Journal of Arboriculture 31(4): July 2005 175 Table 2 (continued). Mean total root numbers (n), mean total root dry weights (g), and mean individual root size (g/n) of 16 species in all trench zones by year, The Los Angeles County Arboretum & Botanic Garden, Arcadia, California, 1997–2000. Species Syagrus romanzoffiana 1997–1998 1998–1999 1999–2000 Trachycarpus fortunei 1997–1998 1998–1999 1999–2000 Trachycarpus wagnerianus 1997–1998 1998–1999 1999–2000 Washingtonia robusta 1997–1998 1998–1999 1999–2000 Number (n) 136.4 96.8 67.5 76.8 31.8 69.3 78.8 111.2 124.6 154.0 174.1 161.4 Weight (g) 36.5 44.2 21.3 5.9 2.9 8.7 16.9 15.0 24.4 22.2 21.5 17.3 Size (g/n) 0.243 0.267 0.229 0.090 0.043 0.076 0.189 a 0.116 b 0.113 b 0.144 0.083 0.077 Means in the same column within a species followed by a different letter are significantly different according to LSD, P < 0.05. capitata and Trachycarpus wagnerianus, which decreased over time (Table 2). Although not counted and analyzed, we observed that all species had severed roots that regrew, some rather prolifi- cally, with up to several new roots per severed root. Seasonality of Growth Soil temperatures varied seasonally, averaging 10°C (50°F) in December, 13°C (55°F) in March, 20°C (68°F) in June, and 21°C (70°F) in September. Mean root numbers were significantly higher in the warmer months (spring, summer, and/or fall) for most species (Table 3). Only Brahea edulis, Chamaerops humilis, and Livistona chinensis lacked distinct seasonality, although they tended to have higher numbers in the summer and fall. Maximum mean warm-season root numbers ranged from about 11 in Brahea edulis to 329 in Washingtonia robusta. In most species, maximum warm- season numbers were 70 or less, although they were well over 200 in six species (Livistona decipiens, Phoenix canariensis, P. reclinata, Syagrus romanzoffiana, Trachycarpus wagnerianus, and W. robusta). Seven species (Archontophoenix cunninghamiana, Livistona decipiens, Phoenix canariensis, Sabal etonia, Serenoa repens, Syagrus romanzoffiana, and Trachycarpus wagnerianus) had highest root numbers in the summer, most with two to ten times as many roots in the summer as in other seasons. Six species had highest root numbers in two or more of the warmer seasons: Butia capitata, Caryota mitis, and Rhapidophyllum hystrix in the summer and fall; Phoenix reclinata and Washingtonia robusta in the spring and summer; and Trachycarpus fortunei in the spring, summer, and fall. Mean winter numbers were low in all species except Washingtonia robusta, which averaged nearly 28 roots. Caryota mitis and Serenoa repens averaged less than one root in the winter, the lowest among all species. Mean root weight increased during the warmer seasons except in Brahea edulis and Livistona chinensis, which had no seasonal differences (Table 3). Mean root size increased during the warmer seasons in half the species: Archon- tophoenix cunninghamiana, Livistona decipiens, Phoenix canariensis, Rhapidophyllum hystrix, Serenoa repens, Syagrus romanzoffiana, Trachycarpus wagnerianus, and Washingtonia robusta (Table 3). The remaining species had no seasonal differences in root size. Root Distribution Lateral Distribution. With the exception of Rhapidophyllum hystrix, mean new root numbers were highest in the zones closest to the trunk and lowest in the zones farthest away (Table 4). All species had at least 50% of growth within 30 cm (12 in.) of the trunk, and three (Livistona decipiens, Trachycarpus fortunei, and Washingtonia robusta) had more than 50% of growth within 15 cm (6 in.) of the trunk. Vertical Distribution. Mean new root numbers in 13 of the 16 species was significantly higher (>60% of new roots) in the combined upper zones 0 to 30 cm (0 to 12 in.) deep than in the combined lower zones 30 to 60 cm (12 to 24 in.) deep (Table 5). These 13 species grew at least twice as many ©2005 International Society of Arboriculture
July 2005
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