For the purposes here, we will assume that the tree trunks are cylinders.
Volume of a cylinder = area of base x height = pi x radius2 x height
part 1 in meters
step 1 conversion factor: 1 inch = 0.0254 meters
step 2 Volume of tree = 3.1415 x (2.5in x 0.0254m/in)2 x 15m = 0.19m3
step 3 Total volume of wood = 0.19m3/tree x 595 trees = 113.05m3
step 4 conversion factor 1 foot = 0.305 meters; 5 feet = 1.52m
step 5 volume of stacks = 1.52m x 1.52m x 1.52m = 3.5m3
step 6 number of stacks = total volume ÷ stack volume
113.05m3 ÷ 3.5m3/stack ≅ 32 stacks (actual number will depend on rounding)
part 2 in feet
step 1 conversion factors: 2.5inches = 0.208feet; 15m = 49.2125feet
step 2 volume/tree = 3.1415 x (0.208f)2 x 49.2125feet = 6.69f3/tree
step 3 total volume = 6.69f3/tree x 595 trees = 3,980.55f3
step 4 stack volume = 5f x 5f x 5f = 125f3
step 5 number of stacks = total volume ÷ volume/stack
3,980.55f3 ÷ 125f3 = ≅ 32 stacks (actual number will depend on rounding)