Floristic changes in the herb-layer vegetation of a deciduous forest in the Lorraine Plain under the influence of atmospheric deposition

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1 Forest Ecology and Management, 55 ( 1992 ) Elsevier Science Publishers B.V., Amsterdam Floristic changes in the herb-layer vegetation of a deciduous forest in the Lorraine Plain under the influence of atmospheric deposition A. Thimonier, J.L. Dupouey and J. Timbal INRA-CRF, Laboratoire de Phytokcologie, Champenoux, F Seichamps, France (Accepted 21 January 1992) ABSTRACT Thimonier, A., Dupouey, J.L. and Timbal, J., Floristic changes in the herb-layer vegetation of a deciduous forest in the Lorraine Plain under the influence of atmospheric deposition. For. Ecol. Manage., 55: In 1990, 221 floristic surveys dating from 1971 and 1972 were re-sampled at the intersections of a systematic grid covering a mixed hardwood forest in the Lorraine Plain (northeast of France). The dynamics of the herb-layer vegetation were studied at various levels: first, the individual behaviour of species (modification of their frequency of occurrence in the forest) was analysed, and then, the overall evolution of the vegetation was studied based on Ellenberg's ecological values and chemical richness indices derived from automatic data processing. Three main features were noted: (i) an increase in the frequency of nitrogen-demanding species; (ii) an increase in the Ellenberg's ecological value N (nitrogen); (iii) an enrichment of the site revealed by trophic level indices. These observations highlight a process of eutrophication in the forest. The spatial representation of the floristic changes shows a distinct structure of eutrophication, which mainly appears to concern the edge of the forest. Apart from Nancy city itself, the arable land next to the forest has been a large source of nitrogen, notably since the beginning of the 1970s due to a change in agricultural practices (a substantial increase in fertilisation). With regard to these observations, the eutrophication in this forest is likely to be ascribed to air pollutants, the deposition of which is enhanced through the forest edge effect. INTRODUCTION Until now, studies on the consequences of atmospheric pollution in forest ecosystems have been mainly concerned with the soil system or the physiological reaction of trees. Very few authors focused on changes in the herb-layer vegetation of the forest under the influence of atmospheric deposition. Nevertheless, the few recent papers published on this topic underline the significance of the lower layers as bio-indicators of long- or mid-term changes Correspondence to: J.L. Dupouey, INRA-CRF, Laboratoire de Phyto6cologie, Champenoux, F Seichamps, France Elsevier Science Publishers B.V. All rights reserved /92/$05.00

2 150 A. THIMONIER ET AL. occurring in ecosystems. These authors conclude that the terrestrial environment is presently undergoing acidification and/or eutrophication. If the danger of acidification may appear obvious, that of eutrophication does not a priori. Beyond the possibly positive effect of fertilisation, this process of enrichment of the environment with nutrients, and notably nitrogen, presents a double threat: firstly, nutrient imbalances and deficiencies of basic cations (Roelofs et al., 1985 ); secondly, an increased scarcity or even the disappearance of plant species which normally compete in nitrogen-deficient habitats ( Ellenberg, 1985; Van Breemen and van Dij k, 1988 ). The common method for studying changes in the vegetation is through observations in the field. Two procedures are possible: ( 1 ) The diachronic approach to vegetation consists of a re-sampling of previous floristic surveys. This approach was used by most authors: Wittig et al. ( 1985 ) in Westphalia, Falkengren-Grerup (1986, 1989) in Sweden, Kuhn et al. (1987) in Switzerland, Van Breemen and van Dijk (1988) in Holland. (2) The synchronic approach, that Tyler followed (1987), relies on a comparison between floristic surveys carried out on sites subject to different levels of pollution. A second method is experimentation, whether in the field (monitoring of fertilised plots for example ), or in the laboratory (experiments carried out in order to define the ecology of species, the significance of which was made clear following observations in the field ). The diachronic approach to vegetation, completed by pollution simulation experiments, seems to be an effective method. However, it requires previous data which are not always available. Besides, frequently, only a very limited number of these original surveys for one type of site could be resorted to in each forest, so that it is difficult to obtain statistically reliable results. We had 477 floristic surveys at our disposal, dating from 1971 and 1972, which were made at the intersections of a systematic grid covering a relatively homogeneous forest in the Lorraine Plain: the forest ofamance (Meurthe-et- Moselle, France). We re-sampled almost half these surveys in J 990, in order to determine possible changes in the herb-layer vegetation. The systematic pattern of the sampling allowed us to obtain a spatial representation of the dynamics between 1971 and 1990 throughout the entire forest. This opportunity based on a mid-term study of the dynamics of the flora and the vegetation of a forest is the first of its kind in France, where no systematic network of diachronic vegetation studies has ever been set up. METHODS The Amance State Forest Amance forest extends over a little more than 1000 ha, about 15 km east of Nancy (Meurthe-et-Moselle, France). It is subjected to a semi-continental

