Net below canopy fluxes in Canarian laurel forest canopies

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Bulk precipitation, stemflow and throughfall were collected in a Canarian laurel forest (North Tenerife), and chemically analysed to determine the net below canopy fluxes. Annual negative fluxes were found for H +, NO 3 −, SO 4 − and Cl − and annual positive fluxes for Ca 2+, Mg 2+, K +, Na +, HCO 3 −, PO 4 3− and DOC. Negative fluxes for most of these ions have been reported previously but this is the first time that they have all been measured at the same time in one forest. The relative importance of atmospheric deposition and canopy leaching to net below canopy fluxes was evaluated using two approaches previously employed in this type of study. The first approach consisted of calculating total inputs, taking inputs as bulk precipitation or bulk precipitation plus the additional input from scavenging dry deposition by vegetation surfaces, corrected by the maximum and minimum leaching rate. The values obtained for inputs were very similar, possibly due to the minimum effect on scavenging particles by the canopy. In the second approach, linear regressions were developed for net below canopy fluxes and the variables related to dry deposition (antecedent dry period) and leaching (event quantity and duration, etc.); this showed that NO 3 − in net below canopy fluxes came from dry deposition, the rest of the elements from leaching, and SO 4 2− from both. Similar results were obtained using both approaches. It was found that a simple linear regression model with event quantity as the independent variable can readily substitute the Lovett and Lindberg (1984) [J. Appl. Ecol. 21 (1984) 1013] model. The latter model was applied for separate internal and external sources in the case of SO 4 2−. This forest differed greatly from other, previously studied forests. Large amounts of water were required for a positive flux (leaching), suggesting that the negative net below canopy fluxes of some elements were due to the low water regimes during the study year. Possible causes of this were the high value of the canopy saturation and the pattern of scattered rainfall, which led to extreme values of interception losses. The point at which the line representing the relationship between net below canopy fluxes and event quantity intersected the x axis varied depending on the leachable properties of the canopy ions.