Michael Leers, University of Melbourne
The urban environment maybe considered one of the harshest planting and growing environments (Apple yard, 2000, Craul,1993,Gilman,1993,Simpson,1981). The London Tree Officers Association expected a 50% tree replacement rate within their municipality as a result off ailed establishment or from damage/vandalism (Apple yard, 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 anyone, or a combination of, poor quality or in appropriate planting stock, inadequate planting techniques or in adequate 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, Cutleretal.,1990, Smith, 1997). However, the micro climate 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 unpavedareasor “vegetated”areas, in turn requiring greater scrutiny with species selection (Bassukand Whitlow, 1985, Irfanet al., 2001, Kjelgren and Clark,1992, Rosheidat and Bryan,2010).
Kjelgren and Clark (1992) found that urban micro climates 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 micro climates and the urban heat island effect are proving to have an increasingly negative effect on plant growth (Bassuk and Whitlow, 1985, Irfanetal., 2001, Kjelgren and Clark, 1992, Rennenbergetal., 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 (Costelloetal., 2003, Litzow and Pellett, 1983, Rennenbergetal., 2006,Roppolo and Miller, 2001, Shirazi and Vogel,2007)
High temperature injury (HTI) is caused by extreme or critically high temperatures (Costelloetal., 2003, Larcher, 1995, Levitt, 1956, Pichler and Oberhuber, 2007, Rennenbergetal., 2006, Rosheidat and Bryan, 2010, etc). The HTI threshold temperature range as 44°C to 50°C forever green conifers during the growing season (Larcher 2003 in Pichler and Oberhuber, 2007p. 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. A side 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 photo respiration at the same time as inhibiting photosynthesis and carbon metabolism is strongly connected to stress (heat and drought) compensation mechanisms (Fitter 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-Plazaolaet 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 maybe 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 etal., 2006). Dickinson (2002) discusses the problem with citing threshold temperatures is that cellnecrosis 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 termused 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 (Hermannand Lavender, n.d.). Summersunscald on individual trees occurring in heavily thinned forest stands of trees may not be apparent for several years (Curtis et al.,2000, Kozlowski etal., 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 (Litzowand Pellett, 1983).There is evidence that the pruning technique lion-tailing not only reduces photosynthetic ability, but also increase the potential for summer sun scaldon thin barked trees (Smiley and Kane, 2006). Lion-tailing is the removal of all the smaller branches from the inside portion o fa 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 the above ground parts of the plant, leaves flowers, fruit and bark. Of particular interest is damage to trunk and bark tissues. Mild summer sun scald, or the initial stages of summer sunscald of the trunk, appears as a reddish discolouration (Figure1). As it progresses the bark shrinks, appears sunken then splits, exposing the sapwood (Figure 2 and 3)(Costelloetal., 2003). Craneetal (1994) describe drying and peeling of the bark, branch die back, wood injury and saprophytic fungion deadbark 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.
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 Silvi cultural Handbook (1999) notes the necrosis of over heated cambial tissues as a result of sunburn in the warmer months. This in turn causes flattened sides, “barksloughing” and poor wood quality. The Hand book 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 sun scald. So it can be said summer sun scald is the result of relatively high ambient temperatures that tend to be localised, irrespective of latitude. Observations of the symptoms of summer sun scald 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 sun scald 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 sun scald symptoms were reported. Eleven trees had badly damaged bark or die back of the cambium, all on the west facing side of the trunk (Figure 4 and 5) (Moore, 2010 pers. comm.).
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 sun scald by testing the hypotheses that the greater the symptoms of summer sun scald the less the shoot tip extension,
- the effects of heat re-radiated from different surfaces by;
- testing the hypotheses that the presence of a surface treatment (asphalt,concrete, granitics and or mulch) on the western side of a newly planted tree will affect the temperature of the trunk of that tree and the growth of that tree (measured as shoot tip extension) in the first summer season after planting.
