What Causes Trees to Grow So Tall? Link Between Height and Climate

How tall do trees grow and what causes them to go? That's a good question and now, scientists have found that climate may be the main limiting factor when it comes to the height of trees.

Both resource allocation and hydraulic limitation can play a role when it comes to tree growth. But many scientists have been puzzles when it comes to finding out which factor, or what combination, actually sets maximum tree height. They've also wondered how their relative importance varies in different parts of the world. That's why researchers decided to look a bit more closely.

The researchers studied how tree height, resource allocation and physiology vary with climate in Victoria state in southeastern Australia. There, Eucalyptus species exhibit almost the entire global range of height among flowering trees, stretching from four to 300 feet.

Southern Victoria, Tasmania and northern California all share high rainfall, high humidity and low evaporation rates. Yet this study reveals that rainfall alone can't explain maximum tree height. A second factor, called evaporative demand, can actually determine how far a given amount of rainfall will go to meeting a tree's demands. For example, warm, dry and sunny conditions cause faster evaporation from leaves, and there's a tight correlation between maximum tree height and the ratio of rainfall to evaporation. As the ratio increased, so did the maximum tree height.

Scientists also found that other factors like soil fertility, the frequency of wildfires and length of the growing season could also affect tree height. Yet in the end, the researchers found that hydraulic limitation more strongly constrains maximum tree height under drier conditions. In contrast, resource allocation more strongly constrained height under moist conditions.

The findings reveal a bit more about why some trees can grow so tall while others remain short. This, in turn, could give researchers a bit more knowledge when it comes to enterprises like using trees for carbon capture and storage.

The findings are published in the journal Ecology.