Abstract
We compared the effects of compost-amended backfill and of the size of the exposed surface area on Norway maple (Acer platanoides L.) trees planted in a high-quality backfill on a brownfield area previously occupied by a truck factory. After 3 years, trees with larger and mulched planting areas had higher leaf gas exchange, leaf chlorophyll, and mineral content than those grown under pavement. The effects of the different quantities of compost in the backfill were more difficult to assess within the framework of this study. Although these effects may become significant in the long term, it is difficult to prescribe compost addition as a useful technique to improve plant growth and physiology when the native soil has been completely replaced by good topsoil.
Reclamation of postindustrial landscape in Italy is important not only for housing and commercial development, but also for recreation and nature conservation. However, there are several problems connected to the redevelopment of brown-field, which may prevent the successful establishment of newly planted trees (Dickinson et al. 2005). In fact, because of the lack of topsoil, plant growth will rely on imported topsoil from other sites or on topsoil-forming amendments such as composted materials.
Organic compost materials generally have long-term beneficial effects on soil physical properties, although in the short term, these benefits are less evident (Watson 2002; Ferrini et al. 2005). Some research projects have shown that organic amendments have a potential role in ensuring quality restoration works (Hornick and Parr 1987; WRAP 2006) and their application is beneficial and relatively inexpensive (Vetterlein and Hüttl 1999). This works well with the European Union countries where the target is to decrease the quantity of organic waste going to landfill sites by 20% by 2010 and by 50% by 2050 (Council Directive 1999/31/EC). The follow up of this directive could result in a large increase in composted organic wastes (Crowe et al. 2002).
Besides soil quality, other important components of planting sites are the open soil surface and the surface treatment over the rooting zone (Coder 1996; Trowbridge and Bassuk 2004).
In many urban sites, the soil surface is covered after planting; typical coverings of paving stones, asphalt, and concrete are impermeable to water and oxygen required by both the tree roots and the soil for proper functioning (Bradshaw et al. 1995). Metal grids or grates used in conjunction with concrete allow water and oxygen exchange only if the soil is not compacted and these grid or grates are also expensive.
Planting the area around the trees with small shrubby cover seems to have a very beneficial effect on soil humus and microorganisms (Bernatzky 1978). Benefits are probably the result of the fact that with groundcover, the risk of being walked on is much reduced.
The aims of this study were to evaluate the effects of backfill amendments, the size of the open soil area, and of the surface treatment on posttransplant growth and physiological characteristics of Acer platanoides L. trees planted in a postindustrial area after regeneration for commercial and housing uses.
MATERIALS AND METHODS
Twenty-four uniform, 5-year-old, 4 to 4.5 m (13.2 to 14.85 ft) tall, 14 to 16 cm (5.6 to 6.4 in) in circumference (measured at 1.3 m [4.29 ft] height), balled and burlapped Norway maple (Acer platanoides L.) ‘Deborah’ trees were planted before budbreak in an area in the city of Milan, Italy, which until the early 1990s was occupied by a truck factory. The existing brownfield soil was removed and replaced with a 120 to 150 cm (48 to 60 in) layer of clay-loam soil having an organic matter content of 2.8% (indicated as topsoil). Planting holes were three times the width and one and a half times the depth of the root ball (size of the root ball was 50 to 55 cm [20 to 22 in] in diameter).
Trees were backfilled with: 1) topsoil with 25% composted material (volume/volume); 2) topsoil with 50% composted material; 3) topsoil with 75% composted material; or 4) topsoil only. Composted material was obtained from yard waste through aerobic biostabilization; compost was aerobic biostabilized with characteristics meeting the Italian standards (Table 1).
Average composition (percent on dry matter) of the mixed compost added to the planting pit.
After planting, the soil surface was covered with: 1) an “opus incertum” pavement (this technique uses irregularly shaped, small, smoothed slabs of stone randomly fitted together with sealed joints) and a metal grate (1 × 1 m [3.3 × 3.3 ft]); 2) left free of pavement for an area of 2.5 × 2.5 m (8.25 × 8.25 ft) around the trunk (6.25 m2 [67.5 ft2]), mulched with pine bark, and then planted with Viburnum davidii Franch.
