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.

Leaf architecture and physiological drought tolerance

Patterns of physiological drought tolerance and leaf venation architecture among 10 woody species.

Quick note on a new paper.


Scoffoni et al. determined the physiological drought tolerance and architecture of 10 woody species. The authors test key components of leaf venation architecture to understand the underlying leaf structural mechanisms for drought tolerance. Most work on drought tolerance focuses on stems and highlight xylem geometries, but the authors show that the density of veins in a leaf are the best correlate with its physiological tolerance of drought. High vein density provides insurance against embolism and allows water to continue to be supplied to areas adjacent to veins that have experienced embolisms that necessarily accompany low water potentials. 


The authors highlight the need to separate leaf size and vein density, which were correlated in the study. But, the research raises an interesting question as to whether the need for higher vein densities serves as a constraint on leaf size and ultimately contributes to one of the major biogeographic patterns of plant form.


I also think their figure, shown above, is pretty stunning. 

Scoffoni, C., M. Rawls, A. McKown, H. Cochard, and L. Sack. 2011. Decline of leaf hydraulic conductance with dehydration: relationship to leaf size and venation architecture. Plant Physiology 156:832-843.

C4 photosynthesis and nitrogen

Comparison of foliar N concentrations among clades.

Since the beginnings of our modern understanding of C4 photosynthesis, it has been set that C4's are more efficient with water and nitrogen. Yet, there have long been unexplained patterns for C4's that didn't match the assertion of greater nitrogen use efficiency. For example, C4 grasses in the field often have lower foliar N concentrations, but also lower root N concentrations. Why would this be? If the leaves need less, shouldn't the roots get more? Also, some C3 grasses like Chionochloa can have foliar N concentrations as low as 6 mg g-1. Most C4's have higher concentrations and only a few have been observed to be below that. Also, foliar N concentrations for any given species are highly plastic and dependent on the balance between C and N supplies and demand. If a given species can have N concentrations that range 30 mg g-1, just how important is the C4 photosynthetic pathway.


Turns out, probably not much. Taylor et al. (2010) used a phylogenetically structured screening experiment to measure a number of morphological and physiological traits of grasses. In doing so, they could compare C3 and C4 species controlling for phylogeny. The research upholds the notion that C4 photosynthesis confers greater water use efficiency to plants. Yet, after controlling for phylogenetic relationships, there were no differences between C3 and C4 species in their foliar nitrogen concentrations. 


By no means the last word on the topic. For example, they only measured ~30 species. Yet, the authors have provided the best experiment to date to address the question and evidence to the contrary will have to be weighed against some strong evidence regarding the ecological consequences of the evolution of C4 photosynthesis.


Taylor, S. H., S. P. Hulme, M. Rees, B. S. Ripley, F. I. Woodward, and C. P. Osborne. Ecophysiological traits in C-3 and C-4 grasses: a phylogenetically controlled screening experiment. New Phytologist 185:780-791.

The competing constraints on roots

Cross section of a Dicanthelium acuminatum root.

Roots have a few important jobs. Anchor a plant. Acquire water. Acquire nitrogen. Sometimes store carbon.

There is no reason that a root system that is optimized to acquire water would also be optimized to acquire nitrogen. Yet, what would a root system that was optimized to acquire water look like vs. one that was optimized to acquire nitrogen?

There are some contingencies here, but it's a good segue to think about how roots are built, no less root systems. There are multiple tradeoffs that would be selected for in different environments. Deep or shallow. Narrow vs. extensive. Thick vs. thin. Stele vs. cortex. Large xylem vs. small xylem. Many small cells vs. few large cells.

If form follows function, one should be able to deduce function from form. We can do this with many other traits. A thick waxy cuticle on a leaf generally reduces water or nutrient loss. Thick bark often protects from fire. Thorns deter browsing mammals.

Yet, if we were to look at the cross-section of a root, what could we tell?

Here's a cross-section of a Sorghastrum nutans root:

Now here's one for Penstemon tubiflorus:

Some of the differences are obvious. Penstemon has a much larger cortex. Sorghastrum larger xylem vessels. But can we deduce their differences in ecology? Which one is more drought tolerant? Which one is the better competitor for nitrogen? Is one more dependent on mycorrhizal fungi?

Wahl and Ryser were the first to try to link up root cross-sections with function, finding good linkages with other traits like plant height and RGR. It's been 10 years since they published their paper on grass root cross sections. No one ever followed their work up.

One of the keys, if not linchpins, to understanding the evolution of plants is waiting for us just under the surface.

Wahl, S. and P. Ryser. 2000. Root tissue structure is linked to ecological strategies of grasses. New Phytologist 148:459-471.

The “Nitrogen Throttle” hypothesis of primary productivity

Imagine a global change scenario. Models predict precipitation will decline for a region. Other things are likely to happen, too. For example, N deposition is likely to increase. But, with soil water determining productivity, the model returns the prediction that productivity will decline, too. As a result, ranchers will fail. Forestry will be diminished. But what if the model was structured wrong? What if soil moisture only indirectly determined productivity and N was actually more limiting? The decline in precipitation might be less important that the increase in N. The original prediction might be 100% wrong.

