Most of us know the game “scissors, paper, rock” where on a simultaneous showing of hands the player’s eventual fate is determined. Scissors cut paper, paper covers rock, rock blunts scissors. A similar game is being played out in the streets of Adelaide right now in the midst of the worst drought in history, record heat waves and the tough new water restrictions.
It is called “tree, water, bluestone” and depending on how we as a community play the hand, the fate of many urban trees, scarce water resources and valuable old buildings will very soon be decided.
In my game, during drought and irrigation restrictions, urban trees can extract moisture from the earth to such a degree that the subsoil shrinks. “Tree beats water”.
Changing moisture regimes around the footings of buildings, particularly those with bluestone or poorly engineered footings can cause significant structural damage. “Water beats bluestone.” Buildings are considered to be irreplaceable and of great value unlike most trees, so, whenever there is an actual or perceived conflict, the tree goes. “Bluestone beats tree”.
HOW TREES AFFECT SOIL MOISTURE (TREE BEATS WATER)
Water held in the soil matrix is taken up by the roots and released to the atmosphere through the leaves in the process of evapotranspiration. This is essential for the tree to carry out photosynthesis, transport nutrients, and to regulate temperature. Trees are therefore efficient hydraulic pumps compensating for the natural infiltration of water into the soil from precipitation which would otherwise cause the water table to rise. Large areas of cleared land are now barren as a result of the rising water table bringing salt to the surface. When the infiltration rate of water into the soil is less than the combined evapotranspiration rate of the plants and the direct evaporation from the soil surface, the soil dries out. This happens whenever there is drought and the imposition of irrigation restrictions, and in urban areas as a result of a reduction in permeable surfaces.
It has been estimated that the daily water usage by each tree in the avenue of 70 elms at the Waite Arboretum is 500 litres during summer. Each tree has an annual requirement of around 100,000 litres. Until the recent drought this has come from water stored in the soil during the wetter months, as the policy for the past 60 years in the Arboretum has been not to irrigate. Over the past 12 months the trees have been slowly dying as soil moisture levels have plummeted to record lows. The trees have a canopy diameter of 16m and their roots exploit an area of infiltration of approximately 500m2. In the 15 months since January 2007 they received approximately 300mm less rainfall than average, so we can assume that the drought has resulted in 150,000 litres less available water per tree. The 5 previous dryer than normal years have also contributed to the situation where the suction required to access soil water is close to the limit for elms and so they are suffering severe stress. On the other hand there are many evergreen trees that transpire at least the same daily quantity, but are able to apply greater suction pressures and so survive drought for longer periods of time. Australian trees have evolved in this regime of flooding rains and prolonged droughts and so are considered as ideal candidates for urban planting. It is common practice to promote these as “water wise” and we’re encouraged to plant them in the face of ongoing water restrictions for urban gardens.
HOW SOIL MOISTURE AFFECTS BUILDINGS (WATER BEATS BLUESTONE)
Many of Adelaide’s older buildings have footings which have not been designed to cope with the reactive nature of the clay subsoils common in the early settled suburbs surrounding the city. In particular the pre WW1 bluestone footings and subsequent concrete strip foundations have failed with the natural seasonal swell and shrinkage of the red brown and black earths covering much of the Adelaide plains. This movement may be as much as 100mm of heave when the subsoils around the footings are saturated and 150mm of settlement when they are desiccated. As the foundations move the building follows, and large cracks appear in the walls and openings around windows and doors. The traditional approach of underpinning is expensive and may be inneffective in stabilizing the structure. It is not the absolute degree of soil hydration that determines the stability of the structure but the change as it moves from a wet to a dry state and vice versa. There are many environmental factors which influence these changes in moisture regimes and although trees may not be the principal cause it can be enhanced by the proximity of vegetation. As soils shrink small cracks form, creating pathways for roots to penetrate. It has been demonstrated that where soil penetrometer stress is less than 3000 kPa plant roots can penetrate massive clay. Where cracks develop the soil stress to be overcome reduces to 500 kPa. The natural ability of tree roots to advance along paths of least resistance is one of their important survival mechanisms and one which has tended to demonize them amongst the engineering fraternity, as the fine root hairs which radiate from the collector roots absorb the moisture adjacent to the footings and to a depth often exceeding 4m.
