Ecological differentiation ofEuphorbia lasiocaula andE. sinanensis (Euphorbiaceae) I plant height,phenology and allocation to stems and leaves

Resource allocation and the seasonal change of stem length inEuphorbia lasiocaula Boiss. andE. sinanensis (Hurusawa) T. Kurosawa et H. Ohashi were examined in 10 populations on hills in Miyagi Prefecture, northern Japan. Differences were found in the diameter of stem, stem/leaf ratio of dry weight, vegetative dry weight/leaf area, and the beginning, end and duration of stem growth.Euphorbia lasiocaula has a thicker stem, a larger stem/ leaf ratio, a larger vegetative dry weight/leaf area, a later beginning and end of stem growth and a longer period of stem growth thanE. sinanensis. These differences support the relationships among plant height, resource allocation and phenology predicted by the mathematical models of Givnish (1982) and Sakai (1991, 1994). The tall and thick stems ofE. lasiocaula are considered to be favorable for capturing sunlight in grassy places, causing it to allocate much of its resources to the stems. On the other hand,E. sinanensis is considered to be adapted to deciduous forest floors or forest margins because it completes growing before it is shaded by canopy trees or by tall herbs, which is enabled by the larger allocation of resources to the leaves.