3 ATMOSPHERIC DEPOSITION AND HERB-LAYER VEGETATION 151 climate. It lies in the Lorraine Plain, which corresponds geologically to the most external Liassic and Triassic rings of the Paris Basin. The substratum mainly consists of marls and limestones, frequently covered with silts of variable depth. Leaching and temporary hydromorphy are two important pedogenetic processes in the Lorraine Plain. The soils of Amance forest are usually leached brown soils, except in some poorly drained valley bottoms of insignificant area where pseudogleys predominate. The forest is mainly composed of broad-leaved species. Sessile and pedunculate oak stands (Quercus petraea (Mattus.) Liebl., Quercus robur L. ) with hornbeam ( Carpinus betulus L. ) occupy the major part of the forest ( 650 ha) (all vascular species are named following the Flora Europaea (Tutin et al., ) ). Beech (Fagus sylvatica L. ) tends to extend naturally, and there is an almost pure beech grove covering 120 ha. The management regime used to be coppice with standards until the nineteenth century. It was then progressively converted into high forest. Sampling One of us (J. Timbal) conducted the initial sampling over a period of 2 years ( 1971 and 1972 ). The sampling was of a systematic type and comprised a square grid ( 140 m X 140 m) covering the whole forest. A floristic survey was made at every intersection of this grid, that is to say 477 surveys. It consisted of a comprehensive inventory of the species present within a 400 m e circle. An abundance-dominance coefficient using the classical Braun-Blanquet scale was assigned to each species. In 1990, we re-sampled the 221 sites studied in 1971 which had not been disrupted by too-extensive thinning. These surveys were uniformly distributed throughout the forest, at every second intersection of the initial grid. Besides the floristic survey itself, we collected general information (slope, aspect, topographic position, silvicultural management), as well as soil observations (humus and soil types, upper soil layer ph measured in the laboratory). Two different technical teams were involved in the two different samplings, but they both still belong to the same laboratory so it was possible to do a careful intercalibration of the sampling methods of the two teams, which prevented systematic errors during sampling. Analysis of the dynamics Various aspects of the vegetation dynamics from 1971 to 1990 were considered: (1) the floristic composition aspect: the evolution of the occurrence frequency of each species in the whole forest; (2) the value of the vegetation

4 152 A. THIMONIER ET AL. as a bio-indicator: the evolution of Ellenberg's coefficients and the evolution of the survey position on the factor axis derived from a multivariate analysis of floristic data, which integrates environmental conditions; (3) the vegetation shift mapping. Comparison of species frequencies We compared the frequency of each species in 1971 and in 1990, considering its occurrence in each survey. We analysed trees, shrubs for which we considered only one layer, and herbaceous species separately. Changes in the number of species between 1971 and 1990, divided by the total number of surveys and expressed as a percentage, allowed us to distinguish four groups: ( 1 ) a group of species which appeared for the first time; (2) a group of species the frequency of which increased (variation greater than 5% ); ( 3 ) a group of species the frequency of which remained stable (absolute variation less than 5% ); (4) a group of species the frequency of which decreased (variation less than -5%). Comparison of Ellenberg's indices Ellenberg (1979) expressed the behaviour of western mid-european vascular species with regard to ecological factors in a coefficient form. We kept three of these coefficients for our analysis: L (light), R (ph) and N (nitrogen). The average coefficient per survey was calculated for 1971 and 1990, according to the following formulae: i=n i=n i~n Eabdo = ~ ciei/ ~ c i E+_ = 1/n ~, e i i=1 i=1 i=l where Eabdo is the mean value of Ellenberg's coefficient E calculated with coefficients of abundance-dominance; E+ is the mean value of Ellenberg's coefficient E caldulated considering the presence or absence of each species; n is the total number of herbaceous species in the survey; ci is the abundancedominance coefficient assigned to species i; ei is the value of Ellenberg's coefficient for species i. This is an index ranging from 1 to 9. It gives an indicator value of each species for either light, ph or soil nitrogen. Low values are characteristic of low-demanding species (shade tolerant, adapted to acidic soils or to low nitrogen levels ). Conversely, high coefficients have been attributed to light-demanding, calcicole and nitrogen-demanding species.

