Root Dimensions of Landscape Tree Cultivars

METHODS

Eight cultivars were chosen to represent a wide range of street trees commonly planted under utility lines, and available locally in central Pennsylvania, U.S.: Amelanchier ‘Cumulus’; Amelanchier Autumn Brilliance®; Amelanchier ‘Snowcloud’; Malus Harvest Gold®; Malus ‘Professor Sprenger’; Pyrus calleryana ‘Autumn Blaze’; Pyrus calleryana Chanticleer®; and Syringa reticulata ‘Ivory Silk’. Five trees were randomly selected in each of six cultivars growing in one of three nurseries (Table 1); two sets of five trees of ‘Ivory Silk’ tree lilac were in two of these nurseries. Nursery soils included silt loam at Nittany Trees and PenCor Nursery, and clay loam and sandy loam at College Gardens Nursery. In addition, six trees of ‘Cumulus’ serviceberry were growing in clay loam along a borough street in a 244 cm wide tree lawn. Altogether, 46 trees were excavated and measured. Average sizes of the cultivars ranged from 3.8 to 16.1 cm diameter at breast height (dbh), and 3.0 to 6.4 m in height (Table 1).

The entire root system of each tree was excavated using an Air-Spade™ Series 2000 with a 150 scfm/90 psig nozzle (Concept Engineering Group, Inc., Verona, PA) connected to an air compressor. Its high-speed air jet removed soil without damaging fine roots. Lengths of the longest roots along the row and at right angles to the row were measured in both directions and added to express root spread; also the depth of the deepest roots was measured. The roots of each tree were then cut at the trunk and at the point where they would meet the edge of a hypothetical standard root ball (American Association of Nurserymen 1996); the specified root ball diameters ranged from 56 cm to 211 cm, according to the caliper of each tree. The two groups of root segments from each tree were then oven-dried at 105 ± 3°C for 4 to 5 days to a constant weight. A few of the ‘Cumulus’ serviceberry roots extended underneath the sidewalk or curbing, so their lengths and weights were estimated from dimensions of unimpeded roots that had equivalent diameters at the same distance from the trunk.

The significance of differences in root spread along the row from spread across the row was determined by pairwise t-tests. Regression equations were calculated to examine the relationships of the dependent variables with dbh. Regression lines for root spread were fitted for Amelanchier, Malus, and Syringa, but not for Pyrus because of the narrow range of dbh in this genus.

RESULTS

The spread and depths of roots increased with tree size (Table 1). Also, there were apparent differences in root spread among genera (Figure 1 and Figure 2). The roots of Syringa were longer than those of Malus, which were longer than roots of Amelanchier cultivars. Regressions of root spread on dbh accounted for 72% to 91% of the variation. Root depths were not as closely correlated with dbh (R² = 0.30) as lengths, and depths did not differ among genera (Figure 3).

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Figure 1.

Root spread across the row related to dbh for cultivars of Amelanchier ‘Cumulus’ (AmCu), Autumn Brilliance® (AmAb), and ‘Snowcloud’ (AmSc); Malus Harvest Gold® (MlHg) and ‘Professor Sprenger’ (MlPs); Pyrus calleryana ‘Autumn Blaze’ (PyAb) and Chanticleer® (PyCh); and Syringa reticulata ‘Ivory Silk’ at Nittany (SyIn), ‘Ivory Silk’ at PenCor (SyIp). Regressions for Amelanchier, small dashed line, Y = 11.9 (dbh) + 66.2, R² = 76%; Malus, long dashed line, Y = 14.5 (dbh) + 97.2, R² = 72%; and Syringa, solid line, Y = 21.9 (dbh) + 85.3, R² = 78%.

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Figure 2.

Root spread along the row related to dbh for cultivars of Amelanchier ‘Cumulus’ (AmCu), Autumn Brilliance® (AmAb), and ‘Snowcloud’ (AmSc); Malus Harvest Gold® (MlHg) and ‘Professor Sprenger’ (MlPs); Pyrus calleryana ‘Autumn Blaze’ (PyAb) and Chanticleer (PyCh); and Syringa reticulata ‘Ivory Silk’ at Nittany (SyIn), ‘Ivory Silk’ at PenCor (SyIp). Regressions for Amelanchier, small dashed line, Y = 21.1 (dbh) + 62.1, R² = 86%; Malus, long dashed line, Y = 8.9 (dbh) + 206.7, R² = 91%; and Syringa, solid line, Y = 32.6 (dbh) + 62.0,R² = 75%.

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Figure 3.

Root depths related to dbh for culivars of Amelanchier ‘Cumulus’ (AmCu), Autumn Brilliance® (AmAb), and ‘Snowcloud’ (AmSc); Malus Harvest Gold® (MlHg) and ‘Professor Sprenger’ (MlPs); Pyrus calleryana ‘Autumn Blaze’ (PyAb) and Chanticleer (PyCh); and Syringa reticulata ‘Ivory Silk’ at Nittany (SyIn), ‘Ivory Silk’ at PenCor (SyIp). The regression Y = 2.35 (dbh) + 35.86, slope is significant at = 0.0001, R² = 0.30; root MSE = 15.3; standard error of slope = 0.55.

