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

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.

Revising traditional theory on the link between plant body size and fitness under competition: evidence from old-field vegetation.

The selection consequences of competition in plants have been traditionally interpreted based on a "size-advantage" hypothesis - that is, under intense crowding/competition from neighbors, natural selection generally favors capacity for a relatively large plant body size. However, this conflicts with abundant data, showing that resident species body size distributions are usually strongly right-skewed at virtually all scales within vegetation. Using surveys within sample plots and a neighbor-removal experiment, we tested: (1) whether resident species that have a larger maximum potential body size (MAX) generally have more successful local individual recruitment, and thus greater local abundance/density (as predicted by the traditional size-advantage hypothesis); and (2) whether there is a general between-species trade-off relationship between MAX and capacity to produce offspring when body size is severely suppressed by crowding/competition - that is, whether resident species with a larger MAX generally also need to reach a larger minimum reproductive threshold size (MIN) before they can reproduce at all. The results showed that MIN had a positive relationship with MAX across resident species, and local density - as well as local density of just reproductive individuals - was generally greater for species with smaller MIN (and hence smaller MAX). In addition, the cleared neighborhoods of larger target species (which had relatively large MIN) generally had - in the following growing season - a lower ratio of conspecific recruitment within these neighborhoods relative to recruitment of other (i.e., smaller) species (which had generally smaller MIN). These data are consistent with an alternative hypothesis based on a 'reproductive-economy-advantage' - that is, superior fitness under competition in plants generally requires not larger potential body size, but rather superior capacity to recruit offspring that are in turn capable of producing grand-offspring - and hence transmitting genes to future generations - despite intense and persistent (cross-generational) crowding/competition from near neighbors. Selection for the latter is expected to favor relatively small minimum reproductive threshold size and hence - as a tradeoff - relatively small (not large) potential body size.