FOREST FLOOR WATER-CONTENT DYNAMICS IN A DOUGLAS-FIR STAND

This paper considers the hydrology of the forest floor within a homoge neous Douglas fir forest. Time domain reflectometry measurements show that forest floor water contents have considerable spatial variabiliti es but similar temporal dynamics. Simple linear relations can be used to translate forest floor water content dynamics from one site in a fo rest to another. Forest floor evaporation rates were calculated using a previously developed forest floor evaporation model and a year of so il water and micrometeorological data. For a relatively wet site withi n the stand the calculated evaporation rate was 137 mm year(-1), for a more representative site 112 mm year(-1) and for a dry site 76 mm yea r(-1). These amounts range between 7 and 13% of the total yearly fores t evapotranspiration. Together with throughfall rates and transpiratio n rates, these forest floor evaporation rates served as boundary condi tions to a soil water model with which we simulated forest floor and m ineral soil water content dynamics. The simulations showed that throug hfall and drainage dynamics determine the forest floor water content d ynamics in wet conditions. In dry periods, forest floor evaporation an d, to a lesser extent, root water uptake determine forest floor water content dynamics. The same simulations showed that 25% of the forest f loor evaporation is replenished by capillary rise from the mineral soi l. (C) 1997 Elsevier Science B.V.