Comparing Root Balls on Laurel OAK Transplanted from the Wild with Those of Nursery and Container Grown Trees

Materials and Methods

Laurel oak (Quercus laurifolia) were planted from 3.8 liter containers in Nov. 1987 into an excessively-drained, medium-textured, sandy field soil in central Florida or into 36 cm diameter fabric containers (Gro-Bags, Root Control Inc., Oklahoma City, OK). Irrigation was applied daily to a 36 ft² area around each tree for two years until trees were harvested in Jan. 1990. Fertilization and pest control were conducted according to the current practices of the participating nursery where the study was conducted. Trees were not root pruned during the study. Five laurel oak, which were about the same height as the nursery-g rown trees and were growing naturally in a sandy, non-managed open field nearby (collected trees) were selected in Jan. 1990 for comparison.

At harvest (Jan. 1990), 5 collected and 5 field-grown trees were dug with a tree spade adjusted to make a root ball diameter in accordance with American Association of Nurserymen standards. Five fabric-grown trees were dug by hand and the fabric removed. Soil was washed from the roots, and roots were separated into diameter classes (0 -1 mm,>l -2 mm, >2-5 mm, >5-10 mm, >10 mm). Roots larger than 5 mm diameter which intersected the perimeter of the root balls were counted, and their cross-sectional area was calculated with a Delta T area meter (Decagon Instruments, Pullman, WA). Root number and cross-sectional area were classified into the root-diameter classes >5 - 10 mm, >10-15mm, >15-20 mm, >20 - 25 mm, >25 - 30 mm and >30 mm. Tops (including trunk, branches and leaves) and roots were dried to constant weight, and caliper and height measured at harvest.

Results and Discussion

Trees collected from the wild were the same height as the nursery-grown trees, but had about 1 cm less trunkdiameter (Table 1). Field-grown trees (5889 g) had more top (shoot) weight than collected trees (2548 g).

The root system on wild trees was dominated by a prominent tap root and few large, horizontally-oriented lateral roots growing from the tap root and from the base of the trunk (Figure 1). Tap roots often dominate the root system of young seedling-grown oaks in sandy, well-drained soil (7). Fine roots grew predominantly from the lateral roots, not from the tap root.

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Figure 1.

Root system Inside the root ball of trees collected from the wild was dominated by a single tap root. Large, horizontal lateral roots and fine roots grew from the tap root close to the surface of the soil.

The field-grown nursery trees did not have a single prominent tap root, but several large roots grew straight or at a slight angle down beneath the trunk on most trees (Figure 2). In most cases these vertically-oriented roots originated from roots which had circled the bottom of the container prior to planting in the nursery. Vertically-oriented roots were generally larger in diameter than other roots. There were only a few fine roots growing from these deeper roots; most fine roots grew from the shallower horizontally-oriented laterals. The plastic bottom of the fabric containers prevented the development of roots deeper than the depth of the fabric container (36 cm) (Figure 3). Smaller-diameter, horizontally-oriented, lateral roots grew from the ‘cage’ of roots which formed at the edge of the container prior to planting into the nursery. The ‘cage’ was 15 cm deep and wide (which are the dimensions of the 3.8 liter nursery container from which trees were planted) and was formed when lateral roots circled the side and bottom of the container after hitting the container walls. As the trees grew in the nursery, these roots increased in diameter and grafted to each other as they touched, forming what appeared to be a braided cage of roots.

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Figure 2.

Root system inside the root ball of field-grown nursery trees showing a large number of horizontal lateral roots and some nearly vertical deep roots. Fine roots are located at the top right hand side of the root ball.

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Figure 3.

Root system inside the root ball of trees grown in fabric containers. Roots are oriented horizontally and fine roots are in the top portion of the root ball. The dense mass of tangled roots in the center of the root ball represents those roots which were growing inside the 3.8-liter container before the tree was planted in the nursery.

There was no difference between field- and fabric-grown trees in the number and cross-sectional area of roots larger than 5 mm diameter located at the edge of the root ball (Table 2). Most roots were between 5 and 10 mm in diameter. Wild trees had fewer roots and less cross-sectional area than nursery-grown trees only in the >5-10 mm root diameter class. This could be explained by the lesser shoot weight on the wild trees (Table 1). Root number and cross-sectional area in all other root diameter classes were comparable among wild and nursery-grown trees. Carlson et al. (3) found that root morphology on seeded-in-place trees was similar to that for trees planted in the nursery from containers.

There was no difference between fabric- and field-grown trees in root weight for roots less than 5 mm diameter (Table 3). However, the root ball size on fabric container trees was about 50% smaller than that on field-grown trees, so there is less of a soil moisture reserve in the fabric container root ball. Increased frequency of irrigation is required to maintain trees transplanted from fabric containers(12). Wild trees had less root weight in each of the root-diameter classes than nursery-grown trees, but this was not surprising since the wild trees were somewhat smaller (i.e., had less shoot weight than field-grown trees - Table 1). However, wild trees had a shoot:root ratio similar to field- and fabric-grown trees, indicating that they may transplant equally well (Table 1).

Nursery-grown and wild trees had a deeper root system than fabric-grown trees since the root balls were both taller and wider. There were some roots in the deeper soil layers which may not dry as quickly as those in the shallower soil. This could help maintain a favorable water status inside the tree and give nursery grown and wild trees an advantage in a well-drained landscape soil where there is enough oxygen at the deeper soil depths for root growth. At transplanting, fabric-grown trees have a shallower root system because the root ball is not as tall as on tree spade-dug trees. Fabric-grown trees will require more frequent irrigation in the first weeks after transplanting than trees transplanted from other production methods (12). But fabric-grown trees could have an advantage in a high water table site or compacted soil where deep roots on nursery-grown and wild trees might succumb to low oxygen.

Trees in the current study were not root pruned at any time during the production period. Some nurseries routinely root prune trees when they are growing in the nursery and this has been shown to increase fine root mass inside the root ball of field-grown live oak (Quercus virginiana) trees (10) and enhance tree growth after transplanting (9). Irrigation practices in the nursery can also have a dramatic impact on fine root mass inside the root ball. Confining irrigation to the area within the fabric container more than doubled the fine root density on laurel oak compared to trees irrigated over a larger area (15). Compared to a nonirrigated control, trickle irrigation increased fine-root weight within the root ball in pin oak and sugar maple, but not in honeylocust (16). The results of the current study may have been different if the trees were root pruned prior to transplanting, or if irrigation were managed differently in the nursery prior to transplanting.

There appears to be no best method of tree production. Those choosing trees from a nursery and maintaining them after transplanting must take production practices at the nursery, the site conditions and post-planting irrigation capabilities into account when selecting among tree production methods. Future studies should compare the irrigation requirements and post-transplant growth of trees transplanted from the various tree production methods.

Conclusions

1. Laurel oak trees collected with a tree spade from a non-managed, naturally regenerated field had a similar shoot:root ratio as trees grown in fabric containers or those planted directly i nto field soil in a managed nursery.

2. Root balls of field-grown trees were nearly identical to those grown in fabric containers except that fabric-grown root balls were about 50% smaller and are easier to handle in the landscape. But because of the smaller size of the root ball, there is probably less reserve water in the fabric container root ball than in the larger-sized root ball of the field-grown tree. If water is not applied regularly following transplanting, trees from fabric containers undergo more water stress than field- or plastic container-grown trees (12).