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

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.

The evolution of grasses: phylogeography of C4 photosynthesis

The temperature niches of grasses of the world overlaid onto their phylogenetic relationships.

The two great datasets in biology are the tree of life and the global biogeographic distributions. The first describes the phylogenetic relationships among organisms. The second describes their distributions on our planet. In a rich and well-nuanced paper, Edwards and Smith have brought the two together to shed light on one of the most fundamental questions regarding the evolutionary ecology of plants, namely the origin of C4 photosynthesis. The authors first use an expanded grass phylogeny to describe the origins of C4 photosynthesis in more detail than has done before. They then determine the current distribution of the grass species to determine the climates they occupy.

With regard to the evolution of C4 photosynthesis, the authors conclude that shifts from C3 to C4 photosynthesis did not involve shifts to warmer macroclimates, but instead to drier macroclimates. This results comes as a bit of a surprise--it is less clear that C4 photosynthesis is a response to low water availability as much as high temperatures. Their next logical step is a bit of a leap--namely that these modern geographic differences can be associated with habitat shifts in the past.

As important as the insights into the phylogeography of C4 photosynthesis is that the evolution of cold-tolerance in grasses is more difficult evolutionarily. Cold-tolerance apparently evolved vary early on in the grass radiation and has not been repeated to the degree that C4 photosynthesis has.

In all, this hardly seems like the last word on the topic. The biogeographic data needs to be improved, climatic ranges rather than centers will likely be used, and the grass phylogeny is still relatively unresolved. Also, we still have little understanding of why C4 photosynthesis would benefit plants in dry environments. That said, there is a lot of insight for many types of researchers and a solid step in understanding the strategies of plants to resource scarcity.

Edwards, E. J. and S. A. Smith. 2010. Phylogenetic analyses reveal the shady history of C4 grasses. Proceedings of the National Academy of Sciences 107:2532-2537.

Phylogenetic correctness

It is axiomatic to state that grasslands are dominated by grasses. It is also axiomatic to state that the trait that allows grasses to dominate grasslands is an adaptation to the grassland environment. The identities of the traits are currently unclear. For example, meristem position used to be the oft-cited reason for grass dominance, despite the clear evidence of many less abundant eudicots with similar meristem position. Yet, whenever we discover what the trait is that separates grasses from other species and allows them to dominate grasslands, we will never be able to declare that trait an adaptation. Such is that state of phylogenetic correctness.

There are many important traits that arose only once. True wood, angiosperm xylem vessels, and Rubisco likely only evolved once and cannot be separated from phylogenetic origins. The conditions and the complexity of the trait did not lead to multiple origins, but instead served as the basis of radiations. Any trait that sits at the base of a tree cannot officially be considered an adaptation. Even if a trait had arisen twice, there would be no statistical basis for calling something an adaptation since it cannot be separated from phylogeny.

What are we left with then? We can apply phylogenetic corrections to account for relatedness when examining trait relationships, and it provides useful knowledge. But to what ultimate point if phylogenetic corrections will not ultimately rule out or in something as an adaptation.

It's currently a silent sticky point in our considerations of the evolution and ecology of plants. One to which I don't have a complete answer, but one of which would be helpful to have clearer consideration. In the end, we will likely determine how grasses differ from other species and why they come to dominate what we call grasslands. And when we do, it'd be nice to be able to recognize the adaptations of grasses for what they are.