Relationship between critical suction units and soil/plant condition (approximate).
cm (h) of water
column held by suction
pF log (h)
Soil water condition
Risk to footings
|0||0||0.0001||0.001||Soil saturated: water draining to aquifer||Leaves fully turgid. Possible root rot damage|
|10||1||0.001||0.01||Water still moving down profile||Leaves fully turgid.|
|100||2||0.01||0.1||Field capacity with no more drainage||Leaves turgid. Maximum growth rate||Maximum heave|
|1,000||3||0.1||1||Moist soil able to accept more water||Healthy but slower growth rate|
|10,000||4||1||10||Refill point reached. Critical shortage||Some plants exhibit signs of stress.||Standard equilibium|
|15,000||4.2||1.5||15||Water remaining in soil unavailable||Permanent wilting point of most plants|
|20,000||4.3||2||20||Very dry||Limit for many trees.|
Arid zone trees surviving
|100,000||5||10||100||Extremely dry||Limit for most natives and arid zone exotics|
|1,000,000||6||100||1000||Approaching limit of dessiccation||All trees dead|
HOW BUILDINGS AFFECT TREES (BLUESTONE BEATS TREE)
Research into this interaction commenced in the late 70’s and by the early 80’s the engineering profession was alerted to the influence of vegetation, particularly trees, on the soil moisture regime around buildings. As a result trees were blamed as the principal cause of damage to buildings either through settlement due to soil desiccation by existing or newly planted trees or rebound due to their removal. Although considerable advances were made in the design of footings by engineers there were huge efforts made to limit the presence of trees to prescribed distances from buildings. There were also species lists compiled in an attempt to identify “good” and “bad” trees in respect of building damage but these have been largely ignored by the community. The fact remains though, that under the current circumstances we are going to see a significant increase in the numbers of stressed trees and damaged buildings. Whilst both are considered to be valuable environmental and cultural assets, particularly in the inner suburbs, when it comes to protecting personal assets such as the family home, the tree loses
A NEW GAME
It will take a number of wet years, if ever, for Adelaide’s soil moisture levels to return to normal. The restrictions on garden watering only make the problem more acute.
We love our old houses and our urban trees and we need to start playing another game to save them both. It’s called “Water Sensitive Urban Design” and many of the principals were laid out 20 years ago. TREENET was founded in response to the evident conflict between engineers and urban forest professionals at a “Trees in the Urban Environment” seminar at the Waite Arboretum in 1995. (1) The looming crisis over water has brought both groups together in an effort to bring about change. The results of the 2003 collaboration between civil engineers and horticulturalists in the first TREENET stormwater sequestration trials will be on view in Claremont Avenue during this Symposium.
WHY VERGES ARE THE NEW GENERATION “WETLANDS”
In an average year 160 gigalitres (160 GL) of water flow down Adelaide’s gutters and into Gulf St Vincent. This run off carries a high load of nutrients and sediment which are destroying the marine environment. Further pressure on this delicate ecosystem will arise from the discharge of highly concentrated brine from the proposed desalination plant designed to replace 50 GL of this loss per annum.
The recommendations from the Adelaide Coastal Waters Study final report (ACWS Nov 2007) are for an urgent reduction in the volumes of stormwater discharge to bring about a 75% reduction in nitrates, a 50% load reduction in particulate matter, as well as reduced flows of organic and mineral toxicants to coastal waters. The State Government in its 2004 Waterproofing Adelaide blueprint set a target of 20 GL of stormwater reuse by 2025 which is only a 12% reduction on current outputs. Urban sprawl and infill will only add to the torrent of stormwater so there is little chance that any improvement in the marine environment as envisaged in the ACWS can be achieved.