- testing the hypotheses that heat radiated from a surface (asphalt, concrete, granitic sand or mulch) may induce trunk summer sun scald or worsen the extent of summer sun scald,
Each tree was measured for annual growth, in some cases for up to three consecutive growth years. The measurement sused to evaluate growth were increases in tree trunk caliper and shoot tip extension (shoot growth). Measuring annual shoot tip extension, the distance between the scales cars and the terminal bud, is a suitable method of quantifying a plant’s top growth (Watson,1987, Watsonetal., 1992). Increase in tree trunk caliper and increase in tree height are also considered to be appropriate and suitable measurements of tree growth (Gilman and Grabosky, 2004, Pichlerand Oberhuber, 2007). To calculate the annual shoot tip extension, a ruler was used to measure the distance between the scale scar and the terminal bud, see Figure 6. Calculation of trunk caliper increase was determined by measuring the trunk caliper 100mm above soil level, measuring at the same point a year later and then determining the difference.
Changed tree orientation
A total of 134 trees from 3 different species-Platanus x acerifolia, Melia azedarach and Acer campestre were used in this experiment. Before leaving the nursery, the trees were marked with as mall spot of paint indicating the north facing side on the tree.The trees were then planted into three different street scapes of Hume City Council, north west metropolitan Melbourne. At the time of planting into the streetscape, this paint mark was positioned so that the trees were planted with a previously selected north, east, west or south compass point orientation so that a fully randomised design was created Results showed no effect on shoot tip extension when altering tree orientation at time of planting, so that it is different from that when grown in the nursery. The literature review under taken for this research did not find any support the hypothesis that tree canopies favour growing towards any particular compass point. The incidence and effects of summer sun scald will be discussed later, see Table2.
Heat radiated from a surface may induce trunk summer sun scald
The experiment used 30 field grown, 45 litre root ball, Platanus x acerifolia that were planted along the centre median of Pascoe Vale Road, Broad meadows in the winter of 2007. Directly adjacent to each tree on the west side, a 0.5m 2 surface was installed in a timber plinthbox, with a 50mm space between the tree and plinth. The four surfaces used were asphalt, concrete, granitics and (decomposed granite, commonly used as a pavement treatment in Melbourne) and organic mulch. All are commonly found surrounding or adjacent to urban trees. Tree orientation at planting was changed, as with the previous experiment,by marking the north facing side of the tree. Half the trees were planted with their original north facing orientation facing north and half had their original orientation turned to face south. Therefore, the experiment was setup having 30 trees in total with 14 facing north and 16 facing south. The trees planted along side asphalt and granitics and had four replicates for each orientation. The trees planted along side concrete and mulch each had three trees facing north and four trees facing south. This created a Fully Randomised Design.
Surface temperatures of the tree trunks and of the adjacent surfaces were recorded using thermal imaging (Figure 7). The thermal imaging was conducted in early March 2008 using a 7710 model, NEC®ThermalImage Camera. A Thermo graphic Report presented conventional photographs along side thermal images of each individual tree. Information provided was the location, the tree identification number and the surface,the date and time the photographs were taken and the ambient temperature at the time each photo was taken. A total of three thermal images and photographs were taken for each tree on 3 March 2008, comprising an image and photograph taken in the morning from 6.48 am to 7.28am, afternoon from 2.33pm to 3.04pm and late afternoon from 7.26pm to 7.48pm.
Results showed no significant treatment effect of treatment surface or tree orientation on shoot tip extension. Though, a Two Sample T-test showed a significant treatment effect, with greater caliper increase for those trees without summer sun scald, Figure 8.
Table 1 compares the mean temperatures for each surface at time of measurement. Means are separated using 95% confidence intervals. Standard deviations are shown in brackets. For each time period, means with the same letter beside them are not different (p<0.05). These results clearly indicate that by the afternoon, concrete and asphalt are hotter than granitic sand and mulch. This implies that they will also radiate greater heat than the other surfaces.
Scatter plots with regression lines were prepared to evaluate any relationship between treatment surface temperatures and tree trunk temperatures at the three measurement times. No correlation was present at the morning and afternoon measurements. However, a low positive correlation was present in the late afternoon (r2=46.9%), Figure 9. When considering Figure 9 along side the data presented in Table 1 and Figure 8, planting trees along side paved areas requires consideration of the possible effects of radiated heat.
In order to provide further background about the growing conditions experienced during all of the experiments, Bureau of Meteorology climate data recorded at the Melbourne Airport station and taken from the Bureaus website is presented and discussed.
Figure 10 shows monthly rainfall in millimetres during the growing seasons from 2006 to 2009. These were the years data were collected from the two in-ground experiments; the Changed Tree Orientation Experiment and the Effect of Paving Surfaces on Newly Planted Trees Experiment. The graph indicates six months out of a total of eighteen growing season months received above average rainfall. The remaining thirteen “growth” months received well be low average rainfall. This coincides with the well reported ten year dry period that was experienced in Victoria from the late 1990’s to 2010.
