Transplant Shock: Severity dependent on tree size and restoration of root-rhoot balance
By Dr. Gary Watson, Morton Arboretum
It is common for a large tree to undergo a prolonged period of slow growth after being transplanted. This period of stagnancy can typically last several years. On the other hand, smaller trees transplanted at the same time will experience a shorter period of reduced vigor and in fact, may surpass the larger tree in size before the larger tree has fully recovered its normal growth rate. The reason for this difference involves the balance between the above and below ground portions of the tree.
When a tree is dug for transplanting, as much as 98 percent of its root system is left behind. When transplanted, often less than 5% of the absorbing roots are moved with the tree. While that's true for both large and small trees, larger trees lose a much greater mass and lateral spread of roots than smaller trees. Because roots of large and small trees grow at the same rate (roughly 18 inches a year) it takes the large tree several years longer to regain the size of its original root system. Thus large trees often experience a long period of slow top growth after replanting.
During this time, the reduced root system is unable to supply the quantity of nutrients and water the upper portion of the tree needs for normal growth. This is one reason adequate watering and other maintenance is critical after replanting.
This extended period of reduced vigor often results in overconcern for the survival of the tree. To the contrary this period of slow growth should be expected since the plant is being supported by such a limited root system.
A 4 inch caliper tree will require 5 years to regain the size of its original root system, which was 18 feet in diameter. A 10 inch caliper tree with a 45 foot diameter root system will need 13 years to recover. The root system of the smaller tree will have become nearly as large as that of the 10 inch tree after this 13 year period.
Because the small tree has had several years of vigorous growth while the larger tree was under stress, the smaller tree actually may have overtaken the large tree in growth by the time the larger tree had restored its root-shoot balance.
Transplanting disrupts the natural balance between roots and crown
In an undisturbed, healthy tree, a physiological balance exists between the roots (where water is absorbed) and the crown (where water is utilized), regardless of size. Tree root systems are normally very shallow and without a taproot. Most major structural roots grow nearly horizontally. The root system normally extends far beyond the drip-line. The fine, absorbing roots are located very near the soil surface. When transplanted, often less than 5% of the absorbing roots are moved with the tree (a typical size tree spade rootball is represented by the triangle shaped shaded area).
The principal of root-shoot balance is much broader than this If the root system of an established tree is damaged, it is likely that the crown will also decline.
The diameter required to regenerate the root system is dependent on the diameter of the original root system
Under favorable conditions, roots can be expected to grow approximately 18" per year. There is no evidence that size of tree has any influence on root growth rate. The illustrations below show the predicted lateral spread of the root system when transplanted and during root regeneration.
A 4" landscape tree would have a spread of 18 feet when transplanted. The small circle represents the diameter of the root ball (44" at surface). Less than 5% of the root system is transplanted in the root ball.
First year: Root system diameter increases to 6 feet, 9% of original volume. Root volume doubles in the first year, but with less than 10% of the original absorbing roots are supporting a full crown, the plant is often under severe water stress. Top growth and bud formation are inhibited.
Second year: Root system diameter increases to 9 feet, 23% of original volume. As in first year, a reduced volume of absorbing roots supporting a full crown causes water stress and inhibits seasonal growth.
Third year: Root system diameter increases to 12 feet, 41% of original volume. As balance is gradually restored, reduced water stress allows better growth.
Fourth year: Root system diameter increases to 15 feet, 60% of original volume. As diameter of root system increases, each annual increment of growth represents a greater volume of roots and effective rate of regeneration accelerates.
Fifth year: Root system diameter increases to 18 feet, 100% of original volume. Roots and crown are now in balance and tree should grow with normal vigor.
The equivalent of approximately two years top growth is lost as a result of transplanting a four inch caliper tree.