Influence of Water Stress and Restricted Root Volume on Growth and Development Of Urban Trees

Many factors influence successful growth of trees in containers. These include plant material; type, size, and shape of container; depth, volume, and color of container; type of medium; exposure and site; frequency of watering and fertilizing; time of transplanting; and prior treatment (e.g., whether or not the plants have been root pruned prior to transplanting) (3, 11, 29, 37, 46, 49, 51, 58, 61, 71, 84, 93, 125, 138, 146, 147, 157, 158, 159, 160, 161, 163, 164, 168).

If plants are pruned to maintain a proper balance between root and shoot growth as in the culture of bonsai trees or grape vines (15, 107), they may live indefinitely in a restricted volume. In most urban settings, however, trees in planters must be replaced after several years (41). The balance between transpiration and absorption determines whether or not internal water stresses develop (33, 34). Species with high top-root ratios tend to have low survival rates when outplanted (71). The root-shoot imbalance created by transplanting is one of the primary causes of transplanting shock with other physiological and pathological problems acting as secondary agents. Until the natural rootshoot balance of the tree is restored, some degree of transplanting stress will exist (153).

A well-branched root system is essential for effective water and nutrient uptake in containers (139, 140, 154). The distribution of roots in the soil is determined by both genetic and environmental conditions (35, 65, 86). Because of restrictions imposed by container walls, limited growth medium, and high water holding capacity of the medium, root growth of trees in containers differs from that in the field (61, 132).

Some tree roots are able to grow in containers of any size and shape (8, 9). Others do better in a particular shape or configuration, depending on the nature of their root system (10, 16, 27, 43, 51, 61, 62). Red oak trees grown in containers that have a relatively low diameter to depth ratio outgrow those in containers with higher ratios (63).

Containers currently on the market for growing tree seedlings were developed primarily for the forest industry (50, 51, 104, 147, 152). Conventional containers, such as clay or plastic pots, are unsuitable for growing tree seedlings. They tend to cause poorly formed root systems, which later impede growth and survival of the trees.

Site and exposure are critical factors in the successful establishment of urban trees. Trees located close to the street are more likely to suffer from water deficits than those located in unpaved areas because of the intense amount of heat reradiated from parked cars and pavement and lower absolute humidity of paved sites (T. H. Whitlow, 1986, personal communication). Paved sites also frequently experience low oxygen exchange and high CO₂ levels. Trees that constantly get dessicated from high winds or frequently get waterlogged often develop abiotic leaf scorch (45) and are more likely to succumb to dieback. Street trees also experience salt damage from excess Na and Cl ions (30, 31, 56, 83, 155) which can be relieved to some extent by application of gypsum (5, 31, 116).

The average survival rate for sidewalk trees is about 10 years (41). Construction damage and altered, unsuitable environments left after construction are two of the main reasons for the poor survival rate of urban trees (39, 41). Available soil used in urban sites is often of poor quality, pH is frequently excessively high, and landfill gases are often present in toxic levels (39). The disturbed nature of urban soils with fill, concrete, refuse, and other artificial factors and different degrees of compaction, makes it difficult to draw any generalizations as to soil properties of a site (7).

Various problems can arise from prolonged confinement of woody plants in a restricted root volume (48, 51, 107, 108, 119, 127). These include root distortion, girdling, and in extreme cases, even strangulation and death of the plant (48, 96). Water stress is one of the most serious problems experienced by container-grown plants and may occur from either an excess or deficit of water (67, 69, 73, 74, 90, 136, 137). Because of a perched water table, trees in planters frequently become waterlogged and experience aeration problems (132). These problems may be partly avoided by providing adequate drainage and a porous soil mix (131, 133). Other problems commonly encountered include compaction, deficiency and toxicity of nutrients and accumulation of soil gases (1, 40, 53, 94, 96, 97, 98, 99, 114, 117, 124, 125, 134, 143, 150, 167, 170).

Since plants grown in confined root volumes are frequently subjected to water stress, it is difficult to know how much of the reduction in growth is caused by drought or excess water and how much is caused by root restriction (2, 17, 18, 19, 89, 100, 112, 113, 149, 156). Under natural conditions, it is difficult to answer this question because plants are seldom irrigated frequently enough during the day to eliminate water deficits. In the greenhouse and growth chamber, however, the effects of these two stresses have been separated by use of an automatic watering system (75).

