The Influence of Waterlogging on the Establishment of Four Australian Landscape Trees

MATERIALS AND METHODS

Four common urban trees in southern Australia— spotted gum (Corymbia maculata, syn. Eucalyptus maculata), brush box (Lophostemon confertus), oriental plane (Platanus orientalis), and London plane (Platanus × acerifolia)—were evaluated for the effects of waterlogging on root growth following planting; the timing was designed to coincide with the typical planting season in southern Australia. Three treatments were imposed on the plants, with ten replicates of each species in each treatment.

RESULTS

Analysis of data for root length and shoot dry weight for all species showed that the data were not normally distributed and there was not constant variance between treatments. Data were therefore analyzed via statistical techniques for nonparametric data, analyzing medians, using Kruskall-Wallace and Mann-Whitney tests (Minitab, Inc. 1995). P values were calculated at the 95% confidence level, with P values for comparisons with a statistically significant difference being P < 0.05.

The experimental species (spotted gum, brush box, oriental plane, and London plane), showed considerable differences in their abilities to grow new roots under waterlogged conditions and to recover from a period of waterlogging during late winter and early spring.

Root Length

Spotted gum

There was no significant difference in root length between the treatment or recovery stages for the control (no waterlogging) or partial waterlogged treatments for spotted gum (Table 2). In the fully waterlogged treatment, root growth during the recovery stage was greater than during the treatment stage (Table 2).

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Table 2.

Comparisons of root length between the treatment (T) and recovery (R) phases. T = R denotes no significant difference between medians; T ≠ R denotes a significant difference between medians at P < 0.05.

During the treatment stage, there was a significant difference in root length between the control and fully waterlogged treatment but not between the control and partially waterlogged treatment (Figure 1). Between the partially and fully waterlogged treatments, there was a difference in root length. During the recovery stage, there was no difference in root length between any of the treatment combinations (Figure 2).

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Figure 1.

Root length versus treatment for spotted gum, brush box, oriental plane, and London plane (treatment phase). Differing letters between specific waterlogging treatments for each species denote significant median differences at P < 0.05.

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Figure 2.

Root length versus treatment for spotted gum, brush box, oriental plane, and London plane (recovery phase). Differing letters between specific waterlogging treatments for each species denote significant median differences at P < 0.05.

Brush box

There was a difference in root length between the treatment and recovery stages for all the waterlogging treatments (Table 2). During the treatment stage there was no difference in root length between the control and partially waterlogged treatments (Figure 1). There was however, a difference between the control and full waterlogging and also between the partial and full waterlogging treatments. At the end of the recovery stage, there was no difference in root length between the waterlogging treatments (Figure 2).

Oriental plane

Because oriental plane was bare rooted, it was easy to observe the origin of new root growth. Primary root growth in the fully flooded treatment was evident from the cut surfaces of the roots, the surfaces of woody roots, and also from hypertrophied lenticels spread over the woody part of the root system. Secondary root growth from the primary roots also was evident. Hypertrophied lenticels were visible along the woody lateral roots, with root primordia and new roots emerging from the intumescences.

There was a difference in root length between the treatment and recovery stages for each treatment (Table 1). During the treatment stage, there was no difference between waterlogging treatments (Figure 1). At the end of the recovery stage, there was a difference between full flooding and other treatments (Figure 2).

London plane

As with oriental plane, it was easy to observe the origin of new root growth from the bare-rooted transplants of London plane. Primary root growth in the fully flooded treatment was evident from the cut surfaces of the roots, from the surface of the roots, and from lenticels. Lenticels were visible along the woody lateral roots, with root primordia emerging from them. Collars of hypertrophied lenticels also were visible at the junction of the stem and on the woody lateral roots, with root primordia emerging from the intumescences.

There was a difference in root length between the treatment and recovery stages for the controls and the fully flooded treatments (Table 1). During the treatment stage, there was a difference in root length between the control and the full waterlogging treatment (Figure 1). For the recovery stage, there was no difference in root length (Figure 2).

Shoot Dry Weight

Spotted gum

During the treatment stage, there was a difference in shoot dry weight between the various waterlogging treatment combinations (Figure 3). At the end of the recovery stage, there was a difference between the control and fully waterlogged treatments only (Figure 4).

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Figure 3.

