective harvesting using good forestry practices can provide conditions which promote regeneration and most landowners respond positively to this knowledge about the species.

Alteration of Soil Moisture Regime:

Cucumber Trees are primarily found in moist to mesic forests with imperfect to well-drained soils and they do not tolerate overly wet or overly dry conditions.  Some reports claim that although Cucumber Tree is moderately drought tolerant, prolonged alteration of soil moisture may adversely affect the tree’s survival, especially in mature specimens.  Although there are no reports of a decline in Cucumber Trees resulting from this threat, its potential should be considered when activities are undertaken nearby.

Table 4.  Evaluation of Human-induced Threats to Magnolia acuminata

 

13.1     Habitat Needs:

The generalized habitat characteristics of viable populations of this species in Ontario are an upland moist deciduous or mixed forest, often in a headwaters area with undulating topography of low swampy areas interspersed with rises (Ambrose & Aboud, 1983).  The established trees occur in the better drained areas within this matrix and regeneration is seen where the forest cover is partially open.  Table 5 provides a summary of vegetation community information gathered at each Cucumber Tree population for the purpose of habitat mapping (Dougan & Assoc, 1998; Ambrose, 2000). In addition to overstory and understory species characterization, corresponding ELC codes and general forest community descriptions are provided.  

Table 5 .  Summary of Species Noted in Habitats Occupied by Magnolia acuminata L. in Ontario Populations.

13.2     Critical Habitat: 

SARA defines critical habitat as “the habitat that is necessary for the survival or recovery of a listed wildlife species”.  In order to identify critical habitat, a comprehensive understanding is required of the species habitat requirements at all stages of its life.   Sufficient information is not currently known to accomplish this in its entirety.  As a result, Critical Habitat is identified to the extent currently known and a schedule of studies has been developed to identify the necessary information required to complete the identification (see Section 13.4).  The schedule of studies is not an exhaustive list and it is expected that further questions will arise as research proceeds.  It is expected that modification of the Critical Habitat identification may occur as a result of further research and a population viability analysis (PVA). 

Critical habitat for Cucumber Tree is being identified, to the extent possible, and includes the vegetation communities in which the populations occur, as defined by the characteristics of those populations in the two meta-population areas that have 10 or more mature trees and are showing successful regeneration.  Provincial habitat mapping guidelines (MNR 1998) have been developed for Cucumber Tree for the purposes of the Conservation Land Tax Incentive Program, which could be applied to map critical habitat.  The habitat identified for this exercise included the occupied habitat and the extent of the vegetation communities (based on Ecological Land Classification) in which a Cucumber Tree population occurs (referred to as the Endangered Plant Community in CLTIP guidelines).  If the Endangered Plant Community is less than 100m from the occupied habitat and adjoins a natural plant community, then a buffer is included to extend the area to be protected up to 100m. These areas are critical to the survival of the species and will form the basis of its natural expansion into recovery habitats once threats are alleviated. 

In addition to the habitat description in section 13.1, the critical habitats are further characterized by the presence of the following associated species, listed in order of highest frequency: Acer rubrum, A. saccharum, Betula papyrifera, Fagus grandifolia, Prunus serotina, Quercus rubra, and Tsuga canadensis.  The following are occasional associates: Amelanchier arborea, Betula alleghaniensis, Castanea dentata, Fraxinus americana, Lindera benzoin, Liriodendron tulipifera, Pinus strobus, Quercus alba, Quercus velutina, and Sassafras albidum.  

Cucumber Tree is listed as Endangered under the federal Species at Risk Act and is regulated under Ontario’s Endangered Species Act.  This provides protection to the tree and its habitat.  Ontario’s Provincial Policy Statement also offers protection by not permitting development or site alteration in its significant habitat.

