Plant and Soil 177: 21-31, 1995. © 1995 KluwerAcademic Publishers. Printed in the Netherlands.

Is the Ca:AI ratio superior to pH, Ca or AI concentrations of soils in accounting for the distribution of plants in deciduous forest?

Ursu la Fa lkeng ren -Gre rup , J6rg Brunet , M a u d E. Quis t and G e r m u n d T y l e r Soil-Plant Research, Department of Ecology, University of Lund, Ecology Building, S-223 62 Lund, Sweden*

Received 30 September 1994. Accepted in revised form 16 June 1995

Key words: Ca:A1 ratio, exchangeable Ca and A1, forest plant distribution, pH, soil solution Ca and A1

Abstract

The distributions of vascular plants in south Swedish deciduous forests were related to exchangeable (exc) and soil solution concentrations of H + (pH), Ca, A1 and the Ca:A1 ratios within these fractions. Topsoils (0--5 cm) of 172 sites with a pHKcl of 3.2-3.9 (corresponding to 3.7--4.4 in soil solution) were used. In the soil solution both total Alt and quickly reacting Air were determined. Exchangeable concentrations were generally well related to plant distributions, the highest correlation coefficients usually being given by pHKcl>Caexc>Alexc.>(Ca:Al)exc. The (Ca:A1)exc ratio was clearly inferior. Out of the soil solution variables studied, Ca concentration, followed by pH, was best correlated with plant distributions, Alt, Alr, and the Ca:AI ratios having similar and lower coefficients. It is concluded that the use of Ca:A1 ratios as a general measure of A1 toxicity in controlling plant distributions is rather problematic. It seems difficult to apply evidence for Ca-A1 interactions from solution culture experiments to field conditions when measured as exchangeable or soil solution concentrations of the soil.

Introduction

Soil chemical conditions often play a major role in the field distribution of plants, though a variety of environmental and other conditions are of importance. Aluminium toxicity is a well-known problem when growing crops on acid soils (Foy, 1988; Wheeler et al., 1991). Aluminium has also been suggested to con- trol the distributions of many native plants in natu- ral habitats (Andersson, 1993; Andersson and Brunet, 1993; Grime and Hodgson, 1969; Henrichfreise, 1981; Runge and Rode, 1991). High A1 concentrations are often characteristic of the lower horizons of Podzols and Dystric Cambisols, but are frequently measured also in highly acidic topsoils of different ecosystems (Andersson, 1993; Tyler, 1993). However, H + toxici- ty may constitute a primary limitation in highly acidic soils, especially in soils with a high organic matter con- tent (Andersson, 1992; Falkengren-Grerup and Tyler, 1993b; Runge and Rode, 1991).

Divalent base cations, in particular Ca, are able to modify AI toxicity in nutrient solution (Alva et al.,

* Fax no: +46462224423

1986; Brunet, 1994; Keltjens and Tan, 1993; Kinraide and Parker, 1987; Rengel, 1992). This antagonistic interaction has motivated the development of models on base cation amelioration of A1 toxicity (Blarney et al., 1992; Grauer and Horst, 1992; Noble et al., 1988; Sverdrup et al., 1992).

A few studies have investigated the relations between plant growth and the Ca:A1 ratio of soil solu- tions (Neitzke and Runge, 1985; Wright and Wright, 1987; Wright et al., 1987). However, comprehensive evidence from the field on the predictive value of the Ca:A1 ratio to the distribution of wild plants as com- pared to other soil parameters is still lacking and the importance of the exchangeable Ca:A1 ratio has not been considered in spite of the fact that exchangeable (neutral salt extractable) concentrations are commonly used in characterising soil chemical conditions.

The objectives of this study are to elucidate to what extent the following soil chemical characteristics are able to account for the distributions of vascular plants on acid (pH 0.2 M KCI<4) deciduous forest soils in south Sweden: (a) the exchangeable or the soil solu- tion Ca:AI ratio, (b) the exchangeable or soil solution

22

Table 1. Concentrations of soil exchange- able (exc.) and soil solution (sol.) vari- ables. Exchangeable concentrations expressed as t~mol g - l dry weight (dw) and/zmol g - t organic matter content (OM). Al t= total A1, Air = quickly reacting A1 of the soil solution

Mean Range

Oak/Hornbeam exc.

