My father planted acorns.

This is one of those recollections that arrives many years after the fact and suddenly strikes me as having been unusual. As a child, however, it seemed perfectly normal that we should go out collecting acorns in the autumn. Compared to my father’s other eccentric habits and hobbies, of which there were many*, gathering acorns didn’t appear to be particularly strange or worthy of note.

In our village during mast years the acorns would rain down from boundary and hedgerow oak trees and sprout in dense carpets along the roadside. This brief flourishing was inevitably curtailed by the arrival of Frankie Ball, the local contractor responsible for mowing the verges. His indiscriminate treatment shredded a summer’s worth of growth and ensured that no seedlings could ever survive.

Sprouting acorn (Quercus robur L.) by Amphis.

Enlightened modern opinion would now declare that mowing roadside verges is ecologically damaging; it removes numerous late-flowering plants and destroys potential habitats for over-wintering insects. I’m not going to pass such judgement here though because it was a purely practical decision. Too much growth would result in blocked ditches, eventually flooding fields and properties. Frankie was just doing his job.

My father, however, couldn’t allow himself to see so many potential oak trees perish. His own grandfather had been a prominent forester back in the old country (one of the smaller European principalities that no longer exists), and with a family name like Eichhorn it’s hard not to feel somehow connected to little oak trees. He took it upon himself to save as many of them as he could.

And so it was that we found ourselves, trowels in hand, digging up sprouting acorns from the roadsides and transporting them by wheelbarrow to the wood on Jackson’s farm.  Here they would be gently transplanted into locations that looked promising and revisited periodically to check on their progress. Over the years this involved at least hundreds of little acorns, perhaps thousands.

They all died. This isn’t too surprising: most offspring of most organisms die before they reach adulthood. Trees have a particularly low rate of conversion of seedlings to adults, probably less than one in a thousand. That’s just one of the fundamental facts of life and a driving force of evolution. Why though did my father’s experiment have such a low success rate? He’d apparently done everything right, even choosing to plant them somewhere other trees had succeeded before**. It’s only after becoming a forest ecologist myself that I can look back and see where he was going wrong.

First, oak trees are among a class of species that we refer to as long-lived pioneers. This group of species is unusual because most pioneers are short-lived. Pioneers typically arrive in open or disturbed habitats, grow quickly, then reproduce and die before more competitive species can drive them out. Weeds are the most obvious cases among plants, but if you’re looking at trees then something like a birch would be the closest comparison.

Oaks are a little different. Their seedlings require open areas with lots of light to grow, which means that they don’t survive well below a dark forest canopy. Having managed to achieve a reasonable stature, however, they stick around for many centuries and are hard to budge. In ecology we know this as the inhibition model of succession. Oaks are great at building forests but not so good at taking them over.

The next problem is that oak seedlings do particularly badly when in the vicinity of other adult oak trees. This is because the pests and diseases associated with large trees quickly transfer themselves to the juveniles. An adult tree might be able to tolerate losing some of its leaves to a herbivore but for a seedling with few resources this can be devastating. This set of forces led to the Janzen-Connell hypothesis which predicts that any single tree species will be prevented from filling a habitat because natural enemies ensure that surviving adults end up being spread apart. A similar pattern can arise because non-oak trees provide a refuge for oak seedlings. Whatever the specific causes, oak seedlings suffer when planted close to existing oaks.

This makes it seem a little peculiar that acorns usually fall so close to their parent trees. The reason acorns are such large nuts*** is that they want to attract animals which will try to move and store them over winter. This strategy works because no matter how many actually get eaten, a large proportion of cached acorns remain unused (either they’re forgotten or the animal that placed them dies) and so they are in prime position to grow the following spring. Being edible and a desirable commodity is actually in the interests of the tree.

A Eurasian jay, Garrulus glandarius. Image credit: Luc Viatour.

Contrary to most expectations, squirrels turn out to be pretty poor dispersers of acorns. Although they move acorns around and bury them nicely, they don’t put them in places where they are likely to survive well. Jays are much better, moving oaks long distances and burying single acorns in scrubby areas where the new seedlings will receive a reasonable amount of light along with some protection from browsing herbivores. My father’s plantings failed mainly because he wasn’t thinking like a jay.

My father’s efforts weren’t all in vain. The care shown to trees and an experimental approach to understanding where they could grow lodged themselves in my developing mind and no doubt formed part of the inspiration that led me to where I am today****. From tiny acorns, as they say.

* His lifelong passion is flying, which at various points included building his own plane in the garden shed and flying hang-gliders. It took me a while to realise that not everyone’s father was like this.

** One possible explanation we can rule out is browsing by deer, which often clear vegetation from the ground layer of woodlands. Occasional escaped dairy cows were more of a risk in this particular wood.

*** Yes, botanically speaking they are nuts, which means a hard indehiscent (non-splitting) shell containing a large edible seed. Lots of things that we call nuts aren’t actually nuts. This is one of those quirks of terminology that gives botanists a bad name.

**** Although I’m very sceptical of teleological narratives of how academics came to choose their areas of study.