It's tough at the top of the world's tallest trees

This is the second (and last) of my holiday posts from North America, in this case researched a little before I left Australia. It's a long one, pictorially, due to the elongated plant subject. I've traveled across the country (by plane), from giant topiary in New York last week to giant trees in California today. From the ridiculous to the sublime.

Back in 2005 when I wrote an article for Nature Australia magazine called 'Size matters', I reported that the world’s tallest plant was a 112.7-metre-high Coast Redwood growing in Humboldt Redwoods National Park, California. I think it still is the tallest but presumably plus or minus a metre or two now. It grows a few hundred kilometres north of where I saw and photographed this Giant Redwood.

Just for the botanical record, the Coast (or Californian) Redwood is Sequoia sempervirens, the only species in this genus. A closely related and equally 'big tree' is called the Giant Redwood, Sequoiadendron giganteum, and it is also the only species in its genus.

Despite its common and botanical names, the Giant Redwood doesn't grow quite as tall as the Coast Redwood, but it achieves bigger bulk - that is, the volume of its trunk is the largest in the world (the Montezuma Cypress, Taxodium mucronatum, in Mexico has the largest girth - over 15 metres in diameter). Both Redwood species frequently top 100 metres in height but the tallest conifer ever recorded was a 126-metre Douglas Fir, Pseudotsuga menziesii.

All these conifers are clearly big trees. Flowering trees, like our gums, also grow tall of course. Although there are claims of Mountain Ashes (Eucalyptus regnans) in southern Australia being over 120 metres, the tallest ever measured officially was 107 metres. Today the tallest living specimen is I think still a 97 m tree called ‘Icarus Dream’ in the Styx Valley in Tasmania. It's quite possible this tree will reach 100 m over the next few decades.

But don't expect it, or even the Coast Redwood, to ever get to 130 metres. As I reported back in 2005, what limits their size is a plumbing problem: how do you get water from the ground to a leaf fluttering 100 or so metres above?

George Koch from Northern Arizona University, and his associates, reckon we'll never see a redwood over 130 metres tall. They climbed the world’s tallest trees to measure water potential and photosynthesis in the highest branches, followed by more detailed analysis of leaves transported back to the laboratory. Water is drawn up the tree (what we call transpiration) in a continuous column as it evaporates from leaves into the atmosphere. They found that gravity starts to win out against water cohesion at around 110 metres.

It turns out the leaves most distant from the base of a gigantic Redwood such as this one are under extreme water stress, and their small size and low photosynthetic rates may be due to the plant closing some of its breathing pores (stomata). This would not only retain precious water, but also slow down the rate of water transport through the plant, reducing the possibility of deadly air bubbles being formed—a break in the water chain would be permanent and mean death for a lofty limb.

They also found that to keep one of these big trees alive and transpiring healthily, the surrounding forest must remain intact to maintain high moisture levels and buffer the trees against storm damage. So if we want to see big trees in California, or in Australia, look after the forests that surround them. The recent drought in California is putting some of these big trees under additional stress, so doing what we can to reduce the severity and impact of climate change will also help our botanical giants survive.

Images: All from Mariposa Grove, at the southern end of Yosemite, taken last Wednesday (I'm now back on Melbourne). Lynda very kindly photographed the top picture.