Global allocation rules for patterns of biomass partitioning.

Enquist and Niklas proposed global rules for plant biomass allocation allometry (1). However, early plant ontogeny (from emergence to ;5 g plant dry mass) follows different rules than they propose, and this early stage constitutes a crucial period for establishment, with plant size ranging across six orders of magnitude. At this crucial stage, their model falls short in a number of important respects. Enquist and Niklas first suggested that total leaf area } ML } MT 34, where ML is standing leaf dry mass and MT is total plant dry mass. In other words, specific leaf area (SLA), defined as total lamina area/ML, remains constant. After producing their first true leaves, however, plants commonly decline dramatically in SLA (2–4). For juveniles of seven woody species ranging in MT from 9 mg to 27 g, we found that SLA } MT 20.22 (4). This scaling may change in later ontogeny, but SLA declines further from saplings to trees (5, 6). Enquist and Niklas also proposed that ML } MS 3⁄4 } MR 34, where MS is stem dry mass and MR is root dry mass (1). For small plants, however, their model produces up to a tenfold error. The data for early ontogeny actually support ML 1 MS } MR—a constant shootto-root ratio (2, 4, 7, 8)—as is predicted by the coordinated growth of shoot and root meristems (7, 9). Given MS } MR, the typical pattern is ML } MT in early ontogeny (2, 4, 10, 11). Finally, Enquist and Niklas assumed that gross photosynthesis, B, } MT 34—an analogy with Kleiber’s Law—but data are insufficient to support this assumption for early ontogeny. That pattern does fit realistic ontogenetic allometries, however, if the leaf-area-based photosynthetic rate (Parea) is stable. Parea sometimes increases ontogenetically with plant size, but only slightly, as the leaf mesophyll thickens (5, 6), because fewer photons penetrate additional mesophyll layers (12). If SLA } MT 21⁄4 and ML } MT, then leaf area } MT 3⁄4 and B } MT 34. Here, as in so many processes in early establishment, SLA plays a fundamental role (13–15). Lawren Sack Department of Organismic and Evolutionary Biology Biological Laboratories Harvard University 16 Divinity Avenue Cambridge, MA 02138, USA E-mail: [email protected]