Abstract
BackgroundTheory predicts that plant species win competition for a shared resource by more quickly preempting the resource in hotspots and by depleting resource levels to lower concentrations than its competitors. Competition in natural grasslands largely occurs belowground, but information regarding root interactions is limited, as molecular methods quantifying species abundance belowground have only recently become available.Principal FindingsIn monoculture, the grass Festuca rubra had higher root densities and a faster rate of soil nitrate depletion than Plantago lanceolata, projecting the first as a better competitor for nutrients. However, Festuca lost in competition with Plantago. Plantago not only replaced the lower root mass of its competitor, but strongly overproduced roots: with only half of the plants in mixture than in monoculture, Plantago root densities in mixture were similar or higher than those in its monocultures. These responses occurred equally in a nutrient-rich and nutrient-poor soil layer, and commenced immediately at the start of the experiment when root densities were still low and soil nutrient concentrations high.Conclusions/SignificanceOur results suggest that species may achieve competitive superiority for nutrients by root growth stimulation prior to nutrient depletion, induced by the presence of a competitor species, rather than by a better ability to compete for nutrients per se. The root overproduction by which interspecific neighbors are suppressed independent of nutrient acquisition is consistent with predictions from game theory. Our results emphasize that root competition may be driven by other mechanisms than is currently assumed. The long-term consequences of these mechanisms for community dynamics are discussed.
Highlights
Co-occurring plant species frequently share space and compete belowground for essential soil nutrients [1,2]
Festuca was clearly projected as the superior species in nutrient competition: its root length densities in monoculture were 1.4–2.1 times higher than of Plantago (Fig. 1, C and D), and soil nutrient solution measurements throughout the study period showed that Festuca monocultures more quickly took-up nutrients in the rich soil layer and depleted them to a lower concentration than Plantago monocultures (Fig. 3)
The species projected as the better competitor for nutrients based on the monocultures (Festuca rubra) did not win the competition
Summary
Co-occurring plant species frequently share space and compete belowground for essential soil nutrients [1,2]. Competition theory predicts that plant species win competition for a shared resource by more quickly preempting the resource supply in hotspots, as a result of greater root plasticity [1,3,4,5], and by depleting resource levels to lower concentrations than their competitors [6,7,8]. Mixtures can draw more resources and will produce more biomass than the average of the monocultures (‘‘overyield’’) if species occupy different niches, such as different rooting depths, take up different nutrient sources, or if they segregate in phenology [10,12,13,14,15,16,17,18,19] This classical model of resource competition and plasticity to nutrients does not take into account responses to neighbors independent of responses to nutrients [20,21]. Competition in natural grasslands largely occurs belowground, but information regarding root interactions is limited, as molecular methods quantifying species abundance belowground have only recently become available
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