Forest monitoring in the Netherlands

Dutch forests are becoming more and more valuable:
1. The Netherlands is a densely populated country and hence the degree of urbanisation is high. Forests play an important role in this society because they provide opportunities for people to get out and enjoy nature. 
2. Forests make up for a large part of the country's ecological structure.
3. Dutch forests are getting older and because of the ecologically sound way they are managed the ageing of forests is accompanied by an increase in forest-biodiversity.
4. Dutch forests yield valuable renewable resources (timber and woody biomass for electricity production), are a carbon dioxide sink and filter out pollutants from the air.

Dutch forest managers/owners feel a responsibility for the quality of the natural system they are managing. In modern Dutch society the forest manager is no longer his own king, he has to be able to communicate his approach to and the results of forest management to 'the audience'. Using objective monitoring data has been proven to be an effective way of communication about forest development and the way management is steering and utilising this development.

Monitoring data in forest/woodland management serve two purposes:
1. To support management decisions
2. To be able to communicate about management with third parties
We at Silve have developed a forest monitoring system for modern 21st century forest management. The system goes by the name of Woodstock, but the Dutch Forest Service uses it by the name of SyHI. SyHI / Woodstock is being used since 1990 and has become the standard inventory/monitoring tool throughout Dutch forestry. 

Forest monitoring using SyHI / Woodstock

Forest monitoring is accomplished by repetitive inventorying. In the Netherlands forest inventories are typically carried out every 8-12 years. Measurement procedures are such that two or more 'moments in time' descriptions of the forest can be compared to give information on forest development.

Forest inventory and monitoring focusses on the qualities of a forest as an ecosystem. The idea is that it is no longer necessary to acquire knowledge on all characteristics of each and every stand. By using SyHI / Woodstock information is collected on the level of forest types, that make up a forest, rather than on the individual stand level. To be able to divide the forest into relevant forest types SyHI / Woodstock needs a (digital) forest type map. Forest maps in the Netherlands usually have a rough forest type description (main tree species, age class of main species). SyHI / Woodstock uses this very simple typology as a standard, however other ways of describing forest types are easily incorporated into the system.

A SyHI / Woodstock forest inventory consists of measurements taken at circular sample plots. These plots are laid out over the forested area in a systematical configuration (see map below). To give you an impression: forests which are subject to this monitoring system vary in size from 50 ha to 2500 ha. The number of sample plots used to describe these forests vary from 50 to 500.

Basic measurements on sample plots
A series of standard measurements are taken from the sample plots:
1. All trees within the plot are measured for diameter (breast height 1,3 meters above the ground), dbh must be over 5 cm
2. Each tree's species is recorded as well as its social position
3. A random sample of trees is selected for height measurement
4. Optionally a sample core is taken from the sample trees to be able to measure tree growth
4. Dead trees are measured
5. The number of very young trees is counted
6. Shrub layer characteristics are recorded
7. Stem quality characteristics are recorded for certain sample trees

A sample of sample plots is generally remeasured to check the quality of the measurements.

Some notes on basic data processing
Rough field data are checked using special software that is part of the processing software.

The conversion from a tree's diameter to the volume of the stem's timber is done by calculating species specific parameters of a general parabolic model (see for instance Cunia, T. 1986. On the error of biomass estimates in forest inventories; part 2: The error component from sample plots. SUNY College of Environmental Science and Forestry, Faculty of Forestry Miscellaneous Publication 9 (ESF 86-001)).

Sample plots are grouped by forest type to allow for post stratification.

Volume increment estimates are obtained from either individual tree basal area growth estimators (Schoonderwoerd, H. 1991. Increment estimators for individual forest trees (in Dutch). Silve raport 10. Silve, Wageningen) or from increment core measuremnts taken from sample trees.

The following information can be considered as the standard output of a SyHI / Woodstock forest inventory:

Standing stock (per species(group) and size class)
> Volume of wood in live trees
> Current volume increment
> Number of trees
> Timber quality

Forest composition and structure
> Tree species composition
> Shrub layer composition
> Mixed forest area
> Area with open spots
> Area with regeneration
> Regeneration composition
> Area with big trees
> Area with shrub layer
> Dead wood: standing and fallen, volume per species and size class
> Variation in density of the crown layer

All characteristics are computed for the whole forested area, or for certain forest types within the forest.

SyHI / Woodstock however is a flexible system and can be tapered for the specific information needs a forest manager might have. Some examples of extra information that we have come across in the 15 years that we are using the system are
> mosses, ferns and higher plants monitoring,
> the determination of tree damage by browsing animals,
> the measurement of thickness and structural composition of humus layers and soil profiles,
> root depth and groundwater tables determination.