A Comparative Analysis of Use and Non-Use Value Estimation
3) i,CUales son las caracterfsticas de la distribuci6n espacial de la especie dentro de una asociaci6n vegetal?
3.6 Survey of Biodiversity at the Forest Margin in the Second Swiss NFI
3.6.4 Reproducibility of Plant Diversity at Forest Margins
4 = 5 = 6 = 7 =
rare less than 1 % (average: 0.5%) 1-5%
6-25%
26-50%
51-75%
76-100%
50 m H = Top height of oldest stand at forest margin k;:::;:j Sum of area proportions (ea. 60%)
Fig. 3. Estimation of the area proportion for each species within the vertical projection of the forest margin surface (shrub belt and shelter belt} (by Brandli. Jn: STIERLIN et al. 1994).
3.6.4 Reproducibility of Plant Diversity at Forest Margins
A good reproducibility of data is a prerequisite for detecting changes with successive inventories. A second independent taxation of forest margins is taken by control crews in order to assess the reliability of recorded parameters. Fig. 4 shows distributions of differ
ences in the numbers of recorded species between survey and control crews. These differences are O or 1 in 40% of the cases, if the whole range of 7 coverage classes is taken
Cll 41
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41 1-1 .µ Ol
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� 0 0
2 0
1 5
1 0
5
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- 8 - 7 - 6 - 5 - 4 - 3 - 2 - 1
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Difference ( number of species ) Covering class
Coverage classes:
.,_._. 1 - 7
4 6-25%
1 = very rare (average: lo/oo) 5 26-50%
2 rare (average: 0.5%)
3 1-5% 6 = 51-75%
7 = 76-100%
3 - 7 -0---0, -+ 4 - 7
5 6 7 8
Fig. 4. Number of tree and shrub species at the forest margin. Differences in the numbers of registered species between survey and control crews.
into account (average: 10.5 species per plot). Considering only species with a minimal coverage of 1 % (average: 4.5 spec. per plot), the concordance increases to 65% within the range of -1 to 1. Considering only species covering more than 5% (average: 2.5 spec. per plot), 85% of the counts agree within the mentioned range. Visual inspection of the graphs shows no obvious systematic trends. However, the reproducibility of all species counts is hardly sufficient. We cannot ignore very rare species just to get higher concordances, as most of these rare species are important habitat elements for wildlife.
Recording the numbers of species without taking into account their abundance is not a satisfactory method to describe the diversity of a plant community. Another measure of diversity is needed. The two major components of diversity are species richness (number of species) and evenness (equalitiy of abundance). In ecology, a big range of indices have been created to describe diversity. Diversity indices seek to crystallize richness and relative abundance, i.e. number of species and number of individuals, in a single number.
Two of the most widely used indices are the Shannon index (Dsh) and the Simpson index (Dsi), as denoted e.g. in MAGURRAN (1983) :
where Pi is the proportion of individuals in the ith species. In the forest margin survey of NFI this is the percentage of the area covered by the ith species.
The Simpson index is very sensitive to changes in the abundance of dominant species, whereas the Shannon index is more affected by rare species. That is why Shannon index has a better discriminant ability between sites than Simpson index ( TAYLOR 1978, cit. in MAGURRAN 1988, p. 71 f.). The main criticism of the Shannon index concerns its insensitivity to the character of abundance distributions and the difficulty of a direct biological interpretation of this measure.
A theoretical example in tab. 2 illustrates these interpretation problems. The same indices values can result with quite different combinations of numbers of species and abundances. The first and the second row of the table have the same values for Shannon index, the first and the third row the same values for Simpson index. The last two columns show the percentages of the indices values determined by the dominant species (p1).
Tab. 2. Examples of diversity index values of Shannon index (D Sh) and Simpson index (D Si)-n t: total number of species
number of species per coverage class p1, p2: percentage of area covered per species
% Dsh• % Dsi by p1: percentage of the index value determined by p1 and n 1
"t "1 Pl (%) "2 P2 (%) Dsh Dsi % Dsh by pl
3 3 33.3 1 . 1 0 0.67
50 1 83.3 49 0.34 1 . 1 0 0 . 3 1 1 3 .8
5 0 1 5 7 . 1 4 9 0.88 2.36 0.67 1 3 . 6
% Dsi by pJ
99.9 98.8
The two indices have been applied to forest margin data of the current survey. 39 taxation stretches have been controlled by April 1994. The indices values of survey crews are compared with those of the control crews in fig. 5, fig. 6 and table 3. It shows that Shannon index values are satisfactory reproducible, whereas those of the Simpson index are not. Often, one dominant species covers a high percentage of an area. In over 30% of
the cases, the estimation of the proportion of dominant species at the same forest margin differs at least by one coverage class between survey crew and control crew. Since the Simpson index is very sensitive to high percentages, this explains the poor reproducibility of the Simpson index.
