Science - February 18, 2011

Tough leaves live longer

Toughness is the most essential mechanical trait for a leaf. This conclusion is reached by Wageninger Lourens Poorter and a group of international scientists in an article to be published next month in Ecology Letters.

Hungry caterpillars finished eating this leaf in one day: a soft and short-lived leaf of the Cecropia concolor, a light-loving tree species in the Bolivian rain forest.
The group, under the leadership of Yusuke Onoda from Japan, placed many studies into the mechanical traits of leaves next to one another, including a study by co-author Poorter from Wageningen. The most common methods of measuring the mechanical traits of leaves are cutting, tearing and punching. These have produced a sea of data, definitions and categories. A way has now been found to handle these by using a global meta-analysis in which the details of a total of 2819 different leaf species can be compared. The group has also unravelled the different components which contribute to leaf strength.
Such work has already resulted in noteworthy insights. The leaf strengths of different species of leaves can vary up to a factor of 800. These variations are found mainly in the levels of toughness. Leaves are strong not because they are thick or have a high tissue density, but because of their toughness. According to the researchers, toughness accounts for 56 percent of the 'mechanical resistance' of a leaf. And that seems to be the case all over the world. However, nothing yet has been said about the causes of this toughness. The article in Ecology Letters will go into this aspect. 
According to Poorter, toughness is related to the stringiness, the vein pattern and the chemical composition of leaves. Poorter published a study last year in the New Phytologist about his research into leaves of tropical trees in Bolivia. There, too, toughness is a crucial factor. Poorter: 'Toughness is a very good predictor of the life-span of a leaf, how tasty it is for leaf eaters and whether a tree variety stands in the light or the shade.' 
Poorter says that the mechanical traits of leaves have a lot of influence on the success and the co-existence of species in nature. Toughness determines where each species can be found. Poorter: 'For example, only one percent of the light can penetrate to the understory of tropical rain forests. There is very little photosynthesis there and therefore, very little carbon absorbed per day. To obtain a yield from this in terms of new leaves, you would need leaves with a long life-span.' Toughness increases the chances of long life, such as by making leaves less attractive to leaf eaters. That enables leaves to be successful in the shadow. On the other hand, fast growers with not so tough leaves can be found in open spaces with much sunlight.
Yonoda et al. (2011) Ecology Letters 14(3)