THE INTERSPECIFIC MASS-DENSITY RELATIONSHIP AND PLANT GEOMETRY

We present an a priori theoretical framework for the interspecific all ometric relationship between stand mass and plant population density. Our model predicts a slope of - 1/3 between the logarithm of stand mas s and the logarithm of stand density, thus conflicting with a previous ly assumed slope of -1/2. Our model rests on a heuristic separation of resource-limited living mass and structural mass in the plant body, W e point out that because of similar resource requirements among plants of different sizes, a nonzero plant mass-density slope is primarily d efined by structural mass. Specifically, the slope is a result of (i) the physical size-dependent relationship between stem width and height , (ii) foliage-dependent demands of conductance, and (iii) the cumulat ive nature of structural mass. The data support our model, both when t he potential sampling bias of taxonomic relatedness is accounted for a nd when it is not. Independent contrasts analyses show that observed r elationships among variables are not significantly different from the assumptions made to build the model or from its a priori predictions. We note that the dependence of the plant mass-density slope on the fun ctions of structural mass pro,ides a cause for the difference from the zero slope found in the animal population mass-density relationship; for the most part, animals do not have a comparable cumulative tissue type.