242 Nielsen et al.: Soil Water Dynamics and Growth of Trees study show that the water resource in 1.5 × 4 m (4.95 × 13.2 ft) planting pits is close to becoming depleted during a normal growing season. Furthermore, the variation among planting pits showed that tree growth was reduced with an increasing rate of water loss. How can we deal with this from a practical point of view? The long-term and best solution is to avoid or reduce the water problems in the design phase. If the trees could be placed further away from the street, salt deposition would be reduced and the roots might have room for expansion in uncompressed and nonpolluted adjacent soil volumes. Depo- sition of deicing salt intensifies the effects of drought (Dob- son 1991; Sieghardt 2000; Czerniawska-Kusza et al. 2004), and the possibility for further root extension will reduce the risk of sublethal storm damage to roots and root ball, thereby also conserving a well-functioning root system (Nielsen 1990; Nielsen and Hansen 2004). Technical solutions for improved water balance could also be considered in the design phase. The most obvious solution is to enlarge the surface of the planting pit, because this enhances the amount of precipitation infiltrating into the pit. An enlargement of the planting pit surface (beyond the crown drip zone) by 1 m2 (10.8 ft2) would theoretically enhance the water input by 265 L (68.9 gal) during the growing season (100 dm2 * 2.6 L/dm2, assuming 265 mm [10.6 in]) rain during the growing season). Put in other words, the increase in relative stem growth from 10.9% in the control street trees to 15.1% for the trees under the 640 L (166.4 gal) irrigation treatment (see Bühler et al. 2006) might have been achieved by a 2.4 m2 (25.92 ft2) enlargement of the planting pit (from 6.4 to 8.8 m2 [69.12 to 95.04 ft2]). Managers in the large German cities recommend a permeable surface of between 8 and 14 m2 (86.4 and 151.2 ft2) (Klaus Schröder, park man- ager, City Osnabrück, pers. comm. 2005), which is in accord with our findings. Kristoffersen (1998) described the possi- bility of expanding the rooting zone of street trees by estab- lishing root-friendly load-bearing layers under sealed sur- faces carrying only light traffic as, for example, structural soils. Furthermore, Schröder (2004) described how to extend the rooting zone downward and below traffic lanes by use of a “subsoil” substrate, which is a root-friendly composition of stones (lava), sand, brown coal, and clay granules. This sub- strate can be compacted to a degree enabling the construction of bicycle lanes and parking lots on top. It is obvious that expansion of the root-friendly soil volume and increased in- filtration of water will reduce the effects of drought and thus the need for costly irrigation. However, it is also important to use a soil or substrate with sufficient water retention capacity. The topsoil used in the planting pits in this study was very coarse because sand was added to facilitate good drainage, but the result was an exponential water loss rate during the first days after rainfall or irrigation. Examples for soil mix- ©2007 International Society of Arboriculture tures are provided by Schröder (2004) and the recent German standard for soil substrates (FLL 2004). Most urban tree managers in European cities have a large stock of trees that do not have proper soil and tree water balance. For some of these plantings, a “tree health irrigation” program might be a reasonable course of action to improve survival and the quality of tree function. Two goals for irri- gation programs are discussed: (1) survival on a short-term basis, and (2) normal growth and development and/or high longevity (50 to 80 years). Survival Trees adapt to drought by means of architecture, morphology, anatomy, and biochemistry (Parker 1968; Lyr et al. 1992; Nielsen 1990). Thus, trees may have poor growth but survive under “normal” dry conditions. A survival problem arises after extreme droughts (Mar:Møller 1965) or if additional stress factors arise. Year-ring analysis showed that drought reduced root growth three times that of stem growth and that forest trees continued to die for up to 3 years after the severe drought in 1976 (Nielsen 1990). Even if trees have adapted to low water supply, strong drought will cause mortality in the root system and, as a result of poor growth conditions, the trees often will not recover. Even if survival is the only goal, a monitoring system should be used to determine when irri- gation is necessary for tree survival. It is obvious that urban tree managers would need to assign their trees to various risk groups with various thresholds of precipitation deficiency. TDR equipment with remote reading could be used for such a monitoring system. An emergency irrigation system, in which irrigation is carried out only to prevent root dieback, would doubtlessly reduce tree mortality as well as crown deformations in urban trees. Proper Development and High Longevity The prerequisite for a proper appearance and a long lifespan is absence of severe degeneration events. Ontogenetic degen- eration is accelerated if the carbohydrate or water balance is disturbed over a long period. Root dieback from drought or storm damage, severe crown pruning, or defoliations may initiate the self-reinforcing degeneration cycle (Nielsen and Knudsen 2004), because damage to either the carbohydrate or the water balance is interacting: A disturbed water balance will effect carbohydrate production negatively and vice versa (Figure 12). If longevity and vital tree development is the goal, it is important to maintain a reasonable growth rate in the tree. Among other things, this is a prerequisite for good compart- mentalization and encapsulation of wounds, and it helps the tree protect itself against insects and fungi. A suitable and inexpensive way to monitor the “degeneration status” of trees is by regular diameter or circumference assessments, because the stem increment reflects the carbohydrate status of the tree.
July 2007
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