What the Wind is Shouting to the Trees
The mystery of why leaves take such different shapes is closer to being solved thanks to a new mathematical model that looks at the problem from the perspective of leaf veins. Since plants suck up more of the greenhouse gas carbon dioxide than anything else on the planet, understanding leaf veins is an important part of grappling with the global carbon budget puzzle.
"Across the world leaves take a very large amount of carbon out of the atmosphere each year," said Ben Blonder a doctoral student at the University of Arizona. Leaves absorb more than the oceans and about 10 times more than the amount humans put into the atmosphere. "To understand the carbon flux, you have to understand how leaves work, Blonder told Discovery News. "But not all leaves work the same."
There are basically three things at play in the workings of a leaf: the amount of carbon required to make it, how long the leaf lives and how fast or slow it processes sunlight -- or its rate of photosynthesis. These factors combine in different ways in different plants in different environments to create an incredible diversity of leaf shapes and structures. And veins are the basis of it all. "The really surprising thing is that these things relate to each other in ways that don't change across the globe," Blonder said.
Blonder developed a mathematical model to predict how leaves are balancing these three factors to best serve a plant, using three properties seen in the vein networks of leaves: density, distance between veins and the number regions of smaller veins that resemble capillaries in humans, referred to in this case as loops.
Vein density is a sign of how much a leaf has invested in the network. The distance between the veins is a measure of how well the veins are keeping the leaf supplied with water and nutrients. The number of loops shows how resilient a leaf is and is related to how long a leaf lives, since loops provide ways to re-route supplies in the case a leaf gets damaged.
Veins tell you a lot about a plant. For example, if a plant opens its leaf pores, called stomata, to absorb more carbon dioxide for photosynthesis, the leaf also loses a lot of water to evaporation. That requires lots of plumbing in the leaves to pipe in the water. That, in turn, means lots of big veins. If a plant requires lots of water all the time, it could favor certain geometrical arrangements of veins, which starts to suggest overall leaf shapes. So it's the veins -- the skeleton of the leaf -- which determine whether you will have a classic maple shape or a blade-like willow.
"Veins do all sorts of things," said Blonder. They provide structural support, resist damage, transport nutrients and even help send chemical signals to the plant, similar to nerves in an animal. "There are trade-offs for leaf patterns," he added. "What we've been able to do is synthesize these things so they all make sense on one big picture."
Blonder tested his model -- predicting relationships among photosynthesis rates, leaf lifespan, carbon cost and even nitrogen costs -- on more than 2,500 species worldwide. It worked. Then he and undergraduates assistants Lindsey Parker, Jackie Bezinson and David Cahler tested 25 leaves from the University of Arizona campus. Their initial results suggest the model works on a local scale, although they are expanding their tests to study leaves from species at the Rocky Mountain Biological Laboratory in Colorado. Blonder and his students have published their work on veins in the journal Ecology Letters.
Ultimately, a good understanding of leaves will become incorporated into climate models. This can help not only balance the carbon budget, but also predict evaporation rates and other weather and climate-related matters that are heavily reliant on plants. "It's of fundamental importance to understand how plants relate to global carbon cycles," said Blonder.
Climate change could likely lead to more wind-fallen trees and damaged forests, say researchers studying how forests grow, mature then fall apart in big wind storms. The connections to climate change are easy to miss, however, because trees often live longer than humans.
“When you think like an ecologist and you realize that the lifespan of trees are multiples of human generations, you see that we often miss things,” said forest researcher Steve Mitchell of the University of British Columbia in Vancouver. He's also the editor of the journal Forestry, which had a special edition on wind this month.
If you know what to look for the story of wind storms and the damage they can do, it's there in the forests Mitchell said. “Wind storms leave a signature behind.”
That signature reveals a lot about the surprisingly profound and under-appreciated role winds play in forests. With more intense wind storms expected with global warming, the story of the wind in the trees is only going to get more dramatic.
That story goes like this: when a fire or a wind storm knocks down a forest, new saplings sprout to quickly take advantage of the open space and sunlight. But competition for that light forces trees to grow tall and slender. As the forest matures, trees have brushy upper canopies with long lever-like trunks and shallow root systems – a recipe for trouble when a big storm comes along and blows on the crowns of the trees.
Trees growing together in this manner are particularly vulnerable to the wind, says Mitchell, because the trees shelter each other. When a few trees fall, more wind penetrates the stand to reach taller, even more precarious trees, and causing more to fall.
“We have wind disturbances that are so severe that entire stands are replaced,” said Mitchell. “This is very much the same logic as the disturbance regimes of fire.”
In places where trees are deliberately planted and grown for lumber, such wind storms are an economic catastrophe. Which is why foresters and climatologists in Europe, in particular, are already looking into the details of at how climate change could increase wind events -- which, along with snow loading and other things -- can mess up a lot of trees in very short order.
Climate models for Northern Europe, show that both mean and extreme wind speeds will increase in the southern and eastern parts of Northern Europe, according to the Finnish Meteorological Institute's Hilppa Gregow and her colleagues in a recent paper in the International Journal of Climatology.
“The change over the ocean is pronounced already in 2046–2065, over the continents in 2081–2100,” Gregow reports. Similar changes can be expected in coastal parts of the U.S. and Canada, said Mitchell, which is why the people responsible for managing forests are starting to pay more attention.