We’ve had yet another unusual spring. April and the early parts of May have been
dry and sunny with no April showers or strong winds. This has meant that the blossoms have been
spectacular and long-lasting. It seems
that everything that can flower has flowered better than ever. We’ve even had four bluebells come up in our
lawn/meadow that we didn’t plant and have never seen since we moved here. The bluebells in the field at the end of
Peter James Lane have been wonderful as well.
One thing that you may not have noticed flowering are the oak
trees. Firstly, oak tree flowers are
small, green and inconspicuous, and secondly they don’t flower every year –
every 3 to 5 years is typical. It has
been found that oak trees in a forest all flower at the same time. Beech trees do the same thing. Apparently, the trees communicate using nerve
impulses and chemical signals via their roots and the symbiotic fungi with
which they swap nutrients. The nerve
impulses travel at one third of an inch per minute – slow, but maybe not so
surprising in an organism that lives many hundreds of years. This is all set out in a brilliant book
called ‘The Hidden Life of Trees’ by Peter Wohlleben, a German forester. And it’s not mumbo-jumbo pseudo-science
either, everything is properly referenced to the appropriate scientific
literature.
There are good evolutionary reasons for this sporadic
flowering of the trees (the years that they flower are called ‘mast years’
after the abundance of beech-mast) – firstly, flowering and producing fruit is
expensive in energy terms and diverts resources from growing wood and leaves that
would be used to harvest more energy from the sun. Secondly, producing acorns or nuts every year
would support a population of deer and pigs that would eat them all leaving
none to grow into seedlings. If all the
trees in the forest produce acorns at the same time then there will be too many
for the deer population to eat, especially a population that has been thinned
by starvation or migration during the non-flowering years. This is mitigated to some extent by the jays
and squirrels that try to eke out the harvest by burying acorns or beech nuts,
something that is helpful to the trees, whereas a reduced population of jays
and squirrels is not so helpful. Evolution,
though efficient is often imperfect.
This sporadic flowering of oak trees got me thinking about
the knopper gall-wasps (Andricus
quercuscalicis), whose bizarre life-cycle I wrote about in November
2014. To recap briefly, the female
gall-wasp lays eggs in the male flowers of the turkey oak; males and females
hatch from the oaks, mate, and the females go and lay eggs in the female
flowers of the English oak. The
generation that hatches from the English oaks are all parthenogenetic females
that reproduce asexually and go on to lay eggs in the male flowers of the
turkey oak and so on. This made me
wonder what happens to the gall wasps when there are no oaks in flower. (The
gall wasps are not pollinators of oaks, or predators of species that do pollinate
oaks because oaks are wind pollinated and don’t produce nectar to attract
insects (hence the inconspicuous flowers).
I had assumed that the gall-wasps
life-cycle was annual, but maybe that isn’t the case. So how
do the gall-wasps survive when the oaks
don’t flower – do they stay in their galls, if so, how do they know when to
emerge? Or can they survive using the
isolated oaks that cannot communicate with their neighbours and flower out of
synch? Do turkey oaks communicate with
English oaks? Is the alternate sexual
and parthenogenetic generation life-cycle of the gall-wasp an evolutionary
response to the sporadic flowering of the oaks? Nature is wonderful, but sometimes it just
makes your head hurt.
