The Corner (Ecological) Office

I had read this book awhile ago, but Kendra just got to reading it, so we've had more of a chance to chat about it lately.

In ecology, there are a lot of nuts and bolts to master. Statistics, experimental design, the literature...Our discipline is not a quick one to master. Yet, when you get to the highest levels of the functioning of the discipline, success has less and less to do with the nuts and bolts than the more sociological aspects--basically how to get people to work together. As a scientific society, we recognize this a bit--we have fellowships for interacting with the press or Capitol Hill, but not with each other. It's unfortunate, too, because the failures and dysfunction are costly yet utterly preventable.

Any airport bookstore is filled with similar topics, but the best book I've found that provides advice on how to be a leader is Adam Bryant's The Corner Office. Bryant writes a weekly column for The New York Times Sunday Business section called The Corner Office. Weekly, he interviews a CEO of a different company and asks them essentially the same questions. When did you first have to manage people? Where did you learn to manage people? How do you hire people? What recent insights do you have on leadership? Almost every week the column is interesting and there are a lot of people who have thought carefully about how to manage people. The book is a compilation of his interviews to date.

There are many lessons in the book. There should be. Leading and managing is complex and just one thing wrong can trip up any project no less corporation. You have to take care of people and let them know they are appreciated. You have to provide a vision that can be easily translated to day-to-day activities. You go to the lowest levels for unfiltered information and to learn all the aspects of work that surround you. You have to be fair and even. You have to appreciate failure.

William Bond and I were talking about similar issues on our trip through Afri-homa. He remembered his first year at UCT. He said that his department use to have a brilliant chair. William described how after his first year--a year consumed by teaching courses for the first time along starting a research project--his chair found him and said, "Good job, William. You made it." And you could tell how that one recognition meant a lot to him.

So much dysfunction in any group, whether a project, a department, or a scientific society, comes from not feeling appreciated. And it costs so little for leaders to appreciate others and let oxygen be consumed on more important issues.

There are a host of other aspects of being a leader of a team that need to be carried out well. Bryant might not nail them all, but there are stories in there worth learning that help advance our discipline as much, if not more, than the latest statistical technique or meta-analysis.

A bit of inspiration to travel

Photo from Wildflower Wonders by Bob Gibbons of Tien Shan Mountains in China (Princeton University Press).

Books can be good for inspiration. Some books inspire with ideas. Others with adventure. Sometimes just photos that inspire one to travel to new places.

I had a chance to read Wildflower Wonders: The 50 best wildflower sites in the world over the past few days. The author, Bob Gibbons, shows off some impressive photography of beautiful grasslands from around the world. From the Tien Shan Mountains to the Outer Hebrides to Namaqualand.

It's more than just a coffee table book of pictures, though. Gibbons does an excellent job weaving together cultural and geological history with the ecology of the areas in ways I wish I could. His discussion of the settlement history of the Julian Alps or the unique geology of the Dolomites in Italy that differentiates its flora from nearby limestone grasslands.

I think we forget to be biogeographers too often and forget that we can learn as much from travels as experimentation. And it's our travels that shape our experiments. Even those that work at the global scale often see just pixels and not the uniqueness of place that truly defines geography.

I have little but praise for this book. It would have been nice to see more pictures of grass rather than the less-than-subtle, somewhat ostentatious wildflowers. But, I guess that's my job on the next trip.

[Thanks to Princeton University Press for the advance copy.]

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.

A lack of fertilization from elevated CO2 in forests

The concentration of CO2 in the atmosphere has been rising steady for some time now. There are two main potential direct effects of this fertilization. The first is a direct increase in photosynthesis, the second a reduced use of water.

At its simplest, the reduced use of water should increase plant production in drier habitats by increasing soil water availability. Less water is used by plants for a given amount of photosynthesis, means more water in the soil, and more productivity before soils dry out.

Theoretically this straightforward, but whether this has happened in ecosystems across the Earth or not is an open question.

Peñuelas, Canadell, and Ogaya synthesized data on two parameters for 47 forests across the world. The first was the C isotope composition of tree rings, which can be used to infer instantaneous water use efficiency. The second was the growth rate of trees themselves--limiting measurements to well-established forests.

Their results are pretty clear. Across a wide range of forests, over the past 40 years trees have been 20% more efficient with water when they photosynthesize.

If trees are primarily limited by water, they should be producing 20% more wood. Yet, there was no significant increase in productivity in tree growth in any region.

If plants are more efficient with water, then what could be holding back plants?


The authors write, "Other factors such as increasing temperature, drought, nutrient limitation and/or plant acclimation may preclude such growth increase."

Which might it be?

The next chapter in this question is going to be pretty interesting.

Peñuelas, J., J. G. Canadell, and R. Ogaya. 2011. Increased water-use efficiency during the 20th century did not translate into enhanced tree growth. Global Ecology and Biogeography 20:597-608.

Low resource tolerance traits

General approach for determining physiological drought tolerance. Individual leaves or plants are dried down over time. Periodically the water potential of the soil or leaves, relative water content (RWC) or wilting stage of the plants is assessed. The response variable is generally gas exchange (stomatal conductance or photosynthesis) or hydraulic conductivity of leaves or stems. Different thresholds of the response variable are used as the ultimate metric  for physiological drought tolerance, i.e. A = 0 or KL = 20% of KLmax.

