That got me curious so I looked it up. Here is what someone in Britain said concerning their low acorn yield...
"oak trees (and many other trees) work on cycles where some years they produces lots of acorns and some years very few and some years somewhere in between. It is thought to be an evolved anti herbivore strategy I believe, it prevents a population of squirrels or rodents of other acorn munchers from growing to a size where it always eats all the acorns. - By having starve years this then causes the acorn munchers populations to decline meaning that there's fewer of them to eat acorns if the tree produces a glut in the subsequent year.
Interestingly all the trees in a woodland seem to follow the same cycle so presumably it is climatically influenced or Terry Pratchett has it spot on and tree conversations are just too slow for us to hear.
Apologies if this is a slightly ropey description, it was something that I was taught at Uni which was a good 11 years ago now!"
Found a cool article from New Jersey about this too...
http://www.nj.com/homegarden/garden/index.ssf/2008/11/garden_diaryburning_issue_of_t.html
"Inquiring minds want to know. Looking into this phenomenon, I find that fluctuations in acorn production are poorly understood, even by tree scientists. It's no wonder the rest of us are at a loss.
Data shows that the crop varies from year to year, species to species and region to region. While some here are seeing a lack, California homeowners in Walnut Creek are being driven to extremes this fall by abundant acorns -- or rather, by the woodpeckers that are stashing the bumper crop in the exterior woodwork of their homes. "