Ca ~mol g - l dw 7.4 2-19

Ca/zmol g - l OM 49 13-170

AI ~tmol g- I dw 3.7 0.4-14

A1/zmol g - 1 0 M 24 2-88

Ca:Al 3.7 0.2-29

OM % 17 7--49

Beech exc.

Ca ~mol g - t dw 14.2 5--40 Ca/zmol g - t OM 110 19-300

AI/zmol g - l dw 6.7 <0,1-18

AI/zmol g - 1 0 M 49 1-130

Ca:AI 3.8 0.3-29

OM % 14 6--27

Oak~Hornbeam sol.

pH 4.1 3.7-5.1

Ca/zM 310 50-1600

Air ~tM 62 22-130 Air/~M 12 4-26

Ca:Alt 5.4 1.0-29 Ca:Air 27 5-115

concentrations of Ca or AI, or (c) the soil or soil solu- tion H + concentration/pH. Particular attention is given to the Ca:A1 ratios.

Materials and methods

Study area and ecosystems

The deciduous forest of the south Swedish province of Scania, an area situated just south of the border of naturally occurring coniferous forest in Scandinavia, was the object of the study. Sites (area 500 m 2, usual- ly 20 x 25 m) were selected in pure stands of beech (Fagus sylvatica) and in pure or mixed stands of horn- beam ( Carpinus betulus) and oak (Quercus robur, in a few cases Q. petraea), and then sometimes with a

shrub-layer of hazel (Corylus ave liana). The sites were distributed over most of the province (an area of ca. 10,000 km 2) so as to represent the edaphical variability of these ecosystems. Descriptive surveys and analyses of the vascular plant composition of the sites have been published (Tyler, 1985, 1989; Rtihling and Tyler, 1986). Treated in this study are only sites with pH 0.2 M KC1 3.2-3.9 in the topsoil (corresponding to 3.7- 4.4 in the soil solution according to Falkengren-Grerup and Tyler (1993b), but single higher values were found in this study (Table 1)), lacking a developed mor (raw humus) horizon. The pH limits were chosen to assess the effect of Ca:A1 at a pH where the solubility of A1 is high enough to constitute an ecologically important factor and Ca may ameliorate its effect. The number of sites was 108 for oak/hornbeam and 64 for beech for- est. Only oak/hornbeam forest was considered for the analysis of soil solution relationships to species distri- butions, using two sets of samples, located in different parts of the site.

Being the condition normal to forest in southern Sweden, 79% of the sites had light-textured, mainly sandy-loamy soils with a low clay content (<5% of the top 0-5 cm layer); only 8% of the plots had > 10% clay. The soils varied from acid dystric Cambisols on tills derived from archaean siliceous rocks to less acid cambisols often of a sedimentary rock origin.

The sites are situated between 30 and 150 m (usual- ly 50-100 m) above sea level, on horizontal ground or gentle slopes of various directions of exposure. Sites with an apparent bad drainage (high water table) were not included. Young stands (< ca. 50 yr) were also avoided. All sites were unlimed, unfertilized and with few exceptions ungrazed.

Soil sampling and chemical analysis

Soil sampling was performed during the summer months. Ten (five for soil solutions) samples of 192.5 cm 3 (depth 0--5 cm below the litter layer) regularly distributed points within the 500 m 2 area were pooled. Material at field moisture passing a 6 mm screen was used for electromeric determination of pH and analy- sis of exchangeable A1 (Alexc); Alexc and pHKcl, were analyzed in 0.2 M KC1, soil:solution weight ratio 1:5 and exchangeable Ca (Caexc, 1 M NH4OAc, pH 7.00, soil:solution weight ratio 1:10). Organic matter con- tent was determined as loss on ignition (ca. 600 °C), % dry weight of the samples after drying to constant weight (105 °C).

