Arboriculture & Urban Forestry 33(4): July 2007 241 ally improved after the dry spring period (Figures 4 and 5). This basic difference in long-term trends was not changed by irrigation. Irrigation elevated the soil water level during the application period, but thereafter, irrigated planting pits ex- hibited the same trend as the control sample (Figure 5). The declining time trend of the water saturation index in street tree pits is more serious than indicated in Figure 4, because street planting pit soils also have a poorer water retention capacity than the park soil (Figure 2). Obviously, street tree planting pits are subjected to severe net water losses during the summer period, whereas the opposite was found for park trees. The fact that the daily water loss rate was slightly lower in nonirrigated street tree planting pits than in the park soil (Figures 7 and 8) suggests that the declining time trend of the planting pit water resource is caused by a severely reduced amount of precipitation infiltrating into the soil of the street tree planting pits. Sealed soil surfaces outside the planting pit lead to surface runoff (into sewers). Thus, it becomes difficult for tree roots to find water outside the planting pit. Intercep- tion of precipitation by buildings, bark mulch, and the tree crown itself limit the water input further. Taking the very wet summer period (with precipitation 55 mm [2.2 in] above the average) into consideration, it is likely that the soil water resource in the planting pit will be totally depleted during a normal growing season. This conclusion concurs with the facts that the growing season of the street trees was 28 days shorter than that of the park trees and that irrigation signifi- cantly prolongs the growing season of street trees (Bühler et al. 2006). The negative effect of decreasing water content on the length of the growing season was also found by Nielsen and Jørgensen (2003). Reduced infiltration of summer rain (water input) is part of the problem. However, the fate of soil water in the planting pits (water output) is, according to the presented results, a major issue also. Because the investigated planting pits are surrounded by curbstones, surface runoff is widely negligible, and soil water is primarily lost through evaporation, transpi- ration, and drainage. The generally elevated air and soil tem- perature in urban environments (Kuttler 1993; Meyer 1982) and the proximity to asphalt covers tend to increase evapo- ration from the soil surface, but the layer of bark mulch is likely to reduce evaporation. This is supported by the slower water loss rate during the dry spring period compared with the park soil (Figure 4). Soil hydrology and tree hydrology normally constitute a mutual feedback system, but the tradi- tionally large impact of tree transpiration in old forest eco- systems (Holstener-Jørgensen 1958/59) was not found to be a driving mechanism in the soil hydrology in the street planting pits. As opposed to the park trees, where growth (and tran- spiration) of the trees was shown to significantly contribute to the soil water decline, the soil water dynamics of street tree pits seemingly were not driven by growth of the trees (Figure 10). This does not suggest that street trees did not transpire water, but it shows that street trees adapt to the soil hydrology rather than influence it. Estimates based on data from Holstener-Jørgensen (1958/ 59) show that 23 m (75.9 ft) tall beech trees on a soil type similar to the park soil in our study had an average daily transpiration of 150 L/day (39 gal/day) during the growing season. As shown in Figure 4, the park subsoil was not satu- rated during the growing season, which strongly indicates that downward drainage was close to zero during the growing season, which also concurs with common hydrologic behav- ior in undisturbed soils (Pedersen 1993). Thus, a daily tran- spiration rate of 10 L/day (2.6 gal/day) for the park trees in our study seems fairly reasonably (Figure 7). The linearly decreasing water loss for park trees (Figure 8) is also in accordance with transpiration as the major cause of water loss in the park trees. Even if it deviates from current experience with undisturbed soils (Pedersen 1993), it is judged that drainage from the planting pit is an important cause of water loss, which is also true during the summer period. The soil texture of the planting pits is coarse, which provides good drainage but poor water retention capacity. The surface of the planting pit surroundings was sealed with asphalt and bricks, which caused the precipitation to run off and the soil around the planting pit—and most likely also the soil below the planting pit—to be comparatively dry. It is likely that the gradient in soil water content between the planting pit and the dry soil around and beneath it tends to level out. This expla- nation concurs with the exponential shape of the water loss curves for street trees (but not for park trees; Figures 8 and 9), and it concurs with basic theory of water movement from moist to dry soil (Brady 1974; Figure 7). Further investigations into the fate of planting pit soil water seem promising. We believe that the linear water loss curve of the park soil in Figure 8 is facilitated by “normal” hydrol- ogy in the surrounding soil and by undisturbed subsoil with higher clay content. Because the soil beneath asphalt and bricks around street pits is generally heavily compacted, the contained water is strongly bound by capillary and adsorptive forces. This soil compaction will further drive the smoothing of soil water gradients between the planting pit and the sur- rounding soil. Furthermore, water that is lost from the plant- ing pit to the surrounding soil will often not be available for tree roots, because roots do not grow in this compressed soil (Kristoffersen 1998, 1999). One further problem with the street planting pits is the large variation among pits in water retention capacity. Whether this is the result of different soil texture in the pits or the result of different water exchange conditions with the surrounding soil is not clear. Applied Discussion As stated in the introduction, poor water input is a common problem for urban trees, and the results from this Copenhagen ©2007 International Society of Arboriculture
July 2007
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