The extensive loss of the American elm (Ulmus americana) during the past 5 decades from our communities along with recent Pine Wilt and Emerald Ash Borer infestations, have left gaping holes in many urban and rural landscapes.

Emerald ash borer (EAB) is a non-native insect currently attacking ash trees in Michigan, Ohio, Indiana, Illinois, Pennsylvania, Maryland, West Virginia, and Ontario, Canada. First identified in southeastern Michigan in 2002, EAB has already killed an estimated 15 million ash trees in that state alone. Collectively over 25 million ash trees have been killed by Emerald ash borer (EAB) and more than $100 million has so far been spent by the US Department of Agriculture on research, eradication and reforestation.

The devastation and substantial economic losses caused by, Dutch Elm Disease (DED), Emerald ash borer (EAB) and Pine Wilt has called attention to the dangers of planting monocultures, or extensive plantings relying on only a very few species. Non-diverse plantings become increasingly vulnerable by encouraging the build-up of pests and diseases.

In Nebraska, there currently are an estimated 2.2 million ash trees. Lincoln’s urban forest is approximately 25% ash, which equates to about 30,000 ash trees on city property and an estimated 120,000 ash on private property. Better management practices recommend than no more than 10% of one species is planted. Though Emerald ash borer (EAB) has yet to be confirmed and/or found in Nebraska, the over-planting of ash should be discontinued and a greater diversity of sustainable tree species planted.

Unfortunately, the lessons of Dutch Elm Disease (DED) and Emerald ash borer (EAB) are just recently being heeded. During and directly after the loss of the American elm, a replacement was sought to fill the large gaping holes in communities left by dead elms. Instead of looking for a diversity of tree species, many municipalities repeated the mistake of the past by overplanting a few species, including Lincoln. Honey locust, ash, linden, Norway maple, flowering crab, and flowering pear have been extensively overplanted.

Emphasis to plant what has been thought to be ‘perfect urban trees’ more so for aesthetics(e.g. fast growth, not “messy”, spring flowering, fall color, leaf drop all at once, etc.) instead  of the right trees for the right places is short-sighted because it does not take into consideration what trees need. In other words, what environmental factors limit the ability of particular tree species to grow and be healthy is the first and most important factor in selecting a tree.

Proper site assessment should always precede plant selection. The match-up of environmental site limitations (e.g. environmental factors such as plant hardiness, moisture requirements, soil type, soil fertility and pH, drainage, light requirements)  and the amount of rooting space, location of above and below ground utilities, sidewalks, driveways, street lighting, property lines, basement foundations,  and other existing trees and landscaping all needs to considered. The match-up of site limitations with tree adaptability is commonly called the “right tree for the right place”. By carrying out site assessments, good plant selection will make more of an impact and greater diversity will be achieved.

Looking at the native tree species in our area is another way of achieving greater diversity. These trees have developed on their own through years of self-selection to survive in our area. The Nebraska State wide Arboretum web site ( provides an excellent resource for tree selection (‘Guide to Woody Plants for Nebraska’ and ‘Underutilized Trees and Shrubs’). However, native species alone are not completely the answer for greater diversity. Some non-native species and horticulturally-developed cultivars do well in our area and also provide greater diversity.

Currently, a very few species still make up the greatest percentage of Lincoln’s urban tree population, so that the danger of monocultural plantings remains real. The need to have a thriving and sustainable diversity of tree species and ages that creates a contiguous healthy ecosystem is an important component in better managing our “green infrastructure”- Lincoln’s urban forest. There also are four factors needed to assure this happens: site assessment, plant selection/diversity, and site modification where necessary and proper planting techniques. By not following through with any one of these, repeated failures instead of successes will occur.

Copyright 2008