STREET TREE TRIALS –AVENUES OF INTEREST

Maureen See Environment and Parks Branch, Brisbane City Council.

Introduction

Brisbane City Council initiated a formal street tree trials program in June 2000 with the selection of an initial five indigenous rainforest species. Now just over three years later, there are seventeen trial species either planted out or in nursery production.

The aim of the trial program is to continue to expand the suite of suitable street tree species for use in creating attractive streetscapes in Brisbane, as well as to showcase the diversity and potential of southeast Queensland’s diverse rainforest flora.

Increasing the use of high performing, but uncommon indigenous tree species along Brisbane footpaths which offer striking features in their form, flowers, foliage etc. will hopefully encourage the wider commercial availability of such species, leading in turn to greater use in home gardens for both amenity and biodiversity benefits.

Although not completely restricted to SEQ rainforest species, the trial program has focussed on them as it is felt there is considerable untapped street potential among their numbers.  This is especially so for the species of dry rainforest scrubs, which can endure harsh growing conditions, not unlike that experienced by street trees.

Species selection

The trial species have been chosen collaboratively from a combination of staff research, other local government plantings in the region, and suggestions from an experienced community rainforest regeneration group in Brisbane, the Brisbane Rainforest Action and Information Network (BRAIN). This group, whose members have been revegetating creeks and rainforest remnant areas in Brisbane for many years, have extensive knowledge and experience in propagating and growing many of the commercially unavailable species which are of interest to this research. Their assistance is invaluable in proving fresh, viable seeds for particular hard-to-get species such as Dissalaria baloghioides (Lancewood).

Final species selections are made based on the full spectrum of street tree selection criteria including the species’ potential to perform well and remain hardy in footpath conditions, good shade cover, attractive flowers, foliage, or bark, a single, straight trunk, strong branch attachment, reasonable lifespan, and ease of propagation. Known undesirable features that would generally exclude species were root issues, suckering, fleshy, thorny or poisonous fruits, excessively slow growth, and severe insect and disease susceptibility.

As there are few existing small and medium-height street species in the existing palette whose maximum height will not conflict with overhead powerlines, emphasis was also given to ensuring a number of such species were included in the trials. Some species which had shown good potential in this regard from earlier trials from 12 years ago (eg; Alectryon connatus) have also been included.

Final tree heights and canopy dimensions for these species under street conditions are still uncertain for many of these species. Thus estimates of final heights have been scaled down from heights for closed forest-grown specimens which are generally much taller and narrower in form.

It was also important to select for a wide variety of tree forms, foliage textures, heights, colours, flowers and barks to maximise the choice available for varying site conditions such as narrow/wide footpath widths, road hierarchies, and existing landscape character.

The species chosen to date are:

The only introduced selection is the species Bolusanthus speciosus (Tree Wisteria) with attractive mauve, pea flowers and an interesting bark texture.

Further information on the seventeen trial species is given in Appendix 1.

Methodology:

The field trials consist of a minimum of 100 specimens of each species planted in a variety of footpath conditions across the city using at least 10 specimens in any one site. Sometimes, site constraints or residents objections have resulted in fewer trees being planted than planned, although commonly 20 –30 specimens are planted. Stock size is usually around 1.1 –1.3 m tall within a 250mm diameter container.

Stock quality is seen to be a critical element, with planting stock contract grown by a nursery contracted to meet Council’s Nursery Stock Standard. Stock is grown at the nursery from seed specifically collected for the purpose by volunteers or nursery staff, or grown from smaller purchased stock.  Planting takes place within either the Council’s “spring planting window” (September-November) or the “autumn planting window” (March-May). Plants are fertilised at planting with Osomocote and thereafter regularly watered, fertilised and weeded at the same frequency as per standard street tree early care practices (ie; a 12-month period consisting of weekly for the first month and then monthly for the next 11 months). An organic liquid fertiliser (Charlie Carp) is added to the water provided at each maintenance visit.

Planting site details and plant dimensions are recorded at planting, and performance is being monitored at present with the assistance of Horticulture students from Grovely TAFE in Brisbane.

Due to inter-specific differences in growth rates and stock availability, as well as linking plantings to seasonal windows, there is considerable variation between species in both the number and timing of their plantings to date.  Eight of the seventeen species have been installed since March 2001, with the last planting in November 2002.

