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 (r2 > .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.
Study sites and study species
Sampling was conducted as time permitted between May and October 2013 from natural plant populations found in a variety of habitats (old‐field meadows, fence rows, and roadside edges) in southern Ontario, mainly in the vicinity of Kingston, Ontario, Canada (44°15′N, 76°30′W), including Queen's University Biological Station (44°33′N, 76°21′W). Candidate species were chosen to include a wide range of species body sizes (based on visual estimation).
Field sampling and data collection
Populations were sampled when resident plants were at the reproduction stage to ensure that body size was at or approaching maximum for the current growing season. Only populations with a minimum of 20 resident plants were used, and where possible, up to five populations at least 2 km apart were sampled for each species. For each population, the largest individual “rooted unit” (Aarssen, 2014) of each species (based on visual estimation) was chosen for sampling. Individuals showing signs of heavy herbivore damage (e.g., missing stems indicated by breakage points) were avoided. In a few cases, where only one population was located, the five largest individuals were collected from that population.
Ten candidate species with population sizes >100 were also chosen for within‐species analyses. In each case, the largest and smallest resident reproductive plants were sampled as well as 18 additional reproductive plants chosen haphazardly to represent the range of resident body sizes between the largest and smallest (20 per species).
For each sampled individual, height, maximum lateral canopy extent, and the perpendicular lateral canopy extent (at the widest point along the maximum lateral extent) were measured. The above‐ground portion of each individual was then harvested, and fresh mass was recorded in the laboratory using an analytical balance. Each individual was then placed in a drying oven at 80°C for 72 hr and then reweighed to obtain dry mass.