List of Critical Habitats: 

NorfolkCounty:

1.     Smith Tract and adjacent properties

2.     Shining Tree Woods (NANPS)

3.     Long Point National Wildlife Area

RegionalMunicipality of Niagara: 

4.     NorthFenwick Footslope Forest ANSI

5.     FenwickSlough ForestWoodlot

6.     NorthwestFenwick ForestANSI

In addition to the critical habitats identified above there are other extant populations that for various reasons are considered marginal for regeneration.  These include populations of a few or single tree or populations which are restricted by a very limited area of suitable habitat (e.g. Lynedoch and Walsh).  These also include several remaining Element Occurrence sites that are documented in the NHIC database.  However most if not all of these latter populations are now extirpated (Ambrose & Aboud, 1983).  Some of these sites may present opportunities to increase the viability of the population if the habitat remains suitable for re-introduction and the landowners are agreeable.  They may be included in the critical habitat if Cucumber Trees are confirmed or the characterization of the species’ habitat has not been altered and it is determined they are necessary for recovery, based on the results of the research activities identified in Section 13.4.

List of Other Habitats: 

NorfolkCounty

Extant:

1.     Lynedoch

2.     Walsh

3.     Baker Tract

Extirpated:

1.     Fairground, 0.5 km west of, last two trees cut in early 1980s

2.     Forestville, 3.3 km NNW.  Last seen in late 1950s

3.     Green’s Corners, south of (1948)

4.     Lynedoch, 2 km WSW of (1992).  Woodlot logged in 1992. (Dougan, 1998).

5.     Turkey Point (1932) 

RegionalMunicipality of Niagara

Extant:

1.     NorthPelham Valley

2.     Fonthill Kame

3.     Memorial Drive

Extirpated:

1.     Hurlburt’s Woods near Fonthill (1947).

2.     Three Mile Woods (1942)

3.     West Lincoln Tp. (1978)

4.     St. Catharines, near (1897)

5.     St. David’s roadside (1998)

More information is needed on characterizing optimal habitat so suitable unoccupied habitat can be delineated in the regions of known populations to focus management, restoration and possible introduction efforts.  The occurrence of small populations in isolated forest patches also suggests that there is a minimum habitat area to support a viable population.

13.3    Examples of activities that are likely to result in destruction of the critical habitat

The primary activities that will likely result in destruction of the critical habitat are:

·       Forest clearing and fragmentation (although small patches may assist Cucumber tree germination and regeneration).

·       Activities which alter drainage patterns, ground water flow and/or soil moisture levels.

13.4    Schedule of Studies     

Due to the lack of information required to identify the critical habitat of Cucumber Tree in southern Ontario, a schedule of studies has been developed (see Table 6).  This will direct efforts to acquire the necessary information to complete the description of critical habitat.  These activities will be completed under the direction of the Recovery Team and Ministry of Natural Resources staff.

Table 6 .  Schedule of Studies 

 

As a large forest canopy tree, it is an important forest component of the eastern deciduous forest ecosystems of the broader Appalachian regions of the United States.  While very limited in Canada’s Carolinian life zone, it can be locally abundant in mountain valleys in the Carolinas and Tennessee (Sargent, 1922).  Being shade intolerant in wet forest habitats, it is able to fill gaps from wind thrown trees or other disturbance.

The Carolinian Zone of Canada is an area of high interest for naturalists due to its high biological diversity, especially of southern species such as Cucumber Tree, other plants and associated animals, and thus supports a significant ecotourism industry.  The wood of Cucumber Tree is similar to and marketed with that of Tulip Tree (the lumber of which is known as Yellow Poplar).  It is fine grained and smooth, with applications for such things as Venetian blinds, furniture, doors and trim.

Some private landowners may be hesitant to allow protection, and access for management and study.   Landscape restoration will depend on defining need, working with local groups such as the Long Point Biosphere Reserve Restoration Program and the Niagara Natural Heritage Corridor Program, and finding willing landowner participants.  This will be a challenge, considering the intensity of habitat fragmentation and land utilization in these two regions.

