## Materials and Methods  
### Experimental Set-Up  
Based on previous observations made by the Regional Forest Services, which pointed out some scattered mortality events some years ago in several Aleppo pine populations located in central Spain, in June 2017 we selected several declining and non-declining stands in two study areas, Cerros Concejilles (40\({}^{\circ}\)16'47.18'' N; 3\({}^{\circ}\)28'54.43'' W) and Cerro Palomero (40\({}^{\circ}\)5'0.09'' N; 3\({}^{\circ}\)22'37.95'' W) forests. Both areas are the result of old Aleppo pine plantations implemented during the 1970s. Reforestations were carried out with pines coming from the same nursery and seed source, as the Regional Forest Service reported to us.  
The climate in both areas is Mediterranean-temperate characterized by cold winters and intense summer droughs, with an average annual precipitation and temperature of 455 mm and 14 \({}^{\circ}\)C respectively (1951-1999 data period from the Arganda weather station, 40\({}^{\circ}\)18'23.16'' N; 3\({}^{\circ}\)26'52.43'' W, Spanish Meteorological Agency, AEMET), however, the climatic information for the 10 years prior to the experiment shows an increasing trend in the average temperature and a decreasing trend for annual precipitation (Figure 1).  
The area of study are smooth hills with basic soils that come from marly limestone sedimentary rocks. The soil of these areas consists of loamy soils, Gypsic cambisol according to the FAO soil classification. The main traits are low fertilization, high carbonate contents and a basic pH (Table 1). The dominant understory vegetation is dominated by alpha-grass tussocks (_Stipa tenacissima_ L.) associated with other perennial grasses, such as _Brachypodium retusum_ (Pers.) [PERSON]. 1812, and evergreen shrubs, and subshrubs like _Rhamnus lycioides_ L., _Quercus coccifera_ L., _Rosmarinus officinalis_ L., Sp. Pl.,1, 23, 1753 or _Thymus vulgaris_ L.  
Figure 1: Annual precipitation (in continuous grey line) and average temperature (in continuous black line) for the 10 years previous to the experiment. Dashed lines show the tendency through time for both parameters.  
The different degrees of disease in each tree and stand were established by visual criteria: healthy stands with non-symptomatic pines (NA-trees) where no declining processes or pine mortality was observed, and declining stands (i.e., stands with decaying pines and tree mortality) within which it was possible to distinguish between non-symptomatic pines with no evident disease signs (NS-trees), and symptomatic pines with visible disease symptoms (S-trees). Symptoms were related to dry leaves on apparently healthy branches, mortality of branches and twigs, defoliation degree and needle malformations due to abnormal growth in terminal shoots (see Figure S1 in Supplementary Materials for descriptive pictures).  
We selected a total of five forest stands: two healthy stands, Concejiles and Palomero, (C and P, respectively) and three declining stands: Concejiles-1 (C1), Concejiles-2 (C2) and Palomero-1 (P1). Six pines displaying each symptomatology type in all the stands were chosen, which resulted in 12 NA pines, 18 NS and 18 S pines. Thus, the total amount of pine trees considered herein was 48.