Many tree species both deciduous and coniferous do essentially 
alternate between good and bad mast years via a built-in 
physiological mechanism. Although I am no expert, I think indeed 
avoidance of seed predation is assumed to be the most likely 
explanation. Weather is probably less important (though obviously 
could play some role during seasons of extreme conditions).


This topic is very well discussed for the informed non-scientist in 
ecologist Bernd Heinrich's excellent book "The Trees in My Forest". 
(author of "Ravens in Winter" and several other very "readable" 
nature books.)

Alternating years may be an evolutionary/natural selective response 
to keep populations of mast consumers (Squirrels, jays, etc ) from 
exploding by virtue of having a huge food crop available every year.

Greg Grove
Huntingdon Co, Pa.



>I looked on the web last night for more info on why there are mast 
>years. This is what I found:
>
>The question of why there are mast years (and also non-mast years 
>like this past one) is an open one. The best guess is that the 
>occurance of mast years in a species is due to a combination of 
>avoidance of seed predation and pollination of the seeds by 
>wind-bourne pollen (as opposed to insect pollination). These two 
>factors would lead to the occurance of the mast year phenomenon. The 
>production of large quantities of seed leads to predator satiation, 
>and with wind-bourne pollination, synchronizing the mast year over a 
>large area has obvious efficiencies.
>
>The question of when mast years occur is also an open one. There has 
>been no correlation found between weather and mast years. What 
>causes all the trees in a large area to produce lots of mast is 
>unknown, but if not weather, what could it be?
>
>John McKitterick
>Columbia, MD
>jbmck-at-comcast.net
>
>
>>  We saw an unusually high number of migrating red-headed woodpeckers at the
>>  Turkey Point Hawk Watch this past fall, along with hundreds and hundreds
>>  of blue jays, so maybe they all did leave for better foraging grounds.
>>
>>  It would make sense that an oak's production of acorns would vary from
>>  year to year. If they do depend on a dry spring season, then having had
>>  two very wet springs in a row may have resulted in the low yield last fall.
>>  We had a mast year around here just a few years ago--I can't remember
>>  if it coincided with a bad drought or not.
>>
>>
>>  --Pat
>>
>>  Pat Valdata, Elkton, MD | 
>>  "The natural function of the wing is to soar upwards
>>  and carry that which is heavy up to the place where dwells the race of gods.
>>  More than any other thing that pertains to the body
>>  it partakes of the nature of the divine." --Plato


-- 
Greg Grove
Instructor, Biochemistry and Molecular Biology
Penn State
267 North Frear, University Park, Pa. 16802
814-865-2231