17.1   Survey Requirements:

This species was the subject of an intensive mapping project undertaken by the Ontario Ministry of Natural Resources for the purpose of endangered species regulation and qualification under the Conservation Land Tax Incentive Program. Thus, the locations and basic demographic information is known for most if not all populations in Canada (Dougan Assoc., 1998; Ambrose, 2001).  Additional habitat mapping will be undertaken for newly regulated sites, however, closer attention will be given to defining critical habitat.

17.2   Biological/Ecological Research Requirements :

The pollination and reproductive biology of this species is only marginally understood (Ambrose & Kevan, 1990).  Further studies are needed to understand its biology and assess where its reproductive potential is being reduced by factors such as habitat fragmentation and small population size.  Further, the population genetics of this species may shed light on impacts of habitat fragmentation in relation to population viability and where best to focus recovery activity to reduce the impacts.  Additional research is required to evaluate the demographic rates (survivorship, recruitment, dispersal) and the trends and fluctuations in these rates to improve the ability to determine the population and distribution required for Cucumber Tree recovery and to fully identify the Critical Habitat.

17.3   Threat Clarification Research Requirements : 

Preliminary studies in the spring/summer of 2002 and 2003 were undertaken to better understand the pollination biology of this species, especially its pollinators and pollination mechanisms, as well as assess demographic data with site characteristics and thereby better define further research needs (Kevan, 2003).  Analysis of additional information being compiled by the Long Point Region Conservation Authority at the Smith Tract will be useful for understanding habitat definition and optimal conditions of seedling recruitment (LPRCA, 2005)

 

Recovery of the Cucumber Tree is considered both biologically and technically feasible. The mechanics of propagating and growing individual trees is well documented (Kock, 1998), but not likely necessary.  Regeneration has been observed in several sites and OMNR management trials have been successful in enhancing recruitment (Ambrose & Aboud, 1983).   The recovery of populations to a more stable level awaits better understanding of this species’ ecology (especially pollination, seed dispersal and recruitment), including a better quantification of distances for gene exchange and general habitat needs, and how best to manage habitats to promote these ecological functions and the species’ population viability.   Once the genetic composition of populations is better known, increasing the genetic diversity of small or single parent populations may be warranted.

Habitat characterization:

By better understanding the site factors that define an optimal habitat, this species’ habitat could be restored on nearby appropriate physical sites and thereby the potential recovery habitat could be enhanced.   Seedlings could be reintroduced into extirpated sites or other sites identified as appropriate.  However, at this time it is questionable that this latter activity is warranted. 

Landscape level habitat restoration:

Restoration on a landscape scale has the potential to link nearby isolated habitats of populations and improve their long term viability, such as through Carolinian Canada’s Big Picture program (Jalava et al., 2000).  There may be sufficient habitat for occupancy, but not for pollinators, dispersers or maintenance of occasional genetic exchange between sub-populations.  Landscape level restoration could address these concerns and improve long term population viability of this and other Carolinian species in jeopardy.

There are many reasons for considering the recovery of this species in the context of its natural landscape.  It typically occurs in sensitive headwater areas, often in association with other plant species at risk (e.g., Aplectrum hyemale, Castanea dentata, Panax quinquifolium) and likely animal species that depend on interior forest and/or woodland ponds.  While occurring in or near wetlands has likely discouraged land clearing for the expansion of agricultural or other land use, the clearing all around many sites has led to fragmentation of and disconnection from nearby Cucumber Tree populations.  Further, as development or agricultural pressures increase, these low forested habitats may come under pressure to clear and drain to the detriment of the species that depend on them as well as viability and ecological functioning of the land to support adjacent human use. 