5 ATMOSPHERIC DEPOSITION AND HERB-LAYER VEGETATION Comparison of the respective positions of the surveys on the multivariate analysis factor axes At the time of the re-sampling of the 221 surveys, we recorded 161 species. Because of problems of plant identification and seasonal phenological phases, only a limited number of species was retained for the analysis. Thus, all the species of doubtful identification in 1971 or corresponding to plurispecific taxonomic complexes were eliminated. This is notably the case for various species of the Carex genus and of certain grasses. The vernal species were also suppressed from the analysis, and the trees were eliminated. Lastly, the species which were present only once were not retained. All things considered, 98 species participated in the factor analysis as active variables. The ecological variables were treated as illustrative ones. The 2 X 221 surveys of the two sets corresponding to 1971 and 1990 participated in the analysis as active individuals. This array was analysed using reciprocal averaging (Gauch et al., 1977; Gauch, 1982 ), one of the most suitable methods for extracting the main ecological factors affecting vegetation composition. Then, the strength of the shift for each survey between its 1971 position and its 1990 position along the main ecological factors derived from reciprocal averaging was used as an indication of vegetation changes. Vegetation shift mapping All surveys were registered in the Atlas geographic information system (Dupouey et al., 1990) in order to map species distribution and vegetation changes. The initial data were smoothed before mapping using standard bilinear interpolation. RESULTS Species number The average species number per survey, all layers being considered, increased significantly in the years For 25.2_+ 1.6 species per survey (significance level of 1%) in 1990, there were in These values were calculated excluding vernal species and species which were not identified in 1971 (see above). Also, the maximum species number of the floristically richest survey is higher in 1990 (63 species) than in 1971 (38 species ). The floristic richness of some of these surveys, located in small valley bottoms, may possibly be derived from undetected micro-heterogeneities in some isolated cases, but the trend is significant. The floristically poorest survey also displays a few more species in 1990 (six species) than in 1971 (four species ).

6 154 A. THIMONIER ET AL. Species frequency Trees and shrubs A distinct increase in the frequency of all germination is noticeable (Table 1 ), but this increase is probably linked to interannual climatic variations and biological cycles rather than to a mid-term observable trend. The increase in F. sylvatica is striking (Table 1 ). This extension of beech concerns all three layers. On the other hand, oak (Q. petraea and Q. robur) recedes in the shrub layer. The progression of beech expresses a natural phenomenon which concerns the whole of the east of France, where climatic conditions are more favourable for beech than oak. The frequency of shrubs remains fairly constant except for Cornus sanguinea, which decreases in number. TABLE 1 Change in tree and shrub species frequencies. Number of plots where the species was noted in 1971 and The total number of plots was 221 Species ( 1 ) Seedling (2) Shrub ( 1 ) + (2) Acer campestre 24 Acer platanoides 0 Acer pseudoplatanus 0 Betula pendula Carpinus betulus 158 Fraxinus excelsior 61 Fagus sylvatica 103 Prunus avium 20 Populus tremula 21 Quercus petraea plus Q. robur 116 Salix caprea 0 Sorbus torminalis 11 Tilia cordata 53 Corylus avellana 36 Crataegus laevtgata 12 Crataegus monogyna 11 C. laevigata plus C. monogyna 21 Cornus sanguinea 22 Prunus spinosa 5 Ribes alpinum 3 Viburnum lantana 0 Viburnum opulus

7 ATMOSPHERIC DEPOSITION AND HERB-LAYER VEGETATION 155 Herbaceous species New species. Several species which were not found in 1971 were noted in 1990 (Table 2 ). Many of them are mesohygrophilic species which may also be more or less nitrogen demanding. We also observe the appearance of bryophytes absent from 1971 surveys: notably Dicranella heteromalla and Mnium hornum, which are acidiphilous species. However, it would seem that these species were not identified by the authors of the surveys at the time. Therefore, their absence in 1971 might not be real. Increasing species. Increasing species are either hygrocline or indicate nutrient-rich habitats. These species can be acidicline species just as well as neutrocline ones. Some of them are typical of clearing and windfall (Galeopsis tetrahit, Rumex spp., Juncus spp. ) (Table 3 ). A marked increase in the frequency of most bryophytes (Eurhynchium stokesii, Eurhynchium striatum, Atrichum undulatum and Fissidens taxifolius ) and pteridophytes (Athyrium filix-femina, Dryopteris filix-mas and Dryopteris carthusiana) is also noticeable. The frequency of Arum maculatum, Cardamine pratensis, Ornithogalum pyrenaicum, Ranunculus auricomus, Ranunculus ficaria and Phyteuma spicatum increased from 1971 to However, all of them are vernal species which may not have been found when the surveys were carried out in the TABLE 2 New species, i.e. species present in 1990, not in Number of plots where the species was noted Species 1990 Hypericum pulchrum 14 R umex spp 13 Rubus idaeus 10 Galium aparine 7 Adoxa moschatellina 5 Paris quadrifolia 5 Valeriana officinalis 5 Hypericum perforatum 3 Ranunculus repens 3 Lathyrus montanus 3 Rubus caesius 3 Luzula campestris 2 Alliaria petiolata 2 Angelica sylvestris 2 Filipendula ulmaria 2 Ranunculus nemorosus 2 Solanum dulcamara 2

