How many seeds does an 'average' tree produce ?
I've often been asked the simple question "How many seeds does an average tree produce?" It has always been an embarrassment to me as a scientist to answer that I am unaware of any scientifically authenticated data on the subject.
Not surprisingly, any answer will depend on the species / size / age of tree; whether it's a good, bad or average crop year; and of course the size of the fruits - for example, an acorn is ca4000 heavier than a birch seed (4g : 0.001g), therefore similarly sized oak and birch trees are unlikely to bear the same numbers of fruits.
So, in the absence of any truly scientifically authenticated data, here are some figures for oak & birch in what I would call a good crop year (the assumptions used to "guesstimate" the figures are explained below) :
- Question: How many seeds does an oak tree produce ?
- Answer: In a good year, between 70,000 - 150,000 acorns per tree.
- Question: How many seeds does a birch tree produce ?
- Answer: In a good year, between 15 - 17 million seeds per tree.
So how do these figures for oak and birch look on a lifetime basis?
Lets say a typical oak tree survives for 500 years and produces the equivalent of a good crop every 5 years from age 50. That's 90 good crops over 450 years = 6.5-13.5 million acorns.
- Question: How many seeds does an oak tree produce over it's lifetime?
- Answer: About 6.5 - 13.5 million acorns.
And for birch, lets say a typical birch tree lives for 50 years and produces the equivalent of a good crop every other year from age 20. That's 15 good crops over 30 years = 300million to 2.25billion seeds.
- Question: How many seeds does a birch tree produce over it's lifetime?
- Answer: About 300 million to 2.25 billion seeds.
Clearly, a single tree of both these species produces astronomical numbers of seeds throughout its lifespan. But of the millions, hundreds of millions or billions of seeds produced throughout the lifespan of a single tree, the survival of one offspring to adulthood will ensure the survival of the species for another generation. So picture the consequences of two offspring surviving per generation - its rather like the mathematical conundrum of doubling the number of wheat grains on successive squares of a chessboard, (an illustration of exponential growth) - it doesn't take very many generations to overwhelm the planet !
Oak assumptions:
1. The BBC web site http://www.bbc.co.uk/nottingham/sense_of_place/facts/major_oak.shtml reports that:
A 'good' crop of acorns for the Major oak (a famous tree in Sherwood Forest, Nottingham) is ca 150,000 acorns.
(This tree is 800-1000 years old with a trunk that is 10ft in diameter).
2. In 2005, a member of the public emailed me to say that her local radio station had reported a farmer who physically counted 72,744 acorns on a single oak tree.
(I subsequently learned this was a 250 year old tree with a 5 ft diameter trunk)
Are these figures believable?
Well, an average acorn weighs ca4gm, so there are ca 250 acorns per kg. Therefore 72,744 acorns are equivalent to 290kg and 150,000 acorns are equivalent to 600kg. I have little hesitation imagining a medium sized oak tree producing ca300kg of acorns and it is not inconceivable to me that such a large and ancient tree as the Major oak might produce ca600kg. I'll accept 70000 - 150000 acorns per oak tree in a good year.
Birch assumptions:
I haven't discovered any published information on the numbers of birch seed per tree, but in one of my former existences (working for the British Forestry Commission), I once obtained figures from an experienced seed collector who 'guesstimated' that they collected between 10-50 kg of birch seeds per tree. Since there are ca1.5 million seeds per kg of birch this equates to between 15 to 75 million seeds per tree.
