NEIGHBORHOOD GARDEN FOR JANUARY 26, 2008
GREATER TREE DIVERSITY NEEDED
BY STEVE SCHWAB
LINCOLN CITY FORESTER
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.
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.
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.
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
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,
crab, and flowering
pear have been extensively overplanted.
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.
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.
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 (arboretum.unl.edu/plantdata) 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.
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.