The nuts and bolts of transitive limitation

Patterns of soil moisture in the lowlands of an annually burned watershed at Konza Prairie. Soil moisture is expressed on a relative basis at 6 depths for 1993 (wet year) and 1994 (dry year).

Earlier, I had discussed a potentially interesting case of transitive limitation, i.e. when the low availability of one resource reduces the availability of another. In the case of water and nitrogen, it is unclear in grasslands whether the limitation ascribed to water could actually be due to low N availability. N mineralization is known to decrease with decreasing soil moisture. As such, as soil moisture declines, so should N mineralization.

The correlation between soil moisture and N mineralization does not necessarily mean that the two should co-limit across a range of soil moistures. In a given soil profile, soil organic N is generally concentrated in shallow depths, while soil moisture is more evenly distributed throughout the soil profile, if not greater at depth. As such, plants can have access to plenty of water at depth even if shallow soils have dried out. Soil N mineralization and moisture might be correlated for a given volume of soil, but not over the whole soil profile.

Konza is an interesting example. At different times, productivity is said to be limited by water and nitrogen, but the two have never been rectified. Do they simultaneously limit production? Does limitation vary over the course of a season, or across years? Or is it transitive?

If it is transitive, disentangling the two is not easy. Standard factorial resource addition experiments do not work since adding water would also increase N availability. Is there a way to add water without increasing N? Not easily from above. But you could add it from below.

Inferentially, if you look at Konza soil moisture patterns, there is always plenty of water at depth, even in dry times. In the above example, in 1994, soil moistures are depleted in shallow soils, but there is very little draw down of deep soils. Proximally, this could be due to the lack of roots at depth, but we are only talking 1 m. The dominant species could easily produce roots at 1 m--if there was a benefit to doing so. If productivity was water limited, there would be a benefit. Yet, if productivity was actually N limited, accessing deep water provides little benefit when N is not being mineralized.

There are other lines of evidence that support the dominant role of transitive limitation at Konza. For example, regardless of whether you add N or water, the same species--Panicum virgatum--comes to dominate. If N was limiting, wouldn't adding N dry out the soils more and favor a low-water, high-N species?

One of the tough things to demonstrate is the roll that soil water potential plays in productivity. I'll likely expand on this later, but there are no relationships between water potential and productivity, only conductance. If we could show that productivity should not be diminished by lowering soil water potential to say -2 MPa, we might be able to demonstrate that it is not water that is limiting directly, but transitively by reducing N supplies.

There are still multiple pieces to assemble before the story is complete, but transitive limitation is likely a linchpin in understanding grasslands.

Finding the needles in the haystack

There are a fair number of papers that are impressive for the number of times they are cited. “Instant classics” that accrue a hundred citations in a year—most in the first paragraph of a paper—and have helped define some part of a discipline.

These papers are impressive and worthy of study in hopes of replicating them, but I am more interested in papers that are likely just as important but have rarely been cited. Any scientist can use Web of Science to find the most cited paper on a topic and then cite it themselves in order to seem authoritative. But, the true scholar knows the obscure paper, one that might only have been cited a few times a year, but can make the case that the paper is as important as one cited a hundred times a year, if only the obscure one were discovered.

I do not have a comprehensive list, but it is an interesting exercise to think about what are the most important papers never to have been cited. If we restrict the list to the papers published over five years ago and have received less than five citations a year on average. And one cannot put one’s own papers on the list, which is unfortunate since most of my CV is obscure but important. (Except for the one soil CO2 flux paper in GCB. That one deserves to be obscure.) Here are ones that I came up with:

1) Wahl, S. and P. Ryser. 2000. Root tissue structure is linked to ecological strategies of grasses. New Phytologist 148:459-471. If ever there was a golden key to unlocking root function in different environments, this would be it. Why this study has not been replicated a dozen times, I do not understand. (30 cites)

2) Dietz, H. and F. H. Schweingruber. 2002. Annual rings in native and introduced forbs of lower Michigan, USA. Canadian Journal of Botany 80:642-649. The idea that you can dig up grassland plants and age them should have set fire to our understanding of plant population dynamics in grasslands. (12 cites)

3) McManus, W. R., V. N. E. Robinson, and L. L. Grout. 1977. Physical Distribution Of Mineral Material On Forage Plant-Cell Walls. Australian Journal of Agricultural Research 28:651-662. The idea that plants accumulate minerals on their cell walls and might use them for structural purposes fundamentally alters how we think of plant structure and turns plant stoichiometry on its ear. It’s never been followed up on as far as I know. (12 cites)

4) McNaughton, S. J., J. L. Tarrants, M. M. McNaughton, and R. D. Davis. 1985. Silica as a defense against herbivory and a growth promotor in African grasses. Ecology 66:528-535. This one came to mind after the previous one. Silica as structure changes the game. This became cited a bit more in 2006-7, but other than those two years never had more than 5 citations a year. (85 cites)

I’ll give this some more thought later. This is a hard list to compile (and my kids are awake now). I should be able to come up with a top ten list of obscure papers later.