Background photo - Junipers in the gardens of Le Corbusier's celebrated "L'Unite" building, Marseille, France 2008
July 2012 - Leaf strategies
Walking around the loch at Drumpellier Country Park at the weekend I encountered two trees beside each other of about the same size. On the right is a leaf from the first of them, Populus tremula known as the Aspen or Trembling Poplar. It is a native broadleaf tree, and the name hints at the way its leaves seem to tremble, even in the slightest breeze. The long leaf stalk is flattened and flexible, and the trembling in the wind is an aerodynamic function of this and the indented margin and the relative stiffness of the leaf.
On the left is a leaf from the second tree, Alnus glutinosa, the Common Alder, also a native broadleaf tree. The leaf is about the same size and shape as the Aspen's with a more or less indented margin. But the leaf stalk is short and relatively stiff and the leaf fairly floppy. In the wind the leaf gets flattened against or wrapped around the twigs.
We can only guess at why two nativ trees with similar leaves have different strategies for reacting to wind. And of course these trees might not normally be right next to each other in nature, they have been deliberately planted at Drumpellier. Alder in nature is found in wet sites close to rivers where its specialised roots can tolerate water in a way that few other trees can. In nature the Aspen can be found in similar but less extreme situations but also amongst crags and by gorges. It is tempting to assume the trembling is just a way of dealign with more exposed locations. But there may be other reasons such as making the leaf less hospitable to parasitic moths and insects. The stiffness may come from a thicker cuticle to avoid excess water loss in breezy locations but in contrast the trembling on even a slight breeze may ensure a minimum degree of cooling and gas exchange.