Brändli, U. B., Kaufmann, E., & Stierlin, H. R. (1995). Survey of Biodiversity at the Forest Margin in the second Swiss NFI. In M. Köhl, P. Bachmann, P. Brassel, & G. Preto (Eds.), The Monte Verità Conference on Forest Survey Designs. «Simplicity ver

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3.6 Survey of Biodiversity at the Forest Margin in the Second Swiss NFI

Urs-Beat Brandli, Edgar Kaufmann, Hans Rudolf Stierlin

Abstract

The Swiss nature conservation policy aims at the promotion of biodiversity. This requires reliable information on the development of forests, too. The second National Forest Inventory (NFI) survey (1993-95) will provide, among other ecological data, for the first time naionwide information on forest margins. First results with data from 1993 have shown that the NFI sampling methode is suitable for collecting reproducible data concerning diversity of ligneous species.

Keywords: biodiversity, diversity index, national forest inventory, forest margin, sampling, survey

3.6.1 Introduction

Since January 1993 a new Federal Forest Law is in force in Switzerland (Schweiz.

Bundesrat 1991). This law preserves the forested area and should guarantee that forests are in good condition to fulfil their functions (wood production, protection, recreation). A third and important aim of this law is the protection of forest as a near-to-nature ecosystem: manifold habitats and diversity of species should be maintained and promoted.

Therefore reliable information on the state and the current development of the forest is required (B RA NDL! 1993). Consequently a whole new selection of parameters on the forest as a habitat for flora and fauna is recorded in the second NFI (STIERLI N et al. 1994) with terrestrial surveys in the years 1993-95.The 2^{n d} NFI survey includes ecologically valuable small biotopes in the forest such as heaps of branches, water bodies and dry or moist sites. For instance, the proportion of deadwood - fallen, still standing or stumps - will be determined. The structure and species composition of shrub layer, clearings and openings in forests are noted as they furnish favorable conditions for numerous plants and animals.

Measures of diversity are considered as indicators of the condition of ecological systems. The higher the diversity, the more ecological niches are believed to be found at a certain site. In the second NFI, one focus is on describing diversity structure of ligneous plants. It will be analysed whether this diversity of trees and shrubs will allow conclusions concerning aspects of faunistic diversity. However, plant diversity is just one of numerous ecological variables recorded in NFL The majority of these parameters will be needed to estimate the ecological value of forest stands and margins (e.g. KRUG 1992).

The survey of ecological parameters at the forest margin (tab. 1) is a new subject in the 2^ndNFI. The forest margin, ranging from the dense forest to the open land provides a special habitat for plants, birds and insects. In 1996, nationwide information on the state and structure of forest margins will be available for the first time in Switzerland. In this paper the focus is on the sampling method at the forest margin and on the reproducibility of the results. Special attention is payed to the diversity of ligneous plant species (trees and shrubs).

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Tab. ^1.Data on forest margin.

Locating the taxation stretches

Distance [dm] and azimuths [gon] from the sample plot centre to the intersection point

Exposure of forest margin [gon]

Structure of forest margin 1 ⁼no shelterbelt, no shrub belt 2 ⁼no shelterbelt but shrub belt present 3 ⁼shelterbelt abrupt, no shrub belt 4 ⁼shelterbelt with overhanging branches,

usually broadleaved, but no shrub belt 5 = with shrub belt under the shelter belt

6 = with shrub belt extending beyond the shelterbelt 7 = with shrub belt and loose, multistoried shelterbelt

Shelter belt width [m]

Shrubbelt width [ m]

Width or herb border [m]

Shape or forest margin 1 = straight

2 ⁼undula^ting 3 -indented 4 = deeply indented 5 = patchy

Density of forest margin 1 ⁼open (0-25% closed) 2 ⁼with gaps (26-50% closed}

3 = _loose (51-75% closed}

4 = _dense (76-100 %closed}

Condition of forest margin 1 ⁼mown

2 - browsed 3 = pruned

4 = low growth cleared

3.6.2 Sampling Method at the Forest Margin

5 = clear-cut 6 ⁼thinned 7 ⁼tended 8 = no influence Limits or forest margin 1 ⁼none

