## 4 Discussion
### Phytoplasma Infection and Induced Defense Mechanisms  
The presence of _C. P. pini_ was detected in all analyzed samples, which supports the wide distribution of this infection at the forest scale as described in some previous studies [41]. However, visual tree decline signs related to phytoplasma compatible symptomatology were only observed in some stands. In fact, the presence of this phytoplasma in non-symptomatic trees has also been reported by several authors [35; 36; 37] who pointed out an unusual pathological expression of _C. P. pini_[36]. Therefore, two phenomena occur in the studied area. First, a general presence of _C. P. pini_ in healthy and unhealthy trees, and secondly a differential expression of pine disease (phytoplasma symptom compatible), which could be related to other factors such as climatic conditions or a differential degree of water stress affecting pines in the stands.  
Terpenoid compounds are commonly found in nature acting as semi-chemicals of feeding deterrents, fungicides or defensive compounds [19]. Since _C. P. pini_ is a parasite of the phloem, changes in terpenoid composition must be also expected as a reaction to pathogenic infection [42]. Variations in these compounds as a response to biotic attacks have been largely reported [43; 44; 45; 46; 47]. For instance, [PERSON] et al., [48] described a positive relation between the concentration of terpenoids in _Cupressus semiperoviens_ infected by _Seiridium cardinale_, which causes the progressive infection known as cypress canker. Accordingly, in our case, the terpenoid composition in phloem differed depending on the health condition of the stands. Regardless of their symptoms, the pines from declining stands (NS-trees or S-trees) displayed a different composition than pines from healthy stands (NA-trees). Indeed, quantitative terpenoids (monoterpene and sesquiterpene types) formed a clear grouping pattern of trees in two separated clades, related with disease prevalence instead of _C.P.  
Figure 9: Panel **a)** Tree mortality (dead trees/ha) in each stand considering only the trees that had recently died (i.e., trees that died less than two years ago). Panel **b)** Percentage of dead trees evidencing bark beetle attacks in each pine stand. The smooth dark gray stack represents attacks by _Tomicus destruens_; the smooth light gray stack represents attacks by _Orthotomic evosus_; the striped dark gray stack represents mixed attacks by both species. Recent logging did not enable us to determine the mortality and bark beetle attacks in P1.  
_pini_ presence. The same pattern was found in the relative abundance of the eight major secondary compounds described. While the relative abundances of the monoterepenes (-)-\(\alpha\)-pinene, D-limonene, camphene and one unknown compound, and the sesquiterpenes carophyllene and humulene, were higher in the trees from declining stands (S and NS trees), isomer (+)-\(\alpha\)-pinene and sesquiterpene isocaryophyllene were more abundant in trees from healthy stands (NA trees). These differences in the pattern of expression could constitute the initially induced response in those trees (NS trees) that have not evidenced declining symptoms yet. All trees are from the same nursery and seed source, as the Regional Forest Service reported to us, so different pine chemotypes are unlikely. On the other hand, future quantification of _C. P. pini_ in host tree tissues by rt-PCR or Q-PCR could clarify if the induced level of response is correlated with microorganism concentrations.