Abstract
Seedlings of selected tree species were grown in small benchtop rhizotrons filled with fine- and coarse-textured soils representing 2 different urban edaphic conditions in Mexico City. Bulk density was significantly higher and porosity was significantly lower in the coarse-textured soil. The maximum depth of root penetration visible behind the glass was significantly greater in the fine-textured soil for each of the 3 species after 5 months. Roots of Fraxinus uhdei penetrated deeper than roots of both Quercus crassipes and Q. crassifolia in both soils. Fraxinus uhdei root and shoot dry weight were significantly reduced in coarse-textured soil compared to the fine-textured soil, but both Quercus species were unaffected by soil type. In the fine-textured soil, F. uhdei root and shoot dry weight was significantly greater than both Quercus species, but not in the coarse-textured soil. At the end of the study, F. uhdei growing in fine-textured soil were taller than F. uhdei growing in coarse-textured soil and taller than both Quercus species in both soils, though the difference took 6 weeks longer to develop in the coarse-textured soil.
Growing conditions in urban landscapes often limit the number of species that can be grown successfully. Those that are the most successful can usually tolerate a broad range of growing conditions (Ware 1993). Poor species diversity exists in many cities around the world (Bueno 1996; Gilman et al. 1996; Gilman 1997; Nilsson et al. 1998). A tree inventory in Mexico City, showed that 72% of all the trees in the city consisted of only 9 species (7 genera). Nineteen percent of the street trees were of a single species, Fraxinus uhdei (Chacalo et al. 1994).
Several challenges are faced when growing urban trees in Mexico City. Air pollution is high because the city is surrounded by mountains at an altitude of 2,240 m (7,350 ft) above sea level. Rainfall occurs primarily between May and October. However, regional environmental factors are not the primary reason for such poor species diversity along the streets. The very dense and rapidly growing population often results in poor-quality planting sites and extreme people-pressure on trees. The variable quality of urban sites, lack of proper tree care, difficulty of producing some species in the nursery, and lack of knowledge about seldom-used species are also factors that limit the use of desirable trees in urban landscapes (González 1993; Romero 1993; Ware 1993; Chacalo and Fernández 1995; Gilman et al. 1996; Gilman 1997).
High diversity of native tree species exists in Mexico. More than 75 different species of trees are native to the region around Mexico City, including 27 Quercus species. One-hundred fifty of the 500 species of Quercus known worldwide are native to the country (Rzedowski and Rzedowski 1979; Nixon 1993; Romero 1993; Bonfil 1998), of which 64% are endemic (Nixon 1993).
Urban soil conditions can severely limit plant growth (Barnes et al. 1971; Craul 1992; Kozlowski 1998). A recent inventory of street trees demonstrated that site limitations related to limitations in the soil environment are present in Mexico City (Chacalo et al. 1997). The lack of information about root growth characteristics of native Mexican tree species in the local urban soils suggested the need for a study on root development. Rhizotrons were chosen over other methods because they allow repeated nondestructive root observation. This method has been used extensively in agriculture but seldom in arboriculture. Rhizotrons vary in size, construction, and operation (Böhm 1979).
The main objectives of this study were 1) to evaluate the use of small benchtop rhizotrons as a system for simulating urban soil conditions and monitoring the resulting changes in root growth, 2) to compare the penetration and total dry weight of the roots of 2 Quercus species seldom used as urban trees in Mexico City compared to the most common street tree, F. uhdei, and, 3) to determine whether the types of soils occurring in Mexico City can limit root growth of trees.
Materials and Methods
The experiment was conducted between August 1996 and March 1997 at Colegio de Posgraduados (Montecillo, State of Mexico, east of Mexico City). The interior dimensions of the rhizotrons used in this study were 40 × 70 × 5.5 cm (front to back) (15.8 × 27.6 × 2.2 in.). The wooden box of the rhizotron was painted with an oil-based outdoor enamel to protect it from water damage. A grid 5 × 5 cm (2 × 2 in.) was painted on the removable glass side held in place by an aluminum frame. To keep light from influencing the root growth, the edges of the glass were covered with aluminum tape and the glass front was covered with thick aluminum foil. The rhizotrons were held at a 30-degree angle on benches throughout the experiment to encourage the roots to grow against the glass. The rhizotrons were arranged in a randomized design.