No doubt there will be increasing adoption of well established “wetland” technologies to clean some of this polluted rainwater before sending it on to the ocean or preferably to the aquifer. This is very capital intensive strategy suited to a few locations remote from the source where land is available, and the environmental benefits are limited to the creation of aquatic habitats. To treat and store 1000 litres of stormwater it is necessary to create a 1m3 hole in expensive real estate along with the land hungry biofilters before injecting it into the aquifer using capital and energy intensive pumping systems.
What Adelaide needs are new, at source, low cost, readily implemented strategies that deliver multiple benefits to the community and the environment. The same 1000 litres of runoff can be stored and made available to trees in only 3m3 of soil. Enormous volumes of soil are already in abundance in publicly held land with no development value, the verge or “nature strip”. Paradoxically this soil is also the driest in the urban environment. The street trees planted in the verge are solar pumps ready to start up the moment water enters the system.
This alternative to established wetlands technologies is currently being investigated in a growing number of TREENET trials which will be demonstrated during this Symposium.
As already stated, trees can easily transpire 100KL of water each annually so that 10,000 will take up at least 1GL. That is equivalent to the volume of being diverted in the Parafield Stormwater Harvesting Facility. The modest 20 GL reuse envisaged by 2025 could be taken up by 20% of our estimated Local Government tree population by diverting stormwater only a metre or so from the gutter into aggregate filled trenches, swales, or permeable pavements. This removes the road generated pollutants and organic based nutrients at the source and puts them into the subsoil where they are broken down and taken up by the tree or bound tightly to the surfaces of clay particles. After this first flush has been diverted, the water flowing to the Gulf is far less polluted.
Many complex designs for achieving this have been proposed by engineers and landscape architects but their high installation and maintenance costs have until now discouraged widespread adoption by councils. The current drought and restrictions which have drastically reduced the irrigation of front gardens has created the need to trial systems that will replace this traditional source of supplementary water for street trees. TREENET has a nominal target of achieving this for $300 per tree for retrofitting existing kerbs and verges, and negligible costs for new installations. A much needed regeneration of Adelaide’s aging urban forest will provide the opportunity to make these practices standard. There will be significant savings in expenditure on repairing uplifted kerbs and footpaths as the root systems will run parallel to this infrastructure in response to the relocation of water resources to the verge.Trees can provide additional significant benefits other than simply providing an alternative for storm water and pollutant discharge to the marine environment, and as such I believe trees leave all alternative uses of stormwater in the shade.
An Adelaide University study (2) conservatively estimated the value of Adelaide’s street trees in 2002 at $171 each based on the cost of replacing the services they provide. Apart from habitat benefits for humans and wildlife and a myriad of other services, the direct influence of trees on climate and hydrology are standout advantages. In transpiring all that water, trees are like giant evaporative coolers and combine this with shading and controlling air movement to reduce the temperature of the city and suburbs conservatively by 4 deg C. This results in a massive reduction in energy use for air conditioning. They also sequester carbon dioxide, considered to be a major contributor to global warming, and perhaps all the trees in Adelaide may offset 25 day’s energy consumption of the proposed desalintion plant annually.
Finally trees have a direct benefit in the control of storm water by intercepting rain in the canopy thus reducing the amount and rate at which this precious resource threatens our marine environment.
- Stormwater harvesting trials for irrigation of street trees and water quality and quantity improvement. Porch Zanker and Pezzaniti. TREENET Proceedings of the 4th National Street Tree Symposium 2003 *
- The Economic Value of Trees in Urban areas: Estimating the benefits of Adelaide’s street trees. Killicoat, Puzio and Stringe TREENET proceedings 3rd National Street Tree Symposium 2002. *
* Available online at treenet.org