Cloud forest

Abstract: Barro Colorado Island is a 1500-ha. hill protruding 137 m above Gatun Lake. Its broad, flat top is underlain by dense basalt. From it radiate steep ridges and valleys cut into sedimentary rocks containing much volcanic debris. Soils mantling the island are generally less than 50 cm deep and are rich in clay; soils deeper than 1 m occur only on the flat hilltop. Soil properties appear to be greatly influenced by topography. The climate of Barro Colorado is typical in many ways of the lowland moist tropics: in the open, the average annual temperature is 27°C, the average diurnal temperature range is 9C, and potential evapotranspiration (Penman) is about 12 cm a month. Per year, 2600 mm of rain fall, 90% during a rainy season lasting from May through December. During the dry season, trade winds prevent convective storms, thereby dictating the seasonal rhythms of climate on this island. A simple model predicts runoff and soil moisture content from rainfall, potential evapotranspiration, and fixed characteristics of the soil. At the beginning of the dry season, the ground contains about 40 cm of water available to plants. It appears that, early in the dry season, leaffall increases markedly when a third of this moisture is exhausted, while increases in soil moisture content stimulate flowering in many plants. This model may enable one to infer phenological rhythms ofthe forest in years past from rainfall records. The permeability of many tropical soils, especially clays, decreases markedly below the fine root zone. Thus, especially if the soil is nearly saturated, runoff from a heavy rain can be forced to the surface as it travels downslope, forming saturation overland flow. Early in the rainy season, little ofthe rainfall reaching the soil runs off. Instead, the water replenishes the soil moisture depleted by evapotranspiration. Subsequent rainfall enters the soil and travels as a shallow subsurface flow and a deeper groundwater flow to drainage channels. This water reaches the channel slowly and causes stream flow to increase gradually. Later in the rainy season, rainfall saturates the soil in a progressively larger fraction of the catchment, leading to more and faster runoff from the hillslopes by saturation overland flow. We estimate that about 20% of the total runoff is by saturation overland flow. Based on 2 years of suspended sediment measurement and 4 years of water discharge record, the Lutz Creek catchment appears to be eroding at the high rate of 598 t/sq. km/yr. or about 0.75mm/yr. Some of this sediment is derived from erosion of banks cut in landslide deposits, and some is probably generated by surface erosion by rain splash and overland flow.

Abstract: Fog has been viewed as an important source of moisture in many coastal ecosystems, yet its impor- tance for the plants which inhabit these ecosystems is virtually unknown. Here, I report the results of a 3-year investigation of fog inputs and the use of fog water by plants inhabiting the heavily fog inundated coastal redwood (Sequoia sempervirens) forests of northern California. During the study period, 34%, on average, of the annual hydrologic input was from fog drip oA the redwood trees themselves (interception input). When trees were absent, the average annual input from fog was only 17%, demonstrating that the trees significantly in- fluence the magnitude of fog water input to the ecosys- tem. Stable hydrogen and oxygen isotope analyses of water from fog, rain, soil water, and xylem water ex- tracted from the dominant plant species were used to characterize the water sources used by the plants. An isotopic mixing model was employed to then quantify how much fog water each plant used each month during the 3-year study. In summer, when fog was most fre- quent, 19% of the water within S. sempervirens, and 66% of the water within the understory plants came from fog after it had dripped from tree foliage into the soil; for S. sempervirens, this fog water input comprised 13-45% of its annual transpiration. For all plants, there was a significant reliance on fog as a water source, es- pecially in summer when rainfall was absent. Depen- dence on fog as a moisture source was highest in the year when rainfall was lowest but fog inputs normal. Inter- estingly, during the mild El Nino year of 1993, when the ratio of rainfall to fog water input was significantly higher and fog inputs were lower, both the proportion and coeAcient of variation in how much fog water was used by plants increased. An explanation for this is that while fog inputs were lower than normal in this El Nino year, they came at a time when plant demand for water was highest (summer). Therefore, proportional use of fog water by plants increased. The results presented suggest that fog, as a meteorological factor, plays an important role in the water relations of the plants and in the hydrology of the forest. These results demonstrate the importance of understanding the impacts of climatic factors and their oscillations on the biota. The results have important implications for ecologists, hydrologists, and forest managers interested in fog-inundated eco- systems and the plants which inhabit them.