The rhizotrons were kept in a large shelter designed for growing plants. A translucent roof provided filtered sunlight. Wall panels could be raised an lowered for ventilation. Air temperature near the rhizotrons was monitored throughout the experiment. Daily temperature fluctuations were approximately 35°C (95°F). A maximum average daytime temperature of 42°C (107.6°F) was reached in October, and minimum overnight average temperature of −2°C (28.4°F) occurred in January. These temperatures were higher than outdoor temperatures during the day and similar to outdoor temperatures during the night.
The soils were prepared and installed in the rhizotrons during August and September 1996. Coarse- and fine-textured soils were collected from urban sites and represented 2 different types of urban soils found in Mexico City The coarse-textured soil was a loamy sand and the fine-textured soil was a clay loam. Analysis showed that nutrients levels were within acceptable ranges for both soil types (Table 1).
The soil was sieved 3 times through a 2-mm (0.1 in.) mesh and then fumigated with methyl bromide for 5 days. Each empty rhizotron was weighed before adding the soil. To settle the soil, air-dried soil was added slowly and continuously while tipping the rhizotron from side to side and striking it against the ground when returning to center. This method was chosen over tamping the soil surface in order to avoid breaking the glass and creating layers that could interfere with root growth. The rhizotrons were weighed again after filling. The height of the soil was recorded for each rhizotron. Because the volume of soil used was too large to oven dry, samples were oven dried and used to convert air-dry weight to oven-dry weight for bulk density calculations. Average bulk density for the whole rhizotron was estimated by using the total soil weight and volume in the rhizotrons. Attempts were made to measure bulk density variations at different depths in the rhizotrons after the glass was removed at the end of the experiment, but intact cores could not be extracted successfully. Porosity was calculated using the formula: f = (Bd/d) where f = porosity, Bd = bulk density, and d = particle density.
A soil thermometer installed in 1 rhizotron verified that soil temperatures remained above 4°C (39.2°F). The soil in each rhizotron was brought to field capacity before planting with a calculated volume of water delivered by a specially designed drip irrigation system. The number of days required for the wetting front to move all the way to the bottom was recorded. The rhizotrons were maintained near field capacity during the experiment by adding measured amounts of water based on rhizotron weight loss, using the same drip irrigation system.
The criteria for native species selection included native origin, attractive ornamental features, and wide ecological distribution Growth of 2 seldom-used species, Q. crassipes and Q. crassifolia, were compared to F. uhdei, the most commonly planted and successful species planted on the streets of Mexico City.
To avoid problems with inconsistent germination, Quercus seeds were germinated before transplanting them into the rhizotrons on October 1, 1996. Fraxinus uhdei seeds were planted directly into the rhizotrons on October 1 and germination occurred 15 days later. There were 2 plants in each rhizotron. Plants that died during the experiment were not replaced. The plants were grown for 6 months.
Root and shoot growth were recorded weekly. Root growth was traced on the glass with markers, using a different color each week for new growth. Maximum depth of visible root penetration was also recorded weekly. Periodic shoot growth measurements included the total height of the plant when the main stem was held vertically.
The process of removing the plants from the rhizotron began on March 15, 1997. All plants of each species were harvested during the same week. The F. uhdei were harvested last because they were the last to germinate.
A nailboard (Böhm 1979) was used to hold the roots in place as the soil was removed. Nail locations corresponded to the line intersections of the 5 × 5 cm grid on the glass. The nails were pressed completely into the soil before turning the nailboard and rhizotron over together, and then removing the wooden back and sides of the rhizotron. The nailboard and soil were soaked together (2 to 3 hours for the clay loam soil, overnight for the loamy sand soil), and then the softened soil was washed away with a gentle stream of water. The depth of maximum penetration of the root system of each plant was recorded. The stems were then cut at the soil line, dried for 24 hours at 80°C (176°F), and then weighed.
The experimental design was a randomized balanced complete factorial (Hicks 1993). Two soils and 3 species were the treatments, with 10 replications of each combination. Treatment effects were determined by analysis of variance (ANOVA) using Sigma Stat 2.0. Differences among treatment means were separated by the Student-Newman-Keuls (SNK) at P < 0.05. All pairwise multiple comparison procedures using the Student Newman Keuls method were applied to raw data. A Student’s t-test for independent samples was used to compare values between soils.
Results and Discussion
Based on overall size and appearance of the plants and their root systems, plant vigor was generally lower in the coarse-textured soil (Figure 1). Five seedlings (17%) in the coarse-textured soil died during the experiment, while none died in the fine-textured soil.
