When i created a cube that surrounds the whole scene it looked pretty however it decreased my sun lamp's emission effect on my bushes and doing so it decreased glossy level of my plants which its not a thing im comfortable with i tried to increase my sun's emission level but didnt do it
If you are using volume scatter and/or volume absortion, some of the light intensity will be lost due to... scattering or absortion.
There is no need to change the materials for the other objects, just change the light intensity.
It is perfectly normal/valid to have high values as long as you get the results you want.
As a different note, the default values of 1 for volume absortion or volme scatter are usually way to high. Play with them and bring them down to get the effect you want.