The amount of rainfall that an area receives can certainly affect the width of
tree rings, but so can other factors. Factors such as temperature, soil
conditions, and the gradient of the slope can have an effect, as can wind, snow
accumulation, and how much sunlight a tree receives.
Still, the amount of variation in a tree's rings can vary between species of
trees and between trees of the same species.
A tree whose rings vary in width are called sensitive. These trees can
have a wide ring if conditions are conducive to growing and a narrow ring if
Sometimes, a tree's annual rings don't show much variation. The reason can be
simply that it's a species that isn't prone to variation. Or it can be that
it's located in a part of the world where the environment doesn't vary greatly
from season to season. Or it can be that it's growing where the water table is
high enough for it to get all of the water it needs. Trees whose rings don't
show much variation in width are called complacent.
Tree-ring scientists prefer to work with tree-ring samples from sensitive
trees, as the variation in their annual rings allows for crossdating.