Figure 11 shows climatic conditions from December 2008 through to March 2009. Shown are the total monthly temperature and rainfall figures along side their longterm averages, over these figures is total evaporation and longterm average evaporation for each month. Though the month of December 2008 received above average rainfall, 26 days received less than 2mm, with two successive days each receiving above 40mm rainfall. Other than the two above average rainfall events, much of 2008/09 growing season experienced well below average rainfall and above average temperatures with high rates of evaporation, with January and February experiencing a peak in total evaporation, well above the 10 year average, taken from 1999 to 2009. This trend certainly contributed to the extreme and devastating events in Victoria on 7 February 2009,known as Black Saturday where very high temperatures resulted in devastating bush fires.
Table 2 details the total numbers of trees in each experiment along side total numbers of deaths, observations of summer sun scald and numbers of trees receiving various forms of intentional or accidental vandalism (such as mower damage). The table indicates some street plantings received a little more than 10% vandalism while others received none. This is not a typical of tree plantings in the urban environment. Also evident are the high numbers of tree deaths for Platanus x acerifolia compared to Acer campestre and Melia azedarach. Despite this death rate, only 10% of studied trees showed summer sun scald, while 30% of the nursery grown trees received some level of summer sun scald.
|30 x Acercampestre 2008||30||0||0||4|
|46 x Meliaazedarach 2007||46||0||3||0|
|46 x Meliaazedarach 2008||46||0||0||4|
|58 x Platanus x acerifolia|
(Changed Orientation) 2007
|33 x Platanus x acerifolia|
(Changed Orientation) 2008
|30 x Platanus x acerifolia|
(Effects of Paving) 2008
Table 2. Total numbers of trees in each experiment along side total numbers of deaths, trees displaying summer sun scald and trees vandalised.
As could be expected, the climate provided difficult growing conditions, having a negative impact on the growth of all trees used in the two in-ground experiments (Costelloetal., 2005, Gilman, 1993, Hitchmough, 1994, Leers, 2000, Moore, 1997b, Watson, 1997). Considering the below average rainfalls along side the observed data from all the experiments and the death of almost 50% of trees from the experiments, the measured growth responses maybe attributed to these extreme conditions (Allenetal., 2010, Larcher, 1995, McDowellet al., 2008, Street and Öpik, 1984)
Even under the conditions of the experiments, particularly the below average rainfalls and high temperature and evaporation rates experienced during the growing season, the intensity and duration of conditions required to induce summer sun scald were not in evidence to the degree that has been observed elsewhere. However the results from the experiments and associated review have shown that transplanting a nursery grown tree, so that its original trunk and canopy orientation is changed, does not affect tree growth. They have also shown that the surfaces of asphalt and concrete are hotter than mulch and/orgranitic sand and as such can re-radiate more heat than mulch and/orgranitic sand.
The results from the experiments and associated review have not proved that heat re-radiated from paving surfaces negatively affects the growth of newly planted trees. Further they have not proved that heat re-radiated from different paving surfaces induces summer sun scald on newly planted trees. Finally, that tree growth is adversely affected the greater the symptom of summer sun scald has not been proved. Because these hypotheses have not been clearly answered, iti s hoped this document will form the basis for further research into the causes and effects of summer sun scald.
Applying these lessons
The climate of the Perth region is typical Mediterranean with dry, hot summers (mean monthly maximum greater than 30°C) and wet, mild winters (mean monthly minimum greater than 8°C). The average annual rainfall at Perth Airportis 773mm, in Perth’s northern regions average rainfall is 606mm and for the western regions, 721mm while the base of the escarpment canget average rainfall around 820mm. The summers are dry with more than 80% of the annual average rainfall occurring between May and October. The south-west of Western Australia has experienced notice able changes in climate, with a general trend of declining annual rainfall since the mid 1970’s, Figure 12. Climate change predictions for the Perth region are increased mean temperatures and lower rainfall. Further, these declines in winter rainfall will result in a significant decrease in stream flow and ground water recharge (Wilson and Valentine, 2009).
