indicators of plant species and community diversity in grasslands

Agriculture, Ecosystems and Environment 98 (2003) 339–351

Indicators of plant species and community diversity in grasslands

Rainer Waldhardt∗, Annette OtteDivision of Landscape Ecology and Landscape Planning, Justus-Liebig-University of Giessen, IFZ,

Heinrich-Buff-Ring 26-32, D-35392 Giessen, Germany

Abstract

Parameters which are directly related to both land use change and biodiversity may be useful tools to indicate biodiversity inmarginal landscapes. In these landscapes for about five decades abandonment of cultivation, especially in favour of extensivegrassland use and succession on abandoned fields, has led to considerable changes in the landscape structure. Regionssuch as the Lahn-Dill Highlands (Hesse, Germany), formerly characterized by small-parcelled crop and grassland rotation,increasingly feature old grassland communities over large areas.

Impacts of changes in the landscape structure on the floristic–phytocoenotic diversity have been studied in two landscapetracts of this region that today are mainly used as grassland. Reconstruction of land use dynamics based on multitemporalaerial photograph interpretations of the period from 1945 to 1997 confirm the predominance of cultivation until ca. 1960 inboth areas. On the basis of phytosociological surveys in one stand abandoned 3 years before the survey and in each threegrassland stands of different age classes (11–27, 28–38, 39–46 and over 46 years), the floristic–phytocoenotic diversity ofthese stands is characterized as follows:

(i) Flora and vegetation are clearly differentiated in relation to stand age.(ii) The vegetation of the older (>38 years) stands is more comparable among one another than is the vegetation of younger

stands.(iii) 19- to 33-year-old stands have the highest number of exclusive species.(iv) Old stands (>46 years) have the highest�-species richness.(v) The stands can be classified into different vegetation types in relation to age.

The floristic–phytocoenotic diversity is associated with site differences of the above age classes. Older stands are morefrequent at upper slopes and the pH values of their soils are lower.

With a small methodological outlay, grassland stands of different age and species diversity can be differentiated byred–green–blue (RGB) colour tonal values from false-colour infrared (FCIR) aerial photographs.

The results open up possibilities for the qualitative and quantitative indication of floristic–phytocoenotic diversity in grass-lands on the basis of stand age, site factors and also green and blue tonal values from the respective FCIR aerial photographs.

Furthermore the results indicate that it is necessary to retain old grassland stands, as well as a mosaic of extensively usedgrassland stands of different ages to retain plant species and community diversity in the study region.© 2003 Elsevier Science B.V. All rights reserved.

Keywords:Marginal agricultural landscape; Land use change; Vegetation dynamics;Festuca rubra–Agrostis capillariscommunity; Aerialphotographs

∗ Corresponding author. Tel.:+49-641-9937163; fax:+49-641-9937169.E-mail address:[email protected] (R. Waldhardt).

0167-8809/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0167-8809(03)00094-X

340 R. Waldhardt, A. Otte / Agriculture, Ecosystems and Environment 98 (2003) 339–351

1. Introduction

With the signing of the Convention on BiologicalDiversity in 1992, Germany has pledged itself to theconservation of biological diversity. Since virtually theentire land area in Germany is managed, this goal mustbe realized by sustainable land use over as large anarea as possible (BMU, 1998). This especially appliesto agriculturally used landscapes, whose biological di-versity has been in decline for several decades, owingto overly intensive land use or to land use abandon-ment (Burel et al., 1998; MacDonald et al., 2000).

A crucial aspect of the total biodiversity of a land-scape is its floristic diversity, which is discussed as acorrelate thereof (Duelli and Obrist, 1998). As withthe closely associated diversity of plant communities,floristic diversity in agricultural landscapes is essen-tially dependent on the former and current land useforms, intensities, patterns and dynamics present inthese landscapes (Waldhardt et al., 2000).

Until ca. 1950, crop/grassland rotation, often insmall-parcelled mosaics, was a widespread practicein marginal agricultural landscapes in Germany withclimatic and/or edaphic conditions unfavourable forcultivation. There, as in similarly disadvantaged re-gions of Europe, cultivation has been largely aban-doned within the last five decades in favour of grass-land and succession on abandoned fields (Baldocket al., 1996). The landscape structure of these regionshas fundamentally changed. Increasingly older plantcommunities have been able to develop, often overwide areas.