8 156 A. THIMONIER ET AL. TABLE 3 Increasing species: number of plots where the species was noted in 1971 and 1990, and percentage of variation between these two dates out of the total number of plots (221) Species Var (%) Eurhynchium stokesii Fissidens taxifolius Galeopsis tetrahit A trichum undulatum Veronica montana Potentilla sterilis Circaea lutetiana Primula elatior Carex sylvatica Viola reichenbachiana Lamiastrum galeobdolon Carex remota Athyriumfilix-femina Luzula pilosa l 1.0 Hedera helix Geum urbanum Polygonatum multiflorum Milium effusum Dryopteris carthusiana Carex pallescens Dryopterisfilix-mas Juncus spp l 8.1 Poa nemoralis Ajuga reptans Scrophularia nodosa Stellaria holostea Vicia sepium Fragaria vesca Moehringia trinervia Eurhynchium striatum Rumex spp Plagiomnium undulatum Geranium robertianum Stachys sylvatica Glechoma hederacea summer or autumn of 1971, whereas the 1990 sampling took place from spring to the beginning of summer. Therefore, frequencies cannot be compared. Stable species. Species considered to be stable however tend to increase rather than decrease. They are indiscriminately acid-tolerant, neutrophilous or calcicole species (Table 4 ).

9 ATMOSPHERIC DEPOSITION AND HERB-LAYER VEGETATION 157 TABLE 4 Stable species (for further explanation, see Table 3 ) Species Vat (%) Hypericum hirsutum R ubus idaeus Thuidium tamariscinum Convallaria maialis Veronica chamaedrys Epilobium montanum Deschampsia cespitosa Heracleum sphondylium Neottia nidus-avis Galium odoratum Valeriana officinalis Solidago virgaurea Urtica dioica Festuca gigantea Hypericum perforatum Lathyrus montanus Carex flacca Poa chaixii Luzula luzuloides Pulmonaria obscura Polytrichumformosum Melica uniflora Euonym us europaeus Silene dioica Ligustrum vulgare 3 4 O. 5 Campanula trachelium 6 7 O. 5 Rubusfruticosus Brachypodium sylvaticum Veronica officinalis Lonicera periclymenum Festuca heterophylla Rosa spp Dicranum scoparium Carex digitata TABLE 5 Decreasing species (for further explanation, see Table 3 ) Species Var (%) Rhytidiadelph us triquetrus Galium sylvaticum

10 158 A. THIMON1ER ET AL. TABLE6 Ellenberg's mean ecological values R (ph), N (nitrogen), L (light). R, L and N are calculated considering the presence or absence of each species for 1971 and The difference between the values ( ) is tested by the Student t-test R (ph) N (nitrogen) L (light) ns 0.27*** -0.12"** ns, Not significant (P> 0.05 ). ***, Significance level of Declining species. Only two species, Galium sylvaticum and especially Rhytidiadelphus triquetrus, declined distinctly (Table 5 ). Species indicator value (Ellenberg's coefficients) Whether the mean Ellenberg's coefficient is calculated using abundancedominance coefficients or presence-absence data, the same trends may be observed. The mean value of R (ph) increases slightly, but not significantly. The mean value of N (nitrogen) increases whereas that of L (light) decreases. The differences are significant for both values (Table 6). The overall correlation coefficient between the variation of N and that of L is very low and not significant (r= 0.04), but an increase in N concomitant with a decrease in L occurs for a great number of the sites (86 out of 221 ). Evolution along the factor axes Significance of the axes derived from the multivariate analyses First axis. The meaning of the axes of reciprocal averaging can be derived from the examination of species projection. Species which are decreasingly nitrogen demanding and increasingly acid tolerant space out along the first axis (Fig. 1 ). Thus, calcicline species, such as Ligustrum vulgare, Viburnum lantana, Euonymus europaeus, Campanula trachelium and Brachypodium sylvaticum are projected onto the negative abscissa of the first axis. Species such as Galium aparine, Glechoma hederacea, Heracleum sphondylium and Paris quadrifolia, which have an affinity for nitrogen and soils with high base saturation, are also found on this part of the axis. Neutrocline species with a fairly large ecological range are projected onto the middle of the abscissa of the first axis: e.g. Vicia sepium and Viola rei-

