Print this page

Holly greens

HollyConsider the American holly (Ilex opaca). This one species illustrates one aspect of the complexity of conservation. Marlin Corn, Naturalist at the Churchville Nature Center (Bucks County), wrote about American holly—the conundrum of its being both threatened yet occasionally common—in his "Through the Eye of the Dragonfly" column in this month’s CNC newsletter, which prompts me to write about the holly here at Crow’s Nest.

American holly is both listed as a threatened species in Pennsylvania and yet is common locally. There are two simple reasons—and one complex one—why it is listed as threatened in this state. First, its native habitat is mostly in the Coastal Plain physiographic provence in Pennsylvania, most of which is the heavily urbanized Philadelphia metropolitan area. So habitat loss is one simple reason there’s not as much native holly here as there might be.

The second simple reason is that the political boundary of Pennsylvania is arbitrary with respect to plant ranges. There are more American hollies growing in other states, but Pennsylvania has only a little bit of what is thought to be the tree’s native range within its boundaries.

And the complex reason—a social factor—is that the greens are used for decorations. You can help protect American holly by asking for holiday decorations that are nursery-grown.

Yet holly is not entirely uncommon in our region. It is widely planted as an ornamental tree in our yards and—since birds eat and pass the red fruit—holly can spread and "escape" in nearby woods (Rhoads & Block, 2000. The Plants of Pennsylvania, University of Pennsylvania Press, p. 151). Holly has spread beyond the Coastal Plain by this partnership of humans planting trees and birds planting the berries.

For example, Crow’s Nest is located in the Piedmont physiographic provence—a more upland habitat that usually hosts a somewhat different community of plants than the Coastal Plain. But American holly appears here and there throughout the preserve, on the wooded floodplain of French Creek, and they are probably descendants of hollies planted in neighborhood yards.

But there is no guarantee that our "escaped" hollies are the same genotype (genetic variety) of holly that naturally occurrs in Pennsylvania—in fact it is likely they are not. Many hollies sold for landscaping are grown in nurseries in New Jersey and North Carolina and could represent genetically distinct, disjunct populations.

Ann Rhoads, the chief botanist at Morris Arboretum of the University of Pennsylvania, expressed concern about hollies she saw growing at Crow’s Nest some years ago. There is a continuum between the subset of garden plants that escape cultivation and the subset of those that naturalize (establish self-replicating populations in the wild) and then the subset of those that are invasive (grow aggressively and displace other plants to the detriment of these other species). A tree that is threatened in its native habitat can be a pest in another habitat. Wouldn’t it be amazing that a single species could be both threatened and invasive in different habitats within the same region or state?

(The tension here is perhaps in the definition of "species," since that term is a container that holds a variety of related individuals and populations, but sometimes so varied as to want to be split into different species.)

Another factor that could (in theory) threaten Pennsylvania’s native hollies is that their genetic distinctiveness could be lost by interbreeding with imported ones, creating yet other genotypes. This genetic "swamping" occurs with small remnant populations that become surrounded by a related but distinct population of the same species. I don’t know how likely this is to happen with holly, just that it has happened with other species.

One more piece of background is necessary to this story: American holly is diecious: there are male- and female-flowering trees. Only female trees produce the lovely red fruit, but to do this the female’s flowers need to be pollinated by an insect that also visited a tree (just one) with male flowers that are open at the same time.

So with the knowledge that I didn’t want to spread more holly through the preserve but wanting a holly for screening in the yard, a few years ago I bought a small American holly from a Pennsylvania nursery at a native plant conference. It was listed as a male tree—so no fruit—that would have been grown as a cutting from an open-pollinated native male tree. So this tree wouldn’t be setting seed, though of course it could be causing female trees in the same area (within the flight range of a bee, which can be considerable) to be more fruitful.

But look at the photo above. The "male" tree turns out to be bearing fruit! I don’t know much about holly, so I don’t know whether it was a simple mistake at the nursery or something else. There are plants and animals that change gender based on environmental conditions, a factor that can improve the population’s ability to sustain itself. Are these fruit viable? I haven’t experimented with them (yet).

I don’t plan to remove this holly in the near future, and I won’t discourage anyone from growing these beautiful trees, particularly if you can find a local nursery propagating Pennsylvania natives. But it does make you think about how a seemingly simple decision of what to plant could have wide-ranging consequences.


  • expand2017 (41)
  • expand2016 (141)
  • expand2015 (167)
  • expand2014 (197)
  • expand2013 (192)
  • expand2012 (241)
  • expand2011 (244)
  • expand2010 (223)
  • expand2009 (233)
  • expand2008 (201)
  • expand2007 (227)
  • expand2006 (269)
  • expand2005 (187)
  • expand2004 (5)