One aim of our research group is to quantify theimportance of both forms of land use dynamics—rotation and long-term land use change—for thefloristic–phytocoenotic �-, �- and �-diversity(Whittaker, 1972) of marginal agricultural landscapes.On the basis of these and other results, we are devel-oping ecologically and economically sustainable landuse concepts within a research cooperative (Frede andBach, 1999). Our research is being carried out in theLahn-Dill Highlands region, which can be regardedas an especially marginal agricultural landscape. Inorder to quantify the importance of grassland agefor the floristic–phytocoenotic diversity, the researchpresented here was conducted in two typical hillslope areas that have been successively taken out ofcultivation since ca. 1950.

In developing sustainable, landscape-orientatedland use concepts, we are devoting particular atten-tion to current hot spots of floristic diversity. From theanalysis of site factors and from the reconstruction ofthe land use history of these areas, important indica-tions of the origins of the present-day diversity can bederived, as well as demands on future land use qualityin order to retain this diversity. If necessary, however,it is possible—at great expense—to conduct a com-prehensive field survey of the floristic–phytocoenoticdiversity at a landscape scale in order to determinehot spots. Aerial photograph analysis may be an effi-cient procedure for determining hot spots. Hence, wehave examined the question of whether false-colourinfrared (FCIR) aerial photographs at a scale of 1:5000—with a small methodological outlay—permitquantitative conclusions to be drawn about the localfloristic–phytocoenotic diversity.

Possibilities for the indication of the floristic–phytocoenotic diversity of grassland vegetation willbe discussed on the basis of stand age, site factors andcolour spectra of the stands in FCIR-photographs.

2. Study region and sites

The Lahn-Dill Highlands cover about 700 km2 inthe highlands of Hesse, Germany. Owing to the fol-lowing conditions:

(i) topography and soils (altitudinal range: 200–600 m a.s.l.; acidic regosols to moderately deepbrown earths dominating over Devonian clay-stone slates, gravelstone slates and greywackeson arable sites on slopes of up to 20◦ or more;Schotte and Felix-Henningsen, 1999);

(ii) climate (mean annual temperature 5–8◦C, annualprecipitation 700–1200 mm);

(iii) agrarian-structure (mean farm size 14 ha; 78% offarms managed with a secondary means of in-come; low levels of fertilizer and pesticide appli-cation;Frede and Bach, 1999).

Cultivation, especially, is not sufficiently profitableon much of the agriculturally used land area (Fredeand Bach, 1999) anymore. Hence, the visual landscapecharacter has fundamentally changed in comparisonwith that of ca. 1950. In many places, extensive grass-land use has replaced the traditional, small-parcelled

R. Waldhardt, A. Otte / Agriculture, Ecosystems and Environment 98 (2003) 339–351 341

crop production and crop/grassland rotation (Kohl,1978). Sheep grazing and annual mowing predomi-nate.

The research presented here was conducted in twoareas 1 (18 ha) and 2 (12 ha) of the Steinbrücken dis-trict (340–400 m a.s.l.). On nutrient- and base-poorregosols and brown earths, the upper and mid-slopesof these south-facing tracts feature land use dynamicsthat are typical of the landscape. On slopes of about15◦, the soils of area 1 are shallower and more skele-tal in character than those of area 2, present on slopesof only about 7◦.

3. Methods

3.1. Land use dynamics

The reconstruction of the land use followedFuhr-Boßdorf et al. (1999). Stereoscopic interpreta-tion of black-and-white aerial photographs from theyears 1945, 1953, 1961, 1972, 1989 and 1997 at ascale of approximately 1: 10,000, with a resolution tothe level of the allotment, was applied. The followingagricultural land use forms: arable land, grassland andabandoned fields with woody plant succession weredifferentiated. Further information about land use wasacquired through discussions with farmers and localofficials in the study region.

3.2. Floristic–phytocoenotic diversity

Thirteen grassland allotments (plots) of differentage were chosen for the floristic–phytocoenotic stud-ies in area 1 (Fig. 1). Cultivation was first abandonedon one allotment: (i) 3 years before the survey wascarried out in 1999 (age class A). Three allotmentseach were last cultivated: (ii) 11–27 (age class B), (iii)28–38 (age class C), (iv) 39–46 (age class D) or (v)more than 46 years (age class E) ago.

Five vegetation surveys were carried out on eachplot in June, with a standard size of 5 m× 5 m (sub-plots) estimating the percentage of individual speciescover and total vegetation cover. Additionally, therespective growth heights of the vegetation wererecorded. The nomenclature of the botanical names isin accordance withWisskirchen and Haeupler (1998).

Statements concerning ecological characteristics ofspecies are in accordance withEllenberg et al. (1992).