The spread of roots along rows were in all cases longer, from 15% to 60%, than spread across the row (Table 1). These differences were significant according to t-tests.

The proportion of root biomass retained inside standard root balls was highly variable and tended to decrease with dbh (Figure 4). Trees below 6 cm dbh had from 53% to 100% of their root biomass inside the hypothetical root balls. Between 6 and 20 cm dbh, trees had from 29% to 83% of root biomass inside the root balls. The regression of biomass on dbh explained only 37% of the variability.

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Figure 4.

Proportion of root biomass stored inside a standard root ball for individual trees of Amelanchier, Malus, Pyrus, and Syringa cultivars. Proportions regressed against diameters (dbh), Y= −0.0374 (dbh) + 0.97, R² = 0.37, root MSE = 0.210, significant at = 0.

DISCUSSION

The equations for root spread versus dbh of Syringa gave results similar to those of Gilman (1988a), who studied larger types of trees, including red maple (Acer rubrum), green ash (Fraxinus pennsylvanica), and honeylocust (Gleditsia triacanthos). The roots of Amelanchier, especially those of the older trees, were only about half as long as those of Syringa.

Furthermore, the horizontal configuration of the root systems of trees in nurseries appeared to be oval, not round. The roots of every cultivar in all nurseries were shorter across the row than along the row. This finding could be attributed to weed control practices that kept rows free of other vegetation that would compete with tree roots (Harris 1966; Whitcomb and Roberts 1973). If the oval shape of root systems is typical of trees in nurseries, it could have implications for possibly modifying cultural or lifting practices that affect the portions of roots retained within root balls.

The trees along the borough street also had shorter roots at right angles to the row, but for a different reason. The sidewalk and curbing presented physical barriers that interfered with root elongation. Therefore, the spread of the roots was just slightly greater than the distance from curb to sidewalk.

The study was not designed to compare effects of cultivars or nursery conditions on root dimensions, although either or both could have affected them. The significant regressions of root spread appear to depend mainly on genera along with dbh, and less on nursery conditions. ‘Ivory Silk’ tree lilacs had longer roots, and the three serviceberry cultivars had shorter roots than two crabapple cultivars, regardless of their location.

The significant relationship of increasing root depth with tree size, though expected, was dependent mainly on the one cultivar growing along the street and fails to explain most of the variability. Some contrasting depth-to-size relationships between related cultivars were observed. For example, the larger ‘Autumn Blaze’ Callery pears had shallower roots than the smaller Chanticleer® trees. The Callery pear cultivar with more shallow roots was growing in the Nittany Trees nursery, and the other with deeper roots was in the College Gardens nursery. The opposite was true of the two crabapple cultivars, so it is doubtful that soil differences were responsible. ‘Ivory Silk’ tree lilacs gave the clearest indication of a possible nursery effect, because the smaller trees at Nittany Trees had deeper roots than the same cultivar at Pencor Nursery. An alternative but less likely explanation is that different root stocks may have caused differences in root depth; however, nothing is known about any of the root stocks.

The most surprising finding is that such a large proportion of the root biomass, from 53% to 100% of the dry weight, would be contained within the standard root ball of trees with a dbh below 6 cm. Even trees that were 6 to 20 cm would have 29% to 83% of the root biomass within the appropriate size root ball. The discrepancy with earlier studies may be due to differences in methodology, tree size and species, and the ways by which root systems may be characterized. Watson and Himelick (1982) concluded, from a model root system of a 10 cm caliper tree representing 88 excavated trees in seven deciduous species, that the soil volume dug by a tree spade would represent only 2% of the volume occupied by all roots, and therefore the root system would be reduced by up to 98%. Similarly, Gilman (1988b) found that 91% to 95% of root lengths would be outside standard root balls of honeylocust, green ash, and poplar (Populus × generosa) trees 3 years after transplanting. It is difficult to reconcile such large root ball differences between methods based on weight versus length of roots, or weight versus soil volumes occupied by roots. Differences among species in their root structures might account for part of the discrepancy, but probably not most of it.

CONCLUSIONS

The root spread of smaller types of landscape trees in nurseries can be estimated reliably from trunk diameter. The longest roots were 15% to 60% longer along rows than across rows, probably due to practices used to control competing vegetation. Roots of Syringa cultivars were up to twice as long as roots of Amelanchier cultivars, and those of Malus cultivars were intermediate.

Root depths were more variable in relation to trunk diameter than were root lengths. The extent to which cultivars, root stocks, soil characteristics, or nursery practices may have influenced this variability was unclear.

The dry weight of roots retained within standard size root balls was 53% to 100% of the entire root system of trees below 6 cm dbh. Trees 6 to 20 cm in dbh would have 29% to 83% of root biomass within their root balls. These percentages based on weights are much larger than the 2% to 9% within root balls found in previous studies based on root lengths or soil volumes occupied by roots.