Molinia caerulea alters forest Quercus petraea seedling growth through reduced mycorrhization

Abstract

Oak regeneration is jeopardized by purple moor grass, a well-known competitive perennial grass in the temperate forests of Western Europe. Below-ground interactions regarding resource acquisition and interference have been demonstrated and have led to new questions about the negative impact of purple moor grass on ectomycorrhizal colonization. The objective was to examine the effects of moor grass on root system size and ectomycorrhization rate of oak seedlings as well as consequences on nitrogen (N) content in oak and soil. Oak seedlings and moor grass tufts were planted together or separately in pots under semi-controlled conditions (irrigated and natural light) and harvested 1 year after planting. Biomass, N content in shoot and root in oak and moor grass as well as number of lateral roots and ectomycorrhizal rate in oak were measured. Biomass in both oak shoot and root was reduced when planting with moor grass. Concurrently, oak lateral roots number and ectomycorrhization rate decreased, along with a reduction in N content in mixed-grown oak. An interference mechanism of moor grass is affecting oak seedlings performance through reduction in oak lateral roots number and its ectomycorrhization, observed in conjunction with a lower growth and N content in oak. By altering both oak roots and mycorrhizas, moor grass appears to be a species with a high allelopathic potential. More broadly, these results show the complexity of interspecific interactions that involve various ecological processes involving the soil microbial community and need to be explored in situ.

Keywords: Ectomycorrhization rate; fungal symbionts; interaction; lateral root; pot experiment.

Figure 1.

(A) Fresh total weight of oak and moor grass when sole-grown (white) or mixed-grown (grey). The two horizontal lines indicate the total fresh weight at the beginning of experiment for oak and moor grass, respectively. The numbers indicate the increase in biomass between the beginning of the experiment and the harvest. (B) Dry weight of oak and moor grass shoots and roots. Values are reported as means ± SE. *, **, *** correspond to P < 0.05, 0.01 and 0.001, respectively, for Student’s t-test for each organ, n = 6.

Figure 2.

Number of lateral roots (A) and ectomycorrhization rate, i.e. ratio of number of mycorrhizal root tips to number of lateral roots (B) when sole-grown (white) or mixed-grown (grey). Values are reported as means ± SE. *** corresponds to P < 0.001 for Student’s t-test for each organ, n = 120.

Figure 3.

Proportion in percentage of oak root mycorrhizas in sole-grown (white) and mixed-grown (grey) for each taxon. Italic number next bar indicated total number of mycorrhizas sequences. The results of BLAST analyses assigning taxa to each sequence are shown on the left. NA means that the BLAST result did not give more details about the taxon. The last line represents the sequences that did not give any BLAST result.

Figure 4.

Nitrogen content in plant (A) shoots and roots when sole-grown (white) or mixed-grown (grey) and in soil (B) in sole-oak pot (light grey), sole-moor grass pot and (dark grey) in mixed-species pot (black). Values are reported as means ± SE. *correspond to P < 0.05, for Student’s t-test for each organ, n = 6.