Abstract
Tree root defects from current nursery production practices influence short- and long-term tree performance and survivability. The Missouri Gravel Bed (MGB) system, a production method using gravel as a substrate, has been used to prevent many of these defects from occurring. MGB production involves planting bare root stock into a bed of gravel with frequent drip irrigation in order to produce a root system with relatively few defects. MGB production methods have also been purported to allow for summer transplanting of many species, as opposed to traditional dormant transplanting. Because gravel has low water- and nutrient-holding capacity, biochar (5% by volume) was incorporated into one plot as a possible means of improving both water- and nutrient-holding capacity over gravel alone. Wood chip mulch was also investigated as a growing substrate in place of the gravel in a growing system. In 2015, three species, Quercus bicolor (swamp white oak), Taxodium distichum (baldcypress), and Tilia cordata (littleleaf linden), were studied in pea gravel (PG), biochar-amended pea gravel (BC), and wood chip mulch bed (MB) growing environments. Very few differences occurred over the growing season with above- or belowground parameters indicating that the minimal-to-no-cost, more readily available substrate of wood chip mulch should be considered in these growing systems.
INTRODUCTION
The short- and long-term ramifications of root system defects from nursery production are well documented (Marshal and Gilman 1998; Gilman et al. 2010; Watson and Himelick 2013). Many techniques have been tested to combat these defects at the time of planting. Appleton and Flott (2009) proposed techniques to bare root container-grown or balled-and-burlapped (B&B) stock as a way of identifying and rectifying these defects to set up a root system for successful growth and development. To minimize root defects and concerns regarding soil and irrigation treatments following planting, the Missouri Gravel Bed (MGB) production system has been proposed as an alternative method for producing woody nursery plant material (Starbuck et al. 2005). Starbuck et al. (2005) has shown transplanting trees via the MGB method can equal the performance of more traditional production and transplanting methods. Personal scientific observations have verified Starbuck’s experience that MGB-grown trees can be harvested and planted bare root during the growing season while in leaf.
The MGB system consists of bare root planting stock lined into growing beds of pea gravel with frequent irrigation. The porosity of the gravel allows for very regular irrigation to eliminate moisture stress, while remaining well aerated to allow for proper root growth and function (Bohnert et al. 2008). One of the challenges in MGB production is the low water- and nutrient-holding capacity of pea gravel. Bohnert et al. (2008) investigated the use of calcined clay and sand as amendments to the pea gravel in order to increase these properties. Their study found that calcined clay amendment increased multiple performance parameters of Gymnocladus dioecious (Kentucky coffeetree) as a result of a better soil environment. In 2005, Starbuck et al. explored the potential use for mulch as a medium when digging, holding, and transplanting trees. They discovered Quercus rubra (northern red oak) performed slightly better when heeled in with wood chips rather than pea gravel.
Another amendment that could potentially bring water- and nutrient-holding capacity to MGB production is biochar. Biochar is a highly concentrated carbon byproduct of pyrolysis with 70 to 80% carbon and an ability to adsorb both nutrients and water while resisting decomposition (Scharenbroch et al. 2013). Work has shown that adding biochar to the growing medium can benefit shade tree performance and resistance to disease (Zwart and Kim 2012; Scharenbroch et al. 2013).
Scant literature exists regarding the use of a MGB system, so the authors were anxious to investigate a traditional MGB system and alternative systems. In this study, a traditional pea gravel (PG) growing system, a pea gravel system amended with 5% biochar (v/v) (BC), and a mulch bed growing system (MB) were compared to investigate their influence on above- and belowground tree growth.
