Report by Colin Hambidge
The latest in the series of arboricultural seminars which are given free-of-charge to tree care professionals by Barcham Trees was led in late February by Professor Francesco Ferrini from the Department of Agrifood Production and Environmental Sciences of the University of Florence, Italy, where he teaches arboriculture, urban forestry, ornamental arboriculture and green areas planting and management.
He began by explaining that the Pistoia region of Tuscany is a major centre for ornamental plant production, with 5000 hectares being devoted to it, and Tuscany being responsible for more than 40 per cent of Italy’s output. There are 1500 nurseries in Tuscany, with 85 per cent of ornamental production being trees, of which the great majority are exported throughout Europe. There is a long history of production around Pistoia, thanks to its fertile, loamy soil and favourable climate, but at present the region is suffering from a crisis of confidence; Investment is dropping, prices and profits are falling as firms are ‘destructuring’.
Francesco and his team at the University of Florence have undertaken many research projects in recent years, and the first he discussed was the effect of different containers on root system structures and post-transplanting establishment of Tilia cordata and Ulmus minor. The aim was to avoid or reduce root circling by comparing the use of Air-Pots®, 0.9-litre and 3-litre traditional, smooth-sided containers. Air-Pots gave the best results for both species, with standard pots producing deformed roots after holding plants for a long period.
The trees were then transplanted to a field and lifted two years later; once again those transplanted from Air-Pots had done better than those from the other containers. Following further examination after four years in the field, root circling was still much higher in the trees grown in standard container during the nursery phase. This was statistically significant in Tilia. The research showed that container form strongly affects root system quality and that malformed roots continue to be so in subsequent years, with defects such as circling, girdling and structural weakness continuing to show even several years after transplanting.
The team’s second project involved looking at the effect of mycorrhizal inoculation in the nursery and at planting time into the urban environment. Urban soil is often of very poor quality, with few micro-organisms present; these are important in virtually all aspects of soil quality. For example, they are needed to aid nutrient uptake; root elongation is the main means by which plants can increase nutrient uptake.
Roots and root hairs cannot enter the smallest of the pores of the soil, but mycorrhizae can and improve water and nutrient uptake, the storage of nutrients, protect against toxins and soil-borne pathogens, and promote higher stress tolerance. In a bid to produce site-specific and species-specific mycorrhizae, roots were harvested from healthy urban trees, and the fungi present were analysed and identified. Fungal strains were then propagated and after eight months fine roots were harvested and used to produce an inoculum.
Inoculation of mature trees can be an expensive business, but not so with young trees on the nursery. Controlled mycorrhization in the nursery did not enhance the growth of container-grown maples, limes and oak. Inoculation did, however, provide several physiological benefits, such as the maintenance of less negative leaf water potential, a higher apparent carboxylation rate, higher RuBP regeneration and higher quantum yield of PSII under water shortage The overall data suggested that inoculated plants were better able to maintain physiological activity of shade tree species during water stress if compared to non-inoculated plants, and so can be considered more drought-tolerant. In the field, water-stressed trees had a higher leaf gas exchange, and mycorrhization was effective, especially on maples.
When Celtis australis and Fraxinus excelsior were planted in streets and car parks, mycorrhizae were very effective in giving Celtis better trunk growth, shoot growth and chlorophyll content, but in Fraxinus only shoot growth was improved. Carbon assimilation and water use efficiency were improved with mycorrhization in both species.
Moving to a better environment, Quercus robur was planted in an urban park, where mycorrhizae gave much better shoot growth and chlorophyll content compared with controls which had not been inoculated. Francesco reminded delegates measurements need to be taken for at least three years to have real value. Though the photosynthesis only rarely showed significant differences
on a single-leaf basis in inoculated trees, these plants had higher growth and they sequestered a much higher amount of carbon on a whole-plant-basis. In conclusion, the project showed inoculation with selected, native mycorrhizae improved, in general, plant growth and physiology, but the time of response depends on the inoculated species and that the effect of mycorrhizae on host growth is dependent on environmental conditions.
Mulching with compost was the next matter on our agenda. This has been a long-term study for Francesco and his team. We know mulching is good and that mulching with compost is good. We must, however, be wary of over-mulching, which can result in the reduction of gaseous exchange, which in turn can lead to suffocation, root disease, while excess water evaporation is hindered.
One such study looked at the effect of soil management techniques on plant height, trunk growth and shoot length of Tilia x europaea and Aesculus x carnea. The height, trunk diameter and shoot growth of the mulched Aesculus all improved significantly, but similar improvements took much longer with the Tilia. The soil temperature around mulched trees was lower than that of those unmulched, which is an important factor in Italian summers, but less so in British ones. Soil respiration was higher on mulched trees, and biological activity was enhanced by mulching, which was also shown to give a more favourable environment for root growth than bare soil can.
A second mulching study looked at the use of compost on the growth and physiology of Acer campestre and Carpinus betulus. Mulching in the row and natural grass cover between the
rows increased plant growth, leaf gas exchanges and chlorophyll content. Mulching significantly reduced soil temperature in the upper 10 cm of soil. Contrary to previous experiments, no change in soil respiration was observed. A decrease in oxygen availability to the roots can occur, especially if mulches are distributed in thick layers or if the mulching material is not sufficiently stable and mature. Francesco believes the optimum mulch depth is 8-15cm, and the material should be a blend of nutrient-poor large particles and nutrient-rich fine particles.