For a comparison of the age classes B–E, themean�-species richness (species count/25 m2), totalground cover and growth heights of the vegetationof each of the 13 plots were initially calculated. Toquantify the similarity of the vegetation between sub-plots (�-diversity of the plots) Renkonen coefficients(Renkonen, 1938) were calculated. The vegetationof the five subplots of each plot were compared inpairs and the mean similarity was determined. Themean Renkonen coefficients of the plots, as well astheir total numbers of species documented within thefive subplots (�-species diversity of the plots; speciesnumber× 125 m2), were taken to calculate meanvalues of each age class. Finally, a phytosociologicalanalysis of the stands according toBraun-Blanquet(1964)was applied.

Using the described method, botanical surveys andanalyses were carried out for three further allotmentsof differing age (one plot from each of the age classesB–D) in area 2. This was done to determine whetherthe vegetation characteristics in area 1 were alsopresent on similar sites in the region.

3.3. Site diversity

In autumn 1999, soil samples from a depth of5–10 cm were taken from all 65 subplots of area 1 todetermine relations between the vegetation and soilconditions.

The proportion of coarse material (>3 mm) of thesoil was determined, together with total carbon (Ct)and nitrogen (Nt) levels, levels of available phos-phorous (CAL method; PCAL) and pH (in CaCl2) ofthe fine soil (analyses according toSchlichting et al.,1995). Considering the coarse material, carbon, nitro-gen and phosphorous levels of the soil of each subplotwere calculated.

Furthermore the topographic position (tp) of theplots within the slopes was determined. Seven classesof topographic positions from the mid-slope (tp1) tothe upper slope (tp7) were differentiated (Fig. 1).

3.4. Colour spectra in FCIR aerial photographs

Non-georeferenciated FCIR aerial photographs ata scale of 1: 5000 were available from a flight in


Fig. 1. Study plots in area 1. Age (in years since last in cultivation) according to aerial photographs: (A) 3; (B) 11–27; (C) 28–38; (D)39–46; (E) >46. Classes of topographic position: tp1–tp7. In each age class three allotments (plots 1–3) were investigated. Five vegetationsurveys (subplots a–e) were carried out on each plot, with a standard size of 5 m× 5 m.

May 1997. The aerial photographs were scanned inthe red–green–blue (RGB) mode at a resolution of300 dpi. The colour spectra of the photographs werenot calibrated. The analysis of the colour spectra wascarried out with the computer-program Adobe Photo-shop 5.5 (Adobe Systems Inc., 1999).

The former allotment boundaries recognizable inthe aerial photographs enabled a spatial classificationof the study sites to be made. The allotment of ageclass A was not included, since it was still in culti-vation at the time the aerial photograph was taken.Within the five subplots of each site standard-sizedpart-images (144 pixels) were cut out of the aerialphotographs and their spectra of red, green and bluetones were analysed. The mean tonal values of the red,

green and blue components were calculated, togetherwith their standard deviations. A tonal value of be-tween 0 and 255 was assigned to each component inevery pixel, higher values representing brighter colourtones. Mean tonal values and standard deviations weresubsequently calculated for each plot, and finally com-pared between age classes. Relations between mean�-species diversity and both the mean tonal values andtheir standard deviations of the subplots were tested.

3.5. Statistics

We performed a detrended correspondence anal-ysis (DCA) with the species data (main matrix)of all 60 subplots of the age classes B–E from


area 1 using the PC-ORD 4 software (McCune andMefford, 1999). Axes were rescaled and rare specieswere downweighted. Method of detrending was bysegments. To see how well the distances in the ordi-nation spaces represent the distances in the original,unreduced spaces, it was proved in “after-the-fact”evaluations of how well distances in the ordinationspaces match the relative Euclidean distances in themain matrices. The “after-the-fact” evaluation is per-formed by calculating the coefficient of determination(r2) between distances in the ordination space anddistances in the original space.

We created joint plots by having our environmentalvariables in second matrices. The angles and lengthsof environmental lines tell the direction and strengthof the relationships. In order to detect correlations be-tween ordination axes and site conditions Pearsons-r(Krebs, 1999) was calculated based on the samplescores of the first and second ordination axes and en-vironmental variables.

The effect of time (age classes B–E) on total groundcover, growth heights and similarity of the vegetation,as well as on both the mean tonal values and their de-viations of the RGB images, was tested by means ofa one-way ANOVA following Kolmogorov–Smirnov-and Sen & Puri tests. Whenever necessary, data wereln-transformed(ln x+0.1) prior to statistical analyses.For multiple comparisons of mean values the TukeyHSD-test was applied. To quantify the correlation be-tween two parameters we calculated Pearsons-r.