Shoot dry weight versus treatment for spotted gum, brush box, oriental plane, and London plane (treatment phase). Differing letters between specific waterlogging treatments for each species denote significant median differences at P < 0.05.

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Figure 4.

Shoot dry weight versus treatment for spotted gum, brush box, oriental plane, and London plane (recovery phase). Differing letters between specific waterlogging treatments for each species denote significant median differences at P < 0.05.

Brush box

During the treatment stage, there was no difference in shoot dry weight between the control and partial waterlogging, but there was a difference between the control and full waterlogging and the partial waterlogging and full waterlogging (Figure 3). Similarly, at the end of the recovery stage, the fully waterlogged plants had significantly less shoot growth than the control and the partially waterlogged treatment (Figure 4). By the end of the experiment, partially waterlogged plants showed some chlorosis and reddening of the leaves and had shed some leaves. The fully waterlogged plants were almost totally defoliated during the treatment stage and did not initiate any new growth buds till the end of the experiment.

Oriental plane

In this species, bud burst occurred 6 weeks after the experiment began. At the end of the treatment stage, fully waterlogged plants were observed to have leaves that had less than half the area of the controls and had much reduced internode growth. These leaves also exhibited chlorosis, particularly on the leaf margins. During the treatment stage, there was no difference in shoot dry weight between the controls and the partially or fully waterlogged treatments (Figure 3). There was, however, a difference between the partially and fully waterlogged treatments. At the end of the recovery stage, there was no difference between the control and partially waterlogged treatments, but there was difference between the control and fully waterlogged treatments and the partially and fully waterlogged treatments (Figure 4).

London plane

In London plane, bud burst also occurred 6 weeks after the experiment began. Fully waterlogged plants had leaves that were less than half the size of those of the controls, and internode extension also was reduced. These leaves also exhibited chlorosis, particularly on the leaf margins. At the end of the treatment stage, there was a difference between the control and the fully waterlogged treatments (Figure 3). At the end of the recovery stage, there was no difference between the control and fully waterlogged treatments (Figure 4).

DISCUSSION

The responses to winter waterlogging of the four tree species in this experiment can be grouped into three types.

1. Spotted gum and oriental plane were able to make new root growth while waterlogged and to recover well after waterlogging ceased.

2. London plane, while unable to make new root growth while waterlogged, did produce new roots rapidly after drainage.

3. Brush box was unable to make new root growth while waterlogged and could not produce roots rapidly after draining.

These observed responses to waterlogging in different species suggest several mechanisms for tolerance of urban soil conditions.

The production of hypertrophied lenticels on the woody roots of London plane and oriental plane seedlings in this experiment suggests a mechanism whereby mature trees may initiate new root growth on woody framework roots, during waterlogging, and after drainage, when oxygen is available again. Stem hypertrophy and hypertrophied lenticels allow for movement of oxygen into the root system, usually via aerenchymatous tissue from the shoots or the stem (McKersie and Leshem 1994).

The ability to produce new roots under waterlogged conditions is another survival mechanism. Adventitious water roots may be produced on the stems of waterlogged seedlings, and soil water roots may be produced in the soil below the water line (Hook 1984). Plants in this experiment did not grow adventitious roots from hypertrophied stem lenticels formed on London plane or oriental plane.

Soil water roots are often formed after the original roots die back to secondary or primary roots, with the initiation of new roots from these points (Hook 1984). In this experiment, spotted gum demonstrated the ability to form soil water roots. However, it is not known if, or how much of, the original root system died back prior to these roots being formed. River red gum (Eucalyptus camaldulensis) is known to produce soil water roots, and these roots elongate rapidly as the soil begins to dry out, enhancing the survival of mature trees after flooding (Sena Gomes and Kozlowski 1980). Neither spotted gum nor brush box are riparian or floodplain species. Spotted gum is found on well-drained, sloping, and frequently rocky areas on the east coast of Australia from Queensland to Victoria (Elliot and Jones 1986). Brush box is from habitats associated with rain forest along Australia’s east coast (Williams 1995).

Some plants tolerant of waterlogging can increase the oxygen supply to their roots by the modification of their root anatomy with the development of aerenchymatous tissue (Coutts and Philipson 1978b). In this study, aerenchymatous tissue was formed in hypertrophied stem tissue (usually at the base of the stem) and in individual hypertrophied lenticels on stems and roots of oriental plane, London plane, and spotted gum (Figure 5).

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Figure 5.

