Wednesday, October 26, 2011

Why trees aren’t taller

The effect of height-induced drought stress on redwood foliage. From Koch et al. 1994.

The tallest tree in the world is about 120 m. One of the most basic questions we have about trees is whether this height represents the tallest possible tree. Are there some fundamental physical constraints that make growing much beyond this height impossible? Or could we grow a 200 m tree?

In 1997, Ryan and Yoder wrote a Bioscience article “Hydraulic limits to tree height and tree growth”. There, they reviewed 4 hypotheses regarding the limits to tree height. In short, they ruled out that as trees get taller their respiration might become too high, nutrients too hard to acquire, or genetic changes associated with maturation (they get too old) limits their growth. These might come into play, but are only contributing factors.

The hypothesis that was left was hydraulic limitation—it’s just too hard to move water much higher. Here, as trees grow taller, the length of xylem from root to leaf increases. Water flow is a function of the ratio of the difference of water potential and resistance. As tree height increases, resistance to water flow increases requiring lower (more negative) water potentials to move water to the top of the tree. As water potentials decline, xylem at the top of the tree is closer to the point of cavitation. Once the string of water snaps at the top of the tree, it’s hard to get water back up there and that part is dead. To be safe, leaves at the top of the tree close their stomata more frequently, which limits carbon gain. Less photosynthesis slows growth, generating a maximum height.

The evidence at the time for this hypothesis was that stomata in any leaf will close if hydraulic resistance increases, hydraulic resistance increases for older trees, and photosynthesis is reduced in older, taller trees.

They end the 1997 review by saying “we may be close to answering some of our oldest questions about tree height.”

Move forward to 2004. Koch et al. studied the tallest tree known on earth, a 113 m redwood in N California. They showed that as one moved progressively up the tree, water potentials declined, photosynthesis declined, and leaf WUE increased as stomates were closed more frequently. Everything fit the hydraulic limitation model.

Yet, when you go to the top of a redwood tree, the water potentials aren’t that low. It only takes 1 MPa to overcome gravity and move water 100 m. Moving water to the top of the redwood tree takes only -2 MPa due to greater resistance in redwood wood. They argue that at this water potential, photosynthesis is essentially zero for the redwoods, which explains why redwoods aren’t much taller.

But it doesn’t explain why other trees that can photosynthesize at tensions below -2 MPa couldn’t build a taller tree.

Subsequent work seems to reinforce this idea. In 2008, Domec et al. assessed xylem design for 85-m tall Douglas fir trees. There, they showed that with increasing height, Doug fir branches had greater resistance to water movement (less efficient) but could with stand greater tensions (more safety). But still, the water potentials at the top of the theoretically tallest Douglas fir (~130 m) did not push the ultimate bounds for plants.

The authors concluded “Mechanisms governing ultimate tree height must be considered in an evolutionary context, and so it is unlikely that the tradeoffs discussed here are identical to those of all other species. A number of coniferous species adapted to arid and semiarid zones can maintain adequate water transport at substantially greater xylem tensions than those normally experienced by the mesic-environment species Douglas-fir and coast redwood.”

Ultimately, the question of tall trees becomes an evolutionary question. Could nature build a 200-m tree? The current limits to tree height might be evolutionary, not physical. If you built a tree with the same plumbing as a drought-tolerant shrub, a 200-m tree might be possible. 

Domec, J. C., B. Lachenbruch, F. C. Meinzer, D. R. Woodruff, J. M. Warren, and K. A. McCulloh. 2008. Maximum height in a conifer is associated with conflicting requirements for xylem design. Proceedings of the National Academy of Sciences of the United States of America 105:12069-12074.
Koch, G. W., S. C. Sillett, G. M. Jennings, and S. D. Davis. 2004. The limits to tree height. Nature 428:851-854.
Ryan, M. G. and B. J. Yoder. 1997. Hydraulic limits to tree height and tree growth. Bioscience 47:235-242.

1 comment:

  1. 200 meters, makes you wonder...
    I think we could have had trees that tall at some point in the prehistoric past.

    The 2008 study by Domec et al estimated that Douglas fir could grow to between 109 and 138 m, but the maximum upper limit range was 131-145 meters with 95% confidence.
    I think some Douglas fir of this height existed right up to the historic period--although they were rare. In Feb. 10, 2009, I unearthed a news report from 1897 of a 142 m Douglas fir that was cut down at the Nooksack river, Whatcom County, Washington State. The tree was 480 years old, and 10.5 m circumference at the base, and 67 meters to the first branch. The tally of wood scaled 96,345 market board feet, enough lumber to build 8 two story homes. An old photo of the cross section of this tree exists in the Whatcom museum with a placard listing the dimensions (Seattle Times, Sep 4, 2011):

    But even if 142 m is hard to believe, there are credible measurements of fir trees 100 to 127 meters in BC, and Washington State. A giant tree near Mineral Lake, Washington, not far from Mt. Ranier was measured by Dr. Richard Edwin McArdle (who later became chief of US Forest Service) in 1924 with abney level at 69 m to a broken top, and 4.7 m diameter at breast height, but he was unable to measure the fallen top as it was overgrown with underbrush at the time. (Carder, Forest Giants pg 3-4). However, earlier in 1905, Joe Westover, a land Land Engineer of the Northern Pacific Railway measured the standing tree at 70 m, and the wind blown broken top he measure on the ground at 51 meters long. Total height of tree was approx. 120 meters. In 1899, only 15 miles from the Mineral Lake, a fallen Douglas fir was measured near the Nisqually river by Edward Tyson Allen, a state forester who was conducting a survey of that region. His tape measurement showed the tree to be 116 meters long, with a broken top-- suggesting the tree may have once stood even taller. I am in the process of contacting the PNW research station in Portland to obtain a copy of this first hand report for my files.

    In 1902 a Douglas fir was cut down in Lynn Valley, BC with a reported height of 126.5 m and 4.3 m diameter. The tree stood at the present Argyle road, near Hastings creek. After being felled it was measured by Alfred John Nye (who owned the land) at 125 m long, not including the 1.5 m stump. A hand written document from 1912 recounting the dimensions of this tree from Mr. Nye are on file in the library of the late Walter M. Draycott of Lynn Valley. (Carder, pg. 8).

    There is a great deal of evidence to believe the Eucalyptus Regnans, "Mountain Ash" may also have reached 140 meters and greater in Australia. A reported dated 21st February, 1872 to Clement Hodgkinson, ESQ., ASSISTANT-COMMISSIONER OF LANDS AND SURVEY from William Ferguson, Inspector of State Forests, gives the particulars of certain fallen giant Eucalyptus trees in the Watts river valley.

    Ferguson reports:

    "Some places, where the trees are fewer and at a lower altitude, the timber is much larger in diameter, averaging from 6 ft. to 10 ft., and frequently trees of 15 ft. in diameter are met with on alluvial flats near the river. These trees average about ten per acre; their size, sometimes, is enormous. Many of the trees that have fallen through decay and by bush fires measure 350 ft. in length, and with girth in proportion. In one instance I measured with the tape line one huge specimen that lay prostrate across a tributary of the Watts, and found it to be 435 ft. from its roots to the top of the trunk. At 5 ft. from the ground it measures 18 ft. in diameter, and at the extreme end where it has broken in its fall, it is 3 ft. in diameter. This tree has been much burnt by fire, and I fully believe that before it fell it must have been more than 500 ft. high. As it now lies it forms a complete bridge across a deep ravine."

    -- Australia and New Zealand, Volume 2
    By Anthony Trollope, 1876. pg 184.