4. Results

4.1. Land use dynamics of the study sites

The land use in area 1 ca. 1950 was present onabout 60 allotments with an average area of only0.3 ha. About 70% of the total area was in culti-vation (Table 1). In the period between 1961 and1972, cultivation was abandoned over large parts ofthe area, and during the last 10 years on nearly allremaining allotments as well. On 2 ha of the formerarable land, succession with woody species estab-lished, today overgrown withCytisus scoparius. Theremaining area was kept open as grassland. Since1973, this grassland has been used for extensive sheepgrazing and mulched once annually. There has been

Table 1Proportion of land use forms in areas 1 and 2 during the period1945–1998a

Proportion of area(percentage of total area)

1945 1953 1961 1972 1989 1997

Area 1 (total area: 18.2 ha)Arable land 70 67 58 17 16 2Grassland 23 24 27 66 60 72Abandoned fieldsb 0 2 8 8 13 13Other land usec 7 7 7 9 11 13

Area 2 (total area: 11.8 ha)Arable land 73 65 68 30 20 2Grassland 6 14 8 42 46 59Abandoned fieldsb 0 0 2 6 6 7Other land usec 21 21 22 22 28 32

a Black-and-white aerial photographs (scale 1: 10,000 approx-imately) were interpreted (Fuhr-Boßdorf et al., 1999).

b These are overgrown withC. scopariusin both landscapetracts (cf.Simmering et al., 2001).

c Other land uses are mainly roads and tracks.

no fertilizer application since the abandonment ofcultivation.

The land use change followed a similar course inarea 2 (Table 1). Again, a large proportion of the arableland was converted to grassland between 1961 and1972, or between 1989 and 1997. Like area 1, area2, is also mainly used for extensive sheep grazing.Some allotments are grazed by cattle or horses, ormown once or twice annually. Individual allotmentsare fertilized; here, more exact informations are notavailable.

4.2. Floristic–phytocoenotic and site diversityof grassland of differing age

The DCA of the surveyed vegetation in area 1(Fig. 2)—site A, with considerably different vegeta-tion, has not been included here—permits the recog-nition of a distinct species diversity of the stands. Astrong correlation between ordination distance anddistance in the originaln-dimensional space wasfound (r2 = 0.78). The first axis (DCA 1) repre-sents the shift in community structure with increasinggrassland age (correlation between ordination axisDCA-1 and age of vegetation:r = 0.86; P < 0.001).Moreover the older stands are more frequent at the


Fig. 2. Joint plot with sites of grassland in area 1. Age (in years since last in cultivation) according to aerial photographs: (A) 3; (B)11–27; (C) 28–38; (D) 39–46; (E) >46. [H+]: concentration of protons (seeTable 2). tp: topographic position (seeFig. 1). In each ageclass three allotments (plots 1–3) were investigated. Five vegetation surveys (subplots a–e) were carried out on each plot, with a standardsize of 5 m× 5 m.

upper slope and the pH values of their soils are lower(seeTable 2).

The vegetation of the age classes D and E is muchmore similar within these classes than is the vege-tation of younger stands within their respective ageclasses. On the contrary, the stands of the age classes Band C are clearly differentiated along ordination axisDCA-2.

A total of 80 plant species was recorded in the 65subplots of area 1. A total of 27 species are found inone respective age class only. Each age class featuresfour to seven exclusive species. Most of those specieswere recorded just in one or two subplots. With a pres-

ence in five respectively seven subplotsConvolvulusarvensisandHeracleum sphondyliumare common inage class C. Both species prefer to grow on base andnutrient rich soils.

In Table 3, for age classes A–E, plant speciesthat essentially contribute to a differentiation of thevegetation are listed, together with their mean covervalues. The table also includes total species countstogether with the mean cover values, growth heightsand Renkonen coefficients of the vegetation. Witha dominance of the sown speciesLolium perenneand Trifolium repens, 65% total ground cover and agrowth height of less than 40 cm, the vegetation of


Table 2Site characteristics of the topsoil (soil depth 5–10 cm) of grassland allotments of differing age in area 1a

