Why are leaves different sizes?
Autumn brings one of nature's most spectacular displays in the turning of leaf colours, triggered by the tree drawing back resources as it prepares for winter dormancy. As you marvel at the blaze of colour, think for a moment about the huge variation in the shapes and sizes of the leaves and ask...why?
Globally, plant leaf sizes vary from a tiny 1mm2 to a massive 1m2 – a million-fold difference that some of the earliest ecologists noted and attempted to explain. Over a century ago the theory was put forward that leaf size varies with latitude, which to some extent is true. But a walk through any woodland reveals a wide range of coexisting leaf sizes: latitude is clearly not the only variable in play.
Credit: WTML / Michael Cooper
Factors controlling leaf size
A recent study, published in Science, gathered together data from 682 sites around the world to compile measurements for 7670 species. The statistical analysis revealed a complex picture, with upper and lower limits on leaf size constrained to varying degrees by daytime and night-time factors. Daytime constraints included factors such as temperature and availability of water and light, night-time constraints were mainly related to colder temperatures.
Leaves are a tree's powerhouse, the place where photosynthesis takes place, the cascade of enzymes and light that creates the sugars that allow the plant to grow. As ever, a balance is struck between competing tensions: absorbing enough heat for efficient chemical reactions without becoming too hot, having enough ground water to cool down through transpiration, not being susceptible to frost damage – the list goes on. Even a plant's position in the vertical hierarchy of woodland, from ground flora to canopy species, will determine the precise conditions of light, temperature and humidity experienced.
The study's authors claim that the new, more detailed model will help scientists to better predict the likely future ranges of plant species as the climate changes. Such studies require data from multiple sources including our very own Nature's Calendar, which is the UK's longest running phenological dataset, made all the more impressive by the fact that the observations are recorded by members of the public.
Autumn is a great time to be in woodlands, with the variety of tree colours, animals busily preparing for winter, the fungi and fruit. Taking part in recording the timing of natural events will help scientists to understand how the changing climate is going to affect our wildlife, so why not combine your woodland walk with a bit of citizen science?