MATERIALS AND METHODS
The study was conducted in a production bed as described below at the Bartlett Tree Research Laboratories in Charlotte, NC. Pressure-treated 10 cm × 10 cm (4 in × 4 in) lumber was used to construct a 2 m × 12.1 m (6.5 ft × 40 ft) frame with 0.6-m (2-ft) high sides. Dividing the structure lengthwise, a 0.6-m (2-ft) high wall was constructed of 10 cm × 10 cm (4 in × 4 in) lumber to separate the MGB into two distinct sections measuring 2 m × 6 m (6.5 ft × 20 ft). The interior walls and floor of the structure were lined with nursery ground cloth to direct irrigation water and rainfall into a 10-cm (4-in) perforated drain pipe to capture leachate so it could be recycled into the site irrigation pond (Figure 1). For the PG treatment, 6 mm (0.25 in) washed river pea gravel was installed to a depth of 46 cm (18 in). For the BC treatment, 6 mm (0.25 in) washed river pea gravel was installed in 15-cm (6-in) lifts. Between each lift, a layer of biochar was added and raked in with a pitchfork to distribute it within the lower lift. This method was repeated until a 46-cm (18-in) depth was reached. The MB treatment was built alongside the wooden structure of the MGB and measured 1.2 m × 12.1 m (4 ft × 40 ft) (Figure 2). This addition was not connected to the drainage system described for the MGB. Fresh arborist wood chip mulch less than fourteen days old was placed directly on the existing soil surface to a depth of 46 cm (18 in).
Construction of the MGB with 10 cm × 10 cm (4 in × 4 in) lumber, lined with ground cloth, and underdrained with a 10-cm (4-in) diameter perforated pipe.
Completed growing bed with pea gravel (PG), biochar-amended pea gravel (BC) and mulch bed (MB) treatments visible.
Irrigation was provided through Netafilm drip irrigation tubing with 3.4 L (0.9 gal) per hour emitters placed every 30 cm (12 in) in the length of pipe. For the PG and BC treatments, tubing was installed within 15 cm (6 in) of the stem on both sides of the plant, with each line of tubing offset by 15 cm (6 in) to ensure there were irrigation emitters located every 15 cm (6 in) longitudinally along the row. For the MB treatment, a single row of irrigation tubing was placed alongside the plantings. Irrigation was cycled on even hours from 8:00 to 18:00 for 5 minutes per cycle daily. As with Starbuck et al. (2005), the authors recognized, based on prior experience and field knowledge, that the various growing media required differing irrigation to reduce the likelihood of water-logging and anoxia.
Plant material in this experiment was Quercus bicolor (swamp white oak), Taxodium distichum (baldcypress), and Tilia cordata (littleleaf linden), which represent commonly used landscape shade tree plantings. Trees were received bare root and heeled in with mulch piles until they could be placed in the treatments. Within days of arrival in early March 2015, the trees were lined out into the treatment beds. Twenty of each species were included in the study, which allowed for five replicates of each growing bed type and harvest date treatment.
All species were randomized and blocked within each treatment growing bed type based on harvest date. Treatment subsets were harvested in May, July, or September. At the time of harvest, root volume was estimated using a water displacement method. Water displacement was calculated by using either a 10-cm (4-in) or 5-cm (2-in) diameter polyvinyl chloride pipe capped on the bottom as a water chamber modeled after Johnson (1983). A 6-mm (0.25-in) brass drain was installed near the top of each cylinder to allow overflow water to be captured. Before each measurement was taken, the cylinder was filled just above the drain and all overflow was allowed to escape. A tree’s root system would then be submerged in the cylinder, which displaced water through the overflow drain. This water was captured and used as a proxy for root volume. Both Q. bicolor and T. cordata were measured using the 10-cm (4-in) polyvinyl chloride pipe while the T. distichum were measured using the 5-cm diameter polyvinyl chloride pipe due to the smaller size of their root system.
After these data were collected, the above- and belowground tissue were severed at the point just above the highest root to collect dry weight data as a measurement of biomass. Initial weights were taken for above- and belowground tissues before they were placed in a CSE Chicago Surgical and Electrical Company drying oven at a temperature of 48° C (120° F) until all moisture had been removed. All data were analyzed using JMP software and ANOVA run independently for each species and date testing treatment effects. Tukey’s HSD mean’s separation tests were run following a significant ANOVA (P < 0.05). Simple regression analysis was used to model the relationship between water displacement and belowground biomass.