Plot Coarse material (wt.%) Ct (%) Nt (%) PCAL (mg × 100 g−1) pH

A 22 ± 4 1.44± 0.09 0.16± 0.01 13± 2 4.4 ± 0.2B-1 23 ± 4 1.72± 0.05 0.18± 0.01 4± 1 4.3 ± 0.1B-2 21 ± 6 1.74± 0.14 0.18± 0.01 8± 2 4.4 ± 0.1B-3 24 ± 5 1.67± 0.12 0.16± 0.01 6± 2 4.4 ± 0.2C-1 24± 8 1.70± 0.17 0.17± 0.01 10± 2 4.3 ± 0.1C-2 23± 6 1.72± 0.15 0.17± 0.02 6± 1 4.2 ± 0.1C-3 21± 5 1.78± 0.15 0.18± 0.08 7± 2 4.1 ± 0.1D-1 23 ± 7 1.76± 0.21 0.17± 0.02 10± 2 4.1 ± 0.1D-2 27 ± 4 1.69± 0.09 0.16± 0.01 7± 2 4.0 ± 0.1D-3 25 ± 4 1.72± 0.11 0.17± 0.01 7± 1 4.0 ± 0.1E-1 27± 8 1.61± 0.21 0.16± 0.02 4± 1 3.8 ± 0.1E-2 26± 5 1.69± 0.15 0.16± 0.01 7± 2 3.9 ± 0.2E-3 27± 9 1.67± 0.21 0.17± 0.02 7± 2 3.9 ± 0.1

a Age (in years since last in cultivation) according to aerial photographs: (A) 3; (B) 11–27; (C) 28–38; (D) 39–46; (E) >46.

site A is distinctly more sparse and of lower canopyheight than that of the other stands. Twenty furtherspecies attain a total cover of around 10%. The pres-ence of several annual weeds of arable land (includ-ing Capsella bursa-pastoris, Lamium purpureumandViola arvensis) indicates that the vegetation of thisallotment is still at a very early stage of secondarysuccession. With a mean Renkonen coefficient of0.87, the vegetation characteristics of the five subplotsare very similar, while the mean�-species richness(12.0 species/25 m2) is the lowest in comparison withthe other age classes.

In age class B, the sown speciesL. perenneandT.repensalso attain high cover values, yet these valuesare distinctly lower than those for the 3-year-old stand.While L. perenneis absent from older sites (C–E),T.repensis still present in these, although with a lowcover. Age class B is characterized by comparativelyhigh cover values of the ruderal speciesTaraxacumofficinaleandTanacetum vulgare, but also by typicalspecies of nutrient-poor grassland sites, such asBro-mus hordeaceusand Agrostis capillaris. In contrast,more nutrient-demanding grassland species, such asArrhenatherum elatius, Galium mollugoandH. spho-ndylium, attain the highest mean cover values in ageclass C.

Festuca rubradominates in age classes D and E,with more than 65% cover. Low-growing forbs, suchas Rumex acetosellaand Hypochoeris radicata,re-flect nutrient- and base-poor conditions.Anthoxan-

thum odoratumandTeucrium scorodoniacharacterizethe oldest stands (age class E).

In all age classes B–E about 41–48 species/125 m2

(mean �-species richness) were recorded. Themean �-species richness is lower in age classB (17.8 species/25 m2) than is in age class E(24.2 species/25 m2). The variability of the�-speciesrichness, as well as of the total ground cover, thegrowth heights and the Renkonen coefficients, iscomparatively high in age classes B and C.

Hence, owing to its land use history, area 1, whichtoday is predominantly used as grassland, features adifferentiated floristic composition over a small area.In an aggregative fashion, the following vegetationtypes can be distinguished: with increasing standage, there is (i) an early successional stage on aban-doned fields (age class A); (ii) ruderalized grasslandvegetation (age class B); (iii) an oat-grass meadow(Arrhenatheretum elatiorisBr.-Bl. 1915) (age classC) and also (iv or v) red fescue–bent-grass mead-ows (F. rubra–Agrostis tenuiscommunity) with alesser (age class D) or greater number (age class E)of acid-tolerant, respectively. Hence, the�-diversityof the grassland vegetation of this area increases inrelation to its age structure.

The vegetation of the three additional allotmentsstudied in area 2 is similarly differentiated with re-gards to stand age: the age class B allotment ischaracterized by a ruderalized grassland stand. Theage class C allotment features an oat-grass meadow,


Table 3Characteristics of the vegetation of grassland of differing age and differentiating species in area 1a

Class A,3 years

Class B,11–27 years

Class C,28–38 years

Class D,39–46 years

Class E,>46 years

Number of plots 1 3 3 3 3

Species count× 25 m−2

Plot 1 12.0± 3.4 15.8± 0.8a 22.0± 3.0a 23.0± 2.5a 24.2± 1.1aPlot 2 20.2± 1.5b 20.6± 3.3b 20.8± 2.8a 24.6± 1.8aPlot 3 17.4± 1.8a 15.8± 1.8a 22.0± 1.9a 23.8± 2.9aAge class 12.0 17.8± 2.2a 19.5± 3.3a 21.9± 1.1a 24.2± 0.4 b

