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Symbiotic associations between plants and arbuscular mycorrhizal fungi are ubiquitous and ecologically important in many grasslands. Differences in species responses to mycorrhizal colonization can have a significant influence on plant community structure. The growth responses of 36 species of warm- and cool-season tallgrass prairie grasses and 59 tallgrass prairie forbs to arbuscular mycorrhizal (AM) fungal colonization were assessed in greenhouse studies to examine the extent of interspecific variation in host-plant benefit from the symbiosis and patterns of mycorrhizal dependence among host plant life history (e.g., annual, perennial) and taxonomic (e.g., grass, forb, legume, nonlegume) groups and phenological guilds. There was a strong and significant relationship between phenology of prairie grasses and mycorrhizal responsiveness, however this relationship was less apparent in forbs. Perennial warm-season C(4) grasses and forbs generally benefited significantly from the mycorrhizal symbiosis, whereas biomass production of the cool-season C(3) grasses was not affected. The root systems of the cool-season grasses were also less highly colonized by the AM fungi, as compared to the warm-season grasses or forbs. Unlike the native perennials, annuals were generally not responsive to mycorrhizal colonization and were lower in percentage root colonization than the perennial species. Plant growth responsiveness and AM root colonization were positively correlated for the nonleguminous species, with this relationship being strongest for the cool-season grasses. In contrast, root colonization of prairie legumes showed a significant, but negative, relationship to mycorrhizal growth responsiveness.

We examined the role of arbuscular mycorrhizal fungi (AMF) in ecosystems using soil aggregate stability and C and N storage as representative ecosystem processes. We utilized a wide gradient in AMF abundance, obtained through long-term (17 and 6 years) large-scale field manipulations. Burning and N-fertilization increased soil AMF hyphae, glomalin-related soil protein (GRSP) pools and water-stable macroaggregates while fungicide applications reduced AMF hyphae, GRSP and water-stable macroaggregates. We found that AMF abundance was a surprisingly dominant factor explaining the vast majority of variability in soil aggregation. This experimental field study, involving long-term diverse management practices of native multispecies prairie communities, invariably showed a close positive correlation between AMF hyphal abundance and soil aggregation, and C and N sequestration. This highly significant linear correlation suggests there are serious consequences to the loss of AMF from ecosystems.

Summary0 Designs for greenhouse studies of interactions between plants are reviewed and recommendations for their use are provided[ 1 Papers published over a 09!year period showed the replacement series design to be the most popular\ especially in studying cropÐweed interactions[ Fifty per cent of the studies involved only two species\ although studies testing the interaction between di}erent genotypes of only a few species were also popular[ 2 Limitations imposed by the choice of design\ the variables measured\ and the analysis used on the range of inferences that may be validly drawn from the experiment are frequently not well understood or appropriate for the questions that appear to be addressed[ One example is the failure to distinguish the outcome of competition "the long!term outcome of interaction# and the e}ects of species on each other[ 3 Studies in which only _nal yield is measured are severely limited as to the inferences which may be drawn[ E}ects due to interspeci_c interaction during the course of the experiment cannot then be separated from pre!existing di}erences\ and interpretation may be biased towards species whose individuals were initially larger[ In addition\ measurements at several times are necessary to understand the changing dynamics of species interaction[ 4 Simple pair!wise mixtures can assess the e}ect of treatment factors on the outcome of competition[ Replacement series and related diallel designs generally produce results that may be size!biased even when initial interspeci_c di}erences are known[ Additive designs "including targetÐneighbour designs#\ despite confounding density with species proportions\ o}er considerable scope for addressing mechanistic ques! tions about interspeci_c interactions[ Designs that allow response surface analysis can avoid many of the problems inherent in the other methods\ but all need to be adjusted for initial interspeci_c di}erences[ Designs for multiple species experiments are still largely untested\ although several designs have been used[ At the level of the individual plant\ hexagonal fan designs permit study of the e}ects of varying the spatial pattern\ and the densities and the relative proportions of interacting species\ but su}er from lack of independence and lack of randomization[ Keywords] additive\ competition\ competitive hierarchy\ diallel\ experimental design\ hexagonal fans\ interspeci_c interaction\ replacement series\ response surface\ size! bias\ targetÐneighbour design Journal of Ecology "0888# 76\ 0Ð05

Clones of Solidago canadensis were studied to determine the degree of physiological interdependency among interconnected ramets. Ramets severed from their parental clone in the field experienced reductions in growth, survivorship, and flowering, relative to intact controls. The results show that new daughter ramets are physiologically dependent upon resources translocated from their parental clone and that this dependency declines with time.When one of the ramets within each of several connected pairs was shaded to I 0% of full sunlight, rates of photosynthesis increased in the remaining illuminated ramets. This and patterns of ramet survivorship and growth indicate that during periods of light limitation ramets become reintegrated. Shaded ramets become supported by the translocation of assimilates from other ramets within the clone.There is a tradeoff between rhizome number and length. When ramets become resource limited they produce fewer but longer new rhizomes. This results in the placement of daughter ramets at a location distant from the parent and may improve their probability of encountering more favorable conditions.The physiological integration among intraclonal ramets reduces the establishment risks of new daughter ramets and enables a genet to integrate local heterogeneity in resource availability. The result is that establishment and competitive ability are increased equitably among genets, and genets are buffered against localized "patch-specific" selection. This reduction in genetic deaths may be one mechanism resulting in the maintenance of variation in clonal populations.