Soil solutions for electrometric pH determination and analysis of Ca and A1 were obtained by centrifug- ing freshly sampled soil (100 g) at field moisture at 12,000 rpm for 60 min, chiefly according to Adams et al. (1980) and Giesler and Lundstr6m (1990). Usually, 8-15 mL solution was obtained, enough for the anal- yses performed. The beech forests were not included in this study. Other chemical analyses were performed after syringe-ultra filtration (Acrodisc PF, 0.8 #m Pre- filter, 0.2 #m Supor). Total Ca (Ca) and total A1 (Alt) were analyzed by atomic absorption spectrophotome- try, Ca in 1% LaC13 using an air-acetylene flame, AI using a N20:acetylene flame.

Quickly reacting A1 (Air) was determined in cen- trifuged and syringe filtered soil solutions using a flow injection analysis method developed by Clarke et al. (1992). The method involves the reaction of Al with 8-hydroxoquinoline to A1 trioxinate, which is extract- ed in chloroform and analyzed spectrophotometrical- ly at 390 nm. Converted to A1 trioxinate are A13+, A1OH 2+, and probably AI(OH) +. Possibly occurring AlSO 2 may also be included. The less plant-available A1 complexes of the soil solution are not determined in the analysis, which makes this speciation method suit- able for estimating the A1 fraction of primary interest in plant toxicity assays. Aluminium concentrations as low as 0.2-0.4 pmol L -1 may be detected (Clarke et al., 1992).

Description of the vegetation

The floristic composition and stand structural charac- teristics were described once (in July-August). The % cover (vertical projection of the area covered by the biomass) was estimated for each species of the field, shrub and tree layers using 10 regularly distributed plots (5 m 2) within each 500 m e site, also used for the soil sampling. The percentage cover of each vascular plant species in a plot was estimated according to the following scale: 0.5, l, 2, 3, 5, 8, 10, 15, 20, 25, 30, 40 ... 100%. Additional species found within the 500 m 2 site but not encountered in the plots were added and estimated to cover 0.5%. The mean % cover of each species in the 10 (5) subplots is used, after appropriate transformation (see statistical treatment). Species are named according to Oberdorfer (1990).

Statistical treatment

Exchangeable concentrations of elements in the soil were calculated in four different ways: (a)/~mol g- l

23

dry weight, (b) log/~mol g-I dry weight, (c) ~tmol g-I organic matter and (d) log ~tmol g-I organic mat- ter, Soil solution concentrations were calculated as (a) pM and (b) log/zM. The calculation mode of soil vari- ables giving the linear correlation coefficient of high- est significance with the species cover, transformed to x/cover% + 0.5, were tabulated. The square root transformation was chosen as most data were sampled from a distribution closer to a Poisson than to a normal distribution (Zar, 1984, p 241). Only species occurring in > 10 sites were considered.

Data from oak/hornbeam forest and beech forest were treated separately, as were extractable and soil solution data.

Results

The concentrations of soil exchangeable and soil solu- tion variables are tabulated in Table 1.

Relations between exchangeable concentrations and species distributions

Out of the soil variables considered in this study pHKo or Cacxc (expressed as/_tmol g-~ organic mat- ter) proved to give the best account for the distribution of species (Table 2, Fig. 1). Species with distribu- tions positively related to pHKcl (negatively to H +) were usually positively related to Caexc but less con- sistently related to Alexc. The low number of cases with (Ca:A1)exc, especially in oak/hornbeam forests, as the best of the soil variables tested is striking. Totally 69% of the species in oak/hornbeam forest and 77% in beech forest were significantly (p<0.05) correlated to any soil variable.

Species tolerant to moderately acid soils (e.g. Aegopodium podagraria, Geum urbanum, Mercuri- alis perennis) correlated positively and best with either pHKcl, or Caexc, whereas species known to occur in very acid sites (e.g. Carex pilulifera, Deschampsia flexuosa, Maianthemum bifolium, Trientalis europaea) were negatively related to these variables. In the list of species with close correlations to Alexc there is a partly unexpected pattern as to the sign of the corre- lation coefficient (Table 2). Species characteristic of highly acidic soils in oak/hornbeam forests were often negatively related to Alex,, e.g. Deschampsiaflexuosa, Frangula alnus, Trientalis europaea and Vaccinium myrtillus. Some species of moderately acid soils were positively related to Alexc, e.g. Deschampsia cespitosa,

