What does body size mean, from the “plant's eye view”?

Abstract

Alternative metrics exist for representing variation in plant body size, but the vast majority of previous research for herbaceous plants has focused on dry mass. Dry mass provides a reasonably accurate and easily measured estimate for comparing relative capacity to convert solar energy into stored carbon. However, from a “plant's eye view”, its experience of its local biotic environment of immediate neighbors (especially when crowded) may be more accurately represented by measures of “space occupancy” (S–O) recorded in situ—rather than dry mass measured after storage in a drying oven. This study investigated relationships between dry mass and alternative metrics of S–O body size for resident plants sampled from natural populations of herbaceous species found in Eastern Ontario. Plant height, maximum lateral canopy extent, and estimated canopy area and volume were recorded in situ (in the field)—and both fresh and dry mass were recorded in the laboratory—for 138 species ranging widely in body size and for 20 plants ranging widely in body size within each of 10 focal species. Dry mass and fresh mass were highly correlated (r 2 > .95) and isometric, suggesting that for some studies, between‐species (or between‐plant) variation in water content may be unimportant and fresh mass can therefore substitute for dry mass. However, several relationships between dry mass and other S–O body size metrics showed allometry—that is, plants with smaller S–O body size had disproportionately less dry mass. In other words, they have higher “body mass density” (BMD) — more dry mass per unit S–O body size. These results have practical importance for experimental design and methodology as well as implications for the interpretation of “reproductive economy”—the capacity to produce offspring at small body sizes—because fecundity and dry mass (produced in the same growing season) typically have a positive, isometric relationship. Accordingly, the allometry between dry mass and S–O body size reported here suggests that plants with smaller S–O body size—because of higher BMD—may produce fewer offspring, but less than proportionately so; in other words, they may produce more offspring per unit of body size space occupancy.

Species with larger body size do not dominate neighbourhood biomass production in old‐field vegetation

“…Recent evidence also indicates that smaller species generally have more dry mass per unit of space occupancy, suggesting that they may also have more seed production per unit of space occupancy within crowded neighbourhoods (Tracey et al. ).…”

“…Hence, they have greater 'reproductive economy', which means that they are more likely to produce at least some offspring when resident plants are forcedbecause of severe neighbourhood crowdingto remain suppressed to only a fraction of their maximum potential body size (Aarssen 2015). Recent evidence also indicates that smaller species generally have more dry mass per unit of space occupancy, suggesting that they may also have more seed production per unit of space occupancy within crowded neighbourhoods (Tracey et al 2016).…”

Resident species with larger size metrics do not recruit more offspring from the soil seed bank in old‐field meadow vegetation

“…Data for MIN and MAX plant sizes, species abundance and seed bank abundance are available in the Dryad Data Repository: https://dx.doi.org/10.5061/dryad.n6c67 (Tracey, Stephens, Schamp, & Aarssen, ); https://doi.org/10.5061/dryad.0p82s0r (Tracey & Aarssen, ).…”

Neighbourhood crowding severely limits seed offspring recruitment in a temperate mesic old-field meadow

“…Successful offspring production instead requires 'reproductive economy' (sex before death) -i.e., capacity to produce at least one offspring despite severe body size suppression, and hence a high fecundity allocation (number of offspring per unit plant size per unit time) (Aarssen 2008(Aarssen , 2015. Interpretation of the components of reproductive economy, we suggest, will require future studies with particular focus on effects resulting from (i) a relatively small minimum reproductive threshold size Aarssen 2014, 2018); (ii) relatively small seed size (Aarssen 2005), (iii) offspring production through clonality (Aarssen 2008); (iv) a relatively high body mass density (hence fecundity per unit plant body space occupancy) (Tracey et al 2016); and (v) relatively high leafing intensity (associated with relatively small leaf mass) and hence larger bud bank (number of axillary meristems per unit plant body size) available for deployment as flowers/fruits -and thus, higher potential fecundity allocation (Dombroskie et al 2016).…”