Species count× 125 m−2 22 41± 3.5a 46± 4.3a 48± 4.2a 45± 1.5a

Total ground cover (%)Plot 1 65.0± 4 91 ± 2a 87± 8a,b 86± 2a 93± 3aPlot 2 74± 10b 92± 3a 89± 6a 85± 4aPlot 3 94± 2a 80± 5b 85± 4a 85± 8aAge class 65.0 86± 11a 86± 6a 86± 2a 87± 5a

Growth height (cm)Plot 1 37.0± 5 66 ± 2a 47± 7a 59± 7a 53± 6aPlot 2 70± 8a 82± 6b 52± 4a 50± 4aPlot 3 69± 6a 62± 3c 54± 4a 54± 2aAge class 37.0 68± 2a 64± 18a 55± 4a 52± 2a

Similarity (Renkonen coefficient)Plot 1 0.87± 0.06 0.85± 0.06a 0.84± 0.05a 0.87± 0.03a 0.86± 0.05aPlot 2 0.72± 0.08b 0.69± 0.12b 0.90± 0.03a 0.87± 0.02aPlot 3 0.89± 0.04a 0.82± 0.06a 0.89± 0.04a 0.84± 0.03aAge class 0.87 0.82± 0.09a 0.79± 0.08a 0.89± 0.01a 0.86± 0.02a

Mean ground cover (%)L. perenne 38 16.6± 23.0T. repens 29 16.0± 14.8 11.3± 6.1 11.3± 9.3 6.3± 3.1C. bursa-pastoris 0.2 0.1± 0.1V. arvensis 0.2L. purpureum 0.1T. officinale 2.6 1.1± 1.0 0.9± 0.2T. vulgare 1.3 ± 2.0 0.1± 0.1B. hordeaceus 2.6 ± 4.3A. capillaris 22.7 ± 3.8a 13.8± 9.4a,b 6.1± 2.5b 5.2± 1.4bA. elatius 0.2 ± 0.2 3.1± 3.9 0.6± 0.4 0.7± 0.6G. mollugo 1.6 ± 2.1H. sphondylium 1.2 ± 2.1F. rubra 2.8 32.0± 15.7a,b 45.0± 10.4a,b,c 71.0± 1.0c,d 67.7± 2.5b,c,dR. acetosella 6.0 ± 3.5 3.8± 3.1Luzula campestris 2.3 ± 1.6 2.3± 1.3Galium saxatile 0.4 ± 0.5 0.3± 0.4H. radicataA. odoratum 0.1 ± 0.0 1.6± 1.2T. scorodonia 0.4 ± 0.4

a Mean values of plot A1 (age class A), of plots 1–3 of each age classes B–E and of age classes are given with standard deviations(location of plots: seeFig. 1). The mean values of the plots and of the age classes B–E were compared. Values with different letters aresignificantly different (P < 0.05; Tukey-HSD).


Table 4RGB colour values and standard deviations of the colour tonal value spectra of grassland allotments of differing age in areas 1 and 2a

Class B, 11–27 years Class C, 28–38 years Class D, 39–46 years Class E, >46 years

Area 1Number of plots 3 3 3 3Red

Mean colour tonal value 200.0± 7.7 199.0± 13.9 202.6± 2.3 200.2± 6.1Mean standard deviation 3.8± 0.7 4.2± 0.6 4.3± 0.7 4.3± 0.3

GreenMean colour tonal value 116.8± 10.2a 123.9± 27.6a 168.7± 9.0b 156.2± 2.5bMean standard deviation 5.5± 1.3a 8.9± 2.8a 16.3± 1.8b 16.1± 0.6b

BlueMean colour tonal value 168.8± 14.2a 158.4± 20.5a 198.5± 1.4b 186.0± 6.4aMean standard deviation 7.3± 1.1a,b 9.7± 1.4a,b,c 12.7± 1.9b,c,d 13.3± 0.6c,d

Area 2Number of plots 1 1 1Red

Mean colour tonal value 227.5 227.2 221.3Mean standard deviation 2.2 2.8 3.3

GreenMean colour tonal value 173.3 181.6 183.5Mean standard deviation 4.1 6.8 8.9

BlueMean colour tonal value 215.9 219.6 215.9Mean standard deviation 5.1 7.3 8.2

a FCIR aerial photographs from 1997 were analysed using the image-processing program Adobe Photoshop 5.5. The aerial photographswere scanned in the RGB mode at a resolution of 300 dpi. Within the zone of five subplots of each allotment, standard-sized part-images(144 pixels) were cut out of the aerial photographs and their spectra of red, green and blue tones analysed. The mean tonal values of theR, G and B components were calculated, together with their standard deviations. Values with different letters are significantly different(P < 0.05; Tukey-HSD). The values in area 2 refer to one study site each.

and the age class D allotment a red fescue–bent-grassmeadow with few acid-tolerant forbs. Similar to thesituation in area 1, the mean�-species richness islower in age class B (18.3 species/25 m2) than isin age class C, respectively, D (22.6, respectively,22.3 species/25 m2).

