Short and Long-Term Effects of Treeshelters on the Root and Stem Growth of Ornamental Trees

Short-term experiments

A randomized, complete block design experiment (6 blocks, 2 replicates per block) was initiated in May, 1993, in a container nursery whereby each block contained two trees of each species, one in a shelter (122-cm tall, 10-15 cm in diameter, tan Tubex® shelter) and one not in a shelter (control). Seed-propagated liners were used as planting stock including: Sequoia sempervirens (D. Don) Endl., Quercus lobata Née, Quercus agrifolia Née, Lagerstroemia indica L. ‘Watermelon Red’, Ginkgo biloba L., Platanus racemosa Nutt. and Fraxinus Iatifolia Benth. The trees were grown in 1.8-liter (#1) containers filled with UC (University of California) Mix (1:1:1, by volume, sand:redwood sawdust:peat moss). Thirty g of Nutricote Total (18-6-8 with micronutrients, Type: 180, PlantCo, Inc., Brampton, Ontario, Canada) were added to the soil surface at planting time. Treeshelters, held upright by 1 stake, were placed over the appropriate plants after the containers were moved to the nursery bed. The nursery containers were placed on 40-cm centers. An automated irrigation system (spray stakes) was used to add approximately 500 ml of water each day. Half the blocks were harvested after three months and the other half after six months. Measurements of height, caliper and fresh and dry biomass accumulation were taken at each harvest.

Solution culture experiments

Tip cuttings of Populus euamericana cv. Giacometti were rooted in aero-hydroponic units (17) containing de-ionized water. Once rooted, the cuttings were put singly into containers holding approximately 2 liters of Hoagland’s Solution 2 plus micronutrients (10) that were continuously aerated (≈1 L·min^-1). Each cutting was held in place in the lid of the container with rubber stoppers that had holes bored through them. This arrangement allowed the rooted cuttings to be held with only their root systems immersed in the aerated nutrient solution. Each container was equipped with an aeration tube, a drip tube capable of distributing fresh Hoagland’s Solution 2 and an overflow tube (just under the lid). Treeshelters were attached to the upper surface of the lid of half the containers using hot glue creating an airtight seal. There were six containers per treatment arranged in a completely randomized design on a bench in a greenhouse kept at 22-28° C. Every day, fresh nutrient solution was added to each container using an automated irrigation system. When the container became filled during each “irrigation”, excess nutrient solution escaped through the overflow tube. Plants were measured and harvested six weeks after placing the rooted cuttings in the nutrient solution containers.

Long-term experiments

A randomized, complete block design landscape experiment (10 blocks) was initiated in April, 1993, whereby each block contained two adjacent trees of each of the same seven tree species used in the nursery experiments that were planted on 1.8 m centers in rows 1.8 m apart. The trees were irrigated with an automatic drip system distributing ~8.7 L every other day for the duration of the experiment. Trees were fertilized once, four weeks after planting, with Agriform 20-10-5 plus minors fertilizer tablets (Grace/Sierra, Milpitas, CA). The fertilizer tablets were pressed 8-12 cm into saturated soil 6-10 cm from each tree immediately after irrigation. After 12 months height and stem diameter (caliper) were measured and fresh and dry mass were determined for tops (leaves, branches, stems) and roots of trees in half the blocks. Root systems were removed from the ground with a backhoe fitted with a 60-cm wide bucket. The soil attached to the roots was removed with pressurized water so root fresh and dry mass could be determined. A recently rinsed root system of each tree was shaken to remove excess water, weighed (fresh mass), cut into pieces, placed in a paper bag, dried for at least seven days at 70°C and weighed (dry mass). The remaining trees in blocks were harvested after 24 months. Trees were harvested by cutting the trunk at ground level. Twigs with leaves and stems were cut into pieces, weighed (fresh mass), placed in paper bags and dried for at least one week at 70°C to obtain dry mass. Collected biomass data included top (leaves, branches, stems) fresh and dry mass.

