。
Seeing the trees put me at ease.
Literal
Tree [object-を] see [when-と] heart [subject-が] softened.
心が和む ('the heart softens / is put at ease') is a set expression for a gentle, calming feeling — often triggered by nature, children, or small kindnesses. 和む is related to 和 ('harmony, peace') and captures a distinctively Japanese aesthetic of soft emotional warmth rather than ecstatic joy. と here is the conditional/temporal 'when/whenever.'