Supply pre‐emption, not concentration reduction, is the mechanism of competition for nutrients

Abstract

Summary• Concentration reduction theory is the leading theory regarding the mechanism of competition for nutrients in soils among plants, yet it has not been rigorously tested.• Here we used a spatially explicit, fine-scale grid-based model that simulated diffusion and plant uptake of nutrients by plants in soil to test whether concentration reduction theory was appropriate for terrestrial plant competition for nutrients.• In the absence of competition, increasing the rate of diffusion allows a plant to maintain positive growth rates below the lowest average concentration to which it can reduce nutrients in soil solution ( R *). As such, differences among plants in the reduction of soil moisture, which here primarily affects nutrient diffusion, can cause R * to predict competitive success incorrectly. The stronger competitor for nutrients captures the largest proportion of the nutrient supply by ensuring nutrients contact its roots before those of a competitor.• Although the metric derived from concentration reduction theory, R *, might have predictive power for competitive outcomes in terrestrial ecosystems, this evidence suggests that plants outcompete other plants for nutrients by pre-empting the supply, not reducing the average concentration.

Seasonal and scale-dependent variability in nutrient- and allelopathy-mediated macrophyte–phytoplankton interactions

“…The significant sprig growth-nutrient uptake correlation for E. canadensis (r 2 = 0.740, p = 0.002) might have been driven by the wider range of biomass change for E. canadensis than C. demersum ( Table I), suggesting that total nutrient removal (or leaking) might have been a nonnegligible cofactor in E. canadensis -R. subcapitata interactions as well. Despite the coarse estimation basis for nutrient uptake rates, our results suggest that sprigs inhibited algal growth by preemptively sequestering nutrients before algae can do so, supporting Craine et al 's (2005) view of availability-and not uptake-driven competition for nutrients by plants. However, we cannot go beyond a generally stated involvement of competition for nutrients between macrophyte sprigs and algae, as 1. nutrient availability remained overall high; 2. nutrient effects cannot be separated satisfactorily with current methodology (e.g., Lürling et al, 2006;Gross et al, 2007); 3. both macrophyte and algal metabolism quickly adapt to substrate availability (Touchette and Burkholder, 2001;Collos et al, 2005); and 4. competition remains one of the most difficult ecological mechanisms to demonstrate empirically (e.g., Connell, 1980Connell, , 1983Goldberg and Scheiner, 2001).…”

Mass invasion of non-nativeElodea canadensisMichx. in a large, clear-water, species-rich Norwegian lake – impact on macrophyte biodiversity

“…The very slight recent oligotrophication in Steinsfjord, with a decrease in water-column total phosphorus concentration from 11-12 mg L x1 in 1979-80 to 8-9 mg L x1 in 2003-04, and a corresponding increase in Secchi depth from around 5.5 to 6 m (Berge et al, unpublished data), also can hardly explain the reduction in abundance suffered by N. flexilis in the same time frame. E. canadensis growth patterns are highly variable, but this species is able to start growing early in the spring (Nichols and Shaw, 1986), sprouting from old stems, and may pre-empt the available nutrient pool in the sediment (Craine et al, 2005). This could have enabled E. canadensis to greatly expand its presence in Steinsfjord at the expenses of other species.…”

Belowground competition in a broad-leaved temperate mixed forest: pattern analysis and experiments in a four-species stand

“…(1) Different species may differ in their surfacespecific water and/or nutrient uptake rates leading to resource pre-emption by the superior competitor (see the contrasting water uptake rates in trees discussed above, and Craine et al 2005).…”

“…Because the mode of competition substantially influences population and community dynamics (e.g. Łomnicki 1980;Yokozawa et al 1998;Aikio and Pakkasmaa 2003), there is a vital debate as to whether belowground competition is symmetric or asymmetric (de Kroon et al 2003;Schenk 2006). Most experimental evidence suggests that soil resource acquisition by root systems of competing plants tends to be proportional to their sizes ('symmetric'; e.g.…”