On a completely different tack, he then presented the findings of an evaluation of shrubs for slope consolidation in urban settings, with the aim of controlling erosion and the risk of landslides on to main roads. Shrubs have several advantages compared with grass; they require less care, no cutting, increase biodiversity and improve the visual quality of the landscape. Currently only very few shrubs are used for landscaping, with cotoneaster and pyracantha being two of the favourites. Francesco’s project involved the growing of 25 different shrub species and cultivars for three years.
In order to evaluate the weed influence on plant growth and weeding time, two mulching systems – black polypropylene and Isonatural®, a fibrous biodegradable material, – were used in comparison with a no-mulch control. No pruning or pest management was carried out during the trial. Plants were irrigated only in the first year during the driest periods of summer using a drip system.
Chemical weeding was performed prior to planting, but not subsequently. Hand weeding
was carried out twice in the first year and three times in the second and third year. The time needed for weed removal was recorded for each plot. Salix purpurea Nana needed the least time for weeding. Spartium junceum and Coronilla emerus were the two with the highest aerial biomass production at the end of the trial. The mulching materials increased biomass production much more than bare soil, but these are initially more expensive options. No differences in growth or weeding times were seen between the two mulching materials.
Many plants showed a satisfactory ground cover at the end of the second year, but only four species showed the highest cover index in all plots. These were Abelia x grandiflora, Hypericum Hidcote, Spartium junceum and Caryopteris x clandonensis. Not all the species tested were found suitable for growing on slopes. The time need for ground covering, disease resistance, limited height increase and good appearance are the main factors to consider when choosing shrubs for this purpose.
Francesco then moved on to consider the effects of root severance by excavation on the growth and physiology of trees. The roots of urban trees have to cope with many infrastructures, and optical fibres have become the latest problem in recent years. A single trench can remove up to 50 per cent of a root system, and this root damage can increase mortality by 22 per cent in the next eight years. Symptoms many not appear until several years after the damage has been done.
Forty eight uniform Tilia x europaea and 48 Aesculus hippocastanum were planted in 2004 in a loamy, sandy soil and allowed to establish undisturbed for five years. Tilia is believed to better tolerate root manipulation than Aesculus (Matheny, 2005). Some were then trenched on one side, some on two sides and some were left as the control. The trenches were made 70cm deep and 40cm from the root flare. Roots were then excavated with an Airspade®.
From a physiological point of view, root severance on young trees induced similar effects as a mild water stress, characterised by diffusive limitation to photosynthesis (metabolic limitation, typical of more severe stress, rarely occurred) and a moderate change in pre- dawn water relation. Recovery, however, is slower than most abiotic (mild) stresses, particularly in sensitive species such as Aesculus. Tilia displayed greater physiological tolerance of root loss than Aesculus. It should be mentioned the experiment was conducted during reasonably rainy years.
Soil sealing and its effect on newly planted trees was the next topic of the day. Francesco believes the frequency of extreme effects, especially floods, is increasing due to climate change and that there is an urgent need to mitigate the deleterious effects of soil sealing. According to the European Commission in 2012, in Italy 8 sq m is sealed every second. In some concreted areas, water run-off can be as high as 85 per cent.
The consequences of soil sealing can be mitigated by alternative street designs. For example, with porous pavements, the pavement itself is permeable to water across its entire structure, whereas permeable pavements are those made by impervious modular elements, but voids between elements allow water infiltration. We need to understand what happens to soil when it is sealed.
Twenty four plots (50 sq m area) were built in November 2011. Each was separated from the surrounding ones by polypropylene barriers, buried in the soil down to 70cm. Two planting pits (1 sq m area) were left unpaved in each plot. Plastic cylinders were put through the pavements to allow direct soil measurements. Some cylinders were near the planting pit, some other were buried 5m away. Pavement thickness in impermeable, permeable and porous formats was about 15cm in all cases, and an unpaved control was also used.
Twenty four plants of Celtis australis and Fraxinus ornus (14-16 cm circumference) were planted in March 2012, according to a randomized block design with six blocks. Each tree was planted in a 1 sq m planting hole, surrounded by 25 sq m of paved soil. The impermeable pavement allowed low infiltration and evaporation, the permeable pavement medium infiltration and low evaporation, and the porous pavement allowed high infiltration and medium evaporation. The control was high in both instances. The soil at 45cm below the permeable and porous pavements was saturated.
This experiment showed planting trees in paved soils is essential to maintain evapotranspiration in urban areas. Pavements had limited effects on growth and physiology of newly planted trees. Celtis is very tolerant of all types of soil cover during establishment, while Fraxinus in impervious pavements displayed some signs of (very mild) stress after the third year from planting.
We then looked at the effects of pruning methods on Acer pseudoplatanus in a four-year project. The method known as topping (cutting in the middle of an internode) is considered to be bad, and even destructive. So why do we top trees? This can be due to a lack of information, because it is a quick, cheap option, many people actually like hard-pruned trees, and there is no legislation on best practice.
Topping, however, does not make trees easier to maintain and neither does it invigorate them. This method affected branch structure by promoting competition among shoots of the same branch and by determining a shift towards a more pioneer (fast growing) behaviour, but at the
expense of tolerance to environmental stresses.
On the other hand, a removal cut, where branches are cut where they join the stem, taking care not to damage the branch collar, inflicts only minimal disturbance to the tree’s physiology. A reduction cut, where branches are cut back to a lateral with sufficient size to become a leader, preserves the normal branching pattern and has little effect on leaf structure and photosynthesis.
The experiment demonstrated that the pruning method, not only its severity, modulates the morpho-physiological response of trees. The maintenance of apical control and dominance are key issues to preserve a structurally sound trees and the long-term efficiency of the photosynthetic apparatus.