11 - - Potytriehum Lonieera _ Brachypodium ATMOSPHERIC DEPOSITION AND HERB-LAYER VEGETATION 159 AXIS 1 AXIS 1 Dieranum seoparium herb laver <30% act~ mdll 30%=<herb layer<40% Luzula luzuloides formosum 02 Atrichum undulatum Luzula pilosa periclymenum Tilia eordata =~ Poa chaixii Fagus sylvatiea ~ Carpinus betulus Eurhynehium striatum Deschampsia cespitosa Rubus frulicosus 4.75=<pH<4.95 mesotrophic mull 4=<pH<4.75, 4.95=<pH< %=<herb layer<60% pseudogley leached brown soil 5.40= <ph< %=<herb layer<80% eutrophic to mesotrophie mull Viola reichenbachiana bottom of slope Z Vicia sepmm Acer campestre 5 15=<pH<5 40 slightly leaehed brown soil -I Cornus sangulnea Geum urbanum =<p11<6 5{) 5 90=<pI1~.6 50 sylvaticum herb layer> =80% bottom of small valley brown soil Z k.~ Galium odoratum Heracleum sphondylium Campanula t rachelium Glechoma hederacea Solidago virgaurea 4}4 eul rophic mull -2 Euonymus europaeus - Viburnum lantana -0 6 marl<62cm -- Galium aparine Paris quadrlfolia L~guslrum vulgate I ph>=6 5 eqtrophic brown soil Fig. 1. The first axis of reciprocal averaging of the vegetation data: a projection of species and ecological variables.

12 160 A. THIMONIER ET AL. chenbachiana. Lastly, more or less acid-tolerant species are found on the positive abscissa of the axis: Luzula pilosa, Milium effusum, Poa chaixii, Atrichum undulatum, Lonicera periclymenum, Luzula luzuloides, Dicranum scoparium, Polytrichum formosum. Projection of illustrative environmental variables onto the first axis highlights its ecological meaning. In the increasing direction of the axis: --ph values decrease; --shallow carbonated marl and rich topographic locations such as the bottom of slopes and small valleys are projected onto the negative part of the axis; --humus type passes from eutrophic mull to acid mull; --soil type passes from eutrophic brown soils to leached soils; --herb-layer cover decreases. All these observations lead us to consider that the first axis represents a decreasing gradient of chemical richness. Nutrient availability and ph (these factors being closely related themselves) are linked to topographic situation, geological substratum, soil and humus types; they condition the extent of the herb-layer vegetation cover. Second axis. Hygronitrocline and heliophilous species, indicators of disturbances such as forest thinning and windfall, are grouped at the top of the second axis ( Hypericum perforatum, Hypericum pulchrum, Stachys sylvatica, Rumex spp., Juncus spp., Veronica montana, Veronica officinalis, Carex pallescens). The lowest value of the canopy cover is projected onto this end of the second axis and confirms the interpretation of the latter as an axis of canopy opening and the subsequent disturbance of the soil hydric balance. Other axes. The third axis may be interpreted as an anthropogenic axis: it separates a group of species linked to the coppice with standards forestry system, from a second group connected with high forest treatment (Becker, 1979). Further axes do not provide any interpretable information. Evolution of the trophic level Considering the ecological meaning of the first axis, a shift of the surveys along this axis expresses variations in the local conditions concerning ph and nutrient availability. We will refer to 'acidification', or 'eutrophication' if a survey moves towards the right, or to the left, respectively, along the first axis between 1971 and Eutrophication concerns many more sites (155) than does acidification ( 59 ). Also, the range of eutrophication ( _ 0.038, significance 1% ) is

