litter interception loss. If the throughfall amount is known, the expected moisture content of the litter can be determined. It can therefore be said that litter interception loss is the difference between the measured and expected water contents (Jewitt, 1991).
2. Field methods, whereby litter is placed into trays or whereby sheets are placed underneath the forest floor and that permit throughfall in excess of litter interception to drain into the lower container. With the throughfall amount known, the litterflow amount can be determined, and the interception loss can be calculated.
According to Gerrits et al., (2007) interception measurement studies have generally concentrated on canopy interception, but interception by the understorey and forest floor can be as high or even higher.
The presence of litter modifies water and energy exchange between the soil and the air. As soil moisture and temperature are two major factors influencing soil respiration, these modifications have an important impact on the ecosystem carbon cycle (Ogée and Brunet, 2002). The forest floor is a source of heat, water vapour and CO2 and its temperature and moisture regulate the rate of evaporation from the litter, as well as the rate of decomposition. The structural properties of a forest floor are highly variable both horizontally and vertically, but it is possible to consider the litter as a whole for a homogeneous vegetation cover (Ogée and Brunet, 2002). However, to understand the structural properties of the forest floor, a classification of the litter layers is required.
5.1 Forest Litter Classification
Most forests have a developed litter layer on the soil by an accumulation of leaves, twigs, bark and so on (Park et al., 1998). The litter layer has an impact on the infiltration and runoff response of the forested catchment. The type of litter layer also influences these responses. Four layers have been identified by Hoover and Lunt (1952) to classify forest floor litter as shown in Figure 1.9. These are as follows:
L layer - The first is the Litter layer or surface layer which consists of freshly fallen leaves, needles, twigs, stems, bark and fruits. In areas of high temperatures and rainfall here decomposition and incorporation are rapid, this layer may be thin or absent during the growing season. This layer is also referred to as the Ao0 horizon.
F layer - The Fermentation layer consists of partially decomposed material that is still recognisable to the original. This layer is also referred to as the Ao1 horizon.
H layer - The Humus layer is that material that is well decomposed and is no longer recognisable as to the original. This layer is also referred to as the Ao2 horizon.
A1 layer - This is the surface mineral-soil horizon where organic matter is incorporated or infiltrated.
Figure 1.9 Forest litter layer classification according to Hoover and Lunt (1952).
The different litter layers have different water holding capacities and hence different hydrological impacts. Bernard (1963) cited in Jewitt (1991) found that the H layer and top few mm of the A1
horizon are capable of holding up to 900% as much water as the L and F layers, despite having a dry mass of only 150% of these layers. From a hydrological point of view, it would appear that the H-layer of the forest litter plays a particularly important role in forest hydrology, as this layer has the potential to hold water like a sponge and gradually release water into the A1 horizon, allowing almost total infiltration of throughfall for some events (Lowdermilk, 1930 cited in Jewitt, 1991).
To determine the interception capacity of the canopy vegetation, conventional rain gauges and trough gauges can be placed under the canopy. In a forested catchment where forest floor litter has developed on the soil surface, the surface litter will intercept both throughfall and stemflow. Miller (1977) in Putuhena and Cordery (1996) reported that typically, 1-3 kg.m-2 of liquid water can be stored on forest vegetation, and a similar amount can be retained on the forest floor. The hydraulic mechanisms of the forest floor interception are similar to the canopy interception process.
Forest floor interception can be considered as a function of:
1. The accumulated mass of litter per unit area
2. The water retention characteristics of the litter (i.e. storage capacity) 3. The wetting frequency of the litter, and
4. The drying rate of the litter.
Thus the amount of rainfall intercepted is also similarly related to the water storage capacities of the surface components (Putuhena and Cordery, 1996). For forest floor litter however there are some obvious mechanical and spatial difficulties when measuring litter interception. The mechanical difficulty is due to the lack of space between the litter and the mineral soil or the grading of litter (H- layer) into the soil (A1-Layer) (Putuhena and Cordery, 1996; Hoover and Lunt, 1952). In a catchment, the thickness of the litter layer can vary from a few millimetres to a few centimetres. The type and composition of the litter on the forest floor can also vary within the forest. The spatial variability of the amount and composition of the litter layer makes it difficult to measure the forest floor interception for a whole catchment. It is partly due to these inherent difficulties in making measurements of forest floor interception that there is a scarcity of information (Putuhena and Cordery, 1996).