Herbs are not just small plants: What biomass allocation to rhizomes tells us about differences between trees and herbs.

Premise: Biomass accumulation over years in vertical stems of trees leads to hypoallometric scaling between stem and leaf biomass within this growth form, while for herbaceous species, biomass allocation between these organ types typically exhibits isometry. However, biomass accumulation in herbs can occur in belowground perennating organs (e.g., rhizomes) that are, contrary to aboveground parts of herbs, long-lived. Although ecologically important, biomass allocation and accumulation in rhizomes (and similar organs) have mostly not been studied.

Methods: We assembled data on biomass investments into plant organs for 111 rhizomatous herbs based on a literature survey and greenhouse experiment. We estimated the proportion of whole-plant biomass invested into rhizomes and, using allometric relationships, analyzed scaling between rhizome and leaf biomass and whether it is more variable than for other organs.

Results: On average, rhizomes comprise 30.2% of the total plant biomass. The proportion allocated to rhizomes does not change with plant size. Scaling between rhizome and leaf biomass is isometric, and allocation to rhizomes is not more variable than allocation to other organs.

Conclusions: Rhizomatous herbs accumulate substantial biomass in rhizomes, and rhizome biomass scales isometrically with leaves, contrary to the hypoallometric relationship between stem and leaves in trees. This difference suggests that the rhizome biomass is in balance with aboveground biomass-a resource of carbon for rhizome formation that, at the same time, is dependent on carbon stored in rhizomes for its seasonal regrowth.