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Pollination Problems for Southern Ontario’s Rare Trees
Peter G. KevanUniversity of Guelph
13 February, 2009
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What is Pollination?
• Transfer of pollen 1. Self-pollination (autogamy)2. Self-pollination
(geitonogamy)3. Cross-pollination
(xenogamy)
Prelude to fertilization … seeds & fruits
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What is Pollination?
• Plant Sex!
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Problems in Pollination
• 1. Too few pollinators• 2. Mates too few or far away• 3. Competition too stiff
• Conservation & Species Recovery Strategies must provides ways for plants to have sex!– No sex, no reproduction … aging, sterile, plants!
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Cucumber Tree (Magnolia acuminata)Magnoliaceae, magnolias
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Cucumber Tree (Magnolia acuminata)
Open as female phase flowers: stigmas recurved, petals upright
Petals spread and pollen is shed: male phase flowers
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Cucumber Tree (Magnolia acuminata)
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Cucumber Tree’s Sexual Problems
• 1. Self-incompatible (must cross-pollinate for fruit and seeds to be produced)
• 2. Trees mostly isolated• 3. Pollinating beetles seems scarce on flowers• 4. Seeds dispersal– by birds
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Kentucky Coffee Tree (Gymnocladius dioica) Fabaceae (Leguminosea) Pea Family
Valued ornamental, large shade tree
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Kentucky Coffee Tree (Gymnocladius dioica)
• Dioecious: • Male & Female Trees
Pollen
Stigma
Pollination seems at night:Moths with tubular tongues take nectar from tubular flowers
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Kentucky Coffee Tree (Gymnocladius dioica)
Female trees produce pods: cut down by squirrels or fall
Pods open, greenish/reddish jellyHard, hard seeds
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Kentucky Coffee Tree (Gymnocladius dioica)
• Seeds are very hard, scarify with file and they germinate well
• Grow well from root-stock, clones
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Kentucky Coffee Tree’s Sexual Problems
• 1. Plants are of too sexes (dioecious) so must cross pollinate
• 2. Trees form isolated unisexual clones too far apart for pollinators to carry pollen from male to female flowers
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Pawpaw (Asimina triloba)Annonaceae, custard apples
Little known fruit tree in Ontario
Grows in clay, wet places
Tropical family
Clonal form of small trees
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Pawpaw (Asimina triloba)
• Stinky flowers
• Pollination by Flies and Beetles• Corpse in pawpaw stand to harbour pollinators!
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Pawpaw (Asimina triloba)
• Tropical like fruit, banana-like taste & many seeds
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Pawpaw’s Sexual Problems
• 1. Self-incompatible (must cross-pollinate for fruit and seeds to be produced)
• 2. Can form large, intra-sterile, clones• 3. Populations are few and isolated• 4. Fruit and seed dispersers probably large and
medium-sized mammalsPollinators not likely a problem (filth flies and beetles)
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American Chestnut (Castanea dentata)Fagaceae, beeches
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American Chestnut (Castanea dentata)Male flowers
Female flowers
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American Chestnut (Castanea dentata)
Sterile, seedless hulls in most Ontario trees
Cross pollination: seeds
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American Chestnut’s Sexual Problems
• 1. Self-incompatible (must cross-pollinate for fruit and seeds to be produced: some self-fertile trees thought to exist
• 2. Remaining plants isolated (some unhealthy, blighted) … potential mates are too far apart
• 3. Pollinators likely small bees & wind together
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Red Mulberry (Morus rubra)
• The most endangered tree in Canada
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Red Mulberry’s Sexual Problems
• Competition from introduced & easily dispersed White Mulberry (Morus alba) – For rearing silk worms, favoured ornamental, hedge-row
tree, etc.• Both species wind pollinated, but White Mulberry
more common• Pollen from White Mulberry fertilizes Red Mulberry, hybridization occurs (introgression)• Red Mulberry ceases to exist!
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Red Mulberry (Morus rubra)
• Dioecious (trees of separate sexes) • Pollinated by wind
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Hop Tree (Ptelea trifoliata)Rutaceae (citrus family)
Host for Giant Swallowtail
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Hop Tree (Ptelea trifoliata)
Only along Lake Erie shore
Shiny leaves, look like poison ivy
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Hop Tree (Ptelea trifoliata)
Dioecious:
Male & Female Trees
Male inflorescence
Female inflorescence
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Hop Tree (Ptelea trifoliata)Fruits dry out & rattle, then dispersed on wind:Skittering across the snow in winter
Reddish bark: Contrast against snow
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Hop Tree’s Sexual Problems• 1. Grows along the shore in dense stands in
some places• 2. Trees of both sexes are equally common• 3. Lots of pollinators of lots of kinds (Flies, Bees,
Beetles)• 4. Sets lots of seeds• 5. NO SEXUAL PROBLEMS! Hop tree is rare, but grows well where it is planted
(even Montreal, Ottawa, etc). Why is its distribution so localized?
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Conclusion• 1. There is no single explanation for the sexual problems of
our rarer trees• 2. The problems come in combinations
– Too few pollinators– Mates too few or far away– Health– Populations too small, inbreeding depression– Competition too stiff, invasion, hybridization (introgression)
Conservation & Species Recovery Strategies must provides ways for plants to have sex!
No sex, no reproduction … aging, sterile, plants!Extinction!