Transitive limitation and precipitation

Thinking more about N and water, I was looking over the Huxman-Smith et al. 2004 Nature paper. This paper summarizes the sensitivity of ANPP to precipitation. With data from 14 sites, they calculate rainfall use efficiencies across years to see how RUE changes with mean annual precipitation. They find that wetter sites have lower RUE, but all sites converge on a constant RUE. (Above figure is precipitation vs. ANPP (g m-2), each x-axis tick is 500 mm y-1).

The most interesting art of the paper are statements on resource limitation.

Here are some key sentences:

1) the authors predict that “the removal of other resource limitations so that precipitation becomes the primary limiting resource will result in an increase in site-level RUE that approaches RUEmax.”

2) “sites with high production potential in years with greater than average precipitation, soil nitrogen or other limiting resources might transiently limit biological activity.”

3) They also state that “biogeochemical constraints (limitation of activity by resources other than water) can increase with increasing precipitation”.

The approach the authors take to limitation is generally one of serial limitation. First one resource limits, and then another. The authors seem to hold the idea that multiple resources can co-limit ANPP, but they are mute about mechanisms or what the tradeoffs are. There is no evidence they considered substitutability leading to co-limitation, or transitive limitation (water limiting N availability). Looking at statement 3 above, they just as easily could have said that biogeochemical constraints can also decrease with increasing precipitation.

The unstated model they use follows something like this: less rain, greater water stress, less limitation by other resources, less production, greater WUE.

The existence of transitive limitation changes the entire story of ANPP responses to increased precipitation: less rain, less nitrogen mineralized, greater limitation by N, less production.

The relationship between ANPP and precipitation is, here, just a ratio. The important part of research is always to look one level of mechanism below the pattern of interest.

For example, if plant WUE was a key factor in RUE, then wouldn’t sites dominated by C4 vegetation have an inherently higher RUE than sites dominated by C3 vegetation? But, if the observed RUE was driven by N mineralization responses to increased soil moisture, wouldn’t they be the same? Although Cedar Creek (CDR) might show no increase in ANPP with increased precipitation due to strong N limitation, there might be unique patterns of sensitivity of N mineralization to variation in precipitation there. For example, maybe with the sandy soils, soil moisture isn't much greater in a high precipitation year.

At this point, both mechanistic hypotheses should be considered equally (one can’t be favored because it comes first). It’d be interesting to see how much of the patterns could be explained by transitive limitation.

Huxman, T. E., M. D. Smith, P. A. Fay, A. K. Knapp, M. R. Shaw, M. E. Loik, S. D. Smith, D. T. Tissue, J. C. Zak, J. F. Weltzin, W. T. Pockman, O. E. Sala, B. M. Haddad, J. Harte, G. W. Koch, S. Schwinning, E. E. Small, and D. G. Williams. 2004. Convergence across biomes to a common rain-use efficiency. Nature 429:651-654.

Co-limitation by water and nitrogen

Resource limitation is one of the great driving factors that structure ecosystem function and ultimately the evolution of species. Yet, rarely should only one resource be limiting to plants. As I outline in RSWP, co-limitation should be the rule rather than exception. Yet, the paradigm shift in our understanding of limitation is still developing.

One of the most important dual limitations to plants is between water and nitrogen. In many ecosystems, increases in biomass are observed when nitrogen is added as well as water. Yet, why would these two resources co-limit production? For example at Konza Prairie, ANPP frequently responds to water addition and is greater in wetter years. In addition, ANPP increases with N fertilization. Mechanistically, how can this be?

One reason why co-limitation occurs is because of acquisition tradeoffs—allocating resources to acquire one resource does not also lead to the acquisition of the other. Their costs are separate. For example, CO2 and water can co-limit because of their spatial separation in acquisition as well as their direct tradeoff in photosynthesis. Yet, generally the same unit of root can acquire both water and nitrogen as they are coincident in the soil.

Co-limitation can also arise when gain or loss rates from the ecosystem are co-dependent on the resources’ availability, causing the resources to be supplied at the same ratio demanded by plants. For example, N and P will often co-limit production because N is more likely to be lost from the soil when P is limiting. When the ratios of supplies and demand match, increases in productivity occur only when both resources are added, not each individually (see Figure 4.2 in RSWP). Yet, the pattern of co-limitation for water and N just doesn’t support this. Productivity generally responds to the addition of each, not just both. Although N2-fixation might be more likely when soils are wet, which bring supply and demand ratios closer,  N loss can also be greater when soils are wet (dentification, leaching). Plus, N loss is not more likely when soils are dry.

If these two main reasons do not apply, why do N and water frequently co-limit production? The secret might lie in the figure at the top of the post. 35 years ago, researchers incubated a number of soils at a range of soil moistures. They found that N mineralization typically was highest at some high soil moisture. As soil moisture declined, N mineralization declined linearly.