2 4

Table 2. Linear correlation for pHKcl (expressed as H + concentration), Caexc, Alexc and (Ca:Al)exc of the topsoil (0--5cm) on vtcover% + 0.5 of species occurring in > 10 sites are considered in oak/hornbeam and beech forest, respectively. No. = number of sites with presence of respective species. The significance levels for oak-hornbeam sites are (n=108):r=0.192 (p<0.5), r=0.250 (p<0.01), r=0.316 (p<0.001), for beech sites (n=64): r=0,246 (p<0.05), r=0.320 (p<0.01), 1"=0.402 (/9<0.001). Whenever log-transformation of the soil variables improved the correlation this value is presented (indicated by L). The correlation coefficient was non-significant (p>0.05) if no coefficient is given

Species No. H Ca dw Ca OM AI dw A 1 0 M Ca:A1

Oak-hornbeam

Viola riviniana/reichenb. 62

Geum urbanum 24

Aegopodium podagraria 20

Stellaria holostea 30

Poa nemoralis 50

Lamium galeobdolon 40

Mercurialis perennis 21

Hepatica nobilis 13 Acer platanoides 30

Campanula persicifolia 12

Fragaria vesca 17

Moehringia trinervia 37

Veronica chamaedrys 26

Crataegus sp. 19

Fraxinus excelsior 17

Dactylis glomerata 36

Hieracium sp. 10

Rubtus saxatilis 24

Anthriscus sylvestris 14 Campanula rotundifolia 14

Melica uniflora 20

Urtica dioica 20

Polygonatum multiflorum 14

Stellaria nemorum 10

Luzula pilosa 48

Convallaria majalis 64

Holcus mollis 29

Malus sylvestris 23

Rumex acetosa 16

Prunus avium 40

Viburnum opulus 24

Oxalis acetosella 79

Dryopteris dilatata 14

Deschampsia cespitosa 39

Athyrium filix-femina 24

Epilobium angusnfolium 24

Melampyrum pratense 44 Potentilla erecta 21

Agrostis capillaris 54

--0.45

-0 .41

-0 .32

--0.30

-0 .30

-0 .29

-0 .28

-0 .27

-0 .26

-0 .26

-0 .24

-0 .24

-0 .24

-0 .21 -0,21L

-0 .21

-0.21L

-0 .21

-0 ,20

-0 .20

0.29

0.24

-0 .20 L

-0 .20 L

-0 .21

0.23

0.30

0.46

0.37 0.20 L

0.45

0.38

0.23

0.23 L

0.22

--0.19 L

0.27

0.26

0.21

0.20 L

--0.28 L 0.41

-0 .26 -0 .21 0.21L

0.22

0.20 L - 0 . 2 5 L 0.21L

-0 .20

-0 .22

-0.31L

0.23

0.25

0.30

0.40

0.41

0.29

0.33

0.23 L

0.23

0.20

-0 .20

-0 .20

-0 .20

-0 .21

--0.23

-0 .26

0.35

0.25

0.41

0.38

-0 .33 -0 .25

0.20 L

- 0 . 2 0 L

-0 .22 L

--0.28 L

--0.27 L

0.22

25

Table 2. Continued

Dryopteris carthusiana 42

Dryopteris filix-mas 31 Rubus idaeus 59

Carex pilulifera 12 O. 19 Maianthemum bifolium 62 0.23

Rubus fructicosus coll. 23 0.23

Sorbus aucuparia 85 0.23

Picea abies 15 0.24

Betula pendula 19 0.24

Galium harcynicum 34 0.34

Vaccinium vitis-idaea 12 0.35

Frangula alnus 38 0.38

Trientalis europaea 39 0.39

Vaccinium myrtillus 30 0.45

Deschampsiaflexuosa 74 0.50

-0 .19

-0 .27

-0 .29

-0 .23

-0 .22

-0 .19 L

-0.33

-0 .19

-0 .19

-0 .19 L

-0.21

-0.31

-0 .25

-0 .25

-0.31

-0 .24

-0 .38

0.29 L

0.25 L

Beech Viola riviniana/reichenb. 45 -0 .64 0.37 0.62 L -0 .27

Lamium galeobdolon 41 -0 .56 0.39 0.45

Fraxinus excelsior 25 -0 .52 0.44 0.52 L -0 .32

