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«Proceedings of the th 14 National Street Tree Symposium 2013 ISBN: 978-0-9806814-1-3 TREENET Proceedings of the 14th National Street Tree Symposium ...»

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 The Prosperous City 2.2 Deliver high quality urban development incorporating sustainability, connectivity, diversity and integrated urban design principles.

 The Prosperous City 3.5 Build on Council’s investment in water reuse projects to attract investment, contribute to urban amenity and support local firms.

 The Sustainable City 4.1 Further maximise re-use opportunities and mitigate the impacts of storm water inundation and flooding.

 The Sustainable City 5.2 Ensure that existing and future urban environments are able to withstand and adopt to future demands.

 Achieving Excellence 6.3 Use expertise, knowledge and technology to improve and develop alternative modes of service delivery.

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For further reading about Sean Connell’s subject matter refer to the following publications:

Connell SD (2007) Water quality and the loss of coral reefs and kelp forests: alternative states and the influence of fishing. In: Connell SD, Gillanders BM (eds) Marine Ecology. Oxford University Press, Melbourne Gorman D, Russell BD, Connell SD (2009) Land-to-sea connectivity: linking human-derived terrestrial subsidies to subtidal habitat change on open rocky coasts. Ecol Appl 19:1114-1126 Falkenberg LJ, Connell SD, Russell BD (2013) Disrupting the effects of synergies between stressors: improved water quality dampens the effects of future CO2 on a marine habitat. Journal of Applied Ecology 50:51-58 European Commission's Environment Directorate-General highlighted the last paper as providing a unique insight into the vital scientific issues relevant to current EU environmental policy

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Introduction The urban environment may be considered one of the harshest planting and growing environments (Appleyard, 2000, Craul, 1993, Gilman, 1993, Simpson, 1981). The London Tree Officers Association expected a 50% tree replacement rate within their municipality as a result of failed establishment or from damage/vandalism (Appleyard, 2000). Therefore, it is not surprising or uncommon to see newly planted street trees either dead or dying. A number of factors are likely to be responsible and may include any one, or a combination of, poor quality or inappropriate planting stock, inadequate planting techniques or inadequate maintenance (Appleyard, 2000, Gilman, 1993, Leers, 2000, Messenger, 1976, Moore, 1997b, Watson, 1997, Watson and Kupkowski, 1991). It is also clear that the availability of growth resources play a critical role in a tree’s development.

Development can also be modified by the environment (Salisbury and Ross, 1992, Shigo, 2008).

Recommendations such as correct planting depth, effective irrigation and other maintenance regimes responsible for successful tree establishment have been set (Gilman, 1993, Leers, 2000, Moore, 1997a, Smith, 1997, Watson, 1997).

A number of authors believe that the soil condition should be the primary consideration when evaluating urban sites for tree planting (Craul, 1993, Cutler et al., 1990, Smith, 1997). However, the microclimate of an urban planting site also needs to be considered when planting trees because of its effect on photosynthesis and growth (Kjelgren and Clark, 1992). For example, trees growing in areas of extensive paving will experience a higher Vapour Pressure Deficit and require greater evaporative cooling than trees in unpaved areas or “vegetated” areas, in turn requiring greater scrutiny with species selection (Bassuk and Whitlow, 1985, Irfan et al., 2001, Kjelgren and Clark, 1992, Rosheidat and Bryan, 2010).

