The allometry of reproduction within plant populations

P> The quantitative relationship between size and reproductive output is a central aspect of a plant's strategy: the conversion of growth into fitness. As plant allocation is allometric in the broad sense, i.e. it changes with size, we take an allometric perspective and review existing data on the relationship between individual vegetative (V, x-axis) and reproductive (R, y-axis) biomass within plant populations, rather than analysing biomass ratios such as reproductive effort (R/(R+V)). The allometric relationship between R and V among individuals within a population is most informative when cumulative at senescence (total R-V relationship), as this represents the potential reproductive output of individuals given their biomass. Earlier measurements may be misleading if plants are at different developmental stages and therefore have not achieved the full reproductive output their size permits. Much of the data that have been considered evidence for plasticity in reproductive allometry are actually evidence for plasticity in the rate of growth and development. Although a positive x-intercept implies a minimum size for reproducing, a plant can have a threshold size for reproducing without having a positive x-intercept. Most of the available data are for annual and monocarpic species whereas allometric data on long-lived iteroparous plants are scarce. We find three common total R-V patterns: short-lived, herbaceous plants and clonal plants usually show a simple, linear relationship, either (i) passing through the origin or (ii) with a positive x-intercept, whereas larger and longer-lived plants often exhibit (iii) classical log-log allometric relationships with slope < 1. While the determinants of plant size are numerous and interact with one another, the potential reproductive output of an individual is primarily determined by its size and allometric programme, although this potential is not always achieved. Synthesis. The total R-V relationship for a genotype appears to be a relatively fixed-boundary condition. Below this boundary, a plant can increase its reproductive output by: (i) moving towards the boundary: allocating more of its resources to reproduction, or (ii) growing more to increase its potential reproductive output. At the boundary, the plant cannot increase its reproductive output without growing more first. Analysing size-dependent reproduction is the first step in understanding plant reproductive allocation, but more integrative models must include time and environmental cues, i.e. development.