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INCREASES WITH FLOODING DURATION 7ATERLOGGING BEGINS
WHEN DRAINAGE IS NOT SUFFICIENT TO ALLOW WATER TO PASS
FREELY FROM THE ROOT ZONE 4HIS CAN HAPPEN FOR A NUMBER
OF REASONS LIKE COMPACTION POOR SOIL STRUCTURE LAYERS OF
CLAY OR A VOLUME OF WATER ENTERING THE SOIL
3OILS VARY NATURALLY IN THEIR DRAINAGE ABILITY BY COMPOSI
TION CLAY LOAM SAND ETC WHICH CAN BE INFLUENCED BY
THE PRESENCE OF GEOGRAPHIC FEATURES )N URBAN ENVIRON
MENTS DRAINAGE IS NOTORIOUSLY POOR AND TREES ARE OFTEN
SURROUNDED BY IMPERMEABLE SURFACES SUCH AS TARMAC
WHICH FUNNEL WATER TO THE EXPOSED GROUND AROUND THEIR
ROOTS AND PREVENT WET SOIL BENEATH FROM DRYING 5NDER
IDEAL CONDITIONS TREES SHOULD HELP TO PREVENT STRESS ON
URBAN DRAINAGE SYSTEMS BY REDUCING THE VOLUME OF RAIN
WATER REACHING THE GROUND AND HELPING RETURN IT TO THE
ATMOSPHERE (OWEVER THIS FUNCTION REQUIRES THAT TREES
ARE KEPT HEALTHY AND SOIL CONDITIONS PLAY AN IMPORTANT
ROLE IN THIS
4REE ROOTS AND THE MICROORGANISMS MYCORRHIZA BACTE
RIA ARTHROPODS ASSOCIATED WITH THE ROOT SYSTEM REQUIRE
OXYGEN PROVIDED BY AIR FILLED PORES 7ATERLOGGING FILLS
THE PORES IN SOIL WITH WATER PREVENTING THIS 4YPICALLY
OXYGEN MOVES INTO THE SOIL THROUGH THESE PORES AT A
RATE SUITABLE FOR BOTH THE ROOTS AND MICROORGANISMS
HOWEVER WHEN THEY ARE FILLED WITH WATER OXYGEN MOVE
MENT IS REDUCED AND EVENTUALLY INHIBITED )F FLOODING IS
MINIMALIE THE SOIL SURFACE IS STILL EXPOSED ONLY THE
UPPER SOIL LAYERS RECEIVE NOTABLE AMOUNTS OF OXYGEN )F
THE WATER LEVEL IS ABOVE THE SOIL SURFACE OXYGEN LEVELS
ARE FAR LOWER )N COMPACTED SOILS THE PORES ARE EVEN
SMALLER AND AS A CONSEQUENCE THEY FILL UP QUICKERIE
WATERLOGGING IS MORE SEVERE
!S BOTH THE ROOT SYSTEM AND ASSOCIATED MICROORGAN
ISMS ARE OXYGEN DEPENDENT ANY OXYGEN PRESENT IN
THE SOIL LAYERS IS QUICKLY USED UP .ORMAL FUNCTION
AND GROWTH OF ROOTS IS INTERRUPTED /NCE OXYGEN
BECOMES DEFICIENT hNORMALv AEROBIC SOIL MICROORGAN
ISMS DROP IN NUMBERS AND ENTER AN INACTIVE STATE
UNTIL SOIL CONDITIONS BECOME MORE HOSPITABLE AGAIN
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