Bulk density of the coarse- and fine-textured soils in the entire rhizotron was 1.20 and 1.01 Mg/m-3, respectively These values are lower than the “ideal soil” and well below the values of 1.70 and 1.46 Mg/m-3 that are generally accepted as threshold values for root growth restriction for these soils (Craul 1992). The low particle density of volcanic materials in these soils (P. Kelsey, personal communication 1997) contributes to the low bulk density.
Soil porosity of the coarse-textured soil was significantly lower than the fine-textured soil (47% and 54%, respectively). Coarse-textured soils usually have less pore space than fine-textured soils because of the smaller particle surface area in relation to volume, and closer packing of the particles (Hillel 1980; Craul 1992). Lower porosity can result in slower diffusion of soil gasses and less oxygen for roots, especially in deeper soils. Reduced aeration may have contributed to the reduced plant survival and vigor (Drew and Stolzy 1996; Kozlowski 1998).
During the initial irrigation of the dry soil in the rhizotrons, the wetting front moved significantly more slowly through the coarse-textured soil. Completely wetting the coarse-textured soils took an average of 3.5 days longer. Slower water movement through the coarse-textured soil is an indicator of greater compaction, a decrease in porosity, and loss of pore continuity (Kozlowski 1998).
Depth of Root Penetration
Roots of the 2 Quercus species in both soils and the F. uhdei in fine-textured soil were visible behind the glass in the majority of the rhizotrons by day 30 (Figure 2). Fraxinus uhdei roots growing in coarse-textured soil were not visible behind the glass until day 42 and were more shallow when they became visible.
Quercus crassifolia roots penetrated significantly deeper than F. uhdei on days 43 and 55 in both soils; Quercus crassipes roots penetrated deeper than F. uhdei on day 55 in both soils and on day 43 in the fine-textured soil; there was no difference between F. uhdei and Q. crassipes on day 43 in the coarse-textured soil.
The more rapid initial root penetration of the Quercus roots in both soils may be related to large energy reserves in the seed. Large seeds, such as Quercus, often produce strong taproots in the seedling stage (Bonfil 1998). The smaller F. uhdei seeds do not produce taproots, and substantial lateral root growth occurs at an early stage (Yorke and Sagar 1970, in Russell 1977).
After day 55, many weeks followed where there was no difference in depth of penetration between species in either soil type (Figure 2). Fraxinus uhdei root penetration became significantly deeper than both Quercus species in both soils on day 156. At this time, F. uhdei roots had penetrated 74% deeper than both oak species in the coarse soil, and 29% and 20% deeper in the fine soil, than Q. crassifolia and Q. crassipes, respectively. Final depth of visible F. uhdei root penetration was significantly greater than both Quercus species in both soils (Table 2).
Roots of the F. uhdei grew against the glass surface continuously, and there was no difference between visible and actual penetration of F. uhdei roots at the end of the experiment (Table 2). The Quercus roots grew away from the glass at times and reappeared a few centimeters deeper after a few days. Measurements of root penetration obtained when the soil was washed from the roots at the end of the experiment showed that the actual maximum root penetration (Table 2) was significantly deeper (more than 10 cm [3.9 in.]) than visible root penetration for both Quercus species in the fine-textured soil and for Q. crassifolia in the coarse-textured soil. As a result, actual F. uhdei root penetration was not significantly deeper than either Quercus species in the fine-textured soil.
Though the differences between visible and real root penetration in the small benchtop rhizotrons were measurable, they were not sufficient to change the overall perception of the vigor and character of the root systems. Such benchtop rhizotrons may be very useful in practical applications where periodic observation and characterization of overall growth of the roots is needed but may have limitations when precise quantification of root systems is needed.
Visible root penetration of all 3 species was significantly reduced by the coarse-textured soil (Table 2). The reduction was probably caused by higher mechanical impedance, lower soil aeration, or both (Alan and Bennie 1991; Craul 1992). The visible roots of both Quercus species penetrated to a depth of less than 30 cm (11.8 in.) in the coarse-textured soil, while in the fine-textured soil, the roots of both Quercus species penetrated to a depth of approximately 50 cm (19.7 in.). The F. uhdei roots penetrated the coarse-textured soil as effectively as the Quercus species were able to penetrate the fine-textured soil (approximately 50 cm). Even greater F. uhdei root penetration was recorded in the fine-textured soil. The ability of this species to grow on nearly all urban sites in Mexico City may be related to the ability of the root system to grow vigorously in a wide variety of soils.
