лесами

In a sentence

• Мы гуляли по лесам ранним утром.

  We walked through the forests early in the morning.

  In this sentence, 'лесами' is used in the instrumental plural form referring to 'forests' as the geographical area being walked through.

• Лесами нашей страны обитают редкие животные.

  Rare animals inhabit the forests of our country.

  Here, 'лесами' is used to indicate 'forests' as the locations associated with the action of inhabitation.

• Лесами движутся тени от облаков.

  Shadows from the clouds move across the forests.

  In this sentence, 'лесами' describes 'forests' in the sense of the regions affected by the movement of shadows.