13 ATMOSPHERIC DEPOSITION AND HERB-LAYER VEGETATION 161 higher than that of acidification ( _ ). The difference in the average abscissa of the surveys is _ (significance 1% ). Mapping of the trophic factor shift Figure 2 highlights a marked spatial organisation of eutrophication. The phenomenon mainly concerns the forest boundaries, notably the western and N IIver 9 strong eutrophicot,on m strong eutrophication moderote eutrophicotion Foirtg moderote eutroph,cotion stight eutrophicotion 5tobitity I'---I stight ccidificotion modero~e ocidjf,cotlon [Z] Foir~ U strong ocioificotlon 500m Fig. 2. Changes in the herb-layer vegetation in Amance forest. The direction of change corresponds to shifts of the surveys along the gradient of chemical richness expressed by the first axis derived from reciprocal averaging. Blank areas correspond to surveys which were not re-sampled (young plantations or felling).

14 162 A. THIMONIER ET AL. southwestern sides. Conversely, acidified areas are confined to the inner part of the forest. DISCUSSION The following developments were observed between 1971 and 1990: a significant increase in the species number per survey and in nitrogen-demanding species frequency; a significant increase in the average nitrogen indicator value; an overall shift of the surveys towards higher trophic levels along the trophic axis; a strong spatial organisation of this shift, enhanced along the forest boundaries. The two methods of overall vegetation analysis (Ellenberg's indicator values and shift along the factor axis) produce the same conclusion that there has been eutrophication of the environment, but this result appears to be more distinct with the second method than with the first. The positions of the surveys on the factor axis are undoubtedly more suitable biological indicators for our study than Ellenberg's coefficients, as these coefficients only have an indicative value. They were established for a geographical area different to that of our study. Moreover, they cannot take into account phenomena such as genetic differentiation and factor substitution which may interfere with the autoecology of the species. Comparison with data in the literature If we refer to the literature, we notice that the results obtained by different authors are not always consistent. However, some general and significant trends can be observed. An increase in the abundance of nitrophilous plants is observed in several studies. Thus, Van Breemen and van Dijk (1988) notice an increase in the abundance of Galeopsis tetrahit in a Dutch forest, in addition to other species not found in Amance. An increase in the presence of Galeopsis tetrahit, as well as that of other nitrophilous species such as Rubus idaeus, Aegopodium podagraria and Chamaenerium angustifolium, is also noted by other authors (Falkengren- Grerup, 1986; Bfirger, 1987; Kuhn et al., 1987). The decline ofrhytidiadelphus triquetrus is noticed by Kuhn et al. ( 1987 ), Kissling et al. ( 1988 ) and Wilmanns ( 1989 ). Kuhn compared past and present floristic surveys, carried out in Switzerland, and according to him, this bryophyte is an indicator of nutrient-deficient sites. He ascribes its decline to the enrichment of the site with nitrogenous substances. However, Wilmanns attributes it to a pollution-susceptibility effect. Floristic changes may also reveal an acidification of the environment, by an increase in acid-tolerant species: Galeopsis tetrahit, Circaea lutetiana, Athyrium filix-femina, and particularly bryophytes such as Dicranella heter-

15 ATMOSPHERIC DEPOSITION AND HERB-LAYER VEGETATION 163 omalla, Dicranum scoparium (Wittig et al., 1985; Biirger, 1987; Kuhn et al., 1987 ), Plagiothecium undulatum and Lophocolea bidentata (Kissling et al., 1988 ). These expanding species are often also more or less markedly heliophilous species ( Rubus idaeus, Galeopsis tetrahit, Dryopteris filix-mas, Athyrium filix-femina, Juncus spp. ) (B/irger, 1987). In deciduous forest sites in southern Sweden, Falkengren-Grerup ( 1986 ) observes a cov ariation between some species cover and ph changes; thus, Mercurialisperennis, Lamium galeobdolon, Galium odoratum, Oxalis acetosella and Luzula pilosa decrease in cover in sites in the acidic part of the ph range. Lastly, the direction of the evolution of the species number varies according to the authors. Wittig et al. ( 1985 ) observe an overall increase in the species number per survey between 1976 and 1983 on Milio-Fagetum sampling plots in the Westphalian Bight. Falkengren-Grerup (1986) also reports an increase in the species number in spite of a decrease in soil ph. Conversely, Kuhn et al. (1987) and Wilmanns (1989) notice a decrease in the species frequency that they ascribe to a closure of the canopy. In some cases, this closure may be due to an increase in crown volume following an improved nitrogen nutrition. Several hypotheses may be considered to explain the eutrophication observed. We shall discuss two of them: the effects of thinning, and the influence of atmospheric deposition. Influence of thinning Thinning induces microclimatic and edaphic modifications, notably due to an increase in radiation reaching the ground. The activation of humus mineralisation derived from these changes is translated into an increased mineralisation of nitrogen. A more obvious effect is then the emergence of nitratophilous species (Epilobium montanum, Rubus idaeus, Galeopsis tetrahit). Plots which were thinned during the last 3 years should thus have evolved in the direction of eutrophication (enrichment of soils with nitrate through the activation of mineralisation ). Similarly, thinning carried out in the years before the first sampling might have been the origin of a temporary enrichment with nitrogen, which would have been reflected by the floristic composition of the surveys dating from In the absence of the intervention of other factors, the plots concerned should then have become acidified from 1971 to In our study, if we compare the existence of thinning and the direction of the evolution in the plots concerned, no links between them are noticeable. There was also no correlation between the variation of L and N. Thinning, therefore, does not appear to be responsible for the changes in the trophic level observed. This may be due to the relative richness of the soils in the

