A new method of growth analysis for plants that experience periodic losses of leaf mass

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Abstract
Summary: A new method (the iterative approach) is presented by which growth analyses can be conducted on plants that have been subjected to significant losses in biomass and leaf area between harvests. The method is particularly useful to analyse the effects of defoliation on growth and biomass allocation. Values for the following parameters can be estimated: absolute growth rate (g day−1), relative growth rate (RGR, g g−1 day−1), net assimilation rate (NAR, g m−2 day−1), leaf area ratio (LAR, m2 g−1), fraction of newly assimilated mass that is allocated to leaf lamina production (flam, g g−1), and daily fractional change in the average specific leaf area of plants (ρ, day−1). These parameters are determined by means of iterations. We defined a number of growth functions, and the values of NAR, flam and the SLA of newly produced leaves were changed until these functions correctly predicted the measured total plant mass, leaf lamina mass and leaf area at the end of the growth period. This avoids having to assume a constant relationship between leaf area and biomass (as in the ‘classical’ approach), and it avoids the use of polynomial functions to fit growth data (as in the ‘functional’ approach) that are unsuitable for fitting data sets exhibiting discontinuities such as abrupt changes in biomass. The method was applied to a greenhouse experiment in which we analysed the effects of sustained defoliation on growth and biomass allocation in a tropical understorey palm, Chamaedorea elegans Mart. We showed that C. elegans plants respond to defoliation with a considerable increase in the allocation of new assimilates to lamina growth (flam) and that, despite the repeated loss of leaf area and associated reductions in LAR, they had RGR values that were similar to those of undamaged plants.