Kjelgren and Clark (1992) found that urban microclimates affect and influence tree growth and physiological responses. In urban settings with paving and buildings, they found tree growth acclimated physiologically and developmentally to the conditions with decreased trunk growth or canopies being sparse and stunted, compared to other trees in more park-like settings. Urban microclimates and the urban heat island effect are proving to have an increasingly negative effect on plant growth (Bassuk and Whitlow, 1985, Irfan et al., 2001, Kjelgren and Clark, 1992, Rennenberg et al., 2006, Rosheidat and Bryan, 2010, Schiavo, 1991). As a result tolerance of high temperatures and or rapid temperature fluctuations may well become a primary consideration for tree selection in many urban environments (Costello et al., 2003, Litzow and Pellett, 1983, Rennenberg et al., 2006, Roppolo and Miller, 2001, Shirazi and Vogel, 2007) High temperature injury (HTI) is caused by extreme or critically high temperatures (Costello et al., 2003, Larcher, 1995, Levitt, 1956, Pichler and Oberhuber, 2007, Rennenberg et al., 2006, Rosheidat and Bryan, 2010, etc). The HTI threshold temperature range as 44°C to 50°C for evergreen conifers during the growing season (Larcher 2003 in Pichler and Oberhuber, 2007 p. 696). Similarly, Levitt (1956) refers to the accepted range of 45°C to 55°C as the limit for most plants. While Kozlowski (1979) believes direct HTI occurs in the 45°C to 60°C range. The knowledge that trees can survive 60°C as compared to 50°C will be important when selecting appropriate species for some urban environments. Aside from the extreme temperatures, Kozlowski (1979) states that temperatures a little lower than those cited usually cause indirect injuries. He also believes that typical HTI damage (sunscald, bark scorch and desiccation) is accentuated by large fluctuations in diurnal temperatures. It is well known that plant temperatures can rise above the ambient temperature, up to 10°C in some plant organs, so ambient temperatures do not directly relate to HTI of plants (Levitt, 1956). He cites examples of soil temperatures 14°C greater than the air temperature and cambial tissues of Spruce trees being 18°C higher than the air temperature. Though it is clear high temperatures cause damage to plant parts, the exact injury threshold temperatures cannot be determined for trees growing in the urban environment due to the effects of other micro-climate variables. Because of the increased rates of transpiration, high temperatures can cause desiccation leading to reduced growth rates and eventually death (Kozlowski, 1979).

Performance of photosynthesis is strongly connected to nutrient uptake and partitioning and competition for nutrients. Further, high temperatures stimulate photorespiration at the same time as inhibiting photosynthesis and carbon metabolism is strongly connected to stress (heat and drought) compensation mechanisms (Fitter The 14th National Street Tree Symposium 2013 and Hay, 2002, Larcher, 1995, McDowell et al., 2008, Rennenberg et al., 2006, Salisbury and Ross, 1992).

Europe’s heat wave in 2003 led to decreased photosynthesis production in Mediterranean forests and ecosystems (Garcia-Plazaola et al., 2008, Rennenberg et al., 2006). It is clear that heat and drought (though with distinctive effects) are responsible for this reduction. However, the metabolic processes affected that contribute to this reduction are not always apparent. Garcia-Plazaola et al (2008) and Rennenberg et al (2006) discuss a number of processes that may decrease carbohydrate synthesis. Rennenberg et al (2006) hypothesize that rapidly increasing high temperatures and sustained moderate increases in temperature affect the photosynthetic system differently. Heat tolerance of plants may be increased by exposure to sub-lethal high temperatures as the plant synthesizes heat shock proteins, isoprene and antioxidants in order to protect the photosynthetic apparatus (Kozlowski and Pallardy, 2002).

During exposure to surface fires, Dickinson and Johnson (2004) discuss the temperature threshold for vascular cambium tissue mortality as 60ºC during simulations. However, they go on to cite this threshold not being applicable to other tissues. Biological factors such as species, bark thickness and stem diameter significantly affect heat resistance (Dickinson, 2004, Mantgem, 2003) Importantly, tree stem cell and tissue impairment is dependent on the rate of temperature increase and duration of exposure (Dickinson, 2004, Jones et al., 2006).

Dickinson (2002) discusses the problem with citing threshold temperatures is that cell necrosis occurs at lower temperatures with increased exposure. He describes that cambium tissues are damaged at about 43°C.

Sunburn is defined as injury or death of plant tissues as a result of exposure to critically high temperatures from solar radiation (Costello et al., 2003). The discussion and evidence of the symptoms of sunscald, sunscorch and sunburn on the trunks of trees is clear (Bernatzky, 1978, Costello et al., 2003, Kozlowski, 1979, Kozlowski et al., 1991, Leers, 2000, Levitt, 1956, Roppolo and Miller, 2001, Rushforth, 1987, etc). In this paper, summer sunscald will be the term used to describe HTI to the trunks of tees in the warmer months. Trees that have developed in a closed stand undergoing heavy thinning and then exposed are highly susceptible to summer sunscald (Hermann and Lavender, n.d.). Summer sunscald on individual trees occurring in heavily thinned forest stands of trees may not be apparent for several years (Curtis et al., 2000, Kozlowski et al., 1991).

