From Tolkien Gateway

Alders were deciduous trees that grew in Middle-earth, especially in Eriador, such as those that bordered the Water west of Hobbiton,[1] and by the brooks between Weather Hills and Midgewater Marshes.[2] They also were mentioned in the songs of Tom Bombadil to have grown by the banks of Withywindle.[3]

The elves of Rivendell mentioned alders in their song under the window of Bilbo Baggins: "Lullaby! Lullaby! Alder and Willow!".[4]

Moreover, alders were known to have grown in the First Age in Dorthonion by the lake Aeluin.[5]