Journal of Arboriculture 31(3): May 2005 155 part of Beijing removed 1261.4 tons of pollutants from the air in 2002. The air pollutant that was most reduced was PM10 ; the reduction amounted to 772 tons. The carbon dioxide (CO2 ) stored in biomass form by the urban forest amounted to about 0.2 million tons. Future research directions to improve our understanding of the role of individual tree species in air pollution reduction are dis- cussed. (Urban For. Urban Green. 2005. 3:65–78) REGULATION OF NITRATE UPTAKE AT THE WHOLE-TREE LEVEL: INTERACTION BETWEEN NITROGEN COMPOUNDS, CYTOKININS, AND CARBON METABOLISM Arthur Geßler, Stanislav Kopriva, and Heinz Rennenberg Pedospheric nitrate uptake is closely integrated with the nitrogen (N) status and demand of the whole tree. Signaling substances communicating the N demand of the shoot to the roots are required in an integrated regulatory system. Besides phloem mobility, such signal compounds must have the potential to repress or increase nitrate uptake either at the transcriptional or post-transcriptional level. Amino com- pounds cycling within the tree are involved in the regulation of nitrate uptake. In many tree species, inorganic N is generally assimilated in roots, and amino acids—the direct products of N assimilation—are transported in the xylem to the sites of N demand. If the quantity of amino acids trans- ported to the aboveground parts of the tree exceeds shoot N demand, some amino compounds are reallocated to the roots by phloem transport. Particular amino compounds exert transcriptional and post-transcriptional control over nitrate uptake by roots. Induction of nitrate transporters is mediated by nitrate or nitrite, or both, and possibly also by cytokinins, which cycle within the tree and act as both root-to-shoot and shoot-to-root signals. This review focuses on tree-specific requirements for N regulation and signaling, as well as the link between carbon metabolism and nitrate uptake. (Tree Physiol. 2004. 24:1313–1321) POTENTIAL HAZARD CHARACTERISTICS OF TILIA, BETULA, AND ACER TREES REMOVED IN THE HELSINKI CITY AREA DURING 2001–2003 Minna Terho and Anna-Maija Hallaksela In order to study decay, and to improve the management and protection of old urban trees, a total of 256 felled urban trees were examined during 2001–2003: 95 Tilia spp., 74 Betula spp., and 87 Acer spp. Most of the trees (73%) were located in the main parks and along the main streets in the downtown area of Helsinki City, Finland. The mean age of the trees was over 60 years, and the majority (64%) were old park trees. Poor condition and increasing risk of failure were the main reasons for felling in 82% of the cases. Thirty three percent of these trees were degenerated or dead, but the amenity value of 14% of the risk trees was still high. The latter were old, big trees which posed a potential hazard, but had a vital and balanced crown. Some characteristic profiles for potential failure were identified for each of the tree species studied: Ganoderma lipsiense in the butts and hollows in the stems of Tilia spp., weak fork formations together with Rigidoporus populinus on Acer spp., and degeneration together with decay in the stem on Betula spp. Decay fungi most commonly identified were R. populinus, G. lipsiense, Inonotus obliquus and Piptoporus betulinus. In addi- tion, Kretzschmaria deusta was very common in three of the parks, and on every one of the tree species investigated. (Urban For. Urban Green. 2005. 3(2):113–120) DEPENDENCY OF BRANCH DIAMETER GROWTH IN YOUNG ACER TREES ON LIGHT AVAILABILITY AND SHOOT ELONGATION Kosei Sone, Ko Noguchi, and Ichiro Terashima Many biomechanical and theoretical studies have been based on the pipe-model theory, according to which a tree is regarded as an assemblage of pipes, each having the same amount of leaf area or leaf mass. However, the physiological mechanisms underlying the theory have not been extensively examined, particularly at the branch level. We analyzed how branches and trunks thickened in nine young Acer mono Maxim. var. marmoratum (Nichols) Hara f. dissectum (Wesmael) Rehder. and A. rufinerve (Siebold & Zucc.) trees. In particular, we examined the roles of light, allocation of photosynthates and shoot heterogeneity. The cross-sectional area (A) of a branch was proportional to cumulative leaf mass or leaf area of the branch, and cumulative cross-sectional area of the daughter branches (ΣA) above a branching point was equal to the A of the mother branch. These results indicate the validity of the pipe-model theory. However, the theory was invalid for current-year growth of branch cross-sectional area (∆A). The ∆A/Σ∆A for a branching point was greatest (nearly equal to 1) at the crown surface, decreased with crown depth, and tended to increase again at the trunk base, and ∆A strongly depended on light interception and the yearly increment of leaves on the branch. We examined factors that influenced ∆A with multiple regression analysis. The ratio of ∆A of a branch to branch leaf area depended on both relative irradiance and mean current-year shoot length of the branch, suggesting that diameter growth of a branch is determined by the balance between supply of photosynthates, which depends on light interception by the branch, and demand for photosynthates, which is created by the high cambial activity associated with vigorous shoot elongation. (Tree Physiol. 2005. 25:39–48) ©2005 International Society of Arboriculture
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