The model species set

By restricting our own freedom, we gain collective power. It's a tenet of larger society, but also scientific society. 

For some the restriction is in the form of Arabidopsis. Zea for others. Populus, Lotus, Medicago...the list of model organisms that are used to answer fundamental questions about the genetics of plants goes on.

But what about the evolution of plants? To a degree, we can compare the genomes of model organisms to hint at some of the broader evolutionary patterns. But evolutionary patterns are generally derived by comparison with multiple members of a single clade. If one wanted to understand the evolutionary patterns of grass, we couldn't just look at a single model organism. We would need to look at a model set of species.

What would a model species set for grasses look like? It would have to be large enough to cover the major clades (~10), but restricted enough that researchers could measure standardized metrics on every species. Probably about 100 species. For grasses, they should come from different continents, span multiple origins of C3 and C4, and cover a wide range of environmental tolerances. Seed should be readily accessible. Most likely seed sets would have to be collected by a central agency for distribution to willing researchers. A central database would be needed to store all the data for other researchers to use.

Once that happened, an individual researcher that was interested in the cold tolerance of grasses could grow up all 100 species, measure their cold tolerance, and then examine the evolutionary patterns of cold tolerance. The next researcher that wanted to examine stomatal density could do the same, and then would be able to compare it with cold tolerance. Root anatomy, mycorrhizal dependence, genome size, carbonic anhydrase activity, flowering phenology, drought tolerance...the database would build. Each time we would learn more about multivariate trait selection in ways that no one lab could do.

Why doesn't this exist? Hard to say. Part of it is probably some small group just deciding which 100 species to use. Would it be perfect and cover all the potential evolutionary questions? No, but there are researchers that are asking these questions anyways, so they might as well be using the same species. Plus, there always could be a second species set identified to fill the gaps in the first for a second round of measurement.

Why not just keep a database and let researchers work on whatever species they felt best allowed them to examine specific ecological and evolutionary contrasts? Never enough overlap. Brassicaceae has 3700 species and even the Arabidopsis genus has 9 species. But everyone works on thaliana even if other crucifers might be better to answer some questions.

Once the scientific community agrees to encourage the restriction of freedom of inquiry into plant evolution a little more, a large amount of collective power will be realized. How long should it take? A few informed individuals who are not afraid to make political sausage would need to be in the same room for about 2 days. How long will it take to get people in a room for 2 days? Hopefully within a year or two.

Climate-nutrition-performance cascade

Critical climate periods for precipitation for ANPP, flowering of 3 grass species, bison weight gain, and the calving rate of adult females the following year.  

Some ranchers around here know that "a dry June is money in the bank". Supposedly when precipitation in June is low, cattle gain more weight. More cow means more money. I haven't heard it too much around here and I had never seen data to support it (until the work on bison weight gain at Konza), but it exemplifies the climate-nutrition-performance cascade and is a cautionary lesson in understanding climate change.

The performance of grazers--their weight gain and calving rate--is dependent on both the quantity and quality of the grass they eat. The interannual determinants of quantity seem fairly straightforward (for some sites). Quality is less so. Quality encompasses a lot of things, but primarily is protein. And protein is nitrogen.

The cascade links climate to performance through protein. The proximal and distal drivers of variation in protein are complicated enough that they are still being worked out. But as we work through this, it will be important to ask whether there protein at certain times is more important than others. And if so, maybe interannual variation in climate at certain times is more important than others. Rain in June might be more important than May, for example.

We've used the critical climate period approach to begin to tease some of this out at Konza. The figure above shows a broad CCP for ANPP--rain that falls in June or September still is important for determining growing season biomass. The three C4 grass species have different, but overlapping, CCP's. Each is about 80 d. In contrast, bison weight gain and calving rate seems to respond to variation in precipitation for just relatively short periods. One in late June, early July. The other mid- to late August. Calving rate depends on just mid- to late August precipitation.

Between climate and bison performance is protein. And the controls on protein we're still figuring out.

Until we do, we'll have a hard time understanding the dynamics of grazers, no less their fate in a world where climate has changed.

Whole-flora analysis: flowering and community assembly

Distribution of first flowering dates for 265 Konza grassland species (as of mid-May). 

There are some ecological analyses that can only be done by analyzing every species in a flora. Statistical inference aside, if we are to understand ecological sorting and community assembly, we need whole-flora analyses.

For example, we've slowly been accumulating data on first flowering for the Konza flora. For each species,  the day of year it first begins to flower is recorded. We're about half-way to having dates for the entire grassland flora. It's a bit biased now as we're still collecting data this year, but that's why this is on a blog...

There seems like there might be pretty strong selection pressures to flower at times when 1) environmental stress is low and 2) there is little competition for pollinators. I'm not sure if there would be selection against synchrony in flowering time for anemophilous species, but we that would be something to test, too. In this case, deciding what the null model is, is the hardest part.

I like the idea of whole-flora analyses. It's a lot of work though.

N vs. P limitation

Nutrients limit grass growth in native grasslands throughout the world. Yet, which nutrients limit growth should vary. N limitation appears to be pervasive in all nutrient limited grasslands and P is often limiting, too. In Europe, grasslands are often divided into those limited by N vs. those limited by P. In N-limited grasslands some species such as Alopercus predominate, while in low-P grasslands its Molinia. 