Morphological root and shoot characteristics of brush box, oriental plane, London plane, and spotted gum subjected to waterlogging (✓ = possesses characteristic; x = does not possess characteristic; n/a = not applicable to dormant, bare-root transplants).

Adventitious water roots and soil water roots may be able to oxidize their rhizosphere, but this capacity is lost as roots harden off during dry periods. The tendency of wetland species to develop soil water roots may account for their capacity to withstand the alternating waterlogging and drying of the wetland site (Hook 1984).

Under fluctuating water table conditions, considerable death may occur in the actively growing roots of mature trees when the water table rises, but some dormant roots survive, and regrowth of soil water roots is from these (Coutts and Philipson 1978a). Soil water roots were developed on oriental plane, London plane, and spotted gum (Figure 5).

A less desirable response to waterlogging involves the death of all or most of the root system without production of new roots. The response of brush box to waterlogging in this experiment may fall into this category, since no root growth occurred during the waterlogging treatment and the plants were slow to recover once they were drained. This response has also been noted for tuliptree (Liriodendron tulipifera) (Hook and Brown 1973) and for Sitka spruce (Coutts and Philipson 1978a). Species that respond in this way are intolerant to waterlogging and die if waterlogged conditions persist.

Figure 5 summarizes the morphological root and shoot characteristics of the experimental species to waterlogging and attempts to rank their tolerance to waterlogging in conjunction with these characteristics.

Using the relative waterlogging tolerance scale in Figure 5, spotted gum is ranked most tolerant of waterlogging because of its combined morphological root and shoot characteristics and its ability to produce rapid root growth when waterlogging was removed. Brush box is ranked as least tolerant because of its complete defoliation under waterlogged conditions, its inability to produce new root growth under waterlogged conditions, and its very slow recovery of root and shoot growth once waterlogging ceased. It was not possible to distinguish between oriental plane and London plane on the basis of morphological root and shoot characteristics.

While it is difficult to correlate these observed responses to plant performance in the landscape, the success of spotted gum and the two plane species as urban trees in southeastern Australia would seem to be consistent with the observations of this experiment. While brush box is successful as an urban tree in parks and grassed areas in streets, field observation suggests it is less able than the other species to deal with difficult sites.

Leaf fall was notable in the waterlogged treatment. The smaller amount of shoot growth for fully waterlogged spotted gum at the end of the recovery may indicate that carbohydrate was being directed into root development to compensate for root growth lost during waterlogging. Toxic compounds produced under waterlogging may have affected shoot growth, but since root growth was not affected to the same extent, this is probably not so.

CONCLUSIONS

Spotted gum and oriental plane were able to produce new root growth under waterlogged conditions and to recover rapidly after waterlogging. London plane and brush box did not grow roots under waterlogged conditions. London plane was able to recover rapidly from waterlogging and initiate new root growth. Brush box by comparison, recovered more slowly.

Shoot growth for spotted gum was not reduced by waterlogging. Under waterlogged conditions, shoot growth was reduced for brush box, London plane, and oriental plane. While shoot growth of London plane and oriental plane recovered rapidly after waterlogging ceased, as new root growth increased rapidly, brush box was unable to recover shot growth during the experiment.

Where trees are planted in sites with compacted and poorly drained soils, root response differences of the type seen in this experiment can go some way to explaining species differences in tolerance to these sites. In winter rainfall climates like southeastern Australia, tolerance of waterlogging after planting and the ability of a transplant to establish new roots after soils begin to drain will be an important survival response. In addition to tolerance to the direct effects of waterlogging, a transplant that is able to produce new roots while waterlogged, exploiting the lower soil strength of wet soil, has opportunities to explore larger soil volumes before the soil becomes too dry. Species such as brush box, which cannot grow new roots under waterlogging, are less able to exploit this opportunity and will enter the dry summer period with restricted root growth and access to smaller soil water reserves.

It is important that an understanding of soil conditions inform the design process for urban sites, particularly with regard to physical properties such as compaction, soil strength, and drainage. Species selection should reflect the known tolerances of species to critical soil conditions. As a result of this study, it is recommended that for sites where post-planting waterlogging can occur, spotted gum, oriental plane, and London plane can be considered suitable species but brush box cannot. For species less tolerant of waterlogging, subsoil drainage should be installed on sites subject to waterlogging, and will be critical for establishment success.