Carex sylvatica 10 -0 .49 L 0.60 0.55

Carpinus betulus 10 -0 .47 L 0.62 0.58

Galium odoratum 22 -0 .44 0.35 0.49

Mercurialisperennis 17 -0 .44 0.27 0.34 -0 .29 L

Urtica dioica 10 -0 .39 0.28 0.35

Stellaria nemorum 35 -0 .38 0.37 L

Dactylis glomerata 16 -0 .37 0.27 0.32

Poa nemoralis 40 -0 .32 0.26 0.30

Deschampsia cespitosa 31 -0 .32 0.25 L

Acerplatanoides 13 -0 .32

Oxalis acetosella 57 -0.30 0.29 L

Mycelis muralis 10 -0 .25 0.25 L

Stellaria holostea 25 -0 .25

Polygonatum multiflorum 14 -0 .23 -0 .46 L

Meliea uniflora 26

Milium effusum 31 Dryopteris carthusiana 18 0.31

Luzulapilosa 22 0.34 -0 .27 -0 .29 0.37

Maianthemum bifolium 33 0.35 -0 .28

Desehampsiaflexuosa 27 0.48 -0 .42 L -0 .49 L

Carexpilulifera 33 0.57 -0 .44 -0 .59 L

0.32

0.26

-0.21

-0 .22

-0 .40 L

-0 .30

-0 ,37 e

0.28

0.31

0.31

-0 .20

-0 .37 L

-0 .34 L

0.20

0.38 L

0.26 L

0.44

0.34 L

0.34 L

0.34 L

0.26

0.25

0.52

-0 .36 L

-0 .34 L

-0 .37 L

Species without significant correlations: Oak/hornbeam: Anthoxanthum odoratum, Corylus avellana, Epilobium montanum, Fagus sylvatica, Galeopsis bifida/tetrahit, Galium aparine, Hypericum sp., Juniperus communis, Mycelis mural& Lath- yrus linifolius, Lonicera periclymenum, Melica nutans, Milium effusum, Poa pratensis, Polygonatum verticillatum, Polypodium vulgate, Populus tremula, Prunus padus, Pteridium aquilinum, Sambucus racemosa, Scropbularia nodosa, Solidago virgaurea, Tilia cordata, Ulmus glabra, Beech: Athyrium filix-femina, Convallaria majalis, Epilobium angustifolium, Galeopsis bifuta/tetrahit, Prunus avium, Rubus idaeus, Sorbus aucuparia.

26

100

Beech exch, 90

eo

70

~ 6o

't 50 4O

3O I

' °

0 , i

H CaCkvCa(~AIo 'wNOM CON H C a d w ~ O M N d w NOM CaN

OaWhombeam e,,OI.

i H ~ A l l AI r C~AI! Ca:AI r

Fig. 1. Species with significant (p<0.05) linear correlation between their distributions and exchangeable and soil solution concentrations expressed as % of total number of species studied (n-54 for oak/hornbeam forest, exchangeable concentrations; n-24 for beech forests, exchangeable concentrations; n-55 for oak/hornbeam forests, soil solution concentrations). The hatched part of the bar represents cases that attained the highest correlation coeflieient of all soil variables considered. Further explanations in Tables 2 and 3.

400

3 0 0 '

2 0 0 "

100"

CATION EXCHANGE ~ o u ,n CAPACITY (C.E.C.) []

% o

o ¢~b a

° ° ~ - 39,961 + 10,542x R^2.0 ,966

ORGANIC H A T T E R CONTPNI" (96)

° 0 lo 2b 3o 40

Fig. 2. Relationship between C.E.C. (/ae, q g - l dry weight), measured in 1983 by the neutral 1 M NH4OAc method as Y,,(H+Na+K+Mg+Ca) and the organic matter content (loss on igni- tion; % dry weight) of topsoil samples (day content < 10%) from beech and oak/hornbeam forest sites in south Sweden. (Unpublished data by 13 Tyler).

Dryopteris dilatata and Oxalis acetosella in oak/horn beam forests and Melica uniflora and Milium effusum in beech forests.

