。
The trees ease the force of the wind.
Literal
Tree [with-で], wind [possessive-の] force [subject-が] softens.
和らぐ ('to soften, to subside') is another member of the 和 ('harmony, peace') family of vocabulary — cf. 和む ('be put at ease'), 和やか ('peaceful'), 和解する ('reconcile'). で here is instrumental — 'by means of the trees.' Subject-less causal statement typical of descriptive prose.