This may be similar to a tree coming from a relatively sheltered nursery, then being transplanted into an exposed street. Also, there is anecdotal evidence of the canopies of trees growing at a particular orientation then transplanted facing another compass point exhibiting different growth rates (Moore 1998). That is, one side of the tree starts its life facing south, with very little conditioning against solar radiation, then is transplanted with that same side facing northwest where the bark experiences the shock of full sun intensity.

Plant moisture stress is a major factor increasing the potential for sunburn (Costello et al., 2003), but it can still occur on sensitive plants when adequate soil moisture is present (Costello et al., 2003). Not only moisture stress, but declining vigour and the tree canopy shape (vase shaped trees with big scaffold limbs) also affect the occurrence of summer sunscald (Litzow and Pellett, 1983). There is evidence that the pruning technique lion-tailing not only reduces photosynthetic ability, but also increase the potential for summer sunscald on thin barked trees (Smiley and Kane, 2006). Lion-tailing is the removal of all the smaller branches from the inside portion of a bigger branch, leaving the only foliage at the very tip of the branch. Another pruning practice, topping, where the ends of branches are removed, is also responsible for summer sunscald due to the sudden exposure of previously shaded bark (Trask, 1933).

Costello et al (2003) define sunburn as exposure to critically high temperatures from solar radiation leading to the dehydration and death of plant tissues. They state sunburn injury is linked to high ambient temperatures and is injury to Figure 1. Symptom of mild summer sunscald on the above ground parts of the plant, leaves flowers, fruit and Melia azedarach Broadmeadows Vic noted in bark. Of particular interest is damage to trunk and bark tissues.

summer 2007/08 Mild summer sunscald, or the initial stages of summer sunscald of the trunk, appears as a reddish discolouration (Figure 1). As it progresses the bark shrinks, appears sunken then splits, exposing the sapwood (Figure 2 and 3) (Costello et al., 2003). Crane et al (1994) describe drying and The 14th National Street Tree Symposium 2013 peeling of the bark, branch dieback, wood injury and saprophytic fungi on dead bark and wood as a result of sudden exposure to direct sunlight for a prolonged period. They believe it is the over-heating of the cambium that causes these symptoms.

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Bernatzky (1978), Kozlowski et al (1991) and Levitt (1956) also discuss the damaging effects of summertime HTI. The temperatures involved are usually below the thermal death point, with symptoms of scorched leaves and fruits, sunburn, leaf abscission and inhibited growth and scorched bark. Novis et al (2005) cite trees becoming unstable due to the death of the cambium, as a result of sunburn. Similarly, the USDA Silvicultural Handbook (1999) notes the necrosis of overheated cambial tissues as a result of sunburn in the warmer months. This in turn causes flattened sides, “bark sloughing” and poor wood quality. The Handbook discusses sudden exposure due to thinning and topping of tolerant species during the warm season causing sunburn.

Biagorria and Romero (2010), also discuss tolerant species being susceptible to extreme levels of solar radiation causing sunburn, or summer sunscald. So it can be said summer sunscald is the result of relatively high ambient temperatures that tend to be localised, irrespective of latitude. Observations of the symptoms of summer sunscald on the western side of the trunk of transplanted trees in the urban environment may now be attributed to the sudden and extreme high temperature events experienced during the summer months.

An example of summer sunscald on the smooth barked Acer truncatum x platanoides ‘Pacific Sunset’ and Acer truncatum x platanoides ‘Norwegian Sunset’ in metropolitan Melbourne was identified in 2010. Some 27 of these trees were planted as street trees in an east to west layout over a period of one to six years before the summer sunscald symptoms were reported. Eleven trees had badly damaged bark or dieback of the cambium, all on the west facing side of the trunk (Figure 4 and 5) (Moore, 2010 pers.comm.).

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A series of experiments were designed to investigate the causes of sunburn on the trunks of newly planted trees. These investigations also considered the effects of altered orientation when nursery grown trees are planted into the landscape. The experiments were conducted beginning in mid 2006 and ending in 2009 and evaluated the changes in the tree’s growing conditions as a result of;

 altering the orientation of a tree when planted into the streetscape, so that it differs from that when grown in the nursery,  the incidence of summer sunscald by testing the hypotheses that the greater the symptoms of summer sunscald the less the shoot tip extension,  the effects of heat re-radiated from different surfaces by;

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