The Caexc concentrations calculated on organic matter content were superior to calculations on dry weight in accounting for the species distributions (Fig. 1). Organic matter content of the humus horizon is a close approximation of the cation exchange capacity, especially in soil poor in clay, according to previous studies on deciduous forest topsoils (Fig. 2). Therefore,

this mode of calculation essentially expresses the Ca saturation of the negatively charged exchange com- plex of the soils and has been demonstrated to give a better prediction of plant uptake of cations (Tyler, 1976) as well as of local distribution patterns of plants (Falkengren-Grerup et al., 1995) than soil concentra- tions calculated on a dry weight basis.

Relations between soil solution concentrations and species distributions

The number of species with a significant correlation to any soil solution variable is as high as 85%. The Ca concentration of the soil solution accounted for the highest share of variability in the species distri- butions and pH (H + concentration) was the second best predictor, when the highest correlation coeffi- cients are considered (Fig. 1). The A1 concentrations were inferior in accounting for the distributions and, though being significantly correlated to the distribution of many species, the Ca:AI ratios gave the best corre- lation in much fewer cases than Ca and pH. As was the case with Ca~xc, Ca seemed to control the importance of the soil solution Ca:A1 ratios to the distributions. The Air concentration was not a better predictor than Alt.

Strong negative correlations with Ca were found for most species typically occurring on very acid soils, e.g. Carex pilulifera, Deschampsia flexuosa, Fran- gula alnus and Trientalis europaea (Table 3); strong positive correlations were found with Ca for typi-

27

Table 3. Linear correlation for soil solution pH (expressed as H + concentration), Ca, total AI (Alt), quickly reacting A1 (Air), Ca:Alt and Ca:Air of the topsoil (0-5 cm) on x/cover% + 0.5 of species occurring in > I 0 sites are considered in oak/hornbeam forest. No. = number of sites with presence of respective species. The signifi- cance levels are (n=126): r=0.175 (p<0.05), r=0.229 (p<0.01), 1"=-0.290 (p<0.001). Whenever log-transformation of the soil variables improved the correlation this value is presented (indicated by L). The correlation coefficient was non-significant (p>0.05) if no coefficient is given

