There are a variety of ways stemflow volume is measured in the field. The most common direct measurement currently used is the bonding of bisectedPVC or other plastic tubing around the circumference of the tree trunk, connected and funneled into a graduated cylinder for manual or a tipping bucket rain gauge for automatic collection. At times the tubing is wrapped multiple times around the trunk is order to ensure more complete collection.
Determining factors
Precipitation The primary meteorological characteristics of a rainfall event that influence stemflow are:
Rainfall continuity - the more frequent and extended are the gaps during the event where no rainfall occurs, the higher the likelihood that potential stemflow volume is lost to evapotranspiration; this is also governed by air temperature, relative humidity and most significantly, wind speed
Rainfall intensity - the amount of total stemflow is diminished when the amount of rain in a given period surpasses the capacity of the flow paths
Rain angle - stemflow generally starts earlier when rainfall is more horizontal; this is more of a determinant in an open forest with a lesser degree of crown closure
Species s is relatively low, due to their textured bark, few and horizontal branches, and high incidence of absorbent epiphytes. The species of the tree affects the amount of timing and stemflow. The particular morphological characteristics that are key factors are:
Leaf shape/orientation - leaves which are and elevated horizontally above the petiole are able to contribute to stemflow
Branch angle - stemflow potential heightens as the angle of the branches and twigs increases
Flow path obstructions - abnormalities on the flow path, such as detached pieces of bark or scars, on the underside of the branch can divert water from stemflow and become a component in throughfall
Bark - stemflow is affected by the degree of ability and smoothness of the bark alongside the branch and stem
Stand characteristics In addition to the effects of individual tree species, the overall structure of the forest stand also influences the amount of stemflow that will ultimately occur, these factors are:
Species composition - the total stemflow for the stand is determined by the contributions of individuals and their species-specific traits
Stand density - morphological characteristics such as branch angle and thickness are largely determined by the amount of density of competing trees in the stand
Canopy structure - individuals located in the understory in a stand with multiple vertically-stratified stories will have a lessened amount of total stemflow due to the interception of dominant and codominant individuals
Other
Seasonality - in the case of deciduous or mixed forests, stemflow rates are slightly higher in the dormant season when no leaves are present and evapotranspiration is reduced; this effect becomes more pronounced as the stem diameter increases
Diurnality - variations in branch weight influence the amount of stemflow; branches are heavier in the morning and lighter in the afternoon
Influence on soil
Chemistry Nutrients that have accumulated on the canopy from dry deposition or animal feces are able to directly enter the soil through stemflow. When precipitation occurs, canopy nutrients are leached into the water because of the differences in nutrientconcentration between the tree and the rainfall. Conversely, nutrients are taken up by the tree when concentration is lower in the canopy than the rainfall, the presence of epiphytes or lichens also contributes to uptake. The nutrients that enter the soil can also reflect the particular environmental conditions around them, for example, plants located in industrialized areas exhibit higher rates of sulfur and nitrogen, whereas those located near the oceans have higher rates of sodium. Soil acidification can be seen around some stems, for example beech trees from dry deposition. 10.1023/A:1015846906956"> Precipitation and morphological factors that influence stemflow timing and volume also affect the chemical composition; in general, stemflow water becomes more dilute during the course of a storm event, and rough-barked species contain more nutrients than smooth-barked species. Water distribution In forested areas, water is more able to effectively penetrate past the topsoil into deeper layers of the soil horizon along tree roots and their subsequent creation of macropores. The loosening of the soil can result in minor landslides.