Root systems were exposed using hydroexcavation techniques (7,8) (Figure 1). A pneumatic conveyor industrial loader vacuum truck (Vactor model 1645, Federal Signal Co., Streator, IL) removed the resulting soil-water slurry. Water was supplied by a nearby hydrant with a flowing pressure of 64 PSI through a 2-½ in. diameter supply line. The hose size was reduced to 1-¾ in. diameter handline with 1-½ in fittings at the excavation site. A modified “EZ Pup” (Durham Fire and Rescue Specialty Tools, North Tonawanda, NY) was utilized at the end of the handline to reduce operator fatigue and a KK “Thunder fog” nozzle (Task Force Tips, Valpariso, IN) was attached to the “EZ Pup”. The water stream delivered under pressure reduced the soil in the root area to a slurry and the Vactor vacuum hose removed it. Using this technique, undamaged roots as small as 1 mm in diameter were exposed. On deeper, more extensive root systems, hydroexcavation continued until roots as small as 1 cm were exposed. They were then cut at the soil interface before removing the excavated portion from the ground. The root systems were air-dried, painted white, suspended above a backlit background and photographed from above using high-contrast black-and-white film. This set up provided excellent contrast between the shadow-darkened root system and the bright background. Once photographed, the root systems were cut into pieces and dried for at least seven days at 70° C (for dry mass). The black- and-white photographs were used to estimate total root length and total root area. Each root system was photographed with a rod of known length (10, 50 or 100 cm) in the picture. The images of known length and a piece of string arranged into a circular shape (ends not touching) were used to calibrate the video imaging system (AgVision Root and Leaf Analysis, Decagon Devices, Inc.) that made the root length and root area estimations. The string was at least 150 mm in length and produced an image at least 200 pixels long on the computer screen to accurately calibrate the digitized image. By comparing the known length of the string and the known length of the rod in each photograph, the actual dimensions of the root system could be All data for all experiments were analyzed using the General Linear Model (GLM) Procedure of the SAS statistical system (16).

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Figure 1.

Hydroexcavation of tree roots with a high-pressure water and vacuum system

Height increase

All species growing in shelters, except G. biloba, showed height increases over control trees of 62-244% after only three months (May to August) of growth (Table 1). This same result occurred in those tree species measured at six months (Table 1). After one year, L. indica, Q. agrifolia and Q. lobata trees growing in shelters were still taller than their unsheltered counterparts, whereas F. latifolia, G. biloba, P. racemosa and S. sempervirens trees growing in shelters were no taller than controls. After two years, only L. indica and S. sempervirens trees growing in shelters continued to be taller than the controls (Table 1). The unsheltered trees of the other five species all had similar heights by then.

Caliper development

Caliper development was only slightly affected by treeshelters after three months in F. latifolia, G. biloba and S. sempervirens (Table 2). At six months none of the tree species showed differences in caliper and long-term (1 or 2 years) effects of treeshelters on caliper were observed in S. sempervirens and P. racemosa (after 1 and 2 years) and F. latifolia and Q. agrifolia (after 2 years) (Table 2). At no time during this study were caliper differences between control (unsheltered) and sheltered trees observed for L. indica or Q. lobata.

Top dry mass

Top dry mass (TDM) varied in treeshelters during the time of the experiment among most of the species. After 6 months L. indica, Q. agrifolia, Q. lobata and S. sempervirens trees growing in shelters had greater top dry mass than control trees (Table 3). However, after 1 year there was no significant difference between unsheltered and sheltered trees except for P. racemosa and S. sempervirens where unsheltered trees had higher top dry mass than sheltered counterparts (Table 3). After 2 years F. latifolia, P racemosa, Q. agrifolia and Q. lobata dees growing in treeshelters had lower dry mass than control trees.

Root dry mass and dry matter partitioning

Except for Q. agrifolia, all tree species eventually had reduced root dry mass (RDM) when grown in treeshelters (Table 4). The reduction in RDM occurred most rapidly (3 months) for F. latifolia, G. biloba and P racemosa and by six months for L. indica, Q. lobata and S. sempervirens. After two years, RDM of sheltered trees ranged from 9 to 71% that of unsheltered trees. Treeshelters reduced the root:shoot ratio of G. biloba, L. indica, Q. lobata and S. sempervirens (Table 4). Most dramatic was the response observed in L. indica and S. sempervirens where the root:shoot ratios for the sheltered trees were 11% and 8% that of the controls.

Total root length and total root area

Treeshelters reduced total root length by 17-39% and total root area by 38-89% compared to controls for all tree species except Q. agri folia (Table 5).

Dry biomass response of Populus trees in aerated solutions

Leaf, stem and root dry mass and root-shoot ratios of Populus were lower in plants growing in shelters versus those unsheltered (Table 6). The percent reductions were similar for trees growing in aerated nutrient solutions compared to tree species growing in containers and in the landscape. Figure 2 shows the reduction in root volume of Populus trees grown in aerated solutions. The reduction in root system growth is common to most trees studied when grown in shelters. In the aerated nutrient solution experiments none of the root system tissues were lost when the plants were harvested compared to the trees hydroexcavated from landscape sites. Height and fresh mass responses were also similar to those of plants growing in soil (data not shown).

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Figure 2.

Root development of Populus euamericana cv. Giacometti trees grown in aerated solution culture with (left) and without (right) treeshelters