June 2017

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.