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

This paper discusses the ways in which physiology, morphology and biomass p artitioning interact to determine interspecific patterns of relative growth rate (RGR). We measure the correlations between RGR and six growth compone nts based on above-ground and below-ground plant attributes, and use these results to propose a general model of how these interacting growth componen ts combine to determine RGR based on the assumption of balanced growth. The data come from 28 wild herbaceous angiosperm species grown in hydroponic s and culture for 40 days under controlled standardised conditions in a growt h chamber, with 16 hours daily of 550 mu mol/m(-2)s(-1) PAR. Interspecific variation in RGR was largely a result of variation in unit leaf and unit ro ot rates. Variation in specific leaf areas, leaf weight ratios, specific ro ot 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 ether, sug gesting tradeoffs in the different ways that a plant can increase net carbo n gain. Root attributes were also negatively correlated with each other, su ggesting tradeoffs in the different ways nutrient acquisition is increased. Attributes increasing net carbon gain favoured allocation to roots, wherea s attributes increasing net nutrient capture favoured allocation to leaves. We propose a path model relating these variables based on the assumption t hat the growth components interact to equilibrate the net uptake rates of c arbon and a limiting nutrient. This path model was not rejected (chi(2) = 1 4.10, 10 df, p = 0.17) indicating a quantitative agreement with the hypothe sis.