Trees were planted in a completely randomized block design and regularly irrigated during spring and summer of each year. After the first year, some plants, regardless of the treatment, showed some dieback and were dropped from the measurements to obtain reliable data, measurements were performed on three single-plant replicates of four backfilling materials and six single replicates of planting area dimension. A CIRAS-1 portable infrared gas analyzer (PP Systems, Hertfordshire, UK) was used to determine instantaneous net photosynthesis (Pn), transpiration rate (E), and water use efficiency (WUE, calculated by dividing Pn by E) three times in the first and second year and five times in the third year (from mid-June through mid-August). Measurements of leaf gas exchange were started in mid-June. The readings were taken between 8:00 a.m. and 4:00 p.m. during sunny days (photosynthetically active radiation greater than 1000 μmol/m−2/s−1) on ten fully expanded leaves from the outer part of the crown and at different heights per plant per treatment and repeated three times each day of measurements. Chlorophyll content was determined in July and September on the same leaves with a portable chlorophyll meter (SPAD-502; Minolta Corp., Ramsey, NJ, U.S.). Previous calibration curves were established for the species by measuring absorbance at 664, 647, and 625 nm with an Hitachi U-2000 spectrophotometer (Hitachi Ltd., Tokyo, Japan) after extraction with dimethylformamide (Moran 1982). Triplicate readings were taken around the midpoint near the midrib of each leaf sample and the values averaged.
Dry weight was determined on ten leaves per plant per treatment. The leaves were chosen from the outer part of the crown at different heights with similar conditions of shape and weight. Leaves were dried at 80°C (176°F) until constant weight.
In the second and third years of the experiment, leaves chosen from the same part of the tree were analyzed for mineral elements content by a specialized laboratory (MAC–Minoprio Analisi e Certificazioni).
Effects of compost addition and of mulch and paving surface and significant interactions between compost addition and of mulch and paving surface were determined by one-and two-way analyses of variance. Differences between treatment means were separated by the protected least significance difference at the 95% confidence level (P ≤ 0.05).
RESULTS AND DISCUSSION
First Year
The results from the first year indicate that the modification of the planting pit had no influence on leaf dry weight and leaf gas exchange. Only the transpiration rate was higher in the trees backfilled with topsoil +75% compost and in the control ones. No significant differences were found regarding leaf chlorophyll content either (Table 2).
Year 1—Effect of backfill amending and of size of the paved surface on leaf dry weight (g), net photosynthesis (Pn tømol/m−2/s−1 di CO2]), transpiration rate (E [mmol/m−2/s−1 di H2O]), on water use efficiency (WUE [Pn/transpiration rate]), and on chlorophyll content (mg/cm−2) of Acer platanoides L. trees (first year).z
No statistical differences emerged regarding the size of the open soil surface with the exception of the WUE, which was higher in the trees that had mulch and groundcover planting.
The interactions between the two factors showed statistical significance, although not univocal; this is most likely the effect of transplant shock.
Second Year
In the second year after transplanting, leaf dry weight and leaf gas exchanges were not affected by soil amendments. Chlorophyll content was lower in the trees amended with 25% of composted material (Table 3).
Year 2—Effect of backfill amending and of size of paved surface on leaf dry weight (g), net photosynthesis (Pn (μmol/m−2/s−1 di CO2]), transpiration rate (E [mmol/m−2/s−1 di H2O]), on water use efficiency (WUE [Pn/transpiration rate]), and on chlorophyll content (mg/cm−2) of Acer platanoides L. trees (first year).z
The surface treatment had significant effects on all of the parameters measured. Trees growing with the pavement and grate had a lower leaf dry weight, reduced leaf gas exchange, and lower leaf chlorophyll content. These findings are similar to those of Celestian and Martin (2005) who compared four different ornamental species in two parking lot locations in United States, finding, in general, a higher leaf gas exchange and leaf chlorophyll content in the trees with the larger landscape area.
