The Eurasian watermilfoil (Myriophyllum spicatum L.) has partially replaced wild celery (Vallisneria americana Michx.) as a community dominant in the littoral zones of lakes of Madison, Wisconsin. The two species have very different growth forms, with that of M. spicatum corresponding more closely to the optimal growth form simulated by the macrophyte production model WEED. The objective of this research was to investigate the mechanisms by which Vallisneria could compensate for its nonoptimal growth form and coexist with Myriophyllum.A quantification of midsummer growth form for the two species at a rooting depth of 80-90 cm showed that M. spicatum had 68% of its shoot biomass within 30 cm of the surface, whereas V. americana had 62% of its leaf biomass within 30 cm of the bottom. Vallisneria had a light extinction coefficient ranging from 0.013 to 0.019 m·g, much higher than the value (ca. 0.006 m·g) for M. spicatum. This indicates less effective penetration of light to lower leaves of V. americana. Half-saturation constants describing the light-dependence of carbon uptake in "shade" and "sun" tissues ranged from 60-197 microeinsteins·m·s for V. americana, and 164-365 μeinsteins·m·s for M. spicatum. The optimum temperature for photosynthesis was 33.6°C for M. spicatum and 32.6°C for V. americana, but Myriophyllum was nearly twice as effective at carbon uptake at 10°C. Integration of all of the above features with WEED showed that, for midsummer conditions, V. americana more than compensated for apparently disadvantageous morphological features by its greater physiological adaptability to low light regimes. Coupled with the temperature-dependence of photosynthesis, it appears that V. americana is favored by midsummer conditions, whereas M. spicatum is at an advantage at other times.

The uptake of phosphorus, the biomass, and the standing P stock were measured over the course of a year in roots and shoots of the Eurasian watermilfoil, Myriophyllum spicatum, from Lake Wingra, Wisconsin. The resulting data were used to estimate the relative contributions of root and shoot uptake to the phosphorus economy of the plant and to examine the role of the plant in moving phosphorus between sediment and water.The total yearly uptake of P by a square meter of Myriophyllum was 3.0 g P m-*. Root uptake accounted for 2.2 g, shoot uptake only 0.8 g. The rate of P release from healthy shoots was insignificant, but about 2.8 g P m-* yr-l was lost due to shoot turnover.Since most of the P uptake is by the roots and much of the plant P is transferred to and lost by the shoots, Myriophyllum is a potentially important vector in the movement ofP from the sediments to the water. The net transfer of P from the sediments to shoots of Myriophyllum in Lake Wingra is about 2.0 g P m-* yr-l. Release of this P during decay makes Myriophyllum an important source of P for pelagic phytoplankton and can explain much of the previously reported P export from the littoral zone of Lake Wingra.

Temperature, phosphorus, and nitrogen were investigated as possible factors influencing decay of Myriophyllum spicatum shoots.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Surveys relating soil wettability and the establishment of annual plants were conducted on soil hummocks under burned and unburned shrubs and trees in a desert scrub community 4 years following a fire. Hummocks under burned and unburned individuals of Larrea divaricata, Prosopis juliflora, and Cercidium floridum were nearly devoid of established annual vegetation, whereas the surrounding soil was densely populated with several species of annual plants, predominately Sphaeralcea emoryi. Hydrophobic (water-repellent) layers of soil were found at various depths in the barren hummocks. In general, the layers were further below the soil surface under burned shrubs or trees, and the degree of water repellency was greater. The soil surrounding the hummocks did not contain hydrophobic layers. The mosaic of annuals in the desert scrub vegetation is probably the result of a reduction of available soil moisture due to the formation of water-repellent soils under the shrubs. The effect was more pronounced following fire. LITERATURE CITED Bond, R. D. 1964. The influence of the microflora on the physical properties of soils. II. Field studies on water repellent sands. Australian J. Soil Res. 2: 123-131. Bond, R. D., and J. R. Harris. 1964. The influence of the microflora on physical properties of soils. I. Effects associated with filamentous algae and fungi.