Study Background
Since 1994, a number of different studies have been performed that looked at the correlation between depth of soil over roots and the health of trees and root systems. The results showed that soil depths of more than 1 inch over the first main order root of many trees studied have significant, adverse effects on the tree's health and root system. It was found that when tree roots were buried too deeply, trunk and canopy health decline was often a result, i.e., the development of cankers, splitting bark, and frost cracks on trunks and dieback and stunted growth in tree canopies. These high soil depths also created the environment for stem girdling roots and encircling roots to occur.
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Stem girdling roots are roots that encircle the stems of trees below the ground. These roots compress the stem and block off vascular tissues prohibiting water, nutrients, and sap from passing between the canopy and roots. Encircling roots, for the sake of this research, are defined as those that encircle or grow parallel to the stem within 6 inches and may become problematic in a relatively few years. If enough of the stem is compressed, this can result in tree death.
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Because planting depth can have such an adverse effect on tree health and root systems, a current study is being performed that focuses on the prevention of these problems. Stem girdling roots and encircling roots can cause serious decline in tree health. It is important to understand when in the tree's life span these dysfunctional roots form, when in its life span to intervene and try to correct the problem, and who can best benefit from this information whether it be nursery growers or homeowners. This study aims to address these issues in hopes of providing information to tree planters and growers so that declines in tree health and dysfunctional root formation that can be prevented will be recognized in time. This information will help promote practices that will improve the health of the urban forest.
Purpose
The study is being conducted at the Teaching, Research, Extension Nursery on the University of Minnesota campus in St. Paul. There are three main purposes of the study:
1. To observe to what extent depth of soil over roots affects tree health and/or influences the formation of dysfunctional root systems.
2. If dysfunctional root systems develop, to observe how soon this occurs.
3. To observe how the results vary between species.
The principle investigators from the University of Minnesota that are responsible for the research are: Gary R. Johnson, Department of Forest Resources, Jeff Gillman, Department of Horticulture, Chad Giblin, Department of Horticulture, David Hanson, Department of Forest Resources, and Patrick Weicherding, University of Minnesota Extension Services.
Stage One: Tree Species
Two tree species were used in the first stage of the study that came from the same wholesale nursery:
Littleleaf Linden (Tilia cordata)-
This is a lowland species that sends out well-established stem adventitious roots within 30 days. Adventitious roots are roots that grow where they normally are not supposed to, in this case, on the stem. |
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Sugar Maple (Acer saccharum), This is an upland species that normally does not send out stem adventitious roots. |
Using these two species is significant to the study not only because these are two species commonly used in landscape design, but also because adventitious roots are often the primary reason that stem girdling roots develop. Using these two species will further determine the role that adventitious roots play in tree health when soil depth is a factor.
Treatments
180 trees of each species were planted in May and June of 2000. The trees were whips, or unbranched nursery stock. Before they were planted, root pruning was performed on each specimen to ensure no dysfunctional root systems were present. Holes were custom dug for each tree and they were then planted at 3 different depths with 60 trees of each species planted in each treatment.
Treatment 1: 0" Depth (Trees planted with roots at ground line.) 60 Sugar Maple, 60 Littleleaf Linden
Treatment 2: 5" Depth (Trees planted with 5" soil over first main order root.) 60 Sugar Maple, 60 Littleleaf Linden
Treatment 3: 10" Depth (Trees planted with 10" soil over first main order root.) 60 Sugar Maple, 60 Littleleaf Linden
These planting depths were chosen as treatments because 0", 5", and 10" of soil over roots are depths commonly found in landscapes. Soil type, texture, and fertility is uniform on the site leaving the only variables of the study to be tree species, planting depth, and time.
Design
The trees were planted in a complete randomized block design with 12 blocks of 30 trees in each. Mulch was placed around the bases of the trees and pulled away form the stems. The rows of mulched trees were separated by aisles of sod to imitate a landscape setting. Each year, data is taken on each tree regarding: mortality (tree death noted), dieback after winter (dead branches counted), growth rate (stem caliper measured) and sucker production (number produced counted).
Harvest
In June of 2003, 1/3 of the trees planted in 2000 were harvested and data was taken after observing the root systems. 4 of the 12 blocks were randomly chosen and all of the trees in those 4 blocks were harvested. Trees were harvested using an air knife in order to blow soil away from the roots and allow them to be dug up with the root systems still intact. The roots of each tree were then inspected for: stem encircling roots, % stem impacted by stem encircling roots, stem girdling roots, and % stem impacted by stem girdling roots.
This data is currently being analyzed to see if various depths of soil over roots and difference in tree species significantly influenced the formation of dysfunctional root systems or adversely affected the health of the specimens. To see how time influences the results of the study, the remaining 2/3 of the planted trees will be harvested in 3 year intervals; 1/3 in 2006 and the final 1/3 in 2009.
Stage Two
To take advantage of the empty spaces on the site created due to harvest and/or mortality, 4 other tree species of varying calipers were planted in June 2003. These species are: Green Ash (Fraxinus pennsylvanica), Bicolor Oak (Quercus bicolor), Honeylocust (Gleditsia triacanthos), and Bur Oak (Quercus macrocarpa). The trees were planted at 0", 5", and 10" depths and will be harvested at 3 year intervals.
