where are the acorns?

[steve rollnick]

Can someone answer this: we have hundreds of oak trees in out little woodland and in spring, the end of every little branchlet is covered in what looks like hundreds and hundreds of little acorns to be. There must be millions in each tree. Then come autumn, very few acorns. Yes, there is some grey squirrel around, but I cant believe that they are eating so many acorns?

What happens to the acorns?

Steve

[Stephen1]

Hi Steve

Great name by the way!

Oak is a so called "masting" species. Essentially what this means is that in most years it produces relatively few acorns. Then every so often it has a year when it produces huge amounts.

The reason for this is to escape predation. If you produce a regular supply of acorns each winter then species will evolve in the long term, and populations adapt in the shorter term, to be able to make use of that regular autumn food supply. If you produce only a few large energy rich seeds most years then the populations of creatures relying on this energy rich supply each autumn will be relatively small. In the years where you produce a huge amount of acorns, i.e. a Mast year, the populations of acorn eating beasties will still be relatively small and there won't be enough time for the seed eating beasties to expand their populations in response to all that easily available food - so in mast years large numbers of acorns escape predation.

The mast years production of acorns uses energy stored in the trees sap from the non-mast years. So basically an oak could produce 5000 acorns a year every year for five years and have almost all of them eaten each year by a population of predators that can eat the best part of 5000 a year. If the same oak produces just a few for four years then 5 x 5000 = 25000 in the fifth year then the population of acorn eating creatures will be very small and won't be able to respond to the sudden glut of 25000 acorns and so a great number won't get eaten.

What you see in most years are male flowers (the tassels) producing lots of pollen. Pollen is energetically cheap to produce compare to a big starchy seed/acorn. So from an evolutionary point of view there is still something to be gained from thowing out plenty of pollen each year and hoiping for the best. The 'hundreds of little acorns to be' that you describe seeing are what the femal flowers develop into before being ' abscised' by the tree- that is deliberatey shed by a similar active process to that of shedding leaves in autume using a specialised layer of cells.

Predicting mast years isn't perfectly understood, and a handful of books will give you a handful of theories. I believe you need a non-droughty hot sunny summer the year before followed by a dry spring and then a warm wet summer giving a 'mast' autumn that year. I wouldn't bet much on this though.

The thing is for the strategy of masting to work well (in avoiding a build up of autumn acorn dependant predators) they need all the trees in a wood to be synchronised. Nowadays most woods have a mixture of some oaks that are 'native' to that wood but others that have been planted as 'good' oaks by foresters over the years - including oaks which originally came from the continent. These means the whole synchronized masting thing has gone to pot a bit!

(Obviously those numbers are just plucked out of the air to illustrate the point!)

[steve rollnick]

Stephen,

that's one really incredible story. Truly amazing.

I cant wait to tell my 9 year-old daughter who will feels its power with all the might of a fairy tale. Thank you so much.

Now if you fancy it, what about this question: there's an urban avenue of little oaks that produce acorns every year? Its not the same species as that in our Welsh woodland. Its smaller, with dantier leaves.

bye and thanks

Steve

[carlight]

as its been a few days ,i'm going to jump in .

To continue in the evolutionary theme ,each and every offspring ought to be , potentialy , genetically variant . ( This would be how come an oak first had the trait to only set seed once every few years , a tactic that appears to be a good one - that particular genetic variation seems to be popular amongst some trees )

There's no reason why a genetically different population wouldn't find success with setting seed every year . Perhaps local seed eaters never became prolific .

On the nurture side of things , could the local enviroment be causing the urban oaks to supress their 'normal' seeding tactics . ( personally i find urban trees to be sad beings - their fruiting rather pointless - their chances of successful reproduction slim )

Anyway ,i ask forgiveness from all those that have studied these matters professionally .

For more top stories from trees , may i recommend Mr tudge's book - ' a secret life of trees ' (save the middle chapters on each family for a very very rainy day !)