Question
Would you please advise us whether we should cut down our 4 year old birch tree, which is located 3 feet away from the opening (cap)of my septic tank?

If yes, what other tall tree/shrub we can plant at the same location to replace the birch tree?  Many thanks for your advice.

Answer
Birches and sewer lines do not mix well. Trees and shrubs are much riskier choices for the drainage field than herbaceous plants. The woody roots of these plants are more likely to clog and damage drain lines. Especially notorious for line clogging are water-loving trees such as Beeches, Birches, Elms, Poplars, Red Maple, Silver Maple, Willows,  Fagus spp. ,Betula spp.,Ulmus spp.
Populus spp.,Acer rubrum, Acer saccharinum, Salix spp.  willows and poplars . Do not plant these near a leach field unless you are prepared mentally and financially for the possibility of needing to install a new field sometime in the future. If you insist on growing them near a field, at least plant them at the far end where the lines will be drier and less conducive to root growth.
Some smaller and less-aggressive woody species may be suitable for planting over the drain field. Some possibilities include fibrous rooted shrubs such as boxwood or holly, or small trees such as dogwoods. When planting shrubs in a leach field, place them between the lines when possible. Normally, drainage trenches are 3 feet wide with 6 feet between trenches.

Trees and shrubs are risky to plant over or near a drain field. Their roots penetrate more deeply than grasses and flowers. Once their roots find the constant source of moisture and nutrients that a septic pipe affords, the roots will invade the pipe. If the pipe becomes filled with roots, waste water can no longer flow down it. The water may back up into the tank and could even back up into your house! Willow, poplar and pine trees are all known to clog up drain pipes.

The best place to plant a tree is near the end of a line, where it is drier and less attractive to root growth. If you insist on planting a shrub over the drain field, the ones that have fibrous roots, such as holly, boxwood, azalea and rhododendron would not be likely to invade pipes. Even a small tree, such as dogwood or dwarf Japanese maple could be considered. Here are other choices to think about:

NOT RECOMMENDED NEAR DRAIN FIELDS

Beech Elm
Poplar Red maple
Silver maple Willow
Pine Oak

BETTER CHOICES TO PLANT NEAR DRAIN FIELDS

Flowering cherry Crabapple
Dogwood Hemlock
Sourwood Crape myrtle
Black gum Goldenrain tree