No abstract

Soil phosphorus response curves of plants with and without mycorrhizas reflect two different, but complementary, phenomena. The first, plant responsiveness to mycorrhizas, is represented by the difference in growth between plants with and without mycorrhizas at any designated level of phosphorus availability. This is also a measure of mycorrhizal fungus effectiveness. The second, the lowest level of phosphorus availability at which plants can grow without mycorrhizas, is here termed dependence upon mycorrhizas. The latter definition differs from conventional usage which fails to distinguish dependence from responsiveness. Sigmoid curves generated by the three-parameter, logistic equation generally can model the responses of plants to mycorrhizas and phosphorus addition and can be used to assess responsiveness, effectiveness, and dependence. Such curves reveal that plant responsiveness or fungus effectiveness determined at a single level of phosphorus availability may be misleading when used to compare different host species' intrinsic capacities to respond to different mycorrhizal fungus species. Instead, the same relative position should be evaluated among phosphorus response curves for different species combinations. Dependence of a plant species known to benefit from mycorrhizas can be assessed with reference to only the phosphorus response curve of plants without mycorrhizas. Dependence is a constitutive property of plant species that can be used to classify them as facultatively or obligately mycotrophic. Dependence is a plant attribute upon which natural selection can act, but responsiveness and effectiveness cannot be selected directly because they are emergent properties of the interaction between plant and fungus species.

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. In experiments with plants grown in pots, vesicular-arbuscular mycorrhizae increased seedling growth of 23 of 28 species from a lowland tropical rain forest region. Mycorrhizae improved survival in six species and cotyledon retention in five species. Mycorrhizae also increased the size of bacterial nodules and the proportion of modulated plants among three leguminous species.Growth of seedlings lacking mycorrhizae slowed greatly or ceased after attainment of sizes correlated with average seed dry mass. Removing cotyledons from individuals of two species that are dependent on mycorrhizae, however, did not increase their response to inoculation. Thus, seed reserves are important for mycorrhiza formation as well as for seedling growth before infection. Large seeds are advantageous to plants that depend on vesicular-arbuscular mycorrhizae because they provide mineral reserves upon which the seedling can draw while awaiting infection.Seedlings of some species could not grow without mycorrhizae, but inoculation did not affect the growth of other species. Species that are least dependent on mycorrhizae have light seeds and colonize disturbed habitats.

No abstract

Earthquakes occasionally denude large areas of tropical forest: for example, 54 square kilometers in Panama in 1976 and 130 square kilometers in New Guinea in 1935. Earthquake rates in New Guinea, but not in Panama, are sufficiently high so that substantial areas of disturbed, nonclimax forest may accumulate. In New Guinea, earthquake-caused landslides are as important as tree falls in the disturbance regime.