Vegetatio 68: 145-156, 1987 145 © Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands

Ecological gradients of boreal forests in South Finland: an ordination test of Cajander's forest site type theory*

Tapani Lahti 1,** & Risto A. V~iis~inen 2,** 1Botanical Museum, University of Helsinki, Unioninkatu 44, SF-00170 Helsinki, Finland 2Zoological Museum, University of HelsinkL P.. Rautatiekatu 13, SF-O0100 HelsinkL Finland

Keywords: Boreal forest region, Canopy-understory interaction, Climax, Correspondence analysis, Forest site type, Grazing, Nutrient, Ordination

Abstract

Ecological gradients in the field layer of southern boreal forests in South Finland were studied in relation to the dominant tree species and the age of forest stands. The data are from a systematic sample of 529 plots from an area of 150 x 200 km, collected in the Third National Forest Inventory in 1951-53. Detrended cor- respondence analysis (DCA) was applied to log-transformed species cover values. It revealed three main gra- dients: fertility, moisture, and the effect of cattle grazing in forests (still extensive in the early 1950's). The fertility gradient dominated the first axis and the two latter sources of variation confounded with it in a com- plex manner in the first two axes of DCA. The second DCA axis was associated with canopy effects on un- derstory pattern, with Pinus and Picea having opposite and Betula intermediate effects.

These results were compared with an ordination model of Cajander's forest site types, based on DCA of independent, ideal data of 107 indicator species. The fertility gradient recovered by the model was almost identical to that obtained from the field data. The gradient was also stable from intermediate-age (40- 69 yr- old) to older forests. The forest site types showed rather large overlaps with main neighbouring types in com- position of ground vegetation or nutrient status of the humus. Competitively efficient feather-mosses, which are dependent on nutrients released from the tree crowns, are considered important regulators of the under- story vegetation. Accordingly, alternative approaches to the forest site type classification to be used in boreal forests treated by modern intensive forestry should give more weight to the effect of the canopy trees.

Introduction

The Finnish School of Cajander was influential in the early co-ordination of classification and gra- dient analysis (Whittaker, 1962, 1967), by arranging forest vegetation into ecological series of forest site types, and relating these to environmental gra- dients, particularly soil fertility. Lucid reviews of Cajander's approach (e.g. Cajander, 1909, 1926) are

given by Frey (1973) and Mikola (1982). A forest site type was defined by Cajander (1926)

for stands which are exploitable or nearly so, and of normal density, with mainly identical floristic composition and a similar biological nature, in- cluding all those stands the vegetation of which differs from that defined above only in those respects which - resulting from the difference in age of the stand, fellings, change in species of

* Nomenclature follows H/imet-Ahti et al., 1984, Retkeilykasvio, Helsinki, for vascular plants, Koponen et aL, 1977, Flora Fennica 6, for bryophytes and Ahti, 1981, Oulanka Rep. 2 : 4 8 - 6 3 for lichens. ** We thank the Finnish Forest Research Institute, Dept. of Silviculture, for giving us access to the data of the Third National Forest Inventory, Lalli Laine for invaluable advice concerning the data, and Pekka Tamminen for unpublished soil data, Seppo Kuusela for assisting in the coding of the data, and Teuvo Ahti, Leena H~imet-Ahti, Matti Leikola, Peitsa Mikola, Tom Philippi, Arne Rousi and four reviewers for comments on the manuscript. The study was supported by a grant from the Academy of Finland (to R.A.V.).

146

stand, etc. - have to be considered as merely ac- cidental or ephemeral or at any rate not permanent .

A clear distinction was made between temporary ( 'successional ') and permanent ( 'climax') c o m m u - nities (cf. Kalela, 1960). According to Frey (1973), Cajander was the first to take the stable communi - ties as the basis for the classification o f vegetation. Ano the r characteristic idea o f the Finnish School is the identif ication o f site types th rough ground vegetation. This reflects the scarcity o f tree species in boreal forests. The term 'forest site type' is used to distinguish the concept f rom 'forest type ' , which is usually characterized by different tree species, i.e. P i n u s and Picea forests.

Cajander ' s forest site type classification was mainly designed for and is still used in practical forestry. It is applied for estimating the potential product ivi ty o f different sites, as well as for giving general frames for yield tables, nat ional forest in- ventories, forest management , forest taxation, etc. (Hustich, 1960; Mikola, 1982). To some extent, the Cajander approach has also been used in Scandina- vian and Baltic countries and in Canada , but out- side the boreal zone it has been o f little impor tance (see Frey, 1973; Grandtner & Vaucamps, 1982; Kielland-Lund, 1982).

