The Mystery of Masting in Oaks

This is the time of year when acorns are dropping and you can hear them crunch under your feet as you walk through the woods. However, only a few oak trees are dropping acorns. This year at Pleasant Valley Conservancy only one oak species seems to be producing acorns: northern red oak.

Acorns from an oak in Unit 11B at the edge of the gravel road. Listen for the crunch as you walk by!

Mast is a term used to describe the hard fruit of various trees such as beech or oak. Oaks as a group produce lots of acorns only every three or four years, and a high production year is called a good mast year. I discussed the masting phenomenon in 2011 when bur and white oak were heavy acorn producers. 

This year, so far, the only trees I have found dropping acorns are red oaks (Quercus rubra). Other oak species in the red-oak group such as Hill's and black oaks do not (so far) seem to be dropping acorns. The last good mast year for the whole red-oak group was in 2010.

It is not unusual for masting to occur only in a single species, such as we seem to be having this year, although the mechanism by which the trees bring this process about is a mystery. There are lots of papers discussing this mystery, but no concrete data. Look for the work of Walter D. Koenig, although his research has focussed on California oaks.

I was surprised to find that other oaks of the red-oak group were not producing acorns. At the east end of Toby's Prairie there is a large grove of Hill's oak (also in the red-oak group), none of which were dropping acorns. And right at the edge of that grove was a single red oak dropping acorns. (The ground was covered, just under this tree.)

How do they synchronize acorn production? Not by chemical signalling, apparently.