A tale of two buckeyes

When is a native plant not a native?  It might seem like an absurd question, but as more gardeners try to help wildlife by planting native species and more nurseries cater to them by selling “native” plants, I think it is a question worth considering.  Nurseries often apply the “native” label to any plant from North America, but sometimes it may be important to be more precise.  Two of my favorite small spring-flowering trees, the painted buckeye (Aesculus sylvatica) and red buckeye (Aesculus pavia) illustrate the point.

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Red buckeye (Aesculus pavia) flowering in my garden this week

The genus Aesculus includes a number of large trees such as  A. hippocastanum,  the European horse chestnut (conkers, anyone?), and A. flava, the yellow buckeye.  A. sylvatica and A. pavia are built on a much smaller scale and make great understory trees for shade gardens.  A large garden is not required.  After nine years in the ground, my A. pavia tree is only about eight feet tall, and A. sylvatica often can’t decide if it wants to be a tree or a shrub.

A. pavia has tubular red flowers, clearly an adaptation for pollination by ruby-throated hummingbirds on their spring migration up the east coast.

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Aesculus pavia flowers. A hummingbird’s delight.

A. sylvatica flowers–while they are virtually identical in shape–are yellow and have a smaller volume of more concentrated nectar, the better to attract bees [1].

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Aesculus sylvatica buds and flowers flowers are greenish yellow as they open
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Mature Aesculus sylvatica flowers are yellow with a few streaks of orange

A. sylvatica is a plant of the southeastern piedmont, ranging from southern Virginia to northern Georgia and Alabama.  Here in our part of the piedmont, it is  one of the first trees to leaf out in spring, and on moist, shaded slopes above streams and rivers,  its bright green leaves add a splash of color to woods that are still brown.

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Small Aesculus sylvatica trees (green foliage) next to a little creek near our house in late March.

The A. sylvatica trees in my garden grew from seeds that I collected in the woods adjacent to our property.  It doesn’t get much more native than that.

A. pavia, while it is also native to North Carolina, is a plant of the coastal plain.  The range of the two species in NC seems to overlap only in four counties in the south central part of the state:  Chatham, Lee, Harnett, and Cumberland Counties [2].  This overlap suggests that A. pavia‘s range has extended inland along the Cape Fear River.

Although the two species are clearly distinct, they hybridize readily in cultivation.   It would seem that hummingbirds sometimes visit yellow A. sylvatica flowers and/or bees visit the red A. pavia.  I (and the squirrels) have planted seed produced by the trees in my garden, and the first of the second-generation trees to bloom has bicolored red and yellow flowers, suggesting it is just such a hybrid.

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A probable hybrid buckeye (A. pavia x A. sylvatica) grown from seed produced by my Aesculus pavia tree.

So this creates a dilemma for piedmont gardeners who purchase A. pavia as a “native” tree.  By cultivating red buckeyes within the natural range of A. sylvatica, we have the potential to alter the gene pool of the true natives.  Perhaps a truly conscientious native plant grower should only cultivate A. sylvatica in the piedmont.

But I’m not yet ready to cut down my red buckeye for three reasons.

First, I’m not a very conscientious native plant grower.  While I try to avoid potentially invasive species and want my garden to be wildlife friendly, I grow lots of Asian, African, and South American plants that interest me.  For instance, I grow Aristolochia fimbriata, native to Brazil, rather than the native pipevines simply because I like it better and it functions just as well as a host plant for swallowtail butterflies.

Second, judging by the number of A. pavia trees that I see growing locally, in private gardens, public botanical gardens, and arboretums, this ship has already sailed.  If cultivated A. pavia interfere with the genetics of A. sylvatica, removing my little tree won’t have any significant effect.

Third, the same thing happens naturally.  In northern Georgia, an extensive hybrid zone has been identified where naturally occuring hybrids grow.  This hybrid zone overlaps the natural range of A. sylvatica but not A. pavia, suggesting that migrating hummingbirds carry A. pavia pollen from the coastal plain to the piedmont [3].  I would bet a bucket of buckeyes that there is also natural  gene flow from A. pavia to A. sylvatica in North Carolina.

References

  1.  Burke, JM, Wyatt, R., dePamphilis, CW, Arnold, CW. (2000).  Nectar characteristics of interspecific hybrids in Aesculus (Hippcastanaceae) and Iris (Iridaceae).  Journal of the Torrey Botanical Society 127: 200-206
  2. Radford, AE, Ahles, HF, Bell, CR. (1968).  Manual of the Vascular Flora of the Carolinas, University of North Carolina Press, Chapel Hill.
  3. Modliszewski, JL, Thomas, DL, Fan, C, Crawford, DJ, dePamphils, CW, Xiang, Q-Y (2006).  Ancestral chloroplast polymorphism and historical secondary contact in a broad hybrid zone of Aesculus (Sapindaceae).  American Journal of Botany 93:377-388.
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