In view o f its theoretical and practical impor- tance it is astonishing that the Cajander approach has not yet been tested by mode rn ordinat ion methods. Our study aims at such a test with quan- titative field data on boreal upland forests f rom southern Finland where the Cajander approach found its first applications.

We treat forest site types as parts o f a single con- t inuum f rom heath-like P i n u s forests to herb-rich forests. We shall pay special a t tent ion to the varia- bility o f g round vegetation within a forest site type in relation to the role o f subjective judgement in site type classification in the field. Finally we consider whether the forest site types are really independent o f the tree species on a site. M a n y authors have sug- gested that the dominan t tree species should be giv- en larger weight in the forest site type classification than in the practice with the Cajander approach (e.g., Sir6n, 1955; Vuokila, 1956; Sepponen, 1985).

Formulation of a predictive model

Relative frequencies o f a number o f key species

in the ground vegetation will be used to make a dis- t inction between different site types, especially in mature, non-dis turbed stands.

First, we construct an a pr ior i model o f the vege- ta t ion o f upland forests in southern Finland, based on Kujala's (1979) tables (following the Finnish tra- dition, the peat land forests - P i n u s bogs and Picea - hardwood swamps - are excluded). Major forest site types are (in order o f increasing pr imary productivity; soil definitions mainly f rom Mikola, 1982):

Cladina site type (abbreviated C1T). On very dry and barren sandy soils (similar vegetation on rock outcrops excluded here). The only tree species is very poorly growing Pinus sylvestris. Undergrowth is dominated by lichen species, especially Clado- nia sectio Cladina. Mosses are scarce. Dwarf shrubs are present, but their cover is low. Herbs occur only occasionally with very low cover values. (In our field data, an intermediate site type (CCIT) between C1T and CT (the following type) was used, be- cause pure C1T forests are rare in South Finland. However, here it will be referred to as C1T.)

Calluna site type (CT). Typical of coarse, dry sandy soils. Pinus-dominated stands with Calluna vulgaris in undergrowth. Ground layer is dominated by lichens, but mosses (mainly Pleurozium schreberi and Dicranum polysetum) are more fre- quent than in C1T.

Vaccinium site type (VT). Usually on subdry sandy tills and alluvial sands. Tree layer consists of Pinus, occurring in pure stands or mixed stands with Betula pendula and/~ pubescens. Poorly growing Picea abies is common as an undergrowth spe- cies and may be sometimes the dominant tree. The field and bot- tom layers are dominated by Vaccinium vitis-idaea and Pleuro- zium schreberi, respectively. Lichens are scarce.

Myrtillus site type (MT). On mesic till soils. Tree species com- position (Picea, Betula, Pinus) is rather variable, though mature forests are typically dominated by Picea. Also shrubs, especially Juniperus communis and Sorbus aucuparia, are rather com- mon. Flowering Vaccinium myrtillus with vegetatively growing V. vitis-idaea cover the major part of the herb layer. Typical grass and herb species include Deschampsia flexuosa, Calamagrostis arundinacea, Maianthemum bifolium, Melam- pyrum pratense, M. sylvaticum, Orthilia secunda, and Solidago virgaurea. The bottom layer is dominated by Hylocomium splendens, Pleurozium schreberi and Dicranum scoparium.

Oxalis-Myrtillus site type (OMT). On mesic to moist nutrient rich tills. Productive Picea stands, with Pinus and Betula occur- ring mainly on burned or managed sites. The shrub layer in- cludes Juniperus communis, Sorbus aucuparia, Salix caprea and Rubus idaeus. The species-rich field layer is characterized by Vaccinium myrtillus and V. vitis-idaea, and typically includes Calamagrostis arundinacea, Deschampsia flexuosa, Maianthe- mum bifolium, Melampyrum sylvaticum, Orthilia secunda, Solidago virgaurea, Melica nutans, Carex digitata, Luzula pilo- sa, Gymnocarpium dryopteris, Oxalis acetosella, and Rubus saxatilis. The bottom layer is dominated by Hylocomium splen- dens and Pleurozium schreberi.

The following rich nemoral forest site types cover only about 1°70 of the total forest area of southern Finland. They have a fer-

tile mull soil, resembling the brumisols of the temperate zone, whereas the previous dwarf-shrub-dominated site types always have a typical podzol soil profile. The traditionally recognized (but poorly defined) site types of the rich forests are:

Oxalis-Maianthemum site type (OMaT). Mesic to moist forests with a Picea canopy, usually mixed with various hard- wood species. The scarcity of Vaccinium species and the occur- rence of eutrophic herb and moss species are the main diffc;- ences in comparison to OMT.

