Root Distribution of Nursery Trees and its Relationship to Transplanting Success

Methods

Eighty-eight trees of 7 species (Table 1) growing in the Natural History Survey arboretum, Urbana, Illinois, were dug with a Vermeer 44 mechanical tree spade and the root systems examined. The wall of the cone-shaped hole was examined for roots of 1 cm diameter and greater. The location, depth and size of the severed roots were plotted on a map representing the outer sur-face of the root ball. These data were compiled for all trees of each species, with respect to soil depth and quadrants relative to north.

The distribution of fine root development was determined by a core sampling method. Three species, Norway maple, green ash and ginkgo, were sampled in July, 1980. The soil cone dimensions were 7.5 cm in diameter and 20 cm long. Each tree was sampled on four sides at the edge of the original root ball. On one side of each tree, randomly selected, a second core was taken immediately below the first, to a depth of 40 cm. The soil-sampling procedure was repeated for another ring of cores adjacent to the outside edge of the first ring of cores. Each of the upper samples was divided into equal upper and lower portions. For each tree, root samples of similar locations and depths were combined and treated as one sample.

After removing all soil and the roots of herbaceous plants and other woody species, total length measurements were taken on each sample. Newman’s line intersection method (13) was used for estimating the total length of root in the sample. This method is based on the principle that as the length of a sample increases the more intersections will be made with a predetermined set of lines, and the actual length can be calculated mathematically by counting these intersections.

Root System Measurements

Directional distribution of major roots

Based on the cumulative root distribution data, five of the seven species developed the highest count of roots in the northern quadrant with ginkgo and ash roots being as/more abundantly formed in the east quadrant (Fig. 1). Combined totals for all seven species showed that the north quadrant contained the highest root counts. Individual species showed wide variations for root distributions in the east, south and west quadrants. It should be noted that ginkgo was the only tree species growing in a different section of the nursery and was planted with different spacing. The ginkgo were spaced 1.5 m × 2 m with the 1.5 m spacing running north and south and the other species were spaced 2.5 m × 2.5 m.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 1.

Directional distribution of major roots of nursery grown trees.

Vertical distribution of major roots

For all seven species, the vertical distribution measurements of total roots 1.0 cm in diameter and larger, showed the greatest development at the soil depth of 13 to 38 cm (5-15 in) with limited growth in the top 12 cm of soil and with rapidly decreasing root development at depths below 38 cm (Fig. 2). These data were adjusted to allow for variation in the circumference of the cone-shaped root ball at each sampling depth. It was noted that for some species this pattern differed slightly. Red maple root density was higher in the top 13 cm sampling layer than any other species. Green ash and redbud produced more root growth below 38 cm than the other five species.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 2.

Vertical distribution of major roots of nursery grown trees.

Vertical distribution of small fibrous roots

For the three species sampled, Norway maple, green ash and ginkgo, core samples showed that the greatest small fibrous root density (meters of root length/liter of soil) of undisturbed nursery-grown trees is in the upper 20 cm of soil, where soil aeration and soil structure would most likely be optimum for fine root development. Furthermore, the fibrous root densities of the Norway maple and the green ash were greater in the 0-10 cm soil layer than in the 10-20 cm soil layer (Fig. 3). Fibrous root growth of ginkgo was limited in the upper 10 cm of soil. This reduced shallow fibrous root growth appeared to correspond to competition from dense turf and other surface vegetation.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 3.

Fine root distribution of nursery grown trees.

Discussion

The evidence that the north side of the tree develops a more extensive root system is convincing, on the basis of the total data compiled on the seven tree species. The fact that ginkgo developed an equal amount of roots in both the north and east quadrants (Fig. 1), probably resulted because these trees were growing in an area where the space between trees was less to the north and south than east and west. The ash developed stronger lateral growth at greater depths than the other species studied and root competition apparently was not an influencing factor. Thus, root competition between trees may have played an important role. Factors which may cause this asymmetric root development of evenly-spaced trees seem most likely to be environmental in nature, such as sun orientation or wind direction. Until the exact nature of the cause of this asymmetric distribution is determined, replanting trees in the same directional orientation would seem advisable. This would minimize the environmental changes above and below the ground surface.

The literature on root distribution consistently reports the highest densities of roots in the upper soil horizons and the results of this study are in agreement. The large number of roots encountered at depths as great as 38 cm can be attributed to the deep loam soil conditions in the nursery where these studies were made. Since most root regeneration originates near severed root ends, vertical distribution appears to be an important influence for regeneration in particular soil types. It is reasonable to expect that most tree species tending to be more deeply rooted would not be as tolerant to heavy clay soils having seasonal high-water tables, a condition which exists in many urban soils. Also, shallow-rooted tree species would not be ideally suited to soil conditions where the upper soil layers are subject to long periods of low water potential. These generalities are especially true for recently transplanted trees; however, after a few years growth, most trees that survive transplanting develop a substantial root system able to tolerate rather severe environmental soil conditions.

The shallow nature of the fibrous root distribution of these nursery trees prior to transplanting is in agreement with other reports of root distribution (2, 3, 16). These data add to the growing evidence of the shallowness of the development of smaller roots of certain species and the need for minimal disturbance (physical or environmental) of the soil around trees to avoid extensive injury to the root system and the ultimate loss of the tree.

Ginkgo fibrous root density in the upper 10 cm of soil was lower than root densities at the 10-20 cm level. Heavy herbaceous vegetation, primarily blue grass, occurred in this area. Apparently grass root competition was an important factor. This evidence indirectly supports previous work in which mulching the soil around trees increases root growth by reducing or eliminating grass competition and by helping maintain adequate soil moisture and aeration in the upper soil layer (11, 19). Grass competition would be especially detrimental to trees growing in areas where the topsoil is shallow and consequently fine root growth is already somewhat restricted by the nature of the soil.

Using the data presented here in conjunction with field observations from the partial excavation of roots of these trees and other studies, a model of a typical nursery-grown tree was constructed (Fig. 4). This figure illustrates the drastic loss of total root system which occurs when the tree is dug from the nursery. In this model, the original root system was exploiting approximately 20 cubic meters of soil to absorb its water and nutrients. After transplanting, the soil volume available to the tree is 0.35 cubic meters (the volume of the root ball). The root ball then represents 2% of the original soil volume. It is easy to understand why rapid regeneration of the root system of transplanted trees is essential for survival and for re-establishment of a favorable root-shoot ratio.

• Download figure
• Open in new tab
• Download powerpoint

Figure 4.

Illustration of a typical root system developed by using field data collected from trees growing in a nursery. The shaded area represents a tree spade soil ball 112 cm in diameter and 102 cm deep. The tree illustrated has a trunk diameter of 4 inches measured at 6 inches above ground height.