7.6 Optional subprogramme SF: Stemflow

• 7.6.1 Introduction
• 7.6.2 Sampling methods
• 7.6.3 Chemical analyses
• 7.6.4 Quality assurance/Quality control
• 7.6.5 Calculation of stemflow amount in mm from stemflow volumes
• 7.6.6 Data reporting
• 7.6.7 References

7.6.1 Introduction

Stemflow measurements are made as part of the assessment of water and chemical fluxes within a forest stand (deposition). Precipitation reaches the ground directly, by dripping off foliage and branches and by trickling down the stem. It is the last category that is captured by stemflow measurements. In addition to providing information on fluxes, stemflow chemistry has an important influence on the corticolous lichens present on the stem (see subprogramme EP), and their associated microfaunas, and also has a significant impact on the soil properties at the base of the stem.

The amount of stemflow varies markedly between species and is strongly dependent on the branch structure. Trees with branches with upwards orientation (e.g. Fagus sylvatica) tend to have much greater amounts of stemflow (contributing 10-40% of total stand precipitation) than those with drooping branches (e.g. Picea abies) (contributing <1% of total stand precipitation). Consequently, stemflow need not be measured on all trees species.

7.6.2 Sampling methods

Ten trees each of the most important species (i.e. >20% of the basal area in the plot) should be monitored. Trees should be representative of the range of basal areas present in the study area.

Sampling will be made monthly, weekly or at a time interval between the two, e.g. every two or three weeks, depending mainly on climate or method used. It is recommended that samples are taken so that correct monthly values can be derived. The sampling integration time should preferably be the same for all deposition measurements (i.e. throughfall and precipitation chemistry).

A variety of different types of equipment exist for measuring stemflow. The majority are based on a spiral collector wrapped around the stem of the tree. When installing such collectors, a variety of points must be taken into consideration:

• Trees grow, and the design must allow for both the diurnal variations that occur in the circumference of a stem and the annual growth of a tree. Silicone is a good but expensive material for the purpose.
• Under no circumstances should the bark be damaged. Great care should be taken to ensure that the bark remains intact, otherwise sap may seep out, contaminating the stemflow. On species with rough bark (e.g. oak species), care should be taken to ensure that the flow is properly collected.

The collecting bottles must be of sufficient capacity to cope with large quantities of stemflow. For example, in beech, two 60 l containers may be necessary. Alternatively, sampling can be automated and flow measured using a tipping-bucket mechanism.

• The collar should be fixed at of height of 0.5 to 1.5 m above the ground surface.

For the collection and handling of samples please refer to subprogramme Throughfall.

After each sampling period, the volume of each stemflow sample must be determined. If stemflow samples are pooled together, they can only be pooled for trees of the same species and similar size and dominance.

7.6.3 Chemical analyses

If the SF subprogramme is carried out, at least the following parameters should be determined: sulphate, nitrate, ammonium, total N, chloride, sodium, potassium, calcium, magnesium and strong acid (by pH).

It is also recommended to determine the electrical conductivity and to determine alkalinity in the samples if annual median pH>5. The determination of total S and heavy metals is optional.

The same analytical methods as used for throughfall chemistry measurements should be adopted.

7.6.4 Quality assurance/Quality control

As for throughfall measurements.

7.6.5 Calculation of stemflow amount in mm from stemflow volumes

Stemflow amount is calculated for each species separately as:

Total stemflow volume in the plot for the species (in liters) =

[Total amount of SF collected from the species (l) x Stand basal area of the species (m²/ha)] / [Basal area of the SF trees of that species (m²/ha)]

The result is divided by the size of the plot ( m²) to get the stemflow amount for the species in mm.

7.6.6 Data reporting

Example files

SF example Excel file
SF example ASCII file

•  File identifier SUBPROG states the subprogramme.   
•  Stemflow measurements are reported for each species separately. MEDIUM refers to the tree species (from NCC code list B4, see Annex 6). For a list of most common tree species see subprogramme TF.   
•  LEVEL is given as the distance of sampling devices from the ground (cm).   
•  Spatial pool SPOOL refers to the number of individual samplers used for each parameter.   
•  Values from weekly measurements are reported as volume weighted monthly means, status flag is W. If the stemflow amount can not be adequately measured, the concentrations for weekly measurements are reported as monthly means, status flag is X. Monthly measurements are reported without status. Stemflow amount is reported as monthly sum, status flag is S. For calculation of volume weighted means, please see Annex 7. General information on flags is given in Chapter 4.   
•  Sampling year and month are given as YYYYMM, day field is left blank.

7.6.7 References

ICP Forests Manual, 2016
http://icp-forests.net/page/icp-forests-manual

ICP Forests manual, 1997. Manual on methods and criteria for harmonized sampling, assessment, monitoring and analysis of the effects of air pollution on forests, 4th edition. Edited in 1997 by the Programme Coordination Centre Federal Research Centre for Forestry and Forest Products (BFH), Hamburg, Germany.