Interception loss is a key process for hydrological cycle. Hydrological function is one of the most important aspects of forest ecosystem function. Canopy and forest litter can enforce rainfall to be re-distributed spatially. Canopy interception is that proportion of rainfall stored and later evaporated from canopy during and after rainfall. Forest litter interception is the rainfall or throughfall stored and evaporated from forest litter directly. Canopy interception can account for 25%-50% of total precipitation in coniferous forests and 10%-35% in broadleaved forests respectively. It is influenced by canopy characteristics, forest age and climatic conditions and so on. Throughfall does not transfer to soil water or runoff completely because of forest litter interception, which can account for 20% of throughfall. The rainfall intercepted is useless for plant growth. However, rainfall is re-distributed by canopy and forest litter, which is significantly important for the balance of soil water.
To obtain a thorough understanding of canopy and forest litter interception and their hydrological effects, in-situ experiment for canopy interception and rainfall simulation experiment for litter interception were conducted from May to October during 2008 and 2009. Middle-aged Abies fabri, mature Abies fabri, coniferous and broadleaf mixed forest were chosen for our study, which consist of the main forest types in subalpine Mountain Gongga. Large troughs, which were 305×24cm in size, were used to collect throughfall. Many researches indicated that large troughs can reduce observation errors. Forest litter interception was obtained by method of artificially simulated rainfall. The rainfall intensity was 0.35 mm/h. Forest litter interception was calculated as the difference between rainfall and water outflowed.
The result showed that canopy interception was 20.9% for middle-aged Abies fabri during 2008, 23.0% and 23.6% for mixed forest in 2008 and 2009, respectively. Canopy interception was almost invariable between years. Canopy interception was controlled mainly by rainfall characteristic, especially rainfall density and rainfall amount. Wind speed seemed to have no effect on rainfall intercepted by canopy, because of the low wind speed and hard twigs in our study area. Forest litter had a larger water holding capacity than did canopy. The saturated litter interception was 5.1, 5.1 and 5.7 mm for middle-aged Abies fabri, mature Abies fabri, coniferous and broadleaf mixed forest respectively, while the corresponding saturated canopy interception was about 1.21, 3.15 and 1.23 mm. The fully decomposed forest litter had the largest water holding capacity. To reduce the lag time effect, the experimental scale for artificially simulated forest litter interception should be larger than 4 m². Although forest litter had a larger water holding capacity than did canopy, evaporation rate was significantly higher in the canopy than on forest floor. This was because the wind speed was low under the canopy, and forest ground received less radiation energy due to high canopy coverage. These two factors resulted in a weaker driving force of evaporation on forest floor. Therefore, the main component of the evaporation due to interception was from the canopy.