Tab. 3. Shannon diversity index. Number of observations per index class, resulting from survey and control.
index value: index value: control
survey 0---0.5 0.5-1.0 1.0-1.5 1.5-2.0 2.0-2.5
0-0.5 6 0 0 1 0
0.5-1 .0 0 1 1 2 0 0
1 .0-1.5 0 3 5 3 0
1 .5-2.0 0 0 2 3 0
2.0-2.5 0 0 1 1 1
The application of diversity indices to describe structural types of forest margins is not yet definitive. At the end of 1995, when the whole data set will be available, further investigations have to be done to answer the questions, which diversity measure is the most suitable to discriminate between sites and what will be the ecological interpretation of that measure.
.-I 0
Fig. 5. Shannon diversity index of tree and shrub species at the forest margin. Comparison of index values, derived from taxations of survey and control crews.
r-1 0 ,._.
Fig. 6. Simpson diversity index of tree and shrub species at the forest margin. Comparison of index values, derived from taxations of survey and control crews.
3.6.5 Conclusions
Methods for assessing ecological parameters have been developed for the second NFI survey and tested in a pilot inventory in 1992. In this paper focus is on sampling the forest margin, especially the survey of ligneous plants. The good reproducibility of plant diversity allows the following conclusions:
- The taxation method of forest margins is suitable to express the diversity of ligneous plants with the help of the Shannon diversity index which takes species richness and abundance into acount.
- The method is suitable to provide reproducible results.
- Changes of diversity described by the Shannon index may be detected with successive inventories.
- The assessment of the number of very rare species, mostly shrubs, is not reliable enough.
- Often, groups of species (e.g. shrubs with berries or bramble) are of special interest when analysing faunistic habitats. At present, the dataset is too small to prove the reliability of the assessment of groups of species.
The assessment of ligneous species along the forest margin takes an average of 10 minutes per plot. This effort amounts to 1-2% of the whole time spent by a field crew on an average NFI sampleplot. We consider the method of species survey at forest margins to be efficient in relation to the expected gain of information.
3.6.6 References
BRANDLI, U.-B. 1993: The National Forest Inventory . . . a Window on the Swiss Forest. Verified Knowledge thanks to Systematic Observation. Birmensdorf, Federal Institute of Forest, Snow and Landscape Research. 16 p. (German, French, Italian or English.)
EAFV (Eidg. Anstalt flir das forstliche Versuchswesen) und BFL (Bundesaml fi.ir Forslwesen und Landschaftsschutz) (eds.) 1988: Schweizerisches Landesforstinventar: Ergebnisse der Erstaufnahme 1982-1986, 1988. Ber. Eidgenoss. Forsch. anst. Wald Schnee Landsch. 305: 375 pp.
(German or French.)
KRUG, Kathrin, 1992: Okologische Bewertung van Waldrandern im Kanton Solothurn. Diplom
arbeit ETH, Abteilung XA, ausgefi.ihrt an der WSL und am Geobot. Inst. ETH, Zi.irich. 95 pp.
(unpublished).
MAGURRAN, A.E., 1988: Ecological Diversity and its Measurement, London. Croom Helm. 179 pp.
Schweizerischer Bundesrat, 1991: Bundesgesetz i.iber den Wald (Waldgesetz) vom 4. Oktober 1991 (WaG). SR 921.0. AS 1992, Bern.
STIERLIN, H.R.; B RANDLI, U.-B.; HEROLD, A.; Z INGGELER, J .. 1994: Schweizerisches Landes
forstinventar. Anleitung fi.ir die Feldaufnahmen der Erhebung 1993-1995. Birmcnsdorf, Eidge
nossische Forschungsanstalt filr Wald, Schnee und Landschaft. 204 pp.