Table 1: List of trial species planted out as of August 2003

Results:

Performance data from all sites is to be provided to Council by the TAFE students shortly.

In the interim, some preliminary data has been collected by Council staff from a subset of existing sites. Where specimens are less than 12 months old, growth data since planting has been extrapolated to an annual figure.

The latter records have provided the following growth data:

Table 2: Average annual growth rate comparisons of trial species as at August 2003

• This species has only been planted at one site to date.

In reviewing species performance to date, the findings must be viewed in the context of the very dry conditions experienced in Brisbane during the period between March 2001 and the present when the plants were installed and become established.

Bureau of Meteorology data for Brisbane for that period reveals that in the first summer following the initial plantings, (the months of December 2001 and January/February 2002), the average summer rainfall in Brisbane was only 229mm, which is 52% of the average annual rainfall for that period (442mm). The following summer of December 2002 and January/February 2003 was 335mm, 76% of the average summer rainfall.

Rainfall was lower overall throughout the year also. In 2001, the entire annual rainfall for Brisbane was 961mm, (being 80% of the annual average rainfall figure of 1204mm), and in 2002 drier still with 741mm (only 62% of the average).

The year 2003 to date has also been below average, with only 77% of the average rainfall received for the months of January to July.

General findings to date are as follows:

• Rhodosphaera rhodanthema (Tulip Satinwood) has performed the best in terms of the growth rate of all eight species planted so far, with an annual height increase of between .47m/yr and .91m/yr. It also has the appeal of glossy largish leaves with a red flush in the new growth. It is subject to some minor insect attacks, but this does not appear to have an impact on general tree health and performance.

The species is performing best at present on two sites on sandy loam soils, although good results are still found on heavier clay soil as well. pH ranges are from 6.8 to 7.

Some young specimens are demonstrating a juvenile structural issue with multiple, crowded branches arising from one point on the trunk.

• The single planting of Alectryon tomentosus (Hairy Bird’s eye) planted in November 2002 is performing uniformly well in an exposed, windy median environment. This species is possibly the second best-performing species at present, after Rhodosphaera rhodanthema.

After only 9 months in the ground, the trees appear to be growing quite vigorously at 0.4m/yr with the added appeal of reddish-bronze new growth. This species is from the very hardy family Sapindaceae, which boasts some of the best-performing native Brisbane street tree species such as Harpullia pendula (Tulipwood) and Cupaniopsis anacardioides (Tuckeroo). (Interestingly, three other species of the seventeen trial species are also from the Sapindaceae family (Guioa semiglauca, Atalaya salicifolia and Alectryon connatus).

Ripening fruit is evident on approximately 15% of specimens. Expected to grow to approximately 8 metres, it has a rounded, shady canopy.

• Elaeocarpus obovatus (Hard Quandong) was expected to be slower growing in comparison to others, but to be a hardy species. This is based on previous cultivation knowledge in garden and park plantings. While growth rates have been reasonable (.26m/yr and .33m/yr), performance in general at the two existing plantings (May 2002 and November 2002) has been affected by maintenance problems. Further plantings of this species at a number of sites are planned in spring 2003, and it will be interesting to compare their performance with existing plantings.

As at all sites, where residents are providing care to specific trees outside their properties, performance is considerably better. This species often occurs naturally along creeklines in Brisbane and is likely to perform best on sites with good moisture At least two other species of this genus are popular landscape choices in Brisbane, Elaeocarpus reticulatus (Blueberry Ash) and Elaeocarpus obovatus (Blue Quandong). A third, Elaeocarpus eumundii (Eumundii Quandong), has now also been selected for trialling following good performance elsewhere. It bears masses of tiny white flowers and attracts birds to its blue fruits.

A considerable variability in form and leaf size amongst specimens in this species was noted at all sites.