Significant interactions were found between the two factors (substrate × size of paved surface). In general, compost addition did not show any positive effect on the trees with mulch treatment.
Third Year
Three years after transplanting, plants grown in amended soil had lower photosynthesis and transpiration rate throughout the growing season compared with the controls. Leaf dry weight and WUE were not affected, whereas chlorophyll content was lower in the trees that had 25% compost addition to the planting pit (Table 4).
Year 3—Effect of backfill amending and of size of paved surface on leaf dry weight (g), net photosynthesis (Pn tømol/m−2/s−1 di CO2]), transpiration rate (E [mmol/m−2/s−1 di H2O]), on water use efficiency (WUE [Pn/transpiration rate]), and on chlorophyll content (mg/cm−2) of Acer platanoides L. trees (first year).z
Trees with the mulch treatment performed significantly better for all the parameters measured. There are probably several reasons why trees grown under pavement or grates had lower leaf gas exchange activity. Research has shown that woody plant gas exchange is largely controlled by the response of the trees to the environment. Therefore, leaves of trees planted with a paved surface can have greater leaf temperature and lower leaf gas exchange than trees under planted with a vegetative surface which indirectly effects plant physiology (Montague and Kjelgren 2004).
The interactions between the two factors of variability (substrate and size of paved surface) were significant for all the parameters expect leaf dry weight. In general, as observed in the previous year, trees grown under paved soil surface showed higher values for most of the parameters proportionally to the compost amount added to the planting pit, suggesting that compost addition can be a valuable tool to mitigate the negative effects of covering the planting area. On the other hand, as observed in the previous year, trees grown in the larger mulched area had lower values for most of the parameters proportional to the compost amount added to the planting pit, supporting the hypothesis that no real benefit can be derived from the use of soil amendments when a good clay loam soil is present and when the space around the tree is not limiting (Shulte and Whitcomb 1975).
Leaf Mineral Content
During the second and third year after transplanting, there was no apparent effect from adding compost to the backfill soil on leaf mineral content (Table 5). All foliar nitrogen levels were low and did not increase consistently after compost addition. Similarly, no significant differences were detected for the other elements.
Effect of planting pit modifications and of open soil surface on leaf mineral content (percent on dry matter).z
With surface treatments in both years, a higher content of nitrogen, phosphorus, and calcium was detected for the trees grown with the mulched surface. Potassium and magnesium were not affected (Table 5). Similar to what was found for the other parameters, significant interactions emerged between the two factors (size of paved area × backfill composition).
It is probable that the higher soil/atmosphere gas exchange in the mulched treatment has enhanced the availability and uptake of some mineral elements. In fact, pavement decreases gas exchange between atmosphere and soil and, therefore, the oxygen supply to the roots (Koolen and Rossignol 1998). A reduced root growth and activity negatively influences nutrient uptake and physiology; our data confirm this hypothesis.
CONCLUSIONS
The results obtained show that the size and type of the unpaved area determines tree health. A larger mulched area with groundcover planting was superior to a smaller paved area.
No tree growth benefits were observed by adding composted materials to the planting pit at the time of transplanting Norway maple in an area previously covered with good topsoil. This confirms findings by other authors who did not find appreciable effects after compost addition to the planting pit (Gilman 2004). The effects of compost addition can be expected only after some years as a result of soil microbial activity and organic matter mineralization. This study, therefore, suggests that topsoil, even a clay one, is a satisfactory substrate for Acer platanoides and that the amount of compost to be added should be determined according to native soil characteristics.
Acknowledgments
Support for this project came in part from the Regione Toscana (Italy) inside the research project “Research on multipurpose green areas” (RISVEM). We thank Dr. Giovanni Sala and Studio Land SRL for their support to the research.
- © 2007, International Society of Arboriculture. All rights reserved.
Jump to section
Related Articles
Cited By...
- No citing articles found.