From Tree Hugger to Bug Hugger

Is your sleep disturbed by worries about disappearing spotted owls and snail darters? Maybe the objects of your concern are too grand. What you should really be worrying about is bugs: native bugs and the native plants that sustain them. That is the argument advanced by professor of entomology Douglas Tallamy in his recent book, Bringing Nature Home: How Native Plants Sustain Wildlife in Our Gardens.(1) He says that we are starving our insects by replacing the native plants that they eat with exotic plants that they can't even recognize as food.

Native sweetbay magnolia. Native insect? Click to enlarge.

Starving the bugs sounds like a great idea, right? Maybe not. Tallamy presents insects as unloved heroes that transform the stored energy in plants into food (themselves) for the next level up in the food chain. In order to transform themselves into bird food, for example, insects must have food of their own to eat. But we are depriving them of food by replacing the native landscape with foreign plants. The vast majority of our insects are dietary specialists; the only foods they are capable of eating are members of the native plant community with which they have co-evolved over millions of years. Those are the foods to which their anatomy and physiology are adapted. They have as much success eating foreign plants as we would have if we tried to adopt the koala bear's diet of eucalyptus leaves. Without native plants, the insects have nothing they can eat, and they perish. With the insects gone, all of the food chain above insects will collapse.

American elm 1267 Park Avenue

The differences between the numbers of insects that can eat native versus foreign plants can be startling. Native oaks can provide food for over 500 different native insect species. Eucalyptus, an exotic, is eaten by only one native insect species. Native plants in Pennsylvania were found to support 35 times more caterpillar biomass, the preferred source of protein for most bird nestlings, than alien plants supported.

Native northern red oak, New Jersey state tree, West 8th St

How big a problem can this insect starvation be? Biodiversity depends on space. The more space, the larger the number of species that can be supported. The relationship is linear.(2) Only 3-5% of the lower 48 states remains undisturbed habitat for plants and animals. The rest has been paved, farmed, taken over by noxious foreign weeds like kudzu and Japanese honeysuckle, or transformed into suburban gardens dominated by exotic shrubs and vast lawns of non-native grasses. Tallamy points out that suburban lawns cover about 62,000 square miles of this country, an area more than eight times the size of New Jersey that is devoted to alien grasses.(3) Worse, 43,000 square miles of blacktop has been spread over the landscape, equal to five and a half New Jerseys. If we have eliminated much of the native vegetation from 95-97% of the American landscape, we can expect to lose 95-97% per cent of our native flora and fauna over time, as extinction adjusts the number of species to the land area that remains. Tallamy cites the toll of habitat destruction on Delaware, where he teaches. As of 2002, Delaware had lost 78% of its freshwater mussel species, 34% of its dragonflies, 20% of its fish species, and 31% of its reptiles and amphibians. Forty per cent of all native plant species in Delaware are threatened or already lost.

American holly East 9th Street

At this dismal point in his book Tallamy offers a ray of hope: suburban gardeners to the rescue! If suburbanites, who control a large swath of the landscape, were to plant native plants on their properties, the countryside could still support a diverse flora and fauna. Mix some American elms in with all those Zelkovas. Make your lawn smaller, and plant a meadow. Tear out some of your English ivy and Japanese pachysandra and plant mayapples. I repent ever having written that foreign, kousa dogwoods have the advantage of not being attacked by the borers that plague native flowering dogwoods (a good example of just what Tallamy is talking about). How petty of me ever to have planted exotic hollies instead of American hollies because of the Americans' problems with leaf miners! I'm going to be beating myself up for years over this. But as part of my rehabilitation program, I will spread the word about Tallamy's book, essential reading for anyone who plants.

Native black walnut leaf nibbled by, no doubt, native insects

(1) Tallamy, DW. Bringing Nature Home: How Native Plants Sustain Wildlife in Our Gardens. Timber Press, 2007.

