Sunday, February 22, 2009

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.

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