Fog Throughfall at a Seasonal Rain Forest in Xishuangbanna, Southwest China

Abstract

Fog throughfall and related microclimatic factors were measured between November 1998 and February 2003 at a tropical seasonal rain forest in Xishuangbanna, Southwest China. Twelve bottle_funnel collectors were set in a random pattern on the forest floor to determine the daily amount of fog throughfall, and two funnel collectors connected with polyethylene tubing to a mechanical recording siphon_raingauge were used to determine fog throughfall intensity and temporal character. Related microclimatic variables including air temperature, relative humidity, wind speed, solar radiation and rainfall were also recorded by a meteorological observation system mounted on a 72 m meteorological tower in the study site. During the study period, absolute amounts of mean annual fog throughfall were up to (89.4±13.5) mm (mean±SD), contributing 4.9%± 1.7% of the annual precipitation, with 85.9%±6.6% of the fog throughfall collected in the foggy season (November_February) and dry_hot season (March_April). The amounts of annual fog throughfall plus fog interception accounted for 16.4%±1.0% of total annual precipitation which includes fog throughfall, fog interception and rainfall. The annual fog throughfall was negatively correlated with annual rainfall, demonstrating that the dependence on fog as an additional water input was highest in the year when rainfall was lowest but fog throughfall was high. Monthly variation in fog throughfall was different from rainfall patterns and negative correlation was found between monthly average minimum air temperature and monthly fog throughfall while positive correlations were found between monthly fog throughfall and monthly average relative humidity, monthly average wind speed during 0∶00-10∶00, and monthly sum of fog_days. In fog_drip occurring days, the average fog throughfall was (0.38±0.27) mm·d -1 and the amounts of fog throughfall per day and throughfall intensity were negatively correlated with air temperature and positively correlated with wind speed. The results suggest that fog, which not only inputs water into the forest but also partly reduces the evapotranspiration of the forest, plays an important role in the hydrology of the forest, especially in the foggy and dry_hot seasons. Therefore, neglect of the contribution from horizontal precipitation (fog throughfall plus fog interception) will make calculations of the water balance inaccurate in the forest. These results also demonstrate the importance of understanding the impacts of climate factors, and have important implications for ecologists and hydrologists interested in fog_inundated ecosystems and the plants that inhabit them.