Why Native, Local Trees
There are and have been, many threats to the Eastern Forest:
1) habitat loss,
2) the spread of exotic and invasive species,
3) the fall of important species (American chestnut),
4) threats faced by numerous others (oak, ash, elm),
5) changes in landscape management imposed in the last 200 years,
6) the decreased knowledge of individual species even as we appreciate the concept of species diversity, and
7) the tendency to assume that the diversity of plants at retail outlet reflects the diversity of the ecosystem as a whole..
These changes are rendering the Eastern Forest, in both suburban and remote locations, less diverse. The Great Oaks makes small and ever-improving steps at assisting homeowners in fostering diversity in the land they manage. Any landowner is a land manager, and any land in the mid-Atlantic is necessarily immersed in the web of species interactions that are all about. The Great Oaks puts in the hands of citizen foresters species options to maximize productivity of this food web that – like it or not- surrounds us.
Trees native to the Mid-Atlantic do more than any other type of plant in the world to promote diversity. Tables of Food Web Services of Native Versus Non-native Plants.
1) how many lepidopteran – butterfly and moth—species are supported by woody plants. ‘Supported’ here does not refer to providing nectar. Rather ‘supported’ means supplying the insect species with food for its larval (caterpillar) stage. While adult butterflies and moths get nectar rewards from flowers, their immature larvae are too voracious to fly from flower to flower. Like any teenagers they are busy eating, but eating leaves. This is how trees provide food for insects; their leaves are like a restaurant. But many insects have particular tastes. These tables indicate that Oaks are a restaurant that more butterfly and moth species prefer than any other.
This caterpillar has used this leaf to make the stretch to adulthood. This is the food web in action.
2) That butterfly and moth species are supported by a great diversity of trees, and that some individual tree species, such as black cherry, and American beech, support huge numbers of butterflies and moths all by themselves.
3) That non-native species of plants, no matter how long they have been present on the continent, are less able to support herbivores of any type. By extension a landscape full of exotic plants, though it may be green, might as well be concrete rubble with respect to native species looking for a meal.
These Tables were provided us by Doug Tallamy, UMD graduate and professor of insect ecology at the University of Delaware. We are using these tables, of which the first is only about butterfies and moths, to draw conclusions about insects In general. We’ll let Doug make the argument as to why this is permissible.
Why bother to plant trees that promote insect biodiversity?
The answer is that the food web extends beyond insects eating foliage. Insects are the protein source for birds. Hummingbirds visit your sugar-water feeder because they have insects available to provide them with protein, and young song birds are not fed regurgitated fruit, seeds or nuts, but rather: regurgitated insects – almost exclusively. This argument is made extremely well in a book by Tallamy, Bringing Nature Home: How Native plants Sustain Wildlife in Our Gardens, Timber Press. Portland, Oregon, 2007.
The tough part is that the species responsible for propping up the greatest insect diversity, the oaks, are in decline all across the landscape. Forest ecologist Marc Abrams of Penn State has documented this in many of his papers, here’s a quote from one of them.
“There has been almost no white oak recruitment for the last century and almost no recruitment of most of the other major upland oak species for at least 50 years. Strong competitive pressure on oak seedlings now exists from a number of species on all but the [driest] and nutrient-poor oak sites. The impact of the important mast-producing species is likely to reverberate through the ecosystem, as many wildlife and insect species depend on them for their dietary needs” (Abrams, 2003. Where has all the White Oak Gone? Bioscience 53(10):927-939.
But similar lessons can be drawn for the suburban landscape, when homeowners don’t allow volunteer trees from local native populations to grow up, but rather plant retail-purchased trees, which, if non-native, don’t reflect the biodiversity of the area, and don’t support the surrounding food web.
One of the last swamp white oaks in College Park, MD. And its in decline. When these trees are gone from this urban ecosystem, how will their unique ecosystem services be replaced?
We at the Great Oaks want to make available to the public native trees that do perform these food web services.
Pollen rarely travels more than ½ mile. Given that a tree can consider itself successful if it only one of its offspring makes it to maturity, and supposing that trees live on average about 200 years, a couple conclusions can be reached:
It takes about 200 years for tree genetics (assuming no further dispersal of seed by wind, water or animals) to move about ½ mile, and hence tree genetics are relatively local.
If you are going to plant a native tree, why do it with material originally taken from Tennessee or Ohio? The largest white oaks anywhere grown on the Delmarva Peninsula. Why would any Marylander look outside their state for a white oak tree? It beats us.
Really, the best thing a homeowner can do to support the local food web is to nuture a native volunteer of local origin. But, if there are no local volunteers, or none of the species you want, then a project like the Great Oaks may be helpful.