Reproductive Effort in Plants. 2. Does Carbon Reflect the Allocation of Other Resources?

The assessment of reproductive effort (RE) in plants has been hampered by the uncertainty about which resource should be used as the currency to evaluate resource-allocation patterns. In this paper, we argue that carbon, because of its important energetic role in plants, tends to integrate the allocation patterns of other resources and can therefore be used as a common currency to assess allocation patterns. We show that the respiratory cost of plant growth in Agropyron repens increases as the concentration of nitrogen and phosphorus in the tissue increases. Furthermore, the respiratory cost per unit of nutrient generally increases as these nutrients become less available. This means not only that carbon allocation tends to reflect the distribution of other nutrients, but also that the allocation of carbon tends to be biased toward that of the most limiting resources. The second point is particularly important because the concept of resource allocation is relevant in life history studies only if resources are limiting growth. This bias in the allocation of carbon is reflected in the convergence at low levels of applied nitrogen of RE calculated in terms of total carbon (biomass plus respiratory carbon) and that calculated in terms of nitrogen. In our experiments, the overall rank correlation between RE calculated in terms of carbon and nitrogen across various genotypes and resource treatments was 0.804. Convergence was not evident between reproductive efforts calculated in terms of carbon and phosphorus, probably because phosphorus was not a seriously limiting factor in these experiments. However, the overall rank correlation between carbon- and phosphorus-based measures of RE was still high (0.900), suggesting that even when phosphorus is not limiting, carbon is an adequate measure of phosphorus allocation.