Globally, nitrogen is the nutrient that limits primary productivity the most. Also, secondary productivity, since protein is often more limiting to herbivores than energy. Yet, whether nitrogen or water limits productivity is still open. Places with little precipitation have little plant biomass. Yet, even vegetation in dry places still responds to N addition.

Separating between water and nitrogen limitation is complex enough that for all intents and purposes, ecologists have ceased to try. Not that it doesn’t matter any more—those that were interested in the topic have likely just stopped hitting intellectual walls. And some simple scenarios show how important it is to get the fundamentals right.

There are two major hypotheses regarding water and N limitation. First, the “Water Stress” hypothesis states that water directly limits production. As soil moisture declines, water supply to plants declines, water stress increases, and plants reduce stomatal conductance to match supply and demand and/or limit cavitation risk.

The other hypothesis is the “Nitrogen Throttle” hypothesis. As soil moisture declines, water stress increases for plants, but this is not what limits photosynthesis. Instead, microbial N mineralization declines causing N supply to plants to decline. Here, low moisture ‘throttles’ N mineralization. As a result of the lowered N supply, plants decrease their stomatal conductance in order to match C and N supplies. Under more extreme soil moisture stress, plants begin to senesce their leaves to maintain minimum growth requirements if not store N for when favorable growing conditions return.

How important is each hypothesis in explaining patterns of productivity? Hard to know, but as I’ve described before there is evidence that nitrogen throttling happens and is important. For example, why would there be soil moisture left deeper in the soil profile if plants were limited by water? Why is it that plants that can photosynthesize at -9 MPa water pressure begin to senesce at -3 MPa?

Like other competing hypotheses, proving one over the other is all but impossible. Factorial resource additions can’t solve the problem when adding water also increases N supply. In the end, it’ll be parsimony that will be tested as lines of evidence are compared.

Why be efficient? A question for C4 plants

C4 grassland in South Africa with a 1.7 m Carl Morrow for scale.

Species with the C4 photosynthetic pathway are in the minority in terms of species, but fix a large amount of the world's carbon, not to mention world's calories that humans consume.

Species with the C4 photosynthetic pathway differ from C3 species in a number of ways. We know that the C4 photosynthetic pathway evolved, or at least radiated during times of declining atmospheric CO2 concentrations. In accordance, C4 species have higher photosynthetic rates at glacial CO2 concentrations (~200 ppm) than C3 species. Therefore, it is generally thought to be an evolutionary response to low CO2 concentrations. In conditions of high light, low CO2, and warm temperatures, the C4 pathway reduces photorespiration and generates greater photosynthetic rates over C3 species.

Yet, the C4 photosynthetic pathway also confers greater resource use efficiency. The C4 pathway comes with increased energetic costs, but also confers greater photosynthetic water use and nitrogen use efficiency. More carbon is fixed in C4 species per unit water and nitrogen allocated to photosynthesis as internal CO2 concentrations are lower, which drives the greater WUE, and less N is needed for the same amount of photosynthesis, which drives greater NUE.

Some of the characteristics of C4 are a bit mythological. For example, although C4’s can have higher photosynthetic nitrogen use efficiency, many C4’s have high tissue N concentrations and many C3’s have as low an N concentration as the lowest C4. Not everything about plants is destined from photosynthetic properties.

That said, is there selective advantage to being more efficient with resources? Efficiency always comes at a cost. This much we know. You have to be inefficient with one resource to be more efficient with another. Light use efficiency comes at the expense of N use efficiency. N use efficiency comes at the expense of water use efficiency. Efficiency also costs time.

So what is the benefit of being efficient? For C4’s, under what conditions is it beneficial to be more efficient with water or nitrogen than C3’s. In a competitive world, efficiency in and of itself benefits no one but your competitors. The less water or nitrogen you use, the more there is for another. The benefit only comes if efficiency allows one to reduce the availability of the limiting resource below the level needed to sustain a potential competitor. Or tolerate more stressful conditions. Do C4’s reduce water or nitrogen availability to lower levels than C3’s? No evidence of that. Do C4’s tolerate lower water or nitrogen availability than C4’s? No evidence of that, either.

We also know that C4’s span a wide range of water and nitrogen availability. NADP-me type C4’s increase with mean annual precipitation, not decrease. And C4’s like the grasses we use in many lawns and golf courses have high nutrient requirements, not low, having evolved in grazing lawns that have high nutrient availability. In all, there is no evidence that C4’s preferentially occupy low water or low nitrogen habitats.

The efficiency of C4 species is one of the great mysteries of evolution. Is it an interesting by-product of selection for carbon gain under certain conditions? Or is it indirectly linked to success in ways that are not obvious? Likely, until we better understand the fundamental question of “Who wins and why?” in the plant world, that aspect of C4’s will still be a mystery.