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In each Level II plot four subplots (10×10 m) are permanently marked for ground vegetation survey (total area: 400 m²). Ground vegetation is studied by means of vegetation uptakes in the subplots every 5 years. Ground vegetation is subdivided in different layers:
A ground vegetation survey consists of listing all the species in each subplot. For every species total cover (%) is estimated using the scale of Londo (Londo, 1976). Also total cover (%) of every layer is estimated. Also the cover (%) of bare soil and litter is estimated.
The Life+ / FutMon action L2b (C1-NFI) made it possible to assess the amount of deadwood in the regional ICP Level I plots in Flanders.
Reference definitions on dead wood were compared with regional definitions. Attention was paid to the definition of the coarse woody debris and the decay stages. Statistical analysis of deadwood volumes was carried out on data from 72 regional Level I plots. Coarse woody debris was measured in the plots, using national and reference methods, respectively line intersect sampling (LIS) and piecewise measurements (Field-Map/FM).
With piecewise measurements, lying dead wood is lacking in 18% of the plots. With LIS this is much more: 35% of the plots are without coarse woody debris. Downed dead wood volumes vary from 0 to 68.5 m³/ha for both methods. With LIS, there is less than 20 m³/ha of lying dead wood in 85% of the plots, with piecewise measurements in 92% of the plots.
In the Level I plots line intersect sampling results in a non significant overestimation of the total downed dead wood volume compared to dead wood assessment with piecewise measurements. Piecewise measurements and LIS are both unbiased estimators of the downed deadwood volume and statistical analysis of the data prove that the differences are not significant.
On the regional scale of Belgium/Flanders (> 2000 plots in the FFI), bridging seems not to be necessary. To estimate the total volume of dead wood, the use of line intersect sampling could be justified because it is less time consuming compared to a piecewise measurement. But still the method is criticised. Some countries do not recommend to assess deadwood volumes with LIS when there are wood accumulations in the forest and some doubt if it is recommendable on hilly sites. It is also not to be used to estimate the deadwood volume of stumps. Besides these circumstances where LIS should be used with caution, it is still a defensible and cost-efficient technique. This is especially true when the goal is to estimate regional averages of volumes of coarse woody debris. For more detailed studies, such as monitoring in forest reserves and estimates of coarse woody debris at individual plot level, a piecewise measurement is more appropriate.
The regional definition of the decay stages is less detailed than the reference definition. The reference definition relates the decay stage to the volume of a deadwood piece and in the regional definition the deadwood volume is not involved. This makes it difficult to build a bridge between the regional and the reference classification system. Some pieces with a ‘slightly decayed’, ‘decayed’ or even a ‘very decayed’ stage could all be ‘partially decayed’ according to the regional definition.
140 pieces of deadwood were classified according to their stage of decay. The regional definition is simple compared to the reference definition but still confusion is possible. Sometimes wood cannot be penetrated with a knife (definition of stage 2) while the bark is lacking (definition of stage 3).
The reference definition is more detailed. In the regional definition, decay stage 1 concerns living stems and there are only 3 stages left for deadwood. With the reference definition there are 4 stages for deadwood and the percentage of the total volume is involved. This makes reclassification of the data very difficult, especially for decay stage 3.
In the reference definition, a piece of deadwood is considered as part of the litter layer, if the hard texture is less than 5% of the total volume. With the regional definition it is always considered as a deadwood piece. Very decayed deadwood is rare in the plots. During the survey, these pieces of deadwood were always considered as part of the coarse woody debris, not as part of the litter.
The collection of data on woody debris has only recently been started in the regional forest inventory plots. Further research on deadwood is recommended. For instance, in some countries a high proportion of the deadwood volume is represented by stumps. In the Flemish region, stumps are not included in the FFI. At this moment fine woody debris and small (dead) trees are also not included in the forest inventory (min. diameter is 7 cm for both).