Arboriculture & Urban Forestry 36(4): July 2010 bulk density of 1.4 g/cm3 , the roots were few and confined to the upper one-third of the pot (Chiapperini et al. 1978). Root- ing can therefore be restricted by moderate soil compaction for particular soil textures. Zisa et al. (1980) found that depth of root penetration for Pinus nigra in sandy loam soil was re- duced at a bulk density of 1.6 g/cm3 , but at 1.4 g/cm3 Pinus nigra were severely restricted in silt loam soils. For Ailanthus altisima the root growth and penetration in sandy loam was sig- nificantly restricted at a bulk density of 1.64 g/cm3 , with simi- lar restriction in sandy soil at 1.7 g/cm3 (Pan and Bassuk 1985). The objective of this study was to evaluate the growth and development of three tree species during the first six months af- ter being transplanted at different levels of compaction in two soil types. As a working hypothesis, it was expected that trees growing in denser clay soils would have less ability to over- come higher mechanical impedance and strength of compacted soils than looser sandy clay loam soils. The three tree species evaluated are in common use in urban forestry in Puerto Rico. MATERIALS AND METHODS The study was conducted on Alzamora Farm of the Agricultur- al Science Faculty at the University of Puerto Rico-Mayagüez (UPRM) (Mayagüez, Puerto Rico) in 2003–2004. Alzamora Farm is located at 18°12’54.3”N and 67°08’43.3”W. The av- erage annual temperature was 25°C with little monthly varia- tion. The annual average precipitation was 3,200 mm, with a dry season from December to April (monthly average rain- fall 68 mm), and a rainy season from July to September [monthly average rainfall 202.4 mm (ATMOS CARIB 2003)]. Callistemon citrinus [(Curtis) Skeels], commonly known as red bottlebrush, is native to Australia and grows easily in tropical and subtropical regions. It is a small tree or large shrub that grows 2–4 m tall and 2–3 m wide, and likes well-drained soils, preferably san- dy loam soils, but is quite adaptable. Red bottlebrush tolerates any soil except very poor or alkaline soils (Gilman and Watson 1993). Tecoma stans [(L.) Juss. ex Kunth], commonly known as trum- pet brush, is a native tree from the Caribbean (Little and Wadsworth 1964), naturalized in much of the tropical and subtropical regions. It is a medium-size brush, 1.5–1.7 m, with many branches and bas- al stems. Trumpet bush can grow well in pH 6.0–7.8 and in most well-drained soils, including calcareous fill, infertile sands, acidic Ultisols and volcanic regolith. The species is described as a “water spender,” which is able to convert to a water saver (Tipton 1994). Tabebuia rosea (dc), commonly known as pink trumpet, is na- tive of tropical continental America, and grows from wet lowlands to dry uplands, from southern Mexico to Venezuela (Gentry 1992). The pink trumpet is a medium-sized tree with medium to fast growth (Whitmore and Hartshorn 1969; Salas-Estrada 1993). Pink trumpet is found in deep and superficial soils, but grows better in well-drained soils with a light texture (Gentry 1992). Pink trumpet can grow well in pH 5.5 to 8.0, in sandy loam to clay loam soils. Field observations suggest the three tree species tested have the following order of tolerance to urban soil condi- tions including compaction: T. stans > T. rosea > C. citrinus. Same-size seedlings 47 cm in height of three ornamental tropical tree species; trumpet bush, bottle brush, and pink trumpet were trans- planted in 18.52 L pots of sandy clay loam or clay at three levels of compaction: control (no compaction), medium compaction with a bulk density of 1.2 g/cm3 , and high compaction 1.4 g/cm3 bulk density. 165 Plant height, trunk shoot diameter, leaf number and col- or, foliar area, and leaf, shoot, and root dry weight were mea- sured on two plants every two months during six months. The seedlings of the three ornamental tropical tree species were donated by the Department of Natural Resources of Puerto Rico’s Arecibo Nursery. The seedlings were acclimatized in a nursery located inside of Alzamora Farm at UPRM Campus. Construction of the Compacted Soils Tree seedlings planted in containers along sidewalks or in side- walks in urban environments show restricted growth over time due to compacted and limited soil volumes. For this reason the study authors chose a compaction approach of using a Proctor test ham- mer to achieve consistent bulk density through each whole pot. A sandy clay loam soil (45.3% sand, 25.9% silt, and 28.8% clay) of pH 7.1 and 1.5% organic matter (O.M.), and a clay soil (2.1% sand, 17.4 % silt, and 80.5% clay) of pH 4.7 and 2.5% O.M., were used to construct the compacted soils. Prior to com- paction, both soils were brought to 20% volumetric moisture content. Desired bulk densities were obtained by compacting different weights of soil to a single volume, applying different numbers of blows to the soil with a hammer of 2.5 kg with a free fall of 30.4 cm used in ASTM Proctor tests (Felt 1965). The soil was compacted in 18.52 L pots with an upper in- ternal diameter of 28.9 cm, a lower internal diameter of 26 cm, and height of 36.5 cm. The soil was poured inside the 18.52 L pots and compacted to achieve different levels of compaction (different bulk density values). After the compacted soil reached a depth of 24.2 cm in each pot, a metallic pot with an internal diameter of 15.9 cm and height of 15.2 cm created with metal- lic mesh (1.3 cm x 2.5 cm) (Figure 1a; Figure 1b) was placed in the center of the 18.52 L pots. Inside this metallic mesh pot PVC tubes (15.3 cm x 15.3 cm) were inserted with almost the same dimensions of the metallic pots to ensure the level of compaction and avoid loss of soil inside the metallic mesh pots. After the metallic pot with the PVC tubes were placed in the center of the 18.52 L pots, more soil was poured around the metallic pot and the soil was compacted further (Figure 1c). Figure 1. Construction of metallic pot with a metallic mesh (1.27 cm x 2.54 cm); (a) height (15.2 cm), and (b) Internal diameter (15.9 cm). After a PVC pipe was placed inside of the metallic pot, the PV pipe with the pot were placed in the center of the 18.52 L item and the soil was compacted using a hammer, (c), of 2.5 kg with a free fall of 30.4 cm used in ASTM Proctor test. Once desired levels of compaction were achieved, the PVC tubes were removed and seedlings were planted inside the me- tallic mesh (Figure 2a; Figure 2b). The same soil type was used to fill the remaining empty space inside the metallic pot (Fig- ure 2c). To obtain a bulk density value of 1.2 g/cm3 for both ©2010 International Society of Arboriculture
July 2010
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