Why trees die: case example

Understanding mortality in plants is a tangle of proximal and distal as well as competing hypotheses. A recent paper in PNAS tried to disentangle a number of issues for understanding mortality in trembling aspen (Populus tremuloides).

The authors use a mix of gradients and experiments to examine patterns of carbohydrate reserves and hydraulic properties for droughted and non-droughted aspen plants. Plants that were droughted and non-healthy did not have reduced carbohydrate levels in their tissues (leaves or roots). In contrast, dying plants consistently were experiencing loss of hydraulic conductance and cavitation.

What is interesting here is that aspen is the lettuce of trees. It is an isohydric, physiologically drought-intolerant species. The research shows that pot experiments should be pretty good at determining the drought tolerance characteristics of species. Screening experiments (and rated, more involved detailed studies like these) should allow for the type and degree of drought tolerance to be assessed for other  species. hence, models of future mortality could be generated for forests across the world.

Anderegg, W. R., J. A. Berry, D. D. Smith, J. S. Sperry, L. D. Anderegg, and C. B. Field. 2012. The roles of hydraulic and carbon stress in a widespread climate-induced forest die-off. Proceedings of the National Academy of Sciences of the United States of America 109:233-237.

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.

Olympic National Park

Isabel and Micah ascending the world's largest Sitka spruce.

The family and I are on vacation in the Olympic Peninsula of Washington. We’ve spent the past three days at Lake Quinalt, which is on the southwest side of mountains and surrounded by temperate rainforest. A few things struck me while here. First, 15 feet of rain (the record annual precipitation) is a lot, but it can be hot and dry here. Second, it would have been wise to have bought a cooler and fast on smoothies for three days. There are few places to eat around here, especially since we are going back to Seattle to eat at places like Salumi and Pike Place Market.

The Quinalt River Valley has six record trees in it. The world’s largest western red cedar, Douglas fir, mountain hemlock, and Sitka spruce, are all in the one valley. The western red cedar is 19.5 feet across. It’s hollow in the middle and you can see daylight when you look up from within. I’m not sure where the phloem was, but there were green limbs up high. The Sitka spruce is 17 feet across and aside from being stuck between an RV park and a golf course, is impressive.

As we’ve hiked through the forests here, it has been interesting to think about how these trees have been accumulating environmental records for so long. Tree ring width and carbon and oxygen isotopes are the main records examined, but I’ve been thinking more about the nitrogen isotopes. From work I’ve done with Kendra in the past, every tree potentially has a record of nitrogen availability in its rings. The isotopic ratio of nitrogen stored in wood is largely set down initially and has been shown to track N availability. Only a small number of trees have had the N isotopes in wood measured and for the most part we are ignorant about how N availability has changed in these immense forests or others. It’s an important question since we don’t know how elevated CO₂ has affected N availability or how frequently N availability might peak with disturbances, which has important implications for the ecology of these forests.

I am pretty sure we don’t have a 10 foot increment borer in the lab, but there are some long records here just waiting to be read.