4.3. Colour spectra of the survey plots in FCIRaerial photographs in relation to the age of thegrassland vegetation

In the FCIR aerial photographs, there are differ-ences in the colour spectra of the green and bluetones in relation to the age of the grassland vegeta-tion (Table 4). In the aerial photograph of area 1, boththe mean tonal values and their deviations are dis-tinctly lower in the age classes B and C than the cor-responding values in the age classes D and E. Thus

the stands of the red fescue–bent-grass meadow (ageclasses D and E) can be differentiated by the analysisof the colour spectra from younger stands. Further-more, positive quantitative relations between the mean�-species richness and both the mean green respec-tively blue tonal values and their deviations were found(Fig. 3).

In the allotments of area 2, the mean green tonalvalue and the deviations of the green and blue tonesin the FCIR aerial photograph of the youngest grass-land allotments (age class B) are the lowest, whilethose of the oldest grassland allotments (age classD) are the highest. In contrast to the situation inarea 1 the mean blue tonal value is not different be-tween stands of different age. Besides, in comparisonwith area 1, the mean red, green and blue tones ofthe three allotments of area 2 have distinctly highervalues.


Fig. 3. Correlations between�-diversity of plant species and RGB colour spectra of FCIR aerial photographs. Mean species counts per25 m2 of 12 grassland allotments in area 1. The analysis of the colour spectra of the study sites in FCIR aerial photographs was carriedout with the aid of the computer-program Adobe Photoshop 5.5. The aerial photographs were scanned in the RGB mode at a resolutionof 300 dpi. r = Pearsons-r; 95%—confidence intervals.


5. Discussion

The hill slope areas presented here feature landuse dynamics that are typical of the Lahn-Dill High-lands. Interpretation of aerial photographs from theperiod 1945–1997 from other parts of this landscape(Waldhardt et al., 2000) indicates that there, too, theproportion of arable land on upper and mid-slopesdecreased markedly between ca. 1960 and 1970,and again after ca. 1990, particularly in favour ofgrassland use. This development can be attributed toseveral causes: These site types are often so steep thatthey cannot be safely cultivated with modern agricul-tural machines. In addition, the high coarse materialcontent of the shallow soils renders ploughing moredifficult. Hence, increasing mechanization of agricul-ture from ca. 1960, led to a rapid abandonment ofcultivation on such arable land sites (Kohl, 1978).Meanwhile, increasingly unfavourable economic con-ditions for agricultural land use, also due to the EUagricultural reforms of 1993, have led to an almostcomplete abandonment of cultivation.

The land use dynamics presented here can be re-garded as typical of marginal, agriculturally usedregions of central Europe. Similar trends are shownby multitemporal aerial photograph interpretations byGloaguen et al. (1994). Comparable developmentsin northern Europe can be found where agriculturalland use was present in former decades. This is doc-umented by aerial photograph interpretations for arural area in central Finland (Ruuska, 1996).

According toDiquelou and Rozé (1997), land useextensification at a landscape scale leads to increasedbiodiversity, since new habitats, such as abandonedfields with woody plant succession, increase habitatdiversity. In contrast, these authors hold that exten-sification at a local (village) scale leads to a greateruniformity with decreasing biodiversity. This is notthe case in the present study. Although a large propor-tion of both investigated areas 1 and 2 has been grazedover large areas for some 30 years, the diversity ofland use forms has increased during this period. To-day, a considerable proportion of the total area is over-grown withC. scopariussuccession. In the Lahn-DillHighlands, according to our research group’s studies,these successional areas have a surprisingly diversefloristic–phytocoenotic inventory (Simmering et al.,2001). At the same time, owing to the gradual aban-

donment of cultivation, as shown in this study, thereis still a small-scale floristic–phytocoenotic diversitywithin the grassland today.

The heterogeneity of grassland in relation to ageraises both the floristic and the phytocoenotic�- and�-diversity of the studied hill slopes. A similar situa-tion was found byAustrheim and Olsson (1999), whostudied grassland in Norway with a comparable agespectrum. There, intermediate phase stands also havethe highest number of exclusive species. Similarly inboth studies, the oldest stands, with little variation inspecies composition, are richest in species. However,this only applies to the aboveground flora. Followingthe abandonment of cultivation, the soil seed reserve ofthe arable land species is more or less quickly depletedand many grassland species do not develop a persistentseed reserve (Bekker et al., 1997). In the Lahn-DillHighlands, the soil seed reserve of arable land speciesis almost completely depleted within about 20 years(Waldhardt et al., 2001).

