Since the late 1800’s, the concentration of atmospheric carbon dioxide has been steadily rising. It is widely understood that this increase will change the climate of the planet. Some of the forecast changes are already evident. The Australian Bureau of Meteorology has data that shows that since 1900 temperatures in Australia have risen by approximately 1oC. Rainfall has also altered but this pattern is not uniform. The south east of the continent has dried but the northwest is wetter. How might an urban forest manager take climate change into account when planning future planting? Forecasts for likely future climate scenarios vary, and are uncertain, and so it is not possible to say exactly what the climate in any place will be like in 20 or 50 years time. However, the climate may change in a dramatic fashion over the next few decades.
By 2030 temperatures could be up to 1.2oC higher than the period 1985-2005. By 2090, some estimates have the temperature rising by as much as 5oC. A rise of 2oC would give Melbourne a temperature regime like that currently experienced in Mildura. A rise of 5oC gives Melbourne a temperature regime like that of Alice Springs. These changes will affect tree performance. The species most at risk are going to be from regions of the world with cool summers, including northern Europe, Northern China and Japan and the northern USA and Canada.
The effects of climate change on rainfall and drought are less easily predicted. Predictions for Melbourne are for a reduction in winter and spring rainfall of up to 15%. The time spent in drought is projected to increase over the course of the century. It should be borne in mind though that even if rainfall doesn’t change, higher temperatures would increase evaporation rates, which will increase water stress. Trees that do not tolerate drought will be most at risk unless reliable alternative water sources are available for irrigation. As many street tree populations are not irrigated, this issue will need to be addressed when choosing trees for streets. While the Millennium drought event (1997-2009) can’t be identified as an effect of climate change, the climatic conditions that existed during that period can be used for a model for future climates. While many species tolerated those conditions, others did not and tree managers should bear in mind species’ performance in their jurisdictions when thinking about tree planting programs.
Planning for the effects of climate change on tree populations will require that we attempt to think about the conditions that a newly planted tree will be potentially facing in 30 or 40 years, as it reaches its maturity. When thinking about these changes though, it is not possible to come up with a single solution – “These are the trees that you should be growing.” Each job and each location will have their own solutions. It is important that when thinking about the future, the best choices are made, and that doing the same as we have always done is no longer tenable.
Urban Landscape Management 