The results of greenhouse and growth chamber studies indicate that the physiological effects of water stress and restricted root volume may be quite different from one another even though the morphological responses may be similar (76). When soybean plants were subjected to water stress or root restriction, restricted root volume had little or no effect on the rate of leaf initiation or photosynthesis. In contrast, water stress greatly reduced both rate of leaf initiation and photosynthesis. Under both water stress and root restriction, branching of the shoot and total plant dry matter accumulation were greatly reduced (76). Root/shoot ratio was increased in water-stress treated plants but was unaffected by root restriction. These studies have been extended to tomato (M. S. Ruff et al., unpublished) and Euonymus (S. P. Dubik et al., unpublished) with similar results.

Research is needed to identify the most common urban stresses and to evaluate the comparative tolerances of different species and cultivars to specific stresses (82). Greater plant diversity is possible if trees are selected for tolerance to stresses at a specific site, rather than for tolerance to all urban stresses (7). Many municipalities and private companies maintain computer inventories of their tree plantings. However, these systems are usually oriented toward management rather than research. Consequently, biologically important parameters such as soil moisture, soil type, and nutrient content, are often excluded as site characteristics.

The phenomenon of cross-protection in woody (68, 91, 92, 126) and herbaceous plants (77, 78, 110) is well known. Several workers have shown that trees subjected to low temperature (68, 91, 126) or flooding (68, 92) prior to SO₂ fumigation, were less damaged by SO₂ than were unstressed control plants. Krizek et al. (77, 78) observed similar protective effects of water stress and temperature pretreatment against SO₂ injury in selected herbaceous species.

One of the ways in which drought, flooding, low temperature, salinity, and air pollution appear to provide cross protection is by increasing the level of abscisic acid (ABA) in the plant; this in turn closes the stomates and reduces water loss through transpiration (12, 77, 144).

Root restriction has an adverse effect on hormone metabolism in the plant. Since plant hormones such as cytokinins and gibberellins are synthesized in the root system (20, 128, 129, 148), one of the primary ways in which root restriction may suppress plant growth is by altering the synthesis and/or transport of these substances in the plant.

Plants subjected to prolonged periods of drought, flooding, salt damage, and other environmental stresses are frequently predisposed to attack by insects and invasion by various disease causing organisms (21, 55, 57, 70, 85, 103, 115, 120, 121, 122, 123, 166) and may exhibit either biotic or abiotic leaf scorch (45). For example, elm trees subjected to drought or soil compaction are more likely to succumb to Dutch elm disease than unstressed trees. Similarly, sweet gum trees grown in water-logged or compacted soils are likely to be more vulnerable to canker than unstressed trees (R. Hammerschlag, 1986, personal communication). Although quantitative data are generally lacking, empirical observation indicates that the effects of drought, flooding, transplanting shock, extreme temperature fluctuation, or compaction may have significant additive effects in reducing lifespan of urban trees, particularly if they occur for prolonged periods.

A multidisciplinary approach is required to establish and maintain a successful planting of urban trees. This will necessitate enlisting the assistance of persons trained in horticulture, landscape architecture, agronomy, soil science, genetics, plant physiology, and plant pathology. By utilizing various genetic, cultural, morphological, and physiological approaches, one can greatly enhance the growth of trees in a restricted root volume and increase the chances for survival under urban stress.

One of the most important needs is to make a careful site assessment. This should include a thorough study of exposure, climate, size, area usage, history of the site (e.g., type of land fill), drainage conditions, and physical and chemical properties of the soil (e.g., type, pH, electrical conductivity of the soil solution, prior pesticide use). If plants are balled and burlapped, prior knowledge of their cultural conditions may be helpful.