A short note here.

One of the developing trends in functional traits is moving away from traits associated with general stress tolerance syndromes to traits that directly are associated with tolerances of low availability of specific resources.

There has been a lot of work to determine the leaf economic spectrum, for example. Yet, the LES separates fast- and slow-growing species in essence without separating the specific resources that have driven the evolution of the slow-growing species.

For example, drought, shade, and low nutrient availability are all supposed to be associated with the traits on slow-return portion of the LES. Therefore, at its best, the LES would not separate out whether species were adapted to drought or shade, if they aren't adapted to both.

A hopeful trend has been measuring drought-tolerance or shade-tolerance directly. Tolerance of low nutrient availability/competitive ability when nutrients are limiting, not so much, but that might change yet.

Mel Tyree and Tom Kursar's work for tropical species is a good example. Their approach is to let tropical tree seedlings grown in pots wilt and then measure the water potential of plants that are severely wilted. Similar to psi-crit that I've measured.



When you do that, you get a range of drought tolerances:

They've used this pretty successfully to explain patterns of diversity in dry and wet tropical forests.

Again, this is a hopeful trend that should bear fruit in the near future.

Engelbrecht, B. M., L. S. Comita, R. Condit, T. A. Kursar, M. T. Tyree, B. L. Turner, and S. P. Hubbell. 2007. Drought sensitivity shapes species distribution patterns in tropical forests. Nature 447:80-82.

Kursar, T. A., B. M. J. Engelbrecht, A. Burke, M. T. Tyree, B. Ei Omari, and J. P. Giraldo. 2009. Tolerance to low leaf water status of tropical tree seedlings is related to drought performance and distribution. Functional Ecology 23:93-102.

Tyree, M. T. 2003. Desiccation Tolerance of Five Tropical Seedlings in Panama. Relationship to a Field Assessment of Drought Performance. Plant Physiology 132:1439-1447.

Fire in grasslands and savannas: a trip to Afri-homa

William Bond with a Gulliver post oak at Tallgrass Prairie Preserve.  

I spent four days last week driving around Oklahoma with William Bond from South Africa. I'm not sure why we chose the state, except in many ways it is the area most analogous (ecologically) to southern Africa that we have in North America. Old soils, intermediate precipitation, native prairies and savannas, and fire.

I've spent a fair amount of time over the past decade with William learning about things I never thought cared about. This trip quickly became a study on the roles of fire, drought, and herbivory in vegetation. It's hard to summarize William's view of the world, but he spends a lot of time thinking about how ecosystems and floras have developed over the past hundred million years. It's not a narrow topic.

I'll do my best to summarize, but essentially we visited different ecosystems such as the Cross Timbers to look for evidence of the antiquity of fire in grasslands, if not the antiquity of grasslands themselves. We'd go to a place like Tallgrass Prairie Preserve and look at oaks to see whether they promote fire. We traveled to Wichita Mountains, another old landscape, to look at how trees were coping with drought. We traveled to Black Kettle grasslands to look for plants with spines as evidence of histories with browsing mammals.

You can't necessarily look at a place and see back 10 million years, but you can try. For example, you can look under a Cross Timbers canopy and try to understand whether fire would rely on grass or oak leaf litter to work through the savanna. Or look at the canopy to see whether the oaks try to shade out the understory or not. Things like this provides evidence of the evolutionary history between these plants and fire.

A place like Black Kettle isn't too far from the Cross Timbers, but it's a radically different ecosystem. Grass there is grazed short. All the woody plants are structurally defended. I'm still picking out prickers from that place. It was never likely a fire world. It was likely always an herbivore world.

Long story short, there are still some great syntheses to be made. We haven't mixed everything up enough such that an old-fashioned field trip can't provide insight into the forces that have structured our world.

Drought and stress tolerance

Comparison of photosynthetic rates for seedlings of dry- and wet-habitat tropical tree species. On average, photosynthetic rates were ~1/3rd higher for dry-habitat species. 

I wrote a bit on this just the other day, but here is a new paper that raises questions about whether low-water species should be considered "stress-tolerators". Pineda-Garcia et al. grew seedlings of 10 pairs of closely related tropical tree species and measured a suite of traits. Dry-habitat species had higher photosynthetic rates than wet-habitat species. In addition, dry-habitat species retained their leaves longer after watering was ceased.

There are always a number of ways plasticity can alter relationships. For example, I once showed that high resource species can have lower N concentrations and longer leaf longevity than low-nutrient species due to patterns of feedbacks to N cycling after establishment. Here, there are a number of mechanisms that could generate the higher photosynthetic rates and longer leaf longevity in this particular experiment that could be reversed in another. Parsimony accrues slowly.

Yet, overall, this is another example where drought-tolerant species are not necessarily following the general "stress-tolerator" syndrome. It will be interesting to begin to officially tally the evidence to see whether there is much support for the two to be linked.

Pineda-Garcia, F., H. Paz, and C. Tinoco-Ojanguren. 2011. Morphological and physiological differentiation of seedlings between dry and wet habitats in a tropical dry forest. Plant, Cell & Environment 34:1536-1547.