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.

Environmental control of terpene emissions from Cistus monspeliensis L. in natural Mediterranean shrublands.

The large amount of volatile organic compound (VOC) emitted by vegetation modifies air quality contributing to both tropospheric ozone and secondary organic aerosol production. A better understanding of the factors controlling VOC emissions by vegetation is mandatory in order to improve emission estimates derived from tropospheric chemistry models. Although the Mediterranean shrublands are particularly abundant and rich in emitting species, their emission potential is poorly known. Focusing on a VOC-emitting shrub species widespread in the Mediterranean area (Cistus monspeliensis L.), we measured and analysed its emissions of terpenes taking into account the age of individuals, the season of sampling and the soil type. Sampling was done under natural environmental conditions. Species of the genus Cistus are frequently reported to be storing species, although we found only one stored monoterpene and three sesquiterpenes in very low amount. Major emitted compounds were alpha-pinene and beta-myrcene. Total terpene emissions were not influenced by plant age but emission of some individual terpenes was positively correlated with age. A strong seasonal effect was evidenced. A larger amount of terpenes was emitted during spring and summer than during fall and winter. Summer emission rates were nearly 70 times higher than winter emission rates. Total and individual terpene emissions were influenced by soil type; emissions on siliceous substrate were ca. seven times higher than those on calcareous substrate. In conclusion, it appears clearly that environmental factors such as soil nature and season should be taken into account in order to achieve improved modelling of terpene emissions by shrub species.

Water deficit stress induces different monoterpene and sesquiterpene emission changes in Mediterranean species. Relationship between terpene emissions and plant water potential.

The effects of water deficit stress and plant water potential (psi) on monoterpene and sesquiterpene leaf emissions from Rosmarinus officinalis, Pinus halepensis, Cistus albidus and Quercus coccifera were studied over 11 days of water withholding (from t(1) to t(11)), after substrates had achieved their field capacity (control pots: t(0)). Volatile compounds were sampled from the same twig per plant all throughout the study, using a dynamic bag enclosure system. Volatiles, collected in Tenax TA, were studied by means of GC-FID and GC-MS. Monoterpene emissions of water stressed plants (t(1)-t(11)) were either similar to those of control seedlings (R. officinalis and Q. coccifera) or higher (P. halepensis and C. albidus). By contrast, sesquiterpene emissions were strongly reduced or inhibited after four days of water withholding, particularly for R. officinalis, thus altering terpene emission composition. Despite the positive effect of water stress on leaf monoterpene emissions of P. halepensis and C. albidus, the significant correlation between these emissions and psi showed a slow decrease of these emissions over long term water deficit periods. This contrasted with the rapid decline of sesquiterpene emissions of R. officinalis according to lower values of psi. These results provide an overall picture of the different responses of monoterpene and sesquiterpene emissions to progressive water loss. They also reveal the utility of using psi for estimating some emission rates of some species according to drought conditions.

Phenolic compounds content in Pinus halepensis Mill. needles: a bioindicator of air pollution.

Foliar phenol concentrations (total and simple phenols) were determined in Aleppo pine (Pinus halepensis Mill.) needles collected in June 2000, from 6 sites affected by various forms of atmospheric pollutants (NO, NO(2), NO(x), O(3) and SO(2)) monitored during two months. Results show an increase in total phenol content with exposure to sulphur dioxide and a reduction with exposure to nitrogen oxide pollution. p-Coumaric acid, syringic acid and 4-hydroxybenzoic acid concentrations increase with exposure to nitrogen oxide pollution, whereas gallic acid and vanillin decrease in the presence respectively of sulphur dioxide and ozone. This in situ work confirms the major interest of using total and simple phenolic compounds of P. halepensis as biological indicators of air quality.