Paleo Slide Set: Tree Rings: Ancient Chronicles of Environmental Change
False rings in Mexican cypress (Cupressus lusitanica).

Under certain climatic conditions, some species will form intra-annual or false rings. 
If climatic conditions are unfavorable to growth during the growing season, the tree 
may mistakenly sense that the end of the season is near, and produce dark, thick-walled 
latewood cells. Improved conditions will cause the tree to produce lighter, thinner-walled 
cells once again, until the true end of the season. The resulting annual ring looks like 
two rings, but when this first ring is closely inspected it can be identified as false 
because the latewood boundary grades back into the earlywood. False rings occur in a 
number of species such as the Mexican cypress pictured here. Young ponderosa pines 
in southeastern Arizona commonly contain false rings as well. In this region, winter 
and early spring rains provide moisture to trees in the early part of the growing season. 
By May and June, the driest part of the year, trees have used up the available moisture 
and, if stressed enough, will begin to produce latewood cells. However, monsoon moisture 
usually begins to fall in July, and with this moisture, trees will again produce earlywood cells.

Photo Credits:
Peter Brown
Rocky Mountain Tree-Ring Research, Ft. Collins, CO