16 164 A. THIMONIER ET AL. Amance forest, where mineralisation is already active, whereas the effects of thinning are mostly perceptible in acid sites. Influence of atmospheric deposition In this case, the hypothesis of thinning effects cannot be retained; we have, however, seen that evolution oftrophic levels is not random but strongly spatially organised. Any voluntary input of nitrogen being excluded, we suggest the hypothesis of an atmospherically derived nutrient input. Amance forest is subject to nitrogen inputs on account of the presence of agricultural land and the Nancy industrial area, in the immediate proximity. Annual nitrogen inputs in Amance through rainwater were assessed to be kg ha- ~ year- 1 in the beginning of the 1970s (Aussenac et al., 1972). These values were already high, but were in accordance with the results obtained by other authors in very industrialised areas. More recent work concerning the area of northeast France, stated that annual nitrogen throughfall inputs were in the order of 15 kg ha -~ year -1 in the Vosges (Probst et al., 1990), and 50 kg ha- 1 year- 1 in the Ardennes (Nys, 1989). Such amounts of annual nitrogen input are likely to be responsible for the observed floristic changes. Nilsson and Grennfelt (1988 ) propose a critical load of 5-20 kg N ha-~ year-1 for deciduous forests. Above this level, significant changes can be expected. Nitrogen mainly comes from three sources: agriculture, industry and transport. Emissions of ammonia are known to have increased sharply since 1950 (e.g. see Asman et al., 1987). An inquiry amongst the farmers whose fields adjoin our forest concludes that there has been a marked increase in the use of nitrogenous fertilisers since the early 1970s. At that time, the local production system progressively changed from extensive breeding on unfertilised pastures, to cereal and oilseed rape cultivation. At present, kg ha-1 year-1 of nitrogen are applied to this cultivation, in both liquid and solid form (nitrate and urea solution, ammonium nitrate granules ). The spatial organisation of vegetation changes in Amance forest shows two clear patterns. Firstly, eutrophication is particularly marked on the sides of the forest. This observation is consistent with a pollution hypothesis, on account of the filtering effect of forest edges, outlined by several authors (Hasselrot and Grennfelt, 1987; Draaijers et al., 1988; Beier and Gundersen, 1989 ). The forest edge induces air turbulence, which enhances exchanges between the receptor and the atmosphere. This is mainly translated into an increase in dry deposition onto the trees at the edge. Deposition decreases from the border towards the interior of the forest. Secondly, eutrophication is higher on the sides exposed to the prevailing winds, which are westerly and southwest-

