That green expansion, however, occurred as neighbouring northern forests were changing.

The southern forests were ironwood forests, while the central and northern forests were teak.

These northern forests are a vast wilderness of pine and spruce trees mixed with patchy stands of birch and poplar.

These are arboreal birds of northern forests.

Since these northern forests rely on thick layers of decaying organic matter for growth and nutrition, they are diminishing in diversity and young plants struggle in these environments.