The shape of leaves is a very important characteristic that helps plants cope with, and become more competitive in, different climates. For example in cooler climates, leaves tend to be shorter than in hotter climates, due to differences in growing season. In wetter climate leaves tend to be wider, but to stop water loss, narrower leaves are preferred in dryer climates. These basic principles give us recognizable leaf shapes for plants in different biomes. For example in tropical rainforests, where it is both hot and wet, plants have long, wide leaves (some even with ‘drip tips’ to cope with excess water). In alpine areas, where it is cool and wet, plants tend to have circular shaped leaves (short and wide). In dry hot areas though, leaves tend to be long and narrow; think eucalypts.
Interestingly in really dry areas, leaves may be lost altogether (or at least reduced to the point of non-function) – for example acacias where the ‘leaves’ are actually a modified stalk (known as a phyllode), and in she-oaks and cactuses the leaves have been lost and photosynthesis is performed by modified stems (known as cladodes).
Anyway, back to leaves. The shape of leaves and their relationship to climate has been known for a long time, and has been used by paleobotanists to infer what the climate was millions of years ago from the shape of fossilized leaves.
So if leaf shape is a good predictor of climate and we know that the climate has been changing recently, then we would expect that the shape of leaves should have changed in areas that have undergone significant climate change over the last 100 years. This was the hypothesis tested by Greg Guerin and colleagues in worked just published in The Royal Society journal Biology Letters. They measured the width of over 500 specimens of Dodonaea viscosa, the narrow-leaved hop bush, from herbarium and field collections dating back 127 years and found that average leaf width had shrunk by 2 millimetres, or 40 per cent.
In a reanalysis of these data, published yesterday, Greg showed that the change in leaf width before 1950 had been insignificant, but that leaves had become significantly narrower since 1980 – compared to a historical baseline. Whilst other factors (as yet unidentified!) may have led to this observed pattern, the change in leaf width is consistent with an adaptation shift in the narrow-leaved hop bush caused by rapid climate change in South Australia.
This is the first time that a change in an adaptive feature in plants has been linked to climate change, although other work has previously demonstrated such a relationship for flowering time in plants and heat stress in animals.
Guerin GR, Lowe AJ (2012) Leaf morphology shift: new data and analysis support climate link. Biology Letters. Published online 31st October, doi: 10.1098/rsbl.2012.0860
Guerin GR, Wen HX, Lowe AJ (2012) Leaf morphology shift in response to climate change. Biology Letters. Published online 4th July, doi: 10.1098/rsbl.2012.0458.