[2/2/2006 3]
Unique programme protects trees
There are some 4,000 trees in the vicinity of Citytunnel work sites where groundwater levels will fall temporarily. So that the trees will not be adversely affected, a unique inspection programme that measures ground moisture and how much water trees take up has been in progress since the summer.
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A tree fitted with accurate equipment to give continuous measurements of sap flow and the moisture content of the ground around them. |
‘So far, ground moisture has not been affected, but it is not until spring, when sap movement begins, that we will see whether the trees are affected’, says Arne Mattsson of Department of public works in Malmö, who heads the inspection programme.
In 2004, an inventory of the condition of trees near Citytunnel work sites was carried out. There were 4,000 trees in total, of which at least 1,000 were regarded as being in need of special protection. To determine whether trees are being adversely affected, four trees around Triangeln have been subjected to intense scrutiny. These trees represent all the sensitive trees and they have been fitted with accurate equipment to give continuous measurements of sap flow in the trees and the moisture content of the ground around them.
During autumn and winter trees become dormant and take up much less water because sap flow is very low, so at that time they are not too badly affected by lack of water.
‘It is only in spring when sap transport in the trees starts in earnest that we can see whether lowering the water table has affected them’, says Arne Mattsson.
‘If their water consumption is reduced it may be because groundwater is low and water access restricted. If access to water is poor for an extended period, we will have to provide irrigation for the most important trees’.
Trees are of course capable of handling reduced water supply themselves. They can for example extend their root systems to seek water deeper underground. But because lowering the water table at Citytunnel worksites will be temporary, there is a risk that trees will become waterlogged when the water table returns to normal.
‘Trees can die of lack of oxygen, or the roots and trunk can be attacked by rot, which leads to the tree withering wholly or partly’, says Arne Mattsson.
There is also a risk of becoming waterlogged due to re-injection when groundwater is pumped up from the work area and returned to ground a little further away.
‘The water table can then become too high and re-injection must be reduced to prevent waterlogging’, explains Arne Mattsson.
‘Another risk factor that we identified is salt water seepage when groundwater is lowered near the sea, as in the case at Malmö Central’.
The financial value of the thousand or so trees regarded as especially valuable has been estimated at SEK 85 million. The aim is that no trees should be seriously damaged during the construction of Citytunnel connections.
The inspection programme is carried out by the Department of public works and partly financed by the Citytunnel project.