For example an agricultural drain runs through the wet woods just to the east of the Baker Tract and though a sub-population in the North Fenwick Forest ANSI.   As an example of how species’ occurrences can change with land practices, land surveys of 1805-15 in southern Illinois illustrated that Cucumber Tree was one of the main witness trees (Leitner & Jackson, 1981).  Today it has a very limited distribution (and is ranked S?) and critically imperiled (S1) in adjacent Indiana.

Thus, the recommended approach is to take a landscape level approach to natural heritage values and promote the restoration of integrity to the natural landscape.  It will serve to address the impacts of fragmentation on this and other species at risk, and to provide an environmentally sensitive framework for future land development.  This would follow the concepts of the Carolinian Canada Big Picture project, as well as local initiatives within the zone (e.g., the Long Point Biosphere Reserve restoration program, Niagara Natural Heritage Corridor program).

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Ambrose, J.D. & P.V. Kevan, 1990.  Reproductive biology of rare Carolinian plants with regard to their conservation management, in G. Allen et al. (eds.) Conserving Carolinian Canada, proc. of a workshop, Univ. of Waterloo Press, Waterloo.

Barnes, B.V. & W.H. Wagner, 1981.  Michigan Trees, University of Michigan Press, Ann Arbor.

Dougan & Associates, 1998.  Endangered species habitat mapping study & landowner correspondence: Magnolia acuminata (Cucumber Tree)  54pp. + maps.

Fox, W.S. & J.H. Soper, 1952. The distribution of some Trees and Shrubs of the Carolinian Zone of Southern Ontario. Trans. Royal Can. Inst. 29(2):65-84.

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Kevan, P.G.  2003.  Sexual reproduction as a constraint to recovery in Magnolia acuminata.   Final report to OMNR, Species at Risk program.  11pp. 

Kock, H.  1998.  Growing Native Plants from Seed, 10^th edition.  The Arboretum, University of Guelph.

Lee, H.T., W.D. Bakowsky, J.Riley, J. Bowles, M. Puddister, P. Uhlg and S. McMurray. 1998.  Ecological Land Classification for Southern Ontario: First Approximation and Its Application.  Ontario Ministry of Natural Resources, South Central Science Section, Science Development and Transfer Branch.  SCSS Field Guide FG-02.

Long Point Region Conservation Authority.  2005.  Cucumber Magnolia Species at Risk Project, Smith Tract 2004/2005 (project proposal)

Leitner, L.A. & M.T. Jackson, 1981. Presettlement forests of the unglaciated portion of southern Illinois, USA. Am. Midl. Nat. 105(2):290-304.

MNR. 1998.  Guidelines for mapping endangered species habitat under the Conservation Land Tax Incentive Program.  Natural Heritage Section, Lands and Natural Heritage Branch, MNR, Peterborough, Ontario.

Mullin, T.J. & S. Bertrand, 1999.  Forest management impacts on genetics of forest tree species. Ontario Ministry of Natural Resources, Southcentral Science Section, Technical Report 113.

NatureServe Explorer: An online encyclopedia of life [web application]. 2001. Version 1.6 Arlington, Virginia, USA: NatureServe. Available: http://www.natureserve.org/explorer. (Accessed: July 22, 2002).

OMNR, 2000.  A silvicultural guide managing southern Ontario forests, Version 1.1. Ont. Min. Nat. Resour. Queen’s Printer for Ontario, Toronto.  648 p.

Sargent, C.S. 1922. Manual of trees of North America. Dover, New York.

Thien, L.B. 1974. Floral Biology of Magnolia. Am. J. Bot. 61:1037-1045.

Thompson, R.J. 1992.  Status report on Cucumber Tree [internal MNR report update]

Van der Pijl, L.  1969.  Principles of dispersal in higher plants.  Springer-Verlag, New York.

Voss, E. 1985, Michigan, Flora, part II. Cranbrook Institute of Science.  Bloomfield Hills

White, D. J. 1998. Status Report Update, Cucumber Tree, Magnolia acuminata.  COSEWIC, Ottawa.  6pp.