Why the sorting though? What traits would have been selected for in low-N vs. low-P soils? Fujita et al. have a new paper coming out in Oikos that I think provides some good data to separate species and shed light on selection when nutrients are limiting. It's long been known that plants can produce phosphatases to increase P availability. Fujita et al. show that low-P species have higher rates of phosphatase production.

With the experiment examining plant growth and activity at a range of N:P supplies, the research has the potential to help understand not only differences in grassland communities but also the response of grasslands to N deposition. Fertilization with N increased phosphatase activity in ways that should further increase the abundance of low-P species.

The authors do a good job for their eight species in linking plant stoichiometry, plant growth, and resource availability, which might ultimately serve as a key trait in understanding selection for success when nutrients are limiting, as well as the functioning of grasslands.

Fujita et al. 2010. Oikos. doi: 10.1111/j.1600-0706.2010.18427.x

Leaf dry matter content: ecologically relevant?

There is still some contention about whether leaf tissue density (mass per unit volume) leaf dry matter content (LDMC; dry mass per unit wet mass) are equivalent and whether past work has shown LDMC to be ecologically relevant, no less more relevant than specific leaf area (SLA).

I've looked through the literature pretty hard. Here's about all I can find:


1. LDMC and leaf tissue density should be positively correlated and there has been some excellent work investigating the underlying causes of variation in LDMC that are relevant for understanding leaf tissue density (Vile et al. 2005, Roderick et al. 1999, Shipley 1995). I still haven't found the perfect test of the two methods, but they should be pretty strongly related.
2. LDMC can predict plant strategies (Vendramini et al. 2002, Wilson et al. 1999). LDMC does a better job than SLA in predicting CSR placement for example.
3. LDMC can predict relative growth rates within species (Ryser and Aeschlimann 1999) and digestibility (Pontes et al. 2007, Ansquer et al. 2009, Duru et al. 2008).
4. LDMC was not correlated with competitive effect or response (Liancourt et al. 2009).
5. LDMC correlated better with soil fertility and sheep grazing intensity than SLA in Norwegian alpine ecosystems (Rusch et al.2009). [Note I still haven't read this paper--it's on order.]

That's about it.

The use of SLA still outnumbers tissue density or LDMC 50 to 1 and there still are essentially no published tests of the utility of either tissue density or LDMC in explaining abundance.

The tyranny of dominance

If you take a walk through a grassland, you are likely to recognize that a few species are more abundant than others. Walk through a nearby grassland and you'll recognize those species again. Dominance of a few species is a hallmark of grasslands. Especially in humid grasslands, species are thought of as dominant, sub-dominant, or rare.

But do grasses evolve to be dominant? Or rare? Dominance might be a common condition for a few species, but how transient is dominance? Or rarity?

This is a topic that easily belies one's inner model of how the world works. More often than not, it's one's view of human society that paints one's ecological canvas. But that's a topic for another day.

Although unstated, there are likely two competing intellectual frameworks at play with discussing dominant species. On the one hand, it is possible that some species have evolved to be dominant and others rare. There are light-demanding canopy trees and there are shade-tolerant understory herbs.  It is the role of the latter to always be underneath.

On the other hand, dominance and rarity are context specific. All species dominate somewhere and at some time. It all depends on the environmental context. If there are species that are dominating it is only because of the prevailing conditions.

Let's look at Konza Prairie. 86 species of grass. Only a few are considered dominants: Andropogon gerardii, Schizachyrium scoparium, Sorghastrum nutans are the big three. Maybe Panicum virgatum if one is feeling inclusive. But what about the other 82? Do they not dominate because they cannot dominate an area, or because they dominate grasslands under conditions that are not prevalent at Konza.

For example, only a portion of Konza is grazed and most of that area not too heavily. All of Konza's Big 4 grasses are less abundant in areas that are grazed than ungrazed. If Konza were grazed more heavily, one would likely assume another set of species were dominants. Bromus arvensis is likely one of those. It is 10,000 times more abundant in grazed than ungrazed areas. It "dominates" grazing lawn areas.

Along these lines, Gene Towne went and calculated that almost half of Konza's grasses have been found to be abundant in at least one of Konza's ~300 10 m2 permanent plots over the past 15 years. The other half? Some of them, such as Elymus virginicus, dominate in wooded areas which just aren't sampled at Konza. Some of them dominate outside of Konza. For example, Panicum coloratum is a dominant in the southwest mesquite woodlands. Poa arida is not common at all at Konza, but Konza is really at its southern range limit. Go to Alberta to find vast stretches of it. Some of the other Konza-rare species are annuals and would likely be a lot more abundant if the ground was pounded by more hooves or we had recently had a major drought.

In all, there's probably too much Berkeley in me to believe that some species are inherently dominant. It seems like, at least for grasses, each species likely dominates somewhere some of the time. That said, it would help to hear a bit more about the assumptions that underlie the concept of dominance. I could accept that grasses would be tyrannical to one another in their quest for resources and reproduction. The idea that some grasses are inherently more likely to dominate than others is a tyranny of thought I am probably not willing to accept yet.

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.