17 ATMOSPHERIC DEPOSITION AND HERB-LAYER VEGETATION 16 5 erly on annual average. This further supports the pollution hypothesis, since the wind conditions atmospheric deposition through its speed and direction. The distinct spatial structure of eutrophication also allows us to consider other possible factors, such as changes in human frequentation in the forest or modifications of big game density, or the mildness of the two previous winters, to be secondary. Some areas at the centre of the forest have undergone a slight acidification. This process could be partly related to the natural acidification of stands due to ageing (Tamm and Hallb~icken, 1988 ). In the other parts of the forest, this phenomenon, if it exists, is probably hidden by the strong response of the plants to the nitrogen inputs. Moreover, the nitrogen input produces a release of protons during nitrification and the root-uptake of nitrogen in the ammoniacal form. This acidification corresponds to a reduction of the acid neutralising capacity of the soil and is not systematically translated into a decrease in soil ph. The soils of Amance are rich, with a large buffering capacity so that a decrease in ph is unlikely to occur. No conclusion can be drawn in the absence of previous ph measurements. These considerations lead us to conclude that atmospheric pollution has an influence on the dynamics of vegetation in Amance forest, and through its floristic composition, the vegetation reveals an enrichment of the site by nitrogenous nutrients. CONCLUSION Within a relatively short interval of time (19 years), marked changes in vegetation have occurred in the forest studied. Both species frequency and vegetation composition of the forest evolved towards a more eutrophic state which reveals an enrichment of most sites with nitrogenous nutrients. Among the various possible causes, atmospheric pollution seems to be the most probable. The strong spatial organisation of vegetation changes is a major argument in favour of this hypothesis. This pollution could partly be ascribed to the change in agricultural practices which occurred in the early 1970s. In order to quantify deposition fluxes and confirm the forest edge effect, we have begun a 1-year study of the atmospheric deposition of nitrogen. We are monitoring canopy throughfall along two parallel lines from the edge to the centre of the forest. These results are consistent with the few works carried out on the incidence of atmospheric deposition on herb-layer vegetation, notably in the Netherlands, Sweden, Germany and Switzerland. Following these results, the existence of a real process of eutrophication of terrestrial ecosystems, parallel to that of coastal ecosystems and ground water, may be assessed. Evidence of soil acidification due to acid deposition reported by several authors (Wittig et al., 1985; Falkengren-Grerup, 1986, 1987, 1989; Tamm and Hallb~icken,

18 166 A. THIMONIER ET AL. 1988; Falkengren-Grerup and Eriksson, 1990) is not indicated by the vegetation changes in our forest. If there is such an acidification, it is necessarily hidden by eutrophication. Indicator values of the species in our sample do not allow the discrimination of the two processes. In any case, our study is a further argument in favour of the rapid instalment of a permanent network for the large-scale monitoring of ecosystems, which would assure future reference points for further studies in vegetation dynamics. ACKNOWLEDGEMENTS The authors acknowledge the technical collaboration ofp. Behr, G. Doucet, C. Kieffer and R. Schipfer. REFERENCES Asman, W.A.H., Drukker, B. and Janssen, A.J., Estimated historical concentrations and depositions of ammonia and ammonium in Europe and their origin ( ). IMOU Report R IMOU (Instituut voor Meteorologie en Oceanografie), Rijksuniversiteit- Utrecht, The Netherlands, 88 pp. Aussenac, G., Bonneau, M. and le Tacon, F., Restitution des min6raux au sol par l'interm6diaire de la liti6re et des pr6cipitations dans quatre peuplements forestiers de l'est de la France. Oecol. Plant., 7: Becker, M., Influence du traitement sylvicole sur la flore foresti6re: cas de la futaie et du taillis-sous-futaie. Vegetatio, 40 ( 3 ): Beier, C. and Gundersen, P., Atmospheric deposition to the edge of a spruce forest in Denmark. Environ. Pollut., 60 (3-4): Biirger, R., Ver~inderungen tier Bodenvegetation in Wald- und Forstgesellschaften des mittleren und siidlichen Schwarzwaldes. Kernforschungszentrum Karlsruhe, Forschungsbericht KfK-PEF, 52:162 pp. Draaijers, G.P.J., Ivens, W.P.M.F. and Bleuten, W., Atmospheric deposition in forest edges measured by monitoring canopy throughfall. Water Air Soil Pollut., 42: Dupouey, J.L., Drapier, J., Geldreich, P. and Dubuit, M., La cartographie automatique en for6t. Application/l l'am6nagement de la for~t de Betschdorf (Bas-Rhin). Rev. For. Fr., 42(1): Ellenberg, H., Zeigerwerte der Gef~isspflanzen Mitteleuropas. Second Edition, Scripta Geobotanica, Vol. 9, G/Sttingen, Ellenberg, Jr., H., Ver~inderungen der Flora Mitteleuropas unter dem einfluss von Diingung und Immissionen. Schweiz. Z. Forstwes., 136: Falkengren-Grerup, U., Soil acidification and vegetation changes in deciduous forest in southern Sweden. Oecologia (Berlin), 70: Falkengren-Grerup, U., Long term changes in ph of forest soils in southern Sweden. Environ. Pollut., 43: Falkengren-Grerup, U., Soil acidification and its impact on herb vegetation. Ambio, 18 (3): Falkengren-Grerup, U. and Eriksson, H., Changes in soil, vegetation and forest yield be-

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