«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 ...»
The 14th National Street Tree Symposium 2013 Each tree was measured for annual growth, in some cases for up to three consecutive growth years. The measurements used to evaluate growth were increases in tree trunk caliper and shoot tip extension (shoot growth). Measuring annual shoot tip extension, the distance between the scale scars and the terminal bud, is a suitable method of quantifying a plant’s top growth (Watson, 1987, Watson et al., 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, Pichler and Oberhuber, 2007). To calculate the Figure 6. Indicating shoot tip extension from scale scar to terminal bud annual shoot tip extension, a ruler on Platanus x acerifolia.
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 a small spot of paint indicating the north facing side on the tree. The trees were then planted into three different streetscapes of Hume City Council, northwest 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 undertaken 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 sunscald will be discussed later, see Table 2.
Heat radiated from a surface may induce trunk summer sunscald The experiment used 30 field grown, 45 litre rootball, Platanus x acerifolia that were planted along the centre median of Pascoe Vale Road, Broadmeadows in the winter of 2007. Directly adjacent to each tree on the west side, a 0.5m2 surface was installed in a timber plinth box, with a 50mm space between the tree and plinth.
The four surfaces used were asphalt, concrete, granitic sand (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 set up having 30 trees in total with 14 facing north and 16 facing south. The trees planted alongside asphalt and granitic sand had four replicates for each orientation. The trees planted alongside 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® Thermal Image Camera. A Thermographic Report presented conventional photographs alongside thermal images of each individual tree. Information provided was the location, the tree identification number and the surface, the The 14th National Street Tree Symposium 2013 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 6.48am to 7.28am, afternoon from 2.33pm to 3.04pm and late afternoon from 7.26pm to 7.48pm.
Figure 7. A thermal image photograph alongside a conventional photograph of Tree 15 taken at 2.
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 sunscald, 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.
The 14th National Street Tree Symposium 2013 Table 1. Mean temperatures for each treatment surface at time of measurement.
Scatterplots 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 alongside the data presented in Table 1 and Figure 8, planting trees alongside 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.
The 14th National Street Tree Symposium 2013 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 below average rainfall. This coincides with the well reported ten year dry period that was experienced in Victoria from the late 1990’s to 2010.
The 14th National Street Tree Symposium 2013 Figure 11 shows climatic conditions from December 2008 through to March 2009. Shown are the total monthly temperature and rainfall figures alongside their long term averages, over these figures is total evaporation and long term 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 bushfires.
Table 2 details the total numbers of trees in each experiment alongside total numbers of deaths, observations of summer sunscald 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 atypical 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 sunscald, while 30% of the nursery grown trees received some level of summer sunscald.
Table 2. Total numbers of trees in each experiment alongside total numbers of deaths, trees displaying summer sunscald 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 (Costello et al., 2005, Gilman, 1993, Hitchmough, 1994, Leers, 2000, Moore, 1997b, Watson, 1997). Considering the below average rainfalls alongside the observed data from all the experiments and the death of almost 50% of trees from the experiments, the measured growth responses may be attributed to these extreme conditions (Allen et al., 2010, Larcher, 1995, McDowell et 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 sunscald 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/or granitic sand and as such can re-radiate more heat than mulch and/or granitic 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 reradiated from different paving surfaces induces summer sunscald on newly planted trees. Finally, that tree growth is adversely affected the greater the symptom of summer sunscald has not been proved. Because these The 14th National Street Tree Symposium 2013 hypotheses have not been clearly answered, it is hoped this document will form the basis for further research into the causes and effects of summer sunscald.
Applying these lessons Perth conditions 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 Airport is 773mm, in Perth's northern regions average rainfall is 606mm and for the western regions, 721mm while the base of the escarpment can get 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 noticeable 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 groundwater 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 in decreased flow to public water supply dams by more than 50% on average and decreased recharge to aquifers has also occurred due to climate variability. Groundwater falls of the Gnangara Mound of up to 4 m 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 The 14th National Street Tree Symposium 2013 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.