The chemistry of precipitation and groundwater in a coastal Pinus Pinea forest (Castel Fusano area, central Italy) and its relation to stand and canopy structure

Chloride and sodium contents of net precipitation (throughfall + stemflow) and groundwater below a Pinus Pinea coastal forest (Castel Fusano, central Italy) are strongly dependent on the barrier-effect accomplished by canopies to the wind-transported sea salt aerosol. During dry periods, forest canopy holds marine aerosol, successively discharged by rainfall: the longer the dry period, the higher the amount of salts accumulated and released to groundwater. Main winds direction and regimes of precipitation are important variables influencing the chemistry of net precipitation and groundwater. Stemflow is a point-source input and its contribution to groundwater recharge and chemistry is spatially relevant, even if it represents only a small fraction of net precipitation. Chloride and sodium concentration of stemflow and groundwater was very high in the portion of the forest directly exposed to sea winds and decreased progressively at increasing distances from the coastline. The drop was very marked just few tenths of metres from the forest face and the rate of reduction was less significant further inland, where below-canopy enrichment was negligible. Stand and canopy structure differences within the forest (created by a wild fire occurred in the year 2000 and by successive localised practices of forest thinning) had strong effects onto chloride and sodium contents of stemflow and groundwater. On the contrary, chloride and sodium abundance of throughfall was not notably influenced by precipitation regimes and by stand structure, being a diffuse input to ground, only moderately intercepted by canopies. However, forest thinning increased volumes of throughfall.