Species No. H Ca Air Air Ca:Air Ca:Air

Oak-hornbeam

Festuca ovina 15 -0.44 L

Fragaria vesca 11 - 0.42 L

Anthoxanthum odoratum 28 -0.41L

Poa pratensis 17 - 0.41L

Viola riviniana/reichenbachiana 59 -0.38 0.26 L

Lamium galeobodolon 37 -0.36 0.21 -0.32 L -0.26

Lathyrus linifolius 37 -0.34 -0.18

Veronica officinalis 15 -0.33 L -0.20 L -0.30 L -0.20

Melica uniflora 15 -0.32 L 0.20 L

Veronica chamaedrys 16 -0.31

Stellaria holostea 32 -0.29 --0.44 L -0.22

Hypericumperforatum 14 -0.28

Agrostis capillaris 73 -0.27

Potentilla erecta 17 -0.24 L -0.30 L

Oxalis acetosella 78 -0.23 -0.40 L

Deschampsia cespitosa 26 -0.22 -0.23

Festuca rubra 13 -0.21

Juncus effusus 10 -0.19 -0.36 t -0.24 L

Juniperus communis 10 -0.18 L

Ulmus glabra 10 0.52 0.33

Milium effusum 20 0.48 0.34

Prunus avium 27 0.42

Corylus aveUana 45 0.29 0.30

Stellaria nemorum 12 0.25 -0.19

Galium aparine 13 0.24

Dactylis glomerata 25 0.24 L

Stellaria media 48 0.24 L

Poa nemoralis 35 0.23 L

Rubus saxatilis 20 0.23 L

Anthriscus sylvestris 10 0.22 L

Tilia cordata 11 0.22 L 0.32

Rubus idaeus 70 0.21 0.18 L

Dryopterisfilix-mas 12 0.20 0.33

Polygonatum multiflorum 11 0.19 L

Malus sylvestris 21 - O. 19 L

Dryopteris carthusiana 33 -0 . ! 8

Convallaria majalis 63 0.20 L

Athyrium filix-femina 15 0.21

Fagus sylvatica 51 -0.19 -0.18 L -0.26

0.27 L 0.27 L

0.38 0.42

0.22

0.28 0.20

0.18 L

-0.21 -0.23

0.20

0.22

0.49 0.32

0.48 0.29

0.29 0.36

0.18 0.18

0.30

0.24 0.21

0.21 0.21L

0.21L 0.22 L

0.20 L 0.23 L

0.20 L

0.18 0.26

0.19

0,18

0.20 L 0.20

28

Table 3. Continued

Betula pubescens 13 -0.19 L Molinia caerulea 11 -0 .20 L -0 .18 -0 .18 L

Rubusfructicosus 29 -0.21L - 0 . 1 9 L -0 .26 L

Maianthemum bifolium 66 -0.21 -0 .23

Vaccinium vitis-idaea 15 -0 .22 L -0 .19 L - 0 . 2 0 L

Picea abies 14 -0 .22 L -0 .28 L -0 .27 L

Betula pendula 16 -0 .26 L -0 .18 - 0 . 2 4 L

Populus tremula 17 -0 .27 L -0 .22 -0 .19

Luzulapilosa 55 -0 .27 -0 .30 -0 .24 L

Galium harcynicum 35 -0 .30 L -0 .22 -0 .25

Frangula alnus 39 -0 .32 L -0 .23 -0 .25

Carexpilulifera 60 - 0 4 4 L -0 .28 - 0 . 3 4 L -0 .28

Melampyrum pratense 55 -0 .46 L -0 .47 L - 0 . 4 4 L

Trientalis europaea 54 0.22 -0 .42 L -0 .45 L -0 .36

Vaccinium myrtiUus 35 0.26 -0 .38 L 0.19 -0 .43 L -0 .38 L

Deschampsiaflexuosa 99 0.31 -0 .62 L -0 .18 -0 .58 L -0 .55 L

Species without significant correlations: Acer platanoides, Dryopteris dilatata, Epilobium angus- tifolium, Galeopsis bifula/tetrahit, Holcus mollis, Moehringia trinervia, Pteridium aquilinum, Rumex acetosella, Sorbus aucuparia, Viburnum opulus.

cal 'mull' plants, e.g. Milium effusum, Viola rivini- ana/reichenbachiana and Poa nemoralis.

Aluminium (Alt and Air) of the soil solution related mainly negatively to species abundance, also including several species usually occurring on very acid soils (e.g. Deschampsia flexuosa, Carex pilulifera, Luzula pilosa). A few other species of less acid soils were positively correlated to A1 (Corylus avellana, Milium effusum, Dryopteris filix-mas).

There was the same tendency for relationships to Alexc but with several cases when soil solution and exchangeable concentrations of an element gave oppo- site relationships. A negative or positive correlation with pH and Ca is almost consistent for both soil solu- tion and exchangeable concentrations (Tables 2 and 3). The number of species with significant correlation to A1 or Ca:A1 in both soil fractions is lower than for Ca.

Discussion

Experiments in solution culture often show an amelio- rating effect of Ca on A1 toxicity. Competition of Ca and A1 for uptake sites and other common binding sites on the root plasmalemma and in the root apoplast is suggested as the mechanism behind this amelioration. It was, therefore, hypothesized that the Ca:A1 ratio rather than the single element concentration governs

Al-toxicity in plants (Rengel, 1992). However, some studies have not found significant ameliorating effects of Ca (Alva et al., 1986; Clarkson, 1966; Henriksen et al., 1992; Tyler, 1993).

The evidence from work with soils is scarce and often contradictory. The Ca:A1 ratio, calculated from the water-soluble or soil solution concentrations, was well related to growth of Fagus sylvatica seedlings (Neitzke and Runge, 1985), Trifolium subterraneum (Wright and Wright, 1987) and Phaseolus vulgaris (Wright et al., 1987). Relationships to growth were usually better than for concentrations of the single ele- ments. The A1 concentration is often in itself a good predictor of growth, e.g. in the above study of beech seedlings, for Quercus rubra seedlings measured in saturated paste solution (De Wald et al., 1990) or extracted in SrC12 (Joslin and Wolfe, 1989). In the latter study Ca had no ameliorating effect. The A1 con- centration for toxicity was higher when measured in the soil solution as compared to other hydroponic exper- iments (De Wald et al., 1990), which demonstrates the difficulty in translating information from solution experiments to field conditions.

