Effects of biological and meteorological factors on stemflow chemistry within a temperate mixed oak–beech stand

The effects of meteorological variables and tree species on stemflow chemistry were investigated within a mixed oak–beech stand during the leafed and the leafless seasons. Stemflow was collected after each rain event. For each investigated ion (H+, Mg2+, Ca2+, K+, NH4+, Na+, Cl−, NO3−, SO42−), mixed linear models were used to analyse the effects of the rain volume (R) and of the length of the preceding dry period (ADP) on net stemflow ion fluxes as well as to assess the effect of tree size on these relationships. The models generally explained more than 70% of the total variability. The product between trunk circumference and tree height (C130Htot) explained most of the inter-individual variability, except for oak during the leafed season for which the effect of tree size was not significant or very limited. On the other hand, besides R and ADP, other rain event characteristics like wind force and direction were suggested to also partly explain the inter-event variability. For each season, net stemflow ion fluxes tended to increase with increasing R and ADP, whose coefficients were interpreted as leaching and dry deposition rates, respectively; exceptions were negative exchange rates (i.e. absorption) for NH4+ and NO3− during the leafless period and for H+ during the leafed season. Moreover, when it appeared significant, the effect of tree size corresponded to an increase of leaching, absorption and/or dry deposition rates as C130Htot increased. Exchange rate estimates were generally higher for the leafless season compared with the leafed period as well as for beech compared with for oak. Dry deposition rate estimates were generally higher for the leafless season compared with the leafed period. Differences in dry deposition rates between both species were particularly pronounced for the leafless season with much higher estimates for beech compared with oak.