Atmospheric element deposit on tree barks: The opposite effects of rain and transpiration

The elemental composition of the deposit formed on the bark of ash-trees was studied over several months on stems ranging from 3 months to 10 years. For this purpose (1) the total elemental composition of the deposit, (2) the structure and composition of the solid particles and (3) the deposit dry weight per dm2 were studied. Concurrently the part of this superficial deposit washed out by rain during 3.5 months was sampled at each rain event and its elemental composition analyzed. This study shows that the deposit was submitted to an intense turnover, with an average leaching-out flux reaching approximately 27 mg dm−2 month−1 and a very low increase of the deposit weight per dm2 during the first five years and almost null afterwards. The origin of this superficial deposit was investigated. The main part (78 ± 10%) was organic matter originating from the atmospheric deposition or from the tree. The inorganic content originated partly from the atmosphere, with geogenic and anthropogenic particles, and also from the tree, in which it was demonstrated by scanning electron microscopy – energy dispersive X-ray that a non-negligible part was obtained from bark transpiration, inducing a superficial deposit, mainly of Ca or K in the area surrounding the lenticels. All those results demonstrate that this bark superficial deposit is a complex matrix which must not be considered as a simple cumulative archive but which seems to be the source of interesting information targeting mostly recent atmospheric pollution pressures, when compared to the bulk of suber integrated particles.

► The age of trees barks changes from one month for the twigs to decades for the trunk.
► The deposit on the bark can be quantified (mg dm−2) and analyzed (ICP-MS and SEM-EDX).
► One air-transferred part represents a cumulative anthropogenic contamination (≤10%).
► Another part comes from xylem sap after an apoplastic transfer through lenticels.
► The deposit is partly leached by rain giving an approximately three-month-turnover.