Competition of the active roots of Cassia siamea Lam. and Zea mays L.(cv.Katumani Composite B) in an alley cropping trial in semi arid Kenya

A study on root competition in alley cropping was carried out in an agroforestry system, involving Cassia siamea Lam. and maize ( Zea mays L. cv. Katumani composite B). The existence and intensity of root competition in the top soil as manifested by the distribution of the active roots of cassia and maize, in space and time, was assessed. The root length density of maize was far greater than that of cassia in the upper 10 cm, implying that cassia was not competing with maize for water and/or nutrients at that depth. However, at maize crop tasselling and grain filling stages there was a marked overlap of roots of the two plants at lower depths (20—50 cm). This varied with distance from the cassia hedge in a way that there was a tendency for highest overlap near middle maize rows. This partly explained observed yield differences. Therefore cassia may not be a suitable choice for alley cropping with maize under semi\|arid conditions on non\|sloping land, unless most of its active roots can be properly managed to absorb resources below the feeding rhizosphere of the active maize roots.

Experimental evidence for root competition effects on community evenness in one of two phytometer species

Aims Plant-plant interactions,being positive or negative,are rec-ognized to be key factors in structuring plant communities.However,it is thought that root competition may be less impor-tant than shoot competition due to greater size symmetry below-ground.Because direct experimental tests on the importance of root competition are scarce,we aim at elucidating whether root competition may have direct or indirect effects on commu-nity structure.Indirect effects may occur by altering the overall size asymmetry of competition through root-shoot competitive interactions.Methods We used a phytometer approach to examine the effects of root,shoot and total competition intensity and importance on evenness of experimental plant communities.Thereby two different phytom-eter species,Festuca brevipila and Dianthus carthusianorum,were grown in small communities of six grassland species over three levels of light and water availability,interacting with neighbouring shoots,roots,both or not at all.Important Findings We found variation in community evenness to be best explained if root and shoot(but not total)competition were considered.However,the effects were species specific:in Dianthus communities increasing root competition increased plant community evenness,while in Festuca communities shoot competition was the driving force of this evenness response.Competition intensities were influenced by environmental conditions in Dianthus,but not in Festuca phytometer plants.While we found no evidence for root-shoot interactions for neither phytom-eter species root competition in Dianthus communities led to increased allocation to shoots,thereby increasing the potential ability to perform in size-asymmetric competition for light.Our experiment demonstrates the potential role of root competition in structuring plant communities.

Using coloured roots to study root interaction and competition in intercropped legumes and non-legumes

Aims Root interactions between neighbour plants represent a fundamental aspect of the competitive dynamics in pure stand and mixed cropping systems.The comprehension of such phenomena places big methodological challenges,and still needs clarification.The objectives of this work were(i)to test if a species with coloured roots can be used to examine the interaction in a legume-non-legume intercropping system;(ii)to verify the importance of initial root growth on the successive root development ofmixture component plants;(iii)to test if the root interaction in the shallowlayers has consequences for deep root growth and(iv)to compare the effect of intraspecific and interspecific competition on root development and biomass growth.Methods A detailed study on root growth and interaction was carried out using rhizotron tubes where two legume species were grown in pure stands or were intercropped with red beet,a variety of Beta vulgaris L.with clear red roots.Within the rhizotrons,the three species were grown either without competitors,with two plants of the same species to measure intraspecific competition or with one legume and one red beet plant to study interspecific competition.The use of mixtures where one component has clearly coloured roots,together with several scalar measurements of root depth and proliferation,allowed the measurement of the root system of each species when grown in the mixtures.Important findings The use of rhizotron tubes coupled with species with coloured roots represented a valuable method to study the belowground interaction in mixed cropping systems.The initial root growth was a very important feature for the subsequent dominance of a species and it was not related to seed dimension.Initial root growth was also important because the root interactions in the shallower soil layers were found to influence the root growth in deeper soil.The root system of the red beet showed much faster and deeper growth than that of the legumes,and made red beet the dominant component in the mixtures while the legume root system was confined to the shallower soil layer.Intraspecific competition was well tolerated by the legumes,but it was limiting for the highly competitive red beet.The outcome of root interaction between neighbour plants was confirmed to be speciesspecific as it changed according to the intensity of the competitive effect/response of each species of the mixture:both legumes were slightly affected by the intraspecific and highly affected by interspecific competition while red beet was more affected by intraspecific competition but strongly dominant when intercropped with legumes.

Complementarity among species in horizontal versus vertical rooting space

Aims Many experiments have shown a positive effect of species richness on productivity in grassland plant communities.However,it is poorly understood how environmental conditions affect this relationship.We aimed to test whether deep soil and limiting nutrient conditions increase the complementarity effect(CE)of species richness due to enhanced potential for resource partitioning.Methods We grew monocultures and mixtures of four common grassland species in pots on shallow and deep soil,factorially combined with two nutrient levels.Soil volume was kept constant to avoid confounding soil depth and volume.Using an additive partitioning method,we separated biodiversity effects on plant productivity into components due to species complementarity and dominance.Important findings Net biodiversity and complementarity effects were consistently higher in shallow pots,which was unexpected,and at the low nutrient level.These two results suggest that although belowground partitioning of resources was important,especially under low nutrient conditions,it was not due to differences in rooting depths.We conclude that in our experiment(i)horizontal root segregation might have been more important than the partitioning of rooting depths and(ii)that the positive effects of deep soil found in other studies were due to the combination of deeper soil with larger soil volume.

Biodiversity and belowground interactions mediate community invasion resistance against a tall herb invader

Aims Species-rich plant communities are hypothesized to be more resistant against plant invasions because they use resources in a more efficient way.However,the relative contributions of aboveground competition and belowground interactions for invasion resistance are still poorly understood.Methods We compared the performance of Knautia arvensis transplants growing in plots differing in plant diversity both under full competition and with shoots of neighbors tied back to determine the relative strength of aboveground competition in suppressing this test invader without the confounding effect of shading.In addition,we assessed the effects of belowground competition and soil-borne pathogens on transplant performance.Important Findings Both aboveground competition and plant species richness strongly and independently affected invader performance.Aboveground biomass,height,leaf mass per area and flowering of transplanted individuals of K.arvensis decreased with increasing species richness of the host community.Species-rich and species-poor communities both imposed equally strong aboveground competition on K.arvensis.However,belowground interactions(especially belowground root competition)had strong negative effects on transplant performance.In addition,the presence of grasses in a plant community further reduced the performance of K.arvensis.Our results suggest that belowground competition can render species-rich host communities more suppressive to newly arriving species,thus enhancing community invasion resistance.