Species and cultivars should be chosen that have wide, shallow, and highly branched roots because deep-rooted plants, with little branching, are much less adaptable to container growing (8, 9). Since container volume rapidly becomes restricting to root growth, slow growing plants should be chosen over rapidly growing species and cultivars (47). Cultivars tolerant to air pollutants, salt damage, drought, waterlogging, extreme cold, and other urban stresses should be selected wherever possible (22, 59, 60, 70, 130). The Metropolitan Tree Improvement Alliance (METRIA) has served as a clearinghouse for information on selecting trees for urban landscapes since 1973 (42, 60).

Cultural approaches include the use of appropriate media and amendments, addition of mulch, installation of an automated irrigation system, selected pruning during drought to reduce water loss, and possible application of antitranspirants. Ideally, native soil should be used. If too many amendments are added, the soil may act like a sponge reducing aeration and causing flooding injury (96). If trees or long-lived shrubs are to be grown in containers, a high proportion of humus in the mix should not be used. Once the humus is decomposed by soil bacteria and fungi, and disappears finally as carbon dioxide, the soil subsides and becomes compact causing aeration problems (37). Chances for survival are greatest if plants are transplanted while they are still relatively small, and if the size of the root ball is large in relation to plant size (7). Inoculation with mycorrhizae may also be needed when tree seedlings are outplanted (169).

Plant production practices at the nursery contribute to some of the problems involved in transplanting (135). If tree roots are pot bound, fracturing of the roots may be needed prior to planting. If trees are maintained too long in a restricted root volume, the roots may become girdled and root pruning may be necessary. Methods of handling trees in transit, storage, and at the planting site are also important in assuring survival. Plant losses may result from failing to soak the root ball (96). Excessive soil moisture and mounding of soil on roots in the nurseries also contribute greatly to tree losses during transplanting.

Physiological approaches include the use of plant growth regulators to inhibit shoot elongation and antitranspirants to reduce water loss (24, 25, 26, 71). The triazoles and other growth retardants are attractive as possible candidates for tailoring growth and increasing tolerance to urban stress (4, 6, 14, 32, 38, 54, 79, 80, 88, 141, 165). These compounds inhibit gibberellin and sterol biosynthesis, increase root branching, and have been found to be effective in providing protection against SO₂, drought, and other environmental stresses (38, 79, 80, 81). Increasing the Al concentration in the soil has also been shown to reduce shoot growth by causing dwarfing of the root system (13, 64), although this method is not recommended.

It is difficult to extrapolate from data obtained on trees growing in the forest to trees growing in an urban environment. Thus, in-depth studies are needed on urban trees to determine the morphological, physiological, and biochemical effects of water stress (both deficits and excesses) and root restriction on plant growth and development. These should be conducted under both controlled-environment conditions and under actual field conditions. They should be accompanied by careful measurements of such parameters as leaf, air, and soil temperatures, radiation conditions, stomatal behavior, water potential, and mineral status (52, 87, 145).

Studies should be focused on mechanisms of adaptation to water stress, root restriction, and other urban stresses (e.g., high pH, Fe stress, Pb pollution) including both avoidance mechanisms and detoxification mechanisms (102). Experiments should be carried out to determine the extent to which urban trees experience osmotic adjustment and other possible mechanisms of stress adaptation. Information is needed to determine the changes in root permeability of trees during drought stress and recovery (162). Efforts should be made to determine the hormonal basis for differences in growth reduction caused by water stress and root restriction. Studies should be conducted to identify trees that can withstand high temperatures as well as those that continue to transpire, thereby maintaining cooler leaf temperatures which may enable them to avoid leaf injury.

Careful studies are also needed to determine the influence of various stress interactions, e.g., drought and air pollutants, drought and mineral stress, water stress and plant pathogens to determine their possible synergistic and antagonistic effects.

Further efforts should also be made to develop computer models of transpiration from individual tree crowns (151). Such information could be used to predict water use, to schedule irrigation, to monitor plant water stress conditions, and to assess the whole plant energy balance. Additional information is also needed to establish minimum irrigation requirements for urban trees (23, 36, 44, 101, 118).

In order to reduce losses from environmental stress and to increase the longevity of urban plantings, it is clear that an interdisciplinary approach is needed. This should involve the cooperation of researchers and arborists. Greater attention should be given to conducting a thorough examination of the biological and edaphic factors at each proposed site and to selecting species and cultivars that are resilient to environmental stress.