A second factor, evaporative demand, helps determine how far a given amount of rainfall will go toward meeting a tree's demands. Warm, dry and sunny conditions cause faster evaporation from leaves, and Givnish and his colleagues found a tight relationship between maximum tree height in old stands in Australia and the ratio of annual rainfall to evaporation. As that ratio increased, so did maximum tree height.
Other factors like soil fertility, the frequency of wildfires and length of the growing season also affect tree height. Tall, fast-growing trees access more sunlight and can capture more energy through photosynthesis. They are more obvious to pollinators, and have potential to outcompete other species.
"Infrastructure" things like wood and roots that are essential to growth but do not contribute to the production of energy through photosynthesis affect resource allocation, and can 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 constraints on tree height imposed by resource allocation and hydraulics should both increase in drier areas. But Givnish and his team wanted to know the importance of each constraint.
The scientists examined the issue by measuring the isotopic composition of carbon in the wood along the intense rainfall gradient in thei
|Contact: Thomas Givnish|
University of Wisconsin-Madison