Journal of Arboriculture 31(4): July 2005 173 Table 1. Palm species used in root growth study, The Los Angeles County Arboretum & Botanic Garden, Arcadia, California, 1997–2000. Species Archontophoenix cunninghamiana Brahea edulis Butia capitata Caryota mitis Chamaerops humilis Livistona chinensis Livistona decipiens Phoenix canariensis Phoenix reclinata Rhapidophyllum hystrix Sabal etonia Serenoa repens Syagrus romanzoffiana Trachycarpus fortunei Common name king palm Guadalupe palm pindo palm, jelly palm clustered fishtail palm European fan palm Chinese fan palm ribbon fan palm Canary Island date palm Senegal date palm needle palm palmetto, cabbage palm saw palmetto queen palm Chinese windmill palm Using a gasoline-powered trencher, we dug a trench 15 cm wide × 60 cm deep × 90 cm long (6 × 12 × 36 in.) in June on a tangent from the base of each of the three, single-tree replicates of each species. Using a spade and shovel, we manually dug trenches on three additional, single-tree replicates of two species, Archontophoenix cunninghamiana and Livistona chinensis, to determine whether roots cut in this method responded differently than those cut by the trencher. We backfilled each trench with perlite and covered it with 2.5 to 7.5 cm (1 to 3 in.) of mulch. We adapted our technique from that of Broschat and Donselman (1984a, 1984b, 1990). We divided each trench into four zones: 15, 30, 60, and 90 cm (6, 12, 24, and 36 in.) from the trunk base. Each of these zones was divided into two depths of 0 to 30 cm (0 to 12 in.) and 30 to 60 cm (12 to 24 in.), resulting in a total of eight trench zones. Zones A, B, C, and D were the upper depth zones, while E, F, G, and H were the lower depth zones. Palms were well irrigated throughout the study to ensure the perlite was kept moist. At 3-month intervals over the next 3 years, we re-excavated each trench to harvest and count all roots, regardless of size or origin, growing into each of the eight trench zones and determined total mean root number per zone. We oven dried harvested roots at 105°C (221°F) until constant weight was achieved and determined total mean root dry weight per zone. We divided root dry weight by root number to calculate mean individual root size (weight per root) per zone. At each 3-month harvest, we recorded the soil temperature using a thermom- eter placed 10 cm (4 in.) deep in the trench. Although we did not determine the origin of each of the new roots, we made general observations about the presence of regrown severed roots. Because rapid root regrowth is critical to transplant success, we combined data from each upper zone with its corresponding lower zone for analysis. Thus, each palm’s data Origin southeastern Australia Guadalupe Island, Mexico northern Argentina, southern Brazil southeastern Asia, Malaysia, Indonesia southern Europe, northern Africa southern Japan, southern China eastern Australia Canary Islands tropical Africa southeastern United States southeastern United States southeastern United States northern Argentina, southern Brazil southern China from zone A were combined with E, B with F, C with G, and D with H to determine the lateral root distribution. We also compared the combined four upper zones with the combined four lower zones to determine the root vertical distribution. The experimental design was completely randomized with each of the 16 species represented by three, healthy, similar- sized and -aged, single-tree replications. We analyzed seasonal- ity and lateral distribution of roots by one-way ANOVA and separated means using least significant difference (LSD). We analyzed vertical root distribution by paired t-tests. RESULTS Mean annual root growth or numbers varied among species over the 3 years of our study (Table 2). There was no significant difference in new root numbers between machine- dug and hand-dug trenches (data not shown). Species with consistently high numbers, over 50 roots annually, include Livistona decipiens, Phoenix canariensis, P. reclinata, Syagrus romanzoffiana, Trachycarpus fortunei, T. wagnerianus, and Washingtonia robusta—the latter the highest, with over 150 roots. Species with consistently low numbers, fewer than 20 roots annually, include Archontophoenix cunninghamiana, Brahea edulis, Butia capitata, Chamaerops humilis, and Serenoa repens, with the Brahea the lowest—typically only one to two annually. Annual root numbers for a species varied annually, but differences were significant in only four species (Table 2). Root numbers decreased after year 1 in Brahea edulis and Livistona chinensis but increased in year 3 in Phoenix canariensis. Root numbers in Chamaerops humilis had a unique low-high-low pattern over 3 years. Mean root weight did not differ annually except in Brahea edulis and Livistona chinensis, which decreased over time, and in Phoenix canariensis, which increased over time (Table 2). Mean root size differed annually only in Butia ©2005 International Society of Arboriculture
July 2005
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