Interacting determinants of interspecific relative growth: Empirical patterns and a theoretical explanation

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This paper discusses the ways in which physiology, morphology and biomass partitioning interact to determine interspecific patterns of relative growth rate (RGR). We measure the correlations between RGR and six growth components based on above-ground and below-ground plant attributes, and use these results to propose a general model of how these interacting growth components combine to determine RGR based on the assumption of balanced growth. The data come from 28 wild herbaceous angiosperm species grown in hydroponic sand culture for 40 days under controlled standardised conditions in a growth chamber, with 16 hours daily of 550 µmol/m−2s −1 PAR. Interspecific variation in RGR was largely a result of variation in unit leaf and unit root rates. Variation in specific leaf areas, leaf weight ratios, specific root areas and root weight ratios were of secondary importance in explaining the differences in RGR. These components of growth were not independent of each other. Leaf attributes were negatively correlated with each other, suggesting tradeoffs in the different ways that a plant can increase net carbon gain. Root attributes were also negatively correlated with each other, suggesting tradeoffs in the different ways nutrient acquisition is increased. Attributes increasing net carbon gain favoured allocation to roots, whereas attributes increasing net nutrient capture favoured allocation to leaves. We propose a path model relating these variables based on the assumption that the growth components interact to equilibrate the net uptake rates of carbon and a limiting nutrient. This path model was not rejected (c2 = 14.10, 10 df, p = 0.17) indicating a quantitative agreement with the hypothesis.