BIOMASS ALLOCATION AND GEOMETRY OF THE CLONAL FOREST HERB LAPORTEA CANADENSIS: ADAPTIVE RESPONSES TO THE ENVIRONMENT OR ALLOMETRIC CONSTRAINTS?

Using a conceptual model, I predicted the direction of biomass allocation and geometric responses to several environmental variables for Laportea canadensis, a clonal forb dominating the herbaceous stratum of many North American floodplain and mesic forests. Laportea stems and plants, especially dominant ones, generally (60%) respond as predicted to canopy opening, conspecific leaf area and density, and poor drainage, but are merely reduced in growth when growing on sandier soils. However, allometric relationships explain most of the variation in geometry and allocation. Still, variation in geometry and allocation (as great as among 21 species of herbs studied by Givnish 1982]), helps explain the success of Laportea in a range of microenvironments. In upland forests, stems in canopy gaps are tallest but allocate relatively less biomass to leaves than shaded stems, suggesting that interherb competition is the major problem faced under canopy gaps. Leaf morphology also changes with increasing canopy opening—individual leaves are larger, heavier, and thicker and are displayed on more steeply ascending petioles. Floodplain plants respond to light gaps mainly with changes in leaf morphology and display. With increasing conspecific density and leaf production, Laportea stems in both uplands and floodplains grow taller, allocate relatively more biomass to stems, and display leaves higher on the stem. The allocation and geometry of taller stems are more independent of density, and more closely affected by tree-canopy opening, than are small stems. Intermediate soil textures in floodplains promote maximum Laportea production; variations in other factors are less important. Poorly drained soils in floodplains (heavy-textured soils at low elevations) cause decreased Laportea height and absolute leaf weight, but increase relative allocation to leaves and roots, as predicted. On the other hand, Laportea appears poorly adapted to sandier soils. Rather than responding to sandier soils as predicted, Laportea's overall growth is reduced. Geometric responses of Laportea to environment are mediated by allometric realities: an increase in height favored in productive environments produces a concomitant decrease in relative leaf allocation. Although predicted (presumably adaptive) shifts are significant when plant size is accounted for, most of the variation in allocation and geometry is due to allometry.