Filices site type (FT). Moist forests characterized by a Picea canopy with an undergrowth of tall ferns, such as Athyrium filix-femina, Dryopteris carthusiana, D. expansa, and Matteuc- cia struthiopteris.

Vaccinium-Rubus site type (VRT). Subdry rich woodlands on eskers, with Pinus in the canopy. Characteristic of the field layer are species favouring dry eutrophic sites, such as Melica nutans, Carex digitata, Rubus saxatilis, Pteridium aquilinum, Conval- laria majalis, Hepatica nob#is, Actaea spicata, Fragaria vesca, Lathyrus vernus and THfolium medium.

Besides these eight traditional site types, some additional ones occasionally have been recognized to indicate special conditions of the sits One of these is the Pyrola site type (PyT) that occurs on clay soils as a variant of the Myrtillus site type. Due to poor permeability, the soil is often somewhat water-logged. It is characterized by the abundance of various Pyrola species.

We use Kujala's (1979) data on 107 characteristic plant species in nine different forest site types in southern Finland (tables for northern Finland are also included, but we do not use them here). In these tables the occurrence of the species is denoted by -, +, 1, 2 or 3 (meaning 'does not thrive at all', 'occasional', 'thrives rather well', 'thrives well', and 'thrives very well', respectively) for each forest site type. Thus, they are not based on any quantita- tive field data (Kujala, 1979: 31). Kujala's tables may be considered independent of the Third Na- tional Forest Inventory (see Kujala, 1979: 3-4).

We assume that these crude codes are roughly comparable to data obtained by extensive quantita- tive sampling over all of southern Finland, logarithmic transformation of abundances, stan- dardization within the species and transformation into a scale with five values (0-4). Thus, Kujala's symbols +, 1, 2 and 3 (above) were given the values 1 to 4, respectively. Since it has been found (e.g. Gauch, 1982a) that reasonable ordination results are often obtained with rather robust abundance data (i.e., with a large error variance), the resulting samples by species matrix was then ordinated with detrended correspondence analysis (hereafter ab- breviated DCA, see below for the method) to pro- duce a predictive apriori model of the mutual rela- tionships of different forest site types and plant species in southern Finland.

147

Figure 1 shows the first two DCA axes of forest site types. Axis 1 orders the site types into a se- quence of increasing fertility, explaining most of the variance in the data (eigenvalues for DCA1 and DCA2 are 0.643 and 0.088, respectively). The sec- ond main point is that on the DCA1 axis, the dis- tances between the most common forest site types are rather regular, from C1T to CT to VT to MT to OMT to rich forests (OMaT and FT). Thirdly, DCA1 scores of these site types are very highly cor- related with three important components indicat- ing nutrient contents of the forest soil: pH (r=0.981, P<0.01), calcium (r=0.999, P<0.001), and nitrogen (r=0.965, P<0.01; pH for CIT, CT, VT, MT and OMT from Aaltonen, 1925; CaO and total nitrogen (also for OMaT) from Valmari, 1921).

The DCA1 species scores can therefore be used as a predictive model against which our field data are further tested, and the sample scores may serve the same purpose for the Cajander forest site types.

DCA2 reflects contrast between VRT (dry nutrient-rich forests) and PyT (moist forests on clay soils). Not only the understories of VRT and PyT have different indicator species, also the dominant tree species differs: Pinus in VRT and Picea in PyT.

Study area

Our study area is a 150 x 200 km rectangle lo- cated within 61°08'N and 62°27'N and 25°14'E and 28°45'E, representing the continental section

DCA2 (0.088)

oVRT

, CIT

e C T

• VT

eMT e O M T

oPyT

I I I I

1 2 3

DCA1 (0.643)

OMar

Fig./ . DCA ordination of forest site types using 107 indicator species of Kujala (1979). C1T = Cladina, CT = Calluna, VT = Vaccinium, MT = Myrtillus, PyT = Pyrola, VRT = Vaccinium-Rubus, OMT = Oxalis-Myrtillus, OMaT = Oxalis- Maianthemum, and FT = Filices forest site type.

148

of the south boreal zone (Ahti et al., 1968) in Fin- land. The area is a typical section of the so-called Lake Finland, covered by forests with a standing stock of 50°7o Pinus, 2007o Picea, and 30°7o Betula in the 1950's. Annual mean temperature is + 4 ° C and annual rainfall varies between 550 and 700 mm. The length of the thermal growing season with daily mean temperatures over + 5 ° C is 160-170 days (Atlas of Finland, 1960).

