Time to Turn?

Leaf strewn streets are as inevitable as autumn itself.  The discarded detritus of another year’s growth always ends up rasping across pavements, clogging the gutter, or chasing the wakes of cars as we speed towards winter.  The annual transformation of our trees’ canopies, from green to a spectrum of burgundies and browns, is a global spectacle. It’s surprising then, that the trigger for this landscape altering phenomenon remains something of a mystery.

The most obvious trigger is day length:  For a deciduous tree, there’s no benefit to a canopy of energy-sapping leafs in the short days of the winter months.  Better to forgo photosynthesis, avoid frost damage, and discard your leaves.  But it’s not the shortening of autumn days that triggers the transition; if it were, a cascade of autumnal colour would flow from the north every year, turning the leaves of Maryland and Madrid simultaneously, alongside other trees sharing the same latitude.

The real trigger is tied to the weather, though our understanding extends little beyond that.  Temperature is often cited as the crucial event: An average temperature around 11^oC, or a temperature dip to 7 or 8^oC, have been suggested as the signal that makes trees turn.  Summer temperatures have been scrutinised too, to try and decipher autumn’s patterns.  The results are mixed:  a warm June turns leaves early, whilst a warm August or September keeps canopies greener for longer.  But when leaves do start to turn, a warm autumn speeds the process up.  And then there’s the influence of ground moisture, where it’s suggested that a healthy water supply (affected by soil type, terrain, and the surrounding vegetation) might prolong the life of leaves.

Confused?  You’re not alone.  Deciding what makes trees turn is a tricky task.  Part of the problem is how we define the life of leaves; do the first few discoloured specimens signal a tree on the turn?  Or is it perhaps the point when half the canopy comprises gold, brown, or red?  And then there’s the problem of species:  Different types of tree discolour at different times, perhaps even responding to different triggers.

Behind the myriad of studies and suggestions, three things are clear: A warm September will probably stave off autumn’s impressive display; weak winds will prolong the spectacle; and the real beauty isn’t in the science, it’s in the leaves, and on the trees, and rasping along a pavement near you.

Image: The remnants of summer cling on, in Shrewsbury, England.

Text and image copyright: Christopher Lee, 2011 - First published 13^th October 2011.