Total Root Dry Weight
Fraxinus uhdei root dry weight was significantly greater in fine- than in coarse-textured soil (Table 3). The coarse-textured soil reduced F. uhdei total root dry weight more than it reduced maximum depth of penetration (61% versus 21%).
There were no statistically significant differences in Quercus root dry weights between soil types (Table 3). A significant reduction (40%) in root penetration of Quercus species in the coarse-textured soil (Table 2), without a significant decrease in root dry weight, indicates the roots were growing more densely in the upper soil surface where there was still ample room for root growth of these small plants. The Quercus species were much smaller plants than the F. uhdei. If the Quercus species had grown larger (until the shallow soils became filled to capacity with roots), such a restriction of roots to the shallow soils may eventually have reduced total root growth, as it appeared to do for the larger F. uhdei plants.
If the distribution of roots of urban trees is restricted to the surface soil layers, similar to that of F. uhdei in the coarse-textured rhizotron soils, the trees may still be able to survive, though they may be more stressed, smaller, and shorter lived, primarily because water and element availability are less than optimal (Russell 1977).
Shoot Growth
Shoot dry weight of F. uhdei was significantly greater in fine-textured soil, but there was no difference in shoot dry weight of either Quercus species in the 2 soils (Table 3). There were no significant differences in shoot height prior to day 80. Starting on day 80 and continuing until the end of the study F. uhdei growing in fine-textured soil were taller than F. uhdei growing in coarse-textured soil. Soil type had no effect on height of either Quercusspecies (Figure 3).
Between day 80 and 135 Quercus shoot growth virtually stopped (Figure 3). In the fine-textured soil, F. uhdei grew significantly taller than Q. crassifolia starting on day 80, and taller than Q. crassipes starting on day 100. In the coarse-textured soil, F. uhdei became taller than both Quercus species much later, on day 122, due to the slower growth of the F. uhdei in the coarse-textured soil. Because F. uhdei root penetration did not become greater than the Quercus until day 156, it appears that shoot growth is not dependent on root penetration alone.
The lack of Quercus shoot growth between day 80 and 135 while roots continued to penetrate deeper probably indicates that this was a natural period of slow shoot growth while the root system developed further to support future shoot growth (Bonner and Vozzo 1987; J. Kohashi, personal communication 1998).
Conclusions
Root responses to the 2 different soil types in this study show that soil conditions similar to those encountered in urban areas can be created in small benchtop rhizotrons. The vigorous growth of the F. uhdei roots could help to explain why this species is able to grow so readily on nearly all urban sites. Roots of Q. crassipes and Q. crassifolia penetrated the moderately favorable urban soil represented by the fine-textured soil in this experiment as well as those of F. uhdei but did not compare as well in the less favorable coarse-textured soil. Based on this data, Q. crassifolia and Q. crassipes may perform well on some urban sites, but probably not on the most difficult urban sites. The coarse-textured soils of Mexico City may be a substantial cause of poor root growth and low survival of trees in Mexico City. Greater knowledge of soil requirements of native Mexican species will allow better matching of plant to planting sites in Mexico City and allow a wider variety of new species to be successfully introduced.
Acknowledgments
The present work was developed in the facilities of El Colegio de Posgraduados (Postgraduate College, Montecillo, México) and the support of Universidad Autónoma Metropolitana unidad Azcapotzalco (Autonomous Metropolitan University), The Morton Arboretum, and Jardin Botánico del Instituto de Biología UNAM (National University of Mexico). Additional funding was provided by the International Society of Arboriculture’s Research Trust and the program PADEP-UNAM # 003330 and 002355. We would like to thank Pat Kelsey (The Morton Arboretum), Josué Kohashi (Postgraduate College), and Silvia Romero (ENEP-Iztacala) for their valuable advice on urban soils, plant physiology, and native oak species selection, respectively. This study would not have been possible without the support of the technicians and research assistants that participated actively in different stages: Felipe Arreguín, Daniel Aldana, Mario Medina, Alfredo Murguía (UAM-A); Juan Sabino (Botanical Garden-UNAM); Mario García, Angel Sánchez, Eligio Jiménez, Raúl Valencia (Post-graduate College); Susan Milauskas and Patty Sauntry (The Morton Arboretum).
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