On the wet end, tropical rain forest requires high precipitation (more than a year) and temperatures and is dominated by trees.
Moderate rain characterizes mediterranean scrub and savanna; high temperatures during dry spells allow grasses to compete with trees.
Moderate moisture and temperature yields temperate forest and temperate grassland.
As moisture and temperature drop further, boreal forest covers the landscape, dominated by conifers.
Very dry and very cold conditions support tundra with low-growing vegetation and no trees.