Does Prescribed Burning Affect Leaf Secondary Metabolites in Pine Stands?
Prescribed burning (PB) is gaining popularity as a low-cost forest protection measure that efficiently reduces fuel build-up, but its effects on tree health and growth are poorly understood. Here, we evaluated the impact of PB on plant defenses in Mediterranean pine forests (Pinus halepensis and P. nigra ssp. laricio). These chemical defenses were estimated based on needle secondary metabolites (terpenes and phenolics including flavonoids) and discussed in terms of chlorophyll fluorescence and soil nutrients. Three treatments were applied: absence of burning (control plots); single burns (plots burned once); and repeated burns (plots burned twice). For single burns, we also explored changes over time. In P. laricio, PB tended to trigger only minor modifications consisting exclusively of short-lived increases (observed within 3 months after PB) in flavonoid index, possibly due to the leaf temperature increase during PB. In P. halepensis, PB had detrimental effects on physiological performance, consisting of (i) significant decreases in actual PSII efficiency (ΦPSII) in light-adapted conditions after repeated PB, and (ii) short-lived decreases in variable-to-maximum fluorescence ratio (Fv/Fm) after single PB, indicating that PB actually stressed P. halepensis trees. Repeated PB also promoted terpene-like metabolite production, which increased 2 to 3-fold compared to control trees. Correlations between terpene metabolites and soil chemistry were found. These results suggest that PB impacts needle secondary metabolism both directly (via a temperature impact) and indirectly (via soil nutrients), and that these impacts vary according to species/site location, frequency and time elapsed since last fire. Our findings are discussed with regard to the use of PB as a forest management technique and its consequences on plant investment in chemical defenses.
KeywordsFire prevention Secondary metabolism Terpenes Phenolic compounds Flavonoid Pinus halepensis Pinus laricio
This study was funded by the French Ministry of Agricultural and Fisheries (GIS Incendie program – N° E 11/07). The authors are grateful to the “Office National des Forêts” (ONF) for PB, especially Philippe Caramelle, Gilles Planelles, Antonella Massaiu, and Jean-Yves Duret from the forestry service of the South Corsica department. We also thank Lionel Kmiec from the ONF (Bouches-du-Rhône and Vaucluse) for his help with selecting sites in the Luberon and for the technical information on prescribed burning in this region, and Virginie Baldy, Anne-Bousquet Mélou, Sylvie Dupouyet, and Thierry Gauquelin from the IMBE for assistance in the field.
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