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.

用于表征植物体大小变异的替代指标已有多种，但此前针对草本植物的绝大多数研究均聚焦于干重（dry mass）。干重能够较为准确且易于测定，可用于比较植物将太阳能转化为储存碳的相对能力。然而从植物自身的视角来看，其对周边紧邻邻体（尤其是种群拥挤时）的本地生物环境的感知，或许用原位记录的空间占用（Space Occupancy, S-O）指标能更准确地体现，而非经烘箱烘干后测定的干重。本研究针对采自安大略省东部天然草本植物种群的本土植株，探究了干重与空间占用（S-O）类植物体大小替代指标之间的关联。研究对象涵盖138个体型跨度广泛的物种，以及10个目标物种各20株体型跨度广泛的植株；研究人员对这些植株原位（野外）记录了株高、冠幅最大侧向延展度、估算冠面积与冠体积，并在实验室测定了鲜重与干重。干重与鲜重呈高度相关（决定系数r²>0.95）且为等速生长（isometric）关系，这表明在部分研究中，物种间（或植株间）的含水量变异或许并不重要，因此鲜重可替代干重使用。但干重与其他S-O类植物体大小指标间的若干关系却呈现出异速生长（allometry）特征：即S-O体型更小的植株，其干重相对更少。换言之，这类植株拥有更高的体质量密度（body mass density, BMD）——即单位S-O体型对应的干重更多。这些结果对实验设计与研究方法均具有实际应用价值，同时也为解读“繁殖经济（reproductive economy）”——即小型植株产生后代的能力——提供了新的启示：因为繁殖力与干重（同一生长季内产生的生物量）通常呈正相关的等速生长关系。据此，本研究报道的干重与S-O体型间的异速生长关系表明：由于体质量密度更高，S-O体型更小的植株或许产生的后代数量更少，但减少的幅度并非成比例；换言之，它们在单位S-O体型空间占用下，能够产生更多的后代。