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

Invasive plants have a multitude of impacts on plant communities through their direct and indirect effects on soil chemistry and ecosystem function. For example, plants modify the soil environment through root exudates that affect soil structure, and mobilize and/or chelate nutrients. The long-term impact of litter and root exudates can modify soil nutrient pools, and there is evidence that invasive plant species may alter nutrient cycles differently from native species. The effects of plants on ecosystem biogeochemistry may be caused by differences in leaf tissue nutrient stoichiometry or secondary metabolites, although evidence for the importance of allelochemicals in driving these processes is lacking. Some invasive species may gain a competitive advantage through the release of compounds or combinations of compounds that are unique to the invaded community—the “novel weapons hypothesis.” Invasive plants also can exert profound impact on plant communities indirectly through the herbicides used to control them. Glyphosate, the most widely used herbicide in the world, often is used to help control invasive weeds, and generally is considered to have minimal environmental impacts. Most studies show little to no effect of glyphosate and other herbicides on soil microbial communities. However, herbicide applications can reduce or promote rhizobium nodulation and mycorrhiza formation. Herbicide drift can affect the growth of non-target plants, and glyphosate and other herbicides can impact significantly the secondary chemistry of plants at sublethal doses. In summary, the literature indicates that invasive species can alter the biogeochemistry of ecosystems, that secondary metabolites released by invasive species may play important roles in soil chemistry as well as plant-plant and plant-microbe interactions, and that the herbicides used to control invasive species can impact plant chemistry and ecosystems in ways that have yet to be fully explored.

Dicamba (3,6‐dichloro‐2‐methoxybenzoic acid) effectively controls many dicotyledonous weeds, but nontarget species such as soybean [Glycine max (L.) Merrill) are susceptible to spray or vapor drift. Field studies were conducted on a Canfield silt loam (fineloamy, mixed, mesic Aquic Fragiudalf) soil to determine the response of ‘Elf’ and ‘Williams’ soybean to dicamba over a wide range of applied rates, and to evaluate the use of dicamba injury symptoms to predict yield reductions. Soybean yield in response to increasing rates of dicamba was described by equations of the form y = Aexp( −bx), where y = yield, A = maximum yield (rate = 0 g ha−1), b is a constant, and x = rate of dicamba applied. Height reduction, seed number ha−1, and morphological symptoms of dicamba injury were useful in assessing yield reduction. Except for Elf soybean treated at the midbloom stage, there was no yield reduction without height reduction, regardless of foliar symptoms. Seed number ha−1 decreased with increasing rates of dicamba and was closely correlated with yield. Yield reductions greater than 10% were indicated by severe morphological symptoms of injury, such as terminal bud kill, splitting of the stem, swollen petioles, and curled, malformed pods. Other foliar symptoms, such as distinctive crinkling and cupping of the terminal leaves, occurred at rates much lower than those required to cause yield reductions.

No abstract

Allelopathy, a phenomenon where compounds produced by one plant limit the growth of surrounding plants, is a controversially discussed factor in plant-plant interactions with great significance for plant community structure. Common mycorrhizal networks (CMNs) form belowground networks that interconnect multiple plant species; yet these networks are typically ignored in studies of allelopathy. We tested the hypothesis that CMNs facilitate transport of allelochemicals from supplier to target plants, thereby affecting allelopathic interactions. We analyzed accumulation of a model allelopathic substance, the herbicide imazamox, and two allelopathic thiophenes released from Tagetes tenuifolia roots, by diffusion through soil and CMNs. We also conducted bioassays to determine how the accumulated substances affected plant growth. All compounds accumulated to greater levels in target soils with CMNs as opposed to soils without CMNs. This increased accumulation was associated with reduced growth of target plants in soils with CMNs. Our results show that CMNs support transfer of allelochemicals from supplier to target plants and thus lead to allelochemical accumulation at levels that could not be reached by diffusion through soil alone. We conclude that CMNs expand the bioactive zones of allelochemicals in natural environments, with significant implications for interspecies chemical interactions in plant communities.