I have a steep hill and in attempting to control erosion I built a small bricked wall (18" tall") and filled behind it with soil. The wall is within the branches of a 15 year old maple. Will the extra soil damage or kill the tree?
So the extra soil is not around the trunk? If several feet away it shroud not be a problem .It is not a recommend practice but my experience is mixed . One house built 20 ft away from a 18" diameter oak, and the tree died. At this house there is an 10 ft space between house and garage with an 18" diameter oak in between ,so foundations roughly 4 ft away on two sides and the tree was not bothered.
Yes, the additional soil will Very Likely demise the Maple Tree! Maples are exceptionally sensitive to Any weight on their root systems! eg, just driving a vehicle over the side of the root system 1 time can demise an established Maple of any size or age! And a few cm of standing water for a few weeks can also! It may take a couple seasons to become apparent, but its generally unsalvageable by then. The first part to deteriorate tends to be the part over the additional weight, but the deterioration continues to the rest of the tree.
Might be better to set the wall into the eroding soil rather than on top of it, thus it wouldnt be adding weight on top of the roots, yet still be effective at reducing the erosion(if all the soil washed away that would also finish the tree!)
To save the tree, would recommend Immediately removing All the additional soil, And any of the wall within the perimeter thats higher than the original soil level, even if the wall is(as is likely) narrow. The Maple May survive if the All the additional soil & that portion of the wall are removed immediately: Maples have survived having sand eg put over part of their root systems for a Few days but then All of the sand immediately removed. Maybe 1 cm(!) total would be ok if After several Years the tree is flourishing: But attempting to add more than about 1 cm, or trying to add 1 cm per year would likely demise it: Maples dont seem to Ever adjust to additional weight on their root systems, even if very gradually added over several years!
The deterioration doesnt seem apparent right away, as the effects tend to be gradual, but even if removed after a few weeks, & before any affects show, the trees tend to be finished: very old trees may last longer(maybe even 2 - 3 seasons), but gradually go. In order to preserve the Maple Tree, would suggest removing Every cm of the additional soil And the wall Immediately. Excellent question.