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Interpreting plant root responses to nutrients, neighbours and pot volume depends on researchers’ assumptions

2020; Wiley; Volume: 34; Issue: 10 Linguagem: Inglês

10.1111/1365-2435.13517

1365-2435

Gordon G. McNickle,

Plant Parasitism and Resistance

Abstract Some plants respond to both neighbours and nutrient levels below‐ground independently, while others respond only to nutrients. These responses have important ecological implications, but debate about the appropriate control treatments used in experiments has led many to debate results. When plants ignore neighbours, and only respond to nutrient levels this is sometimes called an ideal‐free distribution (IFD). When individual plants respond to neighbours by increasing root production to pre‐empt the uptake of neighbours, and do so at the expense of reproduction, this has become known as a tragedy of the commons (TOC). Here, I describe how the history of competition experiments has largely been a history of experiments that only control two of three critical variables: (a) total nutrients per plant; (b) soil nutrient concentration and (c) pot volume. These confounding effects are the source of debate about results. In one study, split‐root pea plants were grown with and without a below‐ground neighbour at many pot volumes with equal total nutrients per‐plant. The researchers explicitly argued that plant growth responses would not be affected by the concentration of nutrients in soil, and therefore they explicitly confounded soil nutrient concentration with neighbour presence. Here, I reanalyse a subset of their data, controlling for soil nutrient concentration, but confounding pot volume with neighbour presence. When I controlled for soil nutrient concentration, the presence of a below‐ground neighbour had no effects on plant size, or allocation, and the effects of volume also disappeared. The reanalysis suggests that the behaviour of peas in this study was consistent with an IFD model, and that the previously reported volume and neighbour responses may have been due to the confounding of nutrient concentration. The results of the study in question depend very much on whether one does analyses that control for pot volume or whether one does analyses that control for soil nutrient concentration. Ultimately, these analyses lead to two different interpretations of the data. New experimental designs are emerging that fully control pot volume and nutrients across competition treatment, and I discuss possible paths forward in this ongoing debate.