Pedunculate Oaks (Quercus robur L.) Differing in Vitality as Reservoirs for Fungal Biodiversity

Abstract

Ecological significance of trees growing in urban and peri-urban settings is likely to increase in future land-use regimes, calling for better understanding of their role as potential reservoirs or stepping stones for associated biodiversity. We studied the diversity of fungal endophytes in woody tissues of asymptomatic even aged pedunculate oak trees, growing as amenity trees in a peri-urban setting. The trees were classified into three groups according to their phenotypic vitality (high, medium, and low). Endophytes were cultured on potato dextrose media from surface sterilized twigs and DNA sequencing was performed to reveal the taxonomic identity of the morphotypes. In xylem tissues, the frequency and diversity of endophytes was highest in oak trees showing reduced vitality. This difference was not found for bark samples, in which the endophyte infections were more frequent and communities more diverse than in xylem. In general, most taxa were shared across the samples with few morphotypes being recovered in unique samples. Leaf phenolic profiles were found to accurately classify the trees according to their phenotypic vitality. Our results confirm that xylem is more selective substrate for endophytes than bark and that endophyte assemblages in xylem are correlated to the degree of host vitality. Thus, high vitality of trees may be associated with reduced habitat quality to wood-associated endophytes.

Keywords: Quercus robur; endophytes; fungal diversity; network analysis; phenolics; tree vitality.

FIGURE 1

Even aged 15-year-old pedunculate oak trees planted in 2003 in Alnarp. In the picture the three vitality classes: high vitality (A), medium vitality (B), and low vitality (C). Photo Marta Agostinelli.

FIGURE 2

(A) Colonization frequency (CF = proportion of bark and xylem samples with at least one emerging colony) and (B) morphotype diversity (S^MT = number of different MTs recovered per twig) mean values in bark (white bars) and xylem (black bars) of pedunculate oak twigs (n = 32) collected from trees (n = 8) in three vitality classes (H, high; M, medium; L, low). Shown are the mean values of eight replicate trees (±SE) with different letters indicating significant difference among vitality classes (Kruskal Wallis test, p < 0.05 in xylem tissues for both CF and S^MT).

FIGURE 3

Individual-based rarefaction curves of pedunculate oak endophytes indicating the number of MTs isolated per tissue (bark or xylem) and vitality class (H, high; M, medium; L, low) with (A) and without (B) singletons.

FIGURE 4

Venn diagrams representing the distribution of unique MTs in bark (solid line) and xylem (dashed line), and MTs shared between the two tissues (overlapping area) based on the presence-absence (A) and on the similar abundance in the tissue (B). The numbers indicate how many morphotypes were found in each tissue and shared between the tissues. MT30 is not included since it represents the group of singletons.

FIGURE 5

Venn diagrams representing the distribution of unique MTs found in high vitality class (solid line), medium class (dashed line), low vitality class (dotted line), and MTs shared among the three vitality classes (overlapping areas) based on their presence-absence (A) and similar abundance (B) in bark and the presence-absence (C) and similar abundance (D) in xylem. The numbers indicate how many morphotypes were found in each vitality class and shared among vitality classes. MT30 is not included since it represents the group of singletons.

FIGURE 6

Bipartite network graphs representing the isolates relative abundance of fungal isolates, arranged after morphotype (indicated by number) in oak tissues (xylem and bark) from high (A), medium (B), and low (C) tree vitality classes.

FIGURE 7

Vitality class based on canonical discriminant analysis of HPLC data (10 UV-absorbing peaks. The red points represent the high vitality leaves (H), the green points the medium vitality leaves (M), and the blue points the low vitality leaves (L). The internal ellipsoid line represents the normal ellipse region estimated to contain the 50% of the population of the group. The external ellipsoid line represents the confidence region containing the true mean of the group with 95% accuracy. The numbers represent the phenolics from the 10 biggest phenolic peaks.

FIGURE 8

Areas under the curve of the peaks of absorbance (AU/mg) of the ten biggest phenolics peaks for high (dotted line), medium (dashed line), and low (solid line) vitality classes. The concentric circles represent the areas under the curve with an increasing of 10,000 AU/mg at every concentric circle. The numbers represent the phenolics from the 10 biggest phenolic peaks.