These might be old data, but the consequences are still amazing, even today. For example, if soil moisture declines from 80% water-filled pore space to 70%, which is still a lot of water for any plant, N mineralization would decline by 12%. Add that water back and N mineralization increases. If this is the case, then when it rains or an area is irrigated, N-limited plants would see an increase in productivity without ever being directly limited by water.

There is other evidence that water might not be directly limiting to productivity, but seem to be. For example, there is the curious observation that there is often plenty of water in the soil for seemingly water-limited plants right below their lowest roots. Dave Wedin noticed that in the Sandhills of Nebraska, there is often plenty of water right below the roots of the C4 grasses, which extend only 25 cm deep. Turns out same thing happens in Konza. These plants can easily produce roots deeper than they do, they just don’t. Why? Because all the soil organic matter is in the top of the soil profile. The water below that zone is of no use to the plants. By the time the shallow soils dry out, no N is being mineralized for the deeper water to be useful.

There are a number of other questions that need to be answered about co-limitation between N and water. Certainly soils can dry out to the point where neither water nor N is available to plants. How often does this occur and are there cross-overs where water becomes more limiting than N? Also, if deep water is available to plants, why don’t they lift it up to shallow soils to increase N mineralization?

All good questions, but the key is not necessarily the answer with which we end as much as the question with which we start. And we always have to start with understanding the mechanisms the determine the patterns in which we are interested. In this case, we have to start by testing co-limitation from the beginning and then asking why co-limitation occurs.

Figure from: Stanford, G. and E. Epstein. 1974. Nitrogen mineralization-water relations in soils. Soil Science Society of America Journal 38:103-107.

Global maps of nutrient limitation?

One of the keys to understanding the evolution of plants as well as the functioning of ecosystems is understanding the geographic patterns of resource limitation. This means maps. With maps of resource limitation, we can examine patterns of dominant species and infer adaptations to differences in resource limitation. Yet, largely, these maps do not exist. We have good maps of climate, fairly good maps of soil resources, but poor maps of resource limitation.

Why is this? Why can't we open up a map that shows us where nitrogen is the most limiting? There are a few fundamental reasons.

1) We do not understand patterns of limitation at any one site. In part, this can be ascribed to limitation being variable over time and small spatial scales. Yet, the multi-factor resource addition experiments that are necessary to fully describe resource limitation are but a handful.

2) We have not developed proximal indicators of resource limitation. Outside of adding the resources directly and examining responses, there are no reliable indicators of resource limitation. For example, plant N:P ratios have long been thought to be good indicators of the relative limitation of N and P in ecosystems. Yet, for a number of reasons, such as species not having a narrow range of "optimal" N:P ratios, these don't work.

3) We don't have the networks in place to make maps. I've written on this before (Bioscience, 2007), but outside of Europe, there is virtually no infrastructure to make extensive measurements. If it can't be measured with a satellite, we just don't measure it on any meaningful geographic scale. For example, I and coauthors have been analyzing global patterns of foliar N isotopes, which is our best easy index of N availability. Over the past 30 years, there have been a little more than 10,000 measurements of foliar N isotopes. It might sound like a big number, but that's only about $100,000 in analytical costs and there are large regions of the world for which no data have been collected. The map shown above looks extensive, but only because of extrapolation from relationships with climate data.

Resource limitation is critically important for understanding the evolution of plants, but also the ecology of ecosystems. Predicting responses to climate change, nitrogen deposition, and invasive species; estimating carbon sequestration, planning for species conservation...these all ultimately depend on resource limitation. Certainly, there are a number of questions close at hand that we must answer, but global maps of resource limitation has to be the point to which we fix our gaze on the horizon. 

The differences among roots

The diversity of leaf types is well known to most children of grade school age. The needles of pines are different than the broad leaves of oaks. Roots on the other hand are a different matter. What two species represent the opposite ends of the spectrum for roots? How should we classify them? Or are most roots the same, its root systems that differ? Or are the basic differences at the cellular level?

In another post, I'll talk about some of what we're learning about differences among species in the anatomy of roots, but for now it's good to think about the things we can see with our naked eye. I'm currently in the middle of an experiment growing over a 100 species of prairie species in containers and looking differences in how the species are built. It's still amazing to wash out roots and see how different two species that can coexist side by side can look like belowground (see picture on left). 

In RSWP, I talk a bit about some of the section pressures on root systems, such as the overproduction of root length when competing for nutrients. Although I've spent a fair amount of time looking at roots, its hard to know who the pines and oaks are of the root world. Roots differ in thickness, hairiness, woodiness, color, connections to mycorrhizal fungi...If pine needles are representative of tough, long-lived, low activity leaves, and oaks wimpy, short-lived, high activity leaves (although there are more extreme versions than oaks, like the lawngrases), the tough-wimpy axis in roots might look something like what is shown above. Yet, it's obvious that the root systems differ between the two. In the early days of ecology, phytogeographers spent a fair amount of time also characterizing root systems of plants, for example tap rooted species vs. species with more lateral roots. We still don't quite understand the full variation of types of roots systems and the selection pressures behind them. In the meantime, I'll keep washing.