The area was (and remains) far from homogene- ous in vegetation structure. The region was the heart of an extensive area of slash and burn cultiva- tion of forests during the 19th century and in the beginning of the 20th century. This type of cultiva- tion changed the tree species composition of forests in favour of Betula and Alnus. Also, Pinus was favoured over Picea.

Slash and burn cultivation, together with cattle grazing, also altered the species composition of field and bottom layers. In addition to changes in the relative proportions of forest species, a number of species from fields and other man-made biotopes invaded forests. At the time of the collection of our data, the forests were gradually shifting back to their natural, conifer-dominated stages, but traces of strong cultural influence are still clearly seen.

Materials and methods

DCA2 (0.098/

-1

-2

Ant dio Pyr med

Cal vul

Cla ran Cla arb

Viocan Pol jun Hie umb Hie spp

Pol vul Ang syl

Car d~y r rot Fes ovi Epiang

Pyrchl Pteaqu FraveSvio riv GerSylveroff Ver cha Dip corn Cal aru Hie tour Agr cap

Pin syl Con mal Rub sax Melnut Mel pra Pla bif Pot ere Pel aph Goo rep Ort sec

Vac vit Pie sch

Pic abi Tri eur Dic ~1 Lin bor Sol vir pv Dic sco Oes fie Oxa ace

Hyl spl Vac myr Luz pil Dry car

Mai bif Rhy tri Cal epi Lyc ann Des cae

Pti cri

Gym dry

M ~ Pol corn Equ syl Mel syl

F Dic maj

I i I I ~ I , I I I I

-1 0 1 2 3 4

DCA1 (0.257)

Fig. 2. DCA species ordination of 207 mature forest samples. For abbreviations o f species names, see Table 1. Arrows indicate three ecological gradients (F = fertility, M = moisture, G = grazing intensity).

Systematic sampling for the field data was done in connection with the third National Forest Inven- tory of Finland 1951-1953 (Ilvessalo, 1951). The transect lines along which the systematic sample sites were chosen ran parallel from SW towards NE. Data were collected from early spring to late au- tumn. Thus, phenological biases are possible in very early or late records. No early records were in- cluded in the data, but 20°70 of the records were late (inventoried between September 16 and November 10). These had been accepted by an experienced field botanist leading the biological inventory, the late Prof. Viljo Kujala.

A circular sample plot of 100 m z was chosen at each km point along the transect lines. In heter- ogeneous sample plots the midpoint of the circle was displaced in steps of one meter until satisfacto- ry homogeneity was obtained. Within the sample plot, the cover value of each plant species was esti- mated using the values of 0 (zero, or very low cover

value), + (less than 0.5°70 cover), 0.5, and integers from 1 to 100 (in our analysis, cover codes 0 and + were transformed to 0.1 and 0.2 per cent, respec- tively). Plants growing on stones and stumps were excluded. A total of 529 plots was accepted for our analysis. These included 360 taxa (22 at the genus level, o f which only Hieracium is sufficiently im- portant). In addition, forest site type, dominant tree species, and age of the dominant trees were recorded.

In his primary tabulations, Kujala divided the samples into young (15-39 yr), intermediate-age (4 0 -7 9 yr) and old (> 80 yr) tree stands. After a preliminary study of the data we decided to change the age limit of old forests from 80 to 70 yr, be- cause the threshold of 80 yr eliminated almost all of the samples with Betula as a dominant tree spe- cies (according to Kalela, 1960, in South Finland the lower layers of a forest usually reach equilibri-

149

Table 1. Species scores of the first two DCA axes from three separate ordinations: (1) predicted values calculated from Kujala's ideal data, (2) mature (>70 yr old), and (3) intermediate-aged (40-69 yr old) forest sites. Abbreviations refer to Fig. 2. Using the species scores of this table, the score values for the samples of mature and intermediate-age stands may be calculated. (For example, in a hypothetical stand we have only two species, with cover values: Hylocomium splendens 40O70 and Pleurozium schreberi 60o78. If a = ln(10 × 40 + 1) and b = ln(10 × 60 + 1) (see text), then DCAI score of mature forests is calculated as the weighted mean, DCAI = (1.08a + 0.59b)/(a + b) = 10.25/12.39 = 0.83.)

A c h n i l

A g r Cap

Xng S y l

A n t dLo

Ath f l l

Ca1 s~u

Ca1 e p l

Ca1 v U l

C e r d i g

ClS s r b

C l s ~ a n

COn mSJ

DOm COS

