Why do some trees hold onto dead leaves?

26 Oct 2015

Most deciduous trees jettison their leaves in autumn as Chlorophyll turns to Xanthophyll, i.e. green to yellow, (coincidentally this won me a session of University Challenge against my wife a couple of weeks ago 5 points to 4!)

 

But why do trees such as Beech and Hornbeam retain leaves? This is all to do with a physiological condition referred to as ‘juvenility’ where young trees, typically below two metres in height, retain their withered dead leaves to stop predation from deer and rabbits which hate the texture of old dry leaves which protect the young juicy buds beneath. Above this two metre line the trees have no need to protect themselves (not many Giraffes in the UK!), so leaves are jettisoned as normal as the tree loses its juvenility. This is why beech and hornbeam hedges retail their leaves and specimen mature trees jettison them. By routinely cutting these as hedges below 2 metres the plants are kept in their juvenile state, so retaining their dead leaves which get pushed off the tree with emerging new growth in the spring.

A fine example of Charles Darwin’s natural selection evolution theory that also interestingly shows this is Holly, Ilex aquifolium. We all know that holly leaves are prickly but if you look up a maturing tree you will see that the leaves get progressively smoother as the tree has no need to protect itself from grazing animals from about 2 metres upwards. Why bother with expending energy producing spikes if they are not needed?

Deciduous trees are random. You can plant an avenue of trees and they could defoliate in the autumn in different weeks or all at the same time. I’ve found over the years that if you try and second guess nature you will be proved wrong most of the time! Pleached trees on a frame over two metres are a law unto themselves. The pruning triggers all sorts of responses. Even through the tree should be too tall to be in a juvenile state some will still show signs of it while others in the same line do not. This is why you can get a random tree in a pleached row look different to others this time of year. Nothing is necessarily wrong with a tree acting out of sync to its neighbour, it is just wired differently. I can empathize with this, having two teenage daughters!

Some trees have defoliating triggers when planted in leaf. Most deciduous trees planted from August onwards will cast off their leaves earlier than established trees nearby but settle down in the following years. London Plane, Platanus hispanica, hates being planted in the summer and this mostly triggers a complete defoliation within two weeks after planting only to re-flush later on if time allows in the growing season. Otherwise they generally come back good as gold the following spring.

All this is not a precise science, just sit back and enjoy the show! Autumn has got to be one of the most stunning time for trees.

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