tree height and climate
August 16, 2014

How And Why Does Maximum Tree Height Vary With The Environment?

April Flowers for - Your Universe Online

Trees species range in size from the Dwarf Willow at 2.5 inches to the coastal Redwood which averages 50 feet, but can top out at over 300 feet. As the coastal Redwood shows, even within one tree species, there can be a wide range of sizes. What limits the height, even within a single species? Different theories attribute this range to the amount of photosynthetic energy allocated to new leaf production, and the ability to hoist water hundreds of feet into the air to supply the solar-powered energy factory in those leaves.

The theories are not mutually exclusive, they might both play a role. A scientific debate has arisen over which factor, or combination of factors, might actually set the maximum height, and how their relative importance varies in different parts of the world.

A new study led by the University of Wisconsin-Madison uses the variations of tree height, resource allocation and physiology related to climate in Victoria, southeastern Australia, to settle this debate. The findings, published in Ecology, focus on the Eucalyptus species, which exhibit nearly the entire global range of height in flowering trees - from four feet to more than 300 feet.

"Since Galileo's time," Thomas Givnish, a professor of botany, explains, "people have wondered what determines maximum tree height: 'Where are the tallest trees, and why are they so tall?' Our study talks about the kind of constraints that could limit maximum tree height, and how those constraints and maximum height vary with climate."

The tallest flowering tree species in the world is the Eucalyptus regnans, commonly called the mountain ash in Australia (it is unrelated to the smaller mountain ash in the US). The tallest living E.regnans is 330 feet tall and can be found in Tasmania, a particularly rainy part of southern Australia. There is some debate over the world's tallest tree over all. The two final contenders are General Sherman, which according to the Guinness World Records is 271 feet tall, and Hyperion, which according to the researchers who have found it and will not reveal the location, is 379 feet tall. Both are Coastal Redwoods located in California, US.

High rainfall, high humidity, and low evaporation rates are factors that southern Victoria, Tasmania and northern California all share, illustrating the need for moisture to supply ultra-tall trees. The research team, which included Graham Farquhar of the Australian National University, shows that moisture alone cannot be the only factor to explain maximum tree height.

The team identified other factors that affect tree height. For example, evaporative demand helps to determine how far a given amount of rainfall will go toward meeting a tree's demands. Faster evaporation is caused by warm, dry and sunny conditions. The researchers found a tightly linked association between maximum tree height and the ratio of annual rainfall to evaporation in old stands of Eucalyptus in Australia.

The remaining factors include soil fertility, frequency of wildfires, and the length of the growing season. The taller a tree is, the more sunlight can be accessed to create energy through photosynthesis. They are also more attractive to pollinators and can outcompete other species for resources.

The researchers found that “infrastructure” - factors such as wood and roots — are essential to growth but do not contribute to energy production through photosynthesis. They do, however, affect resource allocation, and explain the importance of the ratio of moisture supply to evaporative demand.

"In moist areas, trees can allocate less to building roots," Givnish says. "Other things being equal, having lower overhead should allow them to achieve greater height. And plants in moist areas can achieve higher rates of photosynthesis, because they can open the stomata on their leaves that exchange gases with the atmosphere. When these trees intake more carbon dioxide, they can achieve greater height before their overhead exceeds their photosynthetic income."

The team understood that drier climates should increase the constraints of both resource allocation and hydraulics on tree height, but they wanted to understand how important each constraint was.

To do this, the team measured isotopic composition of carbon in the wood along the intense rainfall gradient in the study zone. The carbon isotope composition should not vary if hydraulic limitation alone sets maximum tree height limits, because all trees should grow up to the point at which hydraulics retards photosynthesis. The same is true for resource allocation. The isotope composition would remain stable because resource allocation does not directly affect the stomata.

If, however, both factors limit tree height, there should be an accumulation of heavier carbon isotopes should be seen in moister areas. In these areas, faster photosynthesis is enhanced by wide-open stomata and can balance the costs of building more wood in taller trees.

The research team found the heavier accumulation of isotopes, indicating that hydraulic limitation more strongly constrains maximum tree height under drier conditions. In contrast, resource allocation has more control under moist conditions.

The majority of tree studies have focused on finding the tallest trees, and explaining why they grow where they do. According to Givnish, "This study was the first to ask, 'How does the maximum tree height vary with the environment, and why?'"


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