• Brachychiton discolor (Lacebark) is a very large ornamental tree with attractive pink star-shaped flowers. A popular related species used in ornamental plantings in Brisbane is the Flame Tree (Brachychiton acerifolius). It appears to be performing quite well with growth rates of .20m/yr and .33m/yr at the two assessed sites. Some insect attack is evident on the leaves at both sites but does not appear to be affecting vigour. This species has quite a thick trunk as a mature tree and is selected for wider footpaths only.
• Hodgkinsonia ovatiflora (Golden Ash) is one of the small to medium-height species with a rounded canopy and small glossy leaves, selected also for its potential use under powerlines. Whilst not growing rapidly to date, the two sites recorded are demonstrating acceptable growth of .26m/yr and .29m/yr, and reasonable health and vigour. There appears to be slightly better performance in more sheltered locations within sites. However, a third planting has performed extremely poorly, with nearly all specimens stunted with yellowed, sparse foliage and many virtually leafless specimens. (The growth data for this site cannot be estimated as the average height at planting was not available). The reasons for this result will be investigated further. The oldest plantings are approximately 21 months.
• There was variable performance evident between individual Streblus brunonianus (Whalebone Tree) specimens. The best-performing specimens demonstrate dark green, dense, healthy growth, and often appear to be those receiving additional water and mulch from adjacent businesses or residents in the post-maintenance period. By comparison, others in the same street, often without this extra care have generally sparser foliage and slower growth rates, although in most cases are still acceptable performers. A number of specimens under stress are exhibiting yellowing of leaves, reduced leaf size and leaf drop.   The species appears to be performing fairly well the same at both sites, with one site being a clay loam and the other a sandy loam. pH values at the sites measured were around 6.8 – 7.

In their initial years, it is observed at both sites that in street plantings at least, this species seems to concentrate most biomass increase into bushy lateral growth. The average annual height increase is between .26m/yr and .28m/yr. Small yellow fruit attractive to birds, was observed on two-year-old specimens.  This species holds promise for use under powerlines if successful. A specimen cultivated in a Brisbane home garden is 4 metres high after nine years and with a dense bushy habit and attractive dark green serrated foliage.

• Rapanea variablis (Muttonwood) is another of the lower-growing species, possibly growing to 6m, with attractive glossy, oblanceolate leaves and rust-coloured new growth flushes. Only one planting has occurred to date (November 2002), with specimens so far appearing to be performing acceptably well on average (.26m/yr), with good form and vigour. Many specimens were observed to be in flower over the winter period, with small cream flowers borne directly on the woody stems.
• Flindersia xanthoxyla (Yellow Wood) is a tall spreading species that has been observed as an attractive amenity tree in park and golf course settings. In the trials on a range of soil types, it appears to be a generally slower-growing tree compared to the other species, with an average annual height increase ranging between .07m/yr and .23m/yr between sites. The three remaining sites all recorded average growth rates of 0.14m/yr. As for Streblus brunonianus, specimens appear to be struggling where additional water and mulch have been limited after the conclusion of the establishment maintenance. A quality issue affecting a limited number of stock may be contributing to poor performance in some earlier plantings.

The remainder of the Year 2 species, together with one of the Year 3 species are scheduled for planting this spring:

• Dissilaria baloghioides ( Lancewood)
• Aphananthe phillipinensis (Native Elm)
• Atalaya salicifolia (Scrub White Wood)
• Alectryon connatus (no common name)

Planting of all Year 3 species is expected to be complete by autumn 2004.

As a new element to the trials program in 2003, Energex, the authority responsible for the power distribution network in SEQ, has provided a selection of grafted Brachychiton cultivars to Brisbane City Council for inclusion in the trial program.

Four cultivars have been provided by Energex for trialling to assess their performance as ornamental, lower-growing species under powerlines. In May 2003, twenty-five specimens of the four cultivars were planted and their progress will be now monitored as part of the overall program.

Conclusion

It is anticipated that it may take up to five years or even longer to determine the true potential of many of these species as suitable street trees in Brisbane.

Species that prove successful in these Brisbane trials could be expected to perform comparably from the north coast of New South Wales through to the Sunshine Coast.

This trial work is planned to continue for the foreseeable future as an integral part of Brisbane City Council’s overall street tree management program.

It is expected there are still many more potentially successful street tree species to be found amongst the diverse rainforest flora of South East Queensland, and enormous benefit to be gained from exchanging information with other Councils undertaking similar investigations.

Appendix 1 General Information on Street Tree Trial Species

Year 1 Species

Year 2 species

Year 3 species