2 = roads 3 ⁼forest roads 4 = tracks 5 = fences 6 ⁼game fences 7 ⁼meadow fences 8 = stone walls 9 ⁼ditches 10 = water bodies

Forest margin surroundings 1 ⁼settlement

2 = arable land 3 = rich meadow 4 = pasture land

5 = pasture land with trees 7 = tall forbs

8 = sparse meadows 9 ⁼marsh, boggy meadows 10 = water bodies - lake, river 11 = cliffs, screes

Percentage or ligneous species along the forest margin 1 = very rare

2 = rare 3 = 1-5%

4 = 6--25%

5 = _26--50%

6 ⁼51-75%

7 ⁼76--100%

The 1 ^stNFI being carried out from 1983 to 1985 was a systematic survey on a 1 x 1 km grid (EAFV und BFL 1988). With the 2nd NFI survey about half of the 12'000 terrestrial plots of the first survey will be assessed in the field. Wherever a forest margin lies within 25 m of the centre of an NFI sample plot -in average every 6^thplot - a taxation stretch of 50 m (2 x 25 m) has to be determined along the stock boundary. The stock boundary runs along the outermost trees and shrubs with a DBH .!!l2 cm as shown in figure 1. The intersection point and the endpoints of the 25 m taxation stretches are temporarily marked. Distance and azimuth from the intersection point to the sample plot centre are measured as well as the azimuths of the taxation stretches.

To gain information about data quality, some 10% of the sample plots and taxed forest margins are checked by control crews. These control crews know the position of the plot centre, the distances and azimuths of the taxation stretches.

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Locating the forest margin

X SPC sample plot centre

@ tree of the normal stand

E

O o outermost trees/shrubs DBH > 1 2cm

+ ⁺ trees/shrubs DBH <12cm stock boundary

0 K Intersection point

E end of 25m taxation stretches - shrub belt

non-forest

e

®x sPc

0

Fig. 1. Locating the forest margin (by Briindli. In: STIERLIN et al. 1994).

The description and survey of the forest margin includes the shelter belt, the shrub belt and the herb border (fig. 2). The survey of the ligneous species along the forest margin includes plants of the shrub belt and the shelter belt. Trees belonging to the shelter belt are distinguished from trees within stands by their asymmetric, often lop-sided crowns.

Transect of forest margin

= ::I u ,:, 'C C u al al -

outermost trees of the normal stand outermost trees/shrubs

with DBH >12 cm

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.0 .0

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shrub belt I I shelter belt forest

I marginal trees ¹

1

stand with or without shrub layer

I with or without

I shrub layer I

stock boundary- - I - - innermost dripborder of the shelter belt

Fig. 2. Transect of forest margin (by Briindli. In: Stierlin et al. 1994).

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3.6.3 Survey method of Ligneous Species along the Forest Margin

The main goals of the survey along the forest margin are the determination of the botanical diversity, the description of important habitats for various animals, especially birds and insects (structural diversity) and the judgement of the aesthetic value for man (recreational diversity).

Some 100 tree and shrub species are registered in the Swiss NFL All ligneous plants of the shrub belt and the shelter belt are recorded, whenever their needles or leaves are part of the forest margin surface. The vertical projection of the forest margin surface is defined as the standheight multiplied by 50 m (H x 50 m ⁼100% ). As fig. 3 shows, the sum of area proportions of all species may be less than 100% if there are gaps in the forest margin.

This vertical area covered by each specie is estimated by the field crews from outside of the forest. Seven coverage classes are distinguished:

1 ⁼ very rare only one individuum or only plants smaller than ^1.3m (about ^1%o) 2 =

3 = 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: ^STIERLINet 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

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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.

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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 ^ithspecies. In the forest margin survey of NFI this is the percentage of the area covered by the ^ithspecies.

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

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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.

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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.

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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.