Other studies emphasize the importance of soil pH rather than A1, Ca or Ca:A1 to plant performance under acid conditions. According to Meyercordt et al. (1989) root development of Milium effusum growing in acidic beech woods was best correlated with soil pH, fol- lowed by the water soluble Ca:H ratio, and exchange-

able Ca. Exchangeable Al was less well correlated and water soluble A1 showed no correlation. The oppo- site results for soil solution and exchangeable A1 were found for root growth of soybeans (Glycine max) but Ca-saturation and pH were again well related (Bruce et al., 1988). Splett et al. (1992) reported a close relation- ship between shoot growth in wheat and soil pH as well as AI saturation. Yield of barley and wheat, however, was more accurately predicted by CaClz-extractable AI than by soil pH (Conyers et al., 1991; Wright et al., 1989). No consistent picture of the role of the Ca:A1 ratio as compared to the single elements or pH in soils is thus obtained by the available experimental results reflecting field conditions. Nor are different soil frac- tions of elements, e.g. soil solution, saturated paste solution or salt extracted concentrations, consistently superior in accounting for growth or other variables related to species performance.

Our study differs from those cited above in several respects. We use field data from natural plant commu- nities covering a large region and considering many species. This is required when the general ecologi- cal significance of soil chemical variables is tested. There are, however, two main limitations in this type of work: (a) biomass assessment (cover percentage) is influenced by species interactions and (b) absence of species from a site may be due to other factors than soil chemistry, e.g. other environmental or historical conditions.

Despite these methodological limitations in using field data, the high number of significant correlations between plant abundance and soil pH or soil Ca sup- ports existing knowledge of the studied species and emphasizes the ecological importance of these factors for plant distributions. The lower significance for A1 and sometimes unexpected positive or negative rela- tionships, as compared to the few results on wild field layer species obtained from solution experiments, seems ecologically less consistent. Within a narrow pH range positive correlations between A1 and Ca in soil solution may occur, at least temporarily, e.g. due to mobilization of both ions by similar mechanisms from the exchangeable pools. However, the soil solu- tion concentrations of AI measured in our study were rather low, partly due to the high soil moisture con- tent, but within levels found to be toxic to beech forest plants (e.g. Andersson, 1993; Andersson and Brunet, 1993). There are certainly difficulties in using soil solu- tion concentrations, as these vary during the season and depend on, e.g. precipitation (Falkengren-Grerup, 1994; Falkengren-Grerup and Bergkvist, 1995). The

29

more stable exchangeable fraction, being in some long- term equilibrium with the soil solution, may therefore be superior to single measurements of soil solution concentrations.

The lower predictive value of soil AI and in par- ticular the soil Ca:A1 ratio may also have other expla- nations. Increasing Ca:A1 ratios in soils with a high organic matter content and a very low pH may have no positive effect on plant growth provided it does not imply a pH increase (Falkengren-Grerup, 1995). In these soils, H-toxicity may be the most important limiting factor (Falkengren-Grerup and Tyler, 1993a). On less acid soils the significance of Ca:A1 ratios for plant growth may be lower than, e.g. the Ca concen- tration due to a generally low A1 solubility. Moreover, many species may be able to detoxify A1 in the rhizo- sphere by exudation of AI complexing organic acids (Horst et al., 1990; Miyasaka et al., 1991; Suhayda and Haug, 1986) or other organic compounds with protect- ing properties. Such mechanisms have no or only little possibility to act in solution culture experiments. There are few species tested so far, but such mechanisms may be of great importance for root growth in acid soils. A pH gradient in the rhizosphere caused by, e.g. uptake of nitrate may also be of importance since it changes the AI speciation (Nye, 1981).

We conclude that use of the Ca:A1 ratio as a gen- eral measure of Al-toxicity under field conditions is rather problematic. Soil pH and Ca seem to the supe- rior in assessing the distribution of woodland plants in our material, within a pH range, where AI solubility is high. The Ca:AI ratios are actually less closely related to the abundance and distribution of species than are pHKcl, Ca~xc, Alexc as well as pH and Ca in soil solu- tion. It seems difficult to apply the evidence for Ca-A1 interactions from solution culture experiments to field conditions. The study of rhizosphere processes may be the key to a better understanding of this problem.

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Section editor: J H Graham