Climate modelling by CSIRO shows that average annual rainfalls are projected to decline in the south-west of Western Australia by as much as 20% by 2030 and 60% by 2070. In the last 35 years, reduced rainfalls have resulted indecreased flow to public water supply dams by more than 50% on average and decreased recharge to a quifers has also occurred due to climate variability. Ground waterfalls of the Gnangara Mound of up to 4m were recorded over the period 1979–2004 Figure 13. Significant rainfall declines combined with increased supply for public and commercial water needs as well as increased evapotranspiration are the contributors to this outcome.
Gnangara groundwater system incorporates a number of aquifers, including the Gnangara Mound or superficial aquifer, which is a shallow unconfined aquifer, the semi-confined Mirrabooka aquifer and theLeederville and Yarragadee aquifers that are deep and more confined aquifers that extend north and south beyond the extent of the Gnangara Mound, Figure 14.
Drought related tree mortality
Moisture stress can affect plant growth, from short term impacts on cells and their processes to longer term impacts on root and shoot growth and even death. It is the disruptions to the photosynthetic apparatus and carbon metabolism that will have the greatest negative impact on the establishment of a newly planted street tree due to the reduction in shoot and root growth, amplified by the internal redirection of allocated resources away from the finer roots (Apostol et al., 2009, McDowell et al., 2008, Pichler and Oberhuber, 2007, Watson, 1997, Werner et al., 2001). Even though some of these drought induced effects are reversed as water again becomes freely available (Fitter and Hay, 2002, Larcher, 1995, Salisbury and Ross, 1992), the establishment of the new street tree is delayed or stopped by drought periods or those extended periods of moderate to severe moisture stress (Gilman, 1997, Smith, 1997, Watson, 1997, Whitcomb, 1987).
McDowell et al (2008) discuss three contributing factors of drought related tree mortality which are consistent with other theoretical and empirical results (Figure 15). Hydraulic failure occurs due to reduced soil water availability and then coupled with high evaporative demand causes xylem cavitation. This stops water flow through the tree and leads to desiccation of tissues and when duration and/or intensity of moisture stress is severe enough, the whole plant desiccates. McDowell et al (2008) mention anecdotal observations of mature tree mortality in the absence of pathogens. However, it is not clear if hydraulic failure alone was the cause. Carbon metabolism is disrupted with stomatal closure and the uptake of carbon diminishes and the tree starves because metabolic demand is still current. This may be exacerbated by photoinhibition or increased respiration associated with increased temperatures typical in drought periods. McDowell et al (2008) cite evidence of the link between tree death and carbon availability – trees dying with decreased stem wood growth rates and increased growth variability, though this may be species related. Finally, environmental drought conditions drive changes in the demographics of plant pathogens (insects, fungi, bacteria). The growth rates, population size and/or mortality of some pathogens will be favoured by these conditions, though the exact dynamics for all species are not yet known. These changes may occur in conjunction with the tree’s physiological responses and condition (McDowell et al 2008).
This model can be applied to the example of Perth’s Araucaria heterophylla (Norfolk Island Pine), many of which have been declining. Surveys of 200 Norfolk Island Pine trees were carried out in 2009 and 2010. Samples were collected and morphological characteristics of the consistently isolated fungus analysed. The isolated fungus was identified as Neofusicoccum parvum (Hossein & Burgess, 2011). These Botryosphaeriaceae are common endophytes of a wide variety of woody plants worldwide. Diseases associated with the Botryosphaeriaceae are often stress related requiring a predisposing incident to trigger disease expression. Environmental stresses include drought, extreme temperature fluctuations, nutrient deficiencies and mechanical injuries (Hossein & Burgess, 2011). As well as the decline of the A. heterophylla are reports of the indigenous Melaleuca lanceolata (Rottnest Island Tea Tree) declining and dieing along the North Fremantle foreshore as a result of a Botryosphaeiraceae endophyte during one of Perths driest winters on record in 2010 (Barber, 2011).
Given the research presented in this paper, along with the changing climate, there needs to be further research into paving materials suitable for paths, road ways and other urban infrastructure that do not reradiate as much heat as the commonly used asphalt and concrete. The results of that research needs to be provided to the landscaping industry (including landscape architects) and local government who recommend and use these paving types and other hard surfaces. Finally, there should always be better scrutiny of tree species selection for paved urban environments, particularly given climate change predictions, population increase in the urban environment, together with the conditions favouring antagonistic and pathogenic biotic agent demographics. This scrutiny requires effort by the academic researchers, end users (industry) and tree suppliers – nurserymen.
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