Stand age, topographic position of stands and pHvalues of their soils are found to be important siteconditions to explain grassland diversity. Effects offurther site conditions (e.g. nitrogen mineralizationrates) on the vegetation could not be analysed in ourinvestigation. We do not know, if the sheep grazinghad been spatially homogenous within the investigatedslopes. Spatial patterns of grazing are known to resultin small-scale patterns of different nitrogen mineral-ization rates (Afzal and Adams, 1992) and of the veg-etation itself (Semmartin and Oesterheld, 1996; Adleret al., 2001).

Several species typical of the species-rich oldgrassland vegetation (age classes D and E) indicatenutrient- and base-poor conditions. Species lossesfrom the vegetation must be assumed in responseto fertilizer application (Bakker, 1989; Burel et al.,1998). In this case changes in vegetation structureand species losses are likely to occur in grasslands ofthe Lahn-Dill Highlands, too. Hence, a low trophiclevel is of importance for maintaining the�-diversityof plant species in old grasslands.

Differences of the flora and vegetation of grasslandsof different age as presented here should not be un-derstood as a successional process. To us, this appearsto be inadmissible for several reasons. It has beendemonstrated that the land use pattern, as well as thatof the entire landscape, changed considerably within


the last five decades. Furthermore, it may be assumedthat, at the time the land use was abandoned, arablesites that had been managed for a longer period hadhigher trophic levels than those that had already beenconverted to grassland around 1950. Since plant suc-cession in abandoned fields and grasslands depends onthe history of the allotments and also on their bioticenvironments (Gloaguen et al., 1994; Bakker et al.,1996), it would be wrong to assume that the presentvegetation succession is the same as it was earlier.

Despite methodological simplification, the analysisof colour tonal values and their deviations from twoFCIR aerial photographs scanned in the RGB modepermitted a differentiation of grassland stands of dif-ferent age and species richness. Since the tonal valueswere not calibrated before the analysis, as described byHolopainen and Wang (1998), differences of the abso-lute tonal values of both aerial photographs cannot beattributed to floristic–phytocoenotic differences. How-ever, data derived from one aerial photograph permitconclusions to be drawn on the floristic–phytocoenoticdiversity. In particular, the quantitative positive rela-tionships between the small-scale variability of greenand blue tonal values of the RGB images and the�-species richness of the vegetation are suitable foran exact indication of hot spots of species diversity ofthe studied site at the level of the allotment.

6. Conclusions

As our investigation shows, the age of grasslandstands is of varying importance, if focussing on spe-cific components and spatial scales of their biologicaldiversity (in sensuNoss, 1990; cf. Waldhardt and Otte,2000). The�-species richness within grassland standsof the studied site is higher in older than is in youngerstands. But there is no difference of the�-speciesrichness between stands of different age classes. The�-species diversity within and between stands and thestructural diversity of the vegetation are comparativelyhigh in the intermediate phase. The�-species and veg-etation diversity within a landscape tract, as well asits structural diversity, are positively effected throughstands of different age within a landscape tract. There-fore, differentiated analyses are necessary to assessthe importance of stand age of plant communities ontheir biological diversity. Those analyses will need to

be considered in the development of sustainable landuse concepts at the landscape scale.

In the present study, surrogates and correlates(Duelli and Obrist, 1998; Gaston and Spicer, 1998) ofthe floristic–phytocoenotic diversity of the grasslandof two typical hill slope areas in a marginal agricul-tural landscape were determined using the qualitativeand quantitative indicators “stand age”, “topographicposition”, “pH value of the soil” and “green and bluetonal values” in FCIR aerial photographs. After val-idation, hot spots of diversity in the region can beidentified without the need for a vegetation survey inthe field. At the same time, the elucidated indicatorsprovide indications for an explanation of the spatiallydifferentiated diversity in the studied region.

The floristic�-diversity of the grassland of the stud-ied site types (south-facing upper to mid-slopes) ishighest in older stands with, at the same time, a mod-erate nutrient supply at best. A mosaic of stands ofdifferent age increases the floristic–phytocoenotic�-and�-diversity of the slopes. Both these aspects willbe taken into account in developing ecologically andeconomically sustainable land use concepts within theresearch cooperative mentioned earlier: it is necessaryto retain old grassland stands, as well as a mosaic ofextensively used grassland stands of different ages, onsouth-facing upper and mid-slopes of the region.

Acknowledgements

We would like to thank the German Research Foun-dation (DFG) for financial assistance.

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