Plasticity of Root Traits under Competition for a Nutrient-Rich Patch Depends on Tree Species and Possesses a Large Congruency between Intra- and Interspecific Situations

Zana A Lak,Hans Sandén,Boris Rewald,Mathias Mayer,Douglas L Godbold

doi:10.3390/f11050528

Abstract

Belowground competition is an important structuring force in terrestrial plant communities. Uncertainties remain about the plasticity of functional root traits under competition, especially comparing interspecific vs. intraspecific situations. This study addresses the plasticity of fine root traits of competing Acer pseudoplatanus L. and Fagus sylvatica L. seedlings in nutrient-rich soil patches. Seedlings’ roots were grown in a competition chamber experiment in which root growth (biomass), morphological and architectural fine roots traits, and potential activities of four extracellular enzymes were analyzed. Competition chambers with one, two conspecific, or two allospecific roots were established, and fertilized to create a nutrient ‘hotspot’. Interspecific competition significantly reduced fine root growth in Fagus only, while intraspecific competition had no significant effect on the fine root biomass of either species. Competition reduced root nitrogen concentration and specific root respiration of both species. Potential extracellular enzymatic activities of β-glucosidase (BG) and N-acetyl-glucosaminidase (NAG) were lower in ectomycorrhizal Fagus roots competing with Acer. Acer fine roots had greater diameter and tip densities under intraspecific competition. Fagus root traits were generally more plastic than those of Acer, but no differences in trait plasticity were found between competitive situations. Compared to Acer, Fagus roots possessed a greater plasticity of all studied traits but coarse root biomass. However, this high plasticity did not result in directed trait value changes under interspecific competition, but Fagus roots grew less and realized lower N concentrations in comparison to competing Acer roots. The plasticity of root traits of both species was thus found to be highly species- but not competitor-specific. By showing that both con- and allospecific roots had similar effects on target root growth and most trait values, our data sheds light on the paradigm that the intensity of intraspecific competition is greater than those of interspecific competition belowground.

Highlights

There is strong evidence that belowground competition is an important structuring force in terrestrial plant communities including forests [1,2,3,4]—and often at least intense as aboveground competition [5,6,7]
In Acer, fine root biomass (FRB) did not vary significantly under intra- and interspecific competition compared to isolation
The relative distance plasticity indices (RDPITotal ; Figure 3a) of FRB and CRB possessed in both species the greatest values across all measured traits (beside RDPITotal (PEANAG ) of Fagus, see below; Figure 3a)

Summary

Introduction

There is strong evidence that belowground competition is an important structuring force in terrestrial plant communities including forests [1,2,3,4]—and often at least intense as aboveground competition [5,6,7]. Intransitive mechanisms (i.e., competition mediated by factors other than resources, e.g., soil biota) are increasingly considered [8,9,10,11]. Competition occurs if negative effects on performance or fitness of a target plant (or components of it) result from the presence of another plant [12]. For a discussion on competitive response and effect components). Over-proportional reductions of fine root biomass in mixed stands and even over-proliferation of fine roots in soil volumes shared by allospecific roots have been reported [15,16,17]—pointing towards either greater or reduced root competition intensities under interspecific competition

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Conclusion