Tree Species Unsuitable for Tree-Ring Dating
Most temperate forest tree species (those growing between 25 and 65 degrees latitude),
produce tree rings. Therefore, the majority of dendrochronological research
occurs in these latitudes. Some species, however, are not suitable for tree-ring
dating, and it is important to know and recognize which species to avoid.
On the right is an image of woody tissue from a saguaro (Carnegeia giganteum),
the large (sometimes up to 50 feet tall) cactuses with many arms that characterize
the American Southwest. Three "ribs" of the cactus are depicted here, emanating from
the center of the cactus (a the top of the image). Although dendrochronologists receive
many inquiries into the age of these spectacular plants, saguaros do not produce
annual rings.
Here is a partial cross section from a fan palm, a monocotolydon (the same type of plant
as grasses), which also does not form annual rings. The inside of a plam tree
is basically all primary xylem (woody) tissue - to form annual rings, plants must
form secondary xylem tissue radially, from the inside to the outside. Have you ever
noticed that palm trees never grow wider around the trunk? This is why.
Some desert species do form annual rings, but the ring structure is very
difficult to discern. To the right is a cross section of a mesquite (Prosopis glandulosa)
which shows very faint rings, and is usually undatable. To enhance the ring structure, and
to perhaps help date this desert species, the darker half was colored with black
marker, then covered with chalk dust. The small vessels then fill up with chalk
to highlight the rings. Mesquite is a type of ring-porous angiosperm.
All graphics and text on these pages © 1996 by Lori Martinez, Laboratory
of Tree-Ring Research, and The University of Arizona. Last updated 18 December,
1996. All rights reserved.