The exploitative segregation of plant roots

Plant roots determine carbon uptake, survivorship, and agricultural yield and represent a large proportion of the world’s vegetation carbon pool. Study of belowground competition, unlike aboveground shoot competition, is hampered by our inability to observe roots. We developed a consumer-resource model based in game theory that predicts the root density spatial distribution of individual plants and tested the model predictions in a greenhouse experiment. Plants in the experiment reacted to neighbors as predicted by the model’s evolutionary stable equilibrium, by both overinvesting in nearby roots and reducing their root foraging range. We thereby provide a theoretical foundation for belowground allocation of carbon by vegetation that reconciles seemingly contradictory experimental results such as root segregation and the tragedy of the commons in plant roots.

Root and shoot competition lead to contrasting competitive outcomes under water stress: A Meta-analysis

BackgroundCompetition is a critical process that shapes plant communities and interacts with environmental constraints. Though important to natural communities and agricultural systems, there are surprising knowledge gaps related to mechanisms that belie those processes: the contribution of different plant parts on competitive outcomes and the effect of environmental constraints on these contributions.ObjectiveStudies that partition competition into root-only and shoot-only interactions assess whether plant parts impose different competitive intensities using physical partitions and serve as an important way to fill knowledge gaps. Given predicted drought escalation due to climate change, we focused meta-analytic techniques on the effects of water supply and competitive outcomes.MethodsWe searched Web of Science for peer-reviewed studies and found 2042 results. From which six suitable studies with 92 effect sizes on 10 species were identified to test these effects.ResultsWater availability and competition treatment (root-only, shoot-only, and full plant competition) significantly interact to affect plant growth responses (p < 0.0001). Root-only and full plant competition are more intense in low water availability conditions than shoot-only competition. Shoot-only competition in high-water availability was the most intense showing the opposite pattern. These results also show that the intensity of full competition is similar to root-only competition and that low-water availability intensifies root competition while weakening shoot competition.ConclusionsThese results emphasize the importance of root competition and these patterns of competition may shift in a changing climate, creating further urgency for further filling knowledge gaps to address issues of drought on plant interactions and communities.

Above- and below-ground interactions between Lotus tenuis and Cynodon dactylon under different fertilization levels

Llobet, M., Vignolio, O. R., Savé, R. and Biel, C. 2012. Above- and below-ground interactions between Lotus tenuis and Cynodon dactylon under different fertilization levels. Can. J. Plant Sci. 92: 45–53. Mixtures of legumes and grasses are recommend for xeric garden fields, landscape restorations and to improve productivity, digestibility and quality of grasslands and pastures, due to the complementarity in the use of resources, their resistance to wide edaphoclimatic conditions and the nitrogen-fixing ability of the legume. However, legumes persistence is poor. Two greenhouse studies examined the relative importance of above- and below-ground competition and root physiological attributes between a legume Lotus tenuis and a grass bermudagrass (Cynodon dactylon) under two fertilization levels. The plants were arranged using the ″Divided pot" technique during 34 wk. Root competition was more important on the performance of both species than shoot competition. Cynodon dactylon had greater nutrient use efficiency than L. tenuis. The low root hydraulic resistance and high specific root length of C. dactylon could enhance its ability to explore the soil and compete for water and nutrients more efficiently than L. tenuis, which compensates for these characteristics with a higher root:shoot ratio than C. dactylon. Although mechanisms involved in root competition between species are not fully understood, our results show that root physiological and morphologic traits contribute to understanding the responses of the species in mixture.

Relative Importance of Shoot and Root Competition in Dry-Seeded Rice Growing with Junglerice (Echinochloa colona) and Ludwigia (Ludwigia hyssopifolia)

Two separate studies were conducted to determine the relative importance of root and shoot competition in dry direct-seeded rice growing with junglerice and ludwigia. By growing rice in pots placed within larger pots such that the roots of the plants were either separated from or free to mingle with those of neighboring weeds, or by growing rice in the same pots but in the absence of weeds, the relative importance of shoot and root competition can be described. When rice was grown together with either weed species, shoot competition reduced the growth and yield of rice more than root competition. Results suggest that shoot competition for light may be the primary mechanism determining competitive outcomes between dry direct-seeded rice and junglerice or ludwigia. Junglerice was more competitive than ludwigia, which may reflect the C4 metabolism efficiency of junglerice compared to the C3 metabolism of ludwigia. Rice grain yield was highly correlated with above- and belowground biomass. The results also suggest the importance of measuring the whole plant when seeking to understand differences in the competitive ability of dry direct-seeded rice.

Above and below-ground competition between Kura clover (Trifolium ambiguum) and meadow bromegrass (Bromus biebersteinii): A greenhouse study

Kura clover (Trifolium ambiguum) is a perennial legume that shows potential for use in pasture mixtures in western Canada. Previous studies have shown that early growth of kura clover is reduced when grown in mixtures with grass species. A greenhouse study examined the relative importance of above- and below-ground competition on growth and development of kura clover when grown with a grass species. Kura clover plants were grown in pots with meadow bromegrass (Bromus biebersteinii) and barriers were put in place to remove all competition, shoot competition, root competition, or to allow full competition. After 12 wk, plants were harvested. Height, leaf area, leaf number, leaf dry weight, and root/crown dry weight were measured. Kura clover leaf number and leaf area were greatest when roots did not interact with meadow bromegrass roots. Vegetative biomass of kura clover doubled when there was no root competition. Shoot competition did not alter leaf number, leaf area, or leaf biomass. Successful establishment of kura clover is dependent on the reduction of root competition during the seedling phase. Measures taken to minimize the below-ground interaction should positively affect the yield potential of both species. Key words: Kura clover, meadow bromegrass, shoot competition, root competition