The abundance of fungal endophytes does vary among six study sites as the p-value of perMANOVA analysis is 0.0001, which is less than 0.05 (table-4). Saskatchewan and Alberta's sites have the highest endophyte abundance with an average ASV of 1120 and 1050, while the New Brunswick site has the lowest with an average ASV of 250 (figure-15). Within six study sites, the Old channel river in Saskatchewan has significantly higher total fungal endophyte abundance compared to the Upper green river in New Brunswick (table-5). In addition, the Slave lake forest in Alberta has significantly higher total fungal endophyte abundance in contrast to the Upper green river in New Brunswick (table-5). 

The Slave lake forest in Alberta is dominated by two endophyte genera (Cladosporium and Chrysomyxa), with around 3700 and 1200 ASVs (figure-14). The dominant endophyte genera in the Upper green river in New Brunswick are Cladosporium, with over 500 ASVs (figure-14). The study site in Twist lake in Ontario has one dominant endophyte genera Cladosporium, with more than 2000 ASVs (figure-14). And this site is the most diverse study site. The abundance data in the Cimon town in Quebec indicates three dominant endophyte genera: Rhizosphaera, Coniozyma, and Cladosporium (figure-14). Cladosporium and Chrysomyxa also dominate the Dasserat study site in Quebec (figure-14). The study site in Saskatchewan is dominated by one endophyte genera Lirula, with the highest abundance of more than 4000 ASVs among all identified fungal endophytes (figure-14).

One interesting finding is that Cladosporium is the most abundant endophyte genera in all six study sites, with a relatively high ASV number (figure-14). This is because some species in Cladosporium are air-bone fungal endophytes that can travel a long distance (Hamayun et al., 2009). So there may be a potential to use Cladosporium as a biological control agent for large-scale pest outbreaks.

Another interesting finding is the boosted high abundance of Lirula in the Old channel river site in Saskatchewan (figure-14). One possible reason is that this study site may contain many trees with needle cast. According to previous studies, some species in the Lirula genera are causal agents for conifer needle cast, and endophytes usually cause non-detectable symptoms (Müller et al., 2001; Stefani & Bérubé, 2006).

The presence of fungal endophytes can explain this difference. The site in the Old channel river in Saskatchewan has a high abundance of genera Lirula (figure-15), and Lirula has a strong positive correlation with monoterpene concentration (figure-22). The three dominant terpenes in this site all belong to monoterpenes. In addition, most sites (except New Brunswick- Upper green river) show similar features of terpene concentration and total fungal abundance (figure-15 and figure-16). As a result, terpene concentration differences may link with fungal endophyte abundance variations.

Direct gradient analysis was performed to test the correlations between fungal endophytes abundance and terpene (monoterpene/sesquiterpene) concentration.

The abundance of four fungal endophyte genera, including Pleurotus, Periconia, Phialocephala, and Gnomoniaceae, indicates a strong positive relationship with total monoterpene concentration (figure-17). The abundance of fungal endophyte genera Chalara and Lirula shows a strong positive relationship with the total concentration of sesquiterpenes (figure-17). Other 23 fungal endophyte genera do not correlate with the total concentration of monoterpene and sesquiterpene (figure-17), which suggests a non-significant correlation between total fungal abundance and the concentration of each terpene (figure-17).

Our study demonstrated that six specific fungal endophyte genera could induce terpene production in white spruce. This finding provides evidence of the potential of fungal endophyte genera Lirula, Pleurotus, Periconia, Phialocephala, Chalara, and Gnomoniaceae to become a biological control agent dealing with spruce budworms via induced terpene biosynthesis. Various studies support this significant finding, and scientists discovered that fungal endophytes could induce host trees to produce monoterpenes and sesquiterpenes that are harmful to the survival of spruce budworms (Chen et al., 2021; Tan & Zou et al., 2001).

Lirula could be an ideal biological agent in Saskatchewan based on their relative abundance (figure-14). At the same time, Chalara genera could be applied in Ontario and New Brunswick (figure-14), but the colonization success needs to be justified. Other four genera have a low abundance among all six locations. As a result, they may not be suitable for becoming biological control agents across Canada. This key finding also provides future study direction to investigate the performance of those two endophyte genera in dealing with spruce budworms.

No evidence supports a correlation between total endophyte abundance and the concentration of any detected terpene compounds (figure-18). Although alpha-phellandrene has some strong positive correlation with total fungal endophyte abundance, the overall ordination is poor and not reliable even we tried four different transformations (figure-18). This is because only six fungal genera correlate with the concentration of monoterpenes and sesquiterpenes (figure-17). Within them, Lirula shows high abundance only in the Saskatchewan-Old channel river site, and Chalara indicates relatively high abundance only in the Ontario-Twist lake (figure-14). All other four fungal endophyte genera show low abundance across all six study sites (figure-14). As a result, it is reasonable that there is no significant correlation between total endophyte abundance and the concentration of any terpene compound.

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