(2) Tallamy cites the work of Michael Rosenzweig at the University of Arizona to support his claim that species diversity decreases in proportion to the loss of available space. His very cursory discussion of this important underpinning for his argument is, for me, the weakest part of his book. I would like to have seen Rosenzweig's data and analysis described in detail.

(3) If you would object that Kentucky bluegrass is a native, you would be mistaken. Its seeds were imported by European settlers in the digestive tracts and droppings of cattle.

Copyright Gregory Palermo

Elm envy

The devastation wrought by Dutch elm disease in the middle of the last century left Americans largely bereft of our favorite street tree. We lost no time in trying to find substitutes that could reproduce the look of American elm and resist Dutch elm disease. Japanese Zelkova, Zelkova serrata, was seized upon as a tree that could replace American elms along roadways.

Zelkova was little known until a few decades ago. It is rare to encounter a mature one. In the last few decades the tree has become a very popular elm substitute. The tree shares the American elm's vase-like habit, but Zelkova's vase is typically squatter. The habit is also stiffer. American elm limbs undulate as they ascend. Not so the limbs of Zelkova, which lacks the elm's sinuous grace.

The tree has a handsome and distinctive bark. The young bark has very prominent lenticels(1), horizontal striations, that are reminiscent of the bark of some cherry trees. Older bark is rough and peeling.

The unusual bark and vase-like habit make this species easily recognizable even when it is leafless.

One peculiarity of Zelkova that has the potential to cause trouble as the tree matures is the density of its branching pattern. An entire forest of limbs emerges from the trunk a short distance above ground level. The Central Avenue tree pictured above illustrates the tremendous branching density. The Putnam Avenue Zelkova pictured below shows how numerous limbs typically originate in a bunch.

One wonders what is going to happen to all those limbs when they get large. You would have to bet that some will be crowded out and will drop to the street. Professor Edward. F. Gilman of the University of Florida (formerly of Rutgers) has drawn attention to this problem. In a presentation at the New Jersey Shade Tree Federation meeting in 2006, Professor Gilman spoke of Zelkova as a special case among shade trees in that it needs careful pruning for the first two or three decades of its life in order to develop a healthy form. Is he being overly pessimistic? Hard to know. There just aren't that many mature Zelkovas around on which to base a judgment. If he is right, the enormous vogue that Zelkova now enjoys is going to lead to large maintenance expenses for the municipalities planting them in great numbers. The only mature Zelkova I know is at 9 Remington Avenue in Edison (off Park Avenue). That tree has a diameter at breast height of about three feet and is quite a beautiful specimen. Pruning history? I haven't a clue.

Another American elm substitute that is growing in popularity, but still a rarity, is lacebark elm, Ulmus parvifolia. Like Zelkova, lacebark elm is resistant to Dutch elm disease. Kathryn Uhrich and Jeff Holmes recently planted two lacebark elms in the curbside strip in front of their house at 1441 Evergreen Avenue. They are the only lacebark elms I know in Plainfield. The peeling bark of this species is exceptionally beautiful. These two elms are an exciting addition to Plainfield's urban forest.

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Siberian elm, Ulmus pumila, is yet another elm with resistance to Dutch elm disease. There is an example of this species near the Muhlenberg Hospital Park Avenue parking lot. The first tree on your right as you turn off of Park Avenue onto Laramie is a mature Siberian elm.

Michael Dirr's Manual of Woody Landscape Plants, the Bible for many gardeners, calls this species "one of...the world's worst trees" because of its susceptibility to damage by wind and insects.(2) I think Dirr is exaggerating. I have seen many attractive Siberian elms (not in Plainfield, where it is uncommon), and I find the coarse, chaotic bark quite attractive.

Finally, some people are still planting American elms. Dutch elm disease-resistant varieties include 'Princeton', 'Liberty', and others.

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(1) Lenticels are loosely bundled groups of cells that permit gas exchange, allowing the tree to "breathe" through its bark.

(2) Manual of Woody Landscape Plants, fifth edition, Michael A. Dirr, Stipes Publishing 1998, p. 1048.

Copyright Gregory Palermo