Monday, May 11, 2009

Countdown#2: Competition and supply preemption

Dense root system of Hordeum pusillum

Previously, I discussed how competition selects for suboptimal allocation patterns—at least suboptimal in the absence of competition. If this wasn’t a regressive list, I would have been getting slightly ahead of myself.

As I said before Tilman made a great advance in linking competitive success to resource availability reduction. Although his work might have been theoretically pure, its application to terrestrial systems was conceptually flawed. Plants do outcompete one another by reducing the availability of resources. Yet, the availability of nutrients in soil is not best conceptualized as the solution concentrations. Instead, it’s supplies. When nutrients are limiting, plants outcompete one another by reducing the supplies of nutrients to neighbors. Independent of reducing mineralization rates, supply reduction comes from supply preemption. Because of diffusion limitation, the plant with the most root length per unit volume of soil acquires the majority of the nutrients supplied. Each unit of root length produced reduces nutrient supplies to neighboring plants. The best competitor for nutrients is the plant that can produce the most root length.

I develop at length in RSWP the shift from concentration reduction hypotheses to supply preemption hypotheses and how it changes our outlook on plant interactions. For example, changes in soil moisture would affect prediction of competitive superiority by altering soil solution concentrations, but do not affect predictions based on supply preemption. There still is more theoretical work switching from R* models to SL* (supply per unit length), but the concepts are now more consistent with our understanding of soil nutrient dynamics, even if the are not as theoretically pure as possible. Better understanding of how plants compete for nutrients not only help us understand how competition has altered the evolution of plants, but sets us up to ultimately better understand the resource strategies of wild plants.

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