RESULTS
When looking across all species, treatments, and times, there were no clear trends with belowground measurements. With Q. bicolor, there were never any differences among treatments within a harvest period for belowground biomass (Figure 3) or volume (Figure 4). By the September harvest date, the trees from the MB treatment had less aboveground biomass than the other treatments (Figure 5). T. distichum had higher root volume in the BC treatment compared to PG in May (Figure 4), while the MB treatment had the highest root dry mass and volume in the July harvest (Figure 3). Upon completion of the study, a correlation between root volume estimates and dry root biomass for all three species resulted in an R2 of 0.819 (Figure 6).
Mean belowground biomass of Quercus bicolor, Taxodium distichum, and Tilia cordata grown in three growing media at three harvest intervals. Growing bed treatments were pea gravel (PG), biochar-amended pea gravel (BC), and mulch bed (MB). Means with different letters are significantly different within a harvest interval and within species (HSD, P < 0.05).
Mean root volume of Quercus bicolor, Taxodium distichum, and Tilia cordata grown in three growing media at three harvest intervals. Growing bed treatments were pea gravel (PG), biochar-amended pea gravel (BC), and mulch bed (MB). Means with different letters are significantly different within a harvest interval and within species (HSD, P < 0.05).
Mean aboveground biomass of Quercus bicolor, Taxodium distichum, and Tilia cordata grown in three growing media at three harvest intervals. Growing bed treatments were pea gravel (PG), biochar-amended pea gravel (BC), and mulch bed (MB). Means with different letters are significantly different within a harvest interval and within species (HSD, P < 0.05).
Volumetric water displacement correlation with root dry weight across all three species: Quercus bicolor, Taxodium distichum, and Tilia cordata.
Aboveground biomass was similar to belowground in that the BC treatment resulted in the highest biomass in May (Figure 5). With T. cordata, the May harvest resulted in BC having higher belowground biomass than PG, but that did not translate into any other differences (Figure 3).
DISCUSSION
As with many data in tree science, there was variability in the data among species and harvest time. No one treatment consistently stood out as the best performer across species and time. However, there was one instance where PG outperformed BC or MB; this was seen in September with Q. bicolor. In this study, this would conclude that there is no downside to using either of these treatments. Certainly, the cost and practicality of using these substrates must be considered. For a grower already using PG, it might be quite simple to add biochar into the gravel system to gain potential benefits from the BC treatment. Since biochar is very stable and resistant to decomposition, it should last in the pea gravel for the life of the growing bed. As cost and availability of biochar has improved, it is an easy way to potentially improve the productivity of a MGB system.
The MB treatment held up very well throughout the trial. Only with aboveground biomass of Q. bicolor in the September harvest was there ever a negative result associated with MB. In all other instances, MB performed as well as or better than the PG or BC treatments. The MB treatment is far easier to install as mulch weighs less than pea gravel and is easier to transport. From an economic perspective, in most cases it can be procured for no or minimal cost from arborists. This is also an opportunity for growers to reuse and recycle wood waste material into a growing medium. The MB treatment received half the water that the PG and BC treatments received. Depending on water source, the water cost savings could be significant over time as well as being a more responsible use of water resources. This reduction in water wasn’t a detriment to the health of the trees and is viewed as a positive by the authors. The MB treatment deserves further investigation as there are likely differences among mulches with regards to species, C:N, size of particle, and other variables.
Although not initially an objective of this project, finding that root volume estimates from the water displacement method correlate well with dry biomass is helpful. Water volume displacement can be measured multiple times on the same plant over time, which could be helpful for certain experiments where this would be desirable. Dry biomass measurements are easier and faster, but require destruction of the plant being measured.
- © 2019, International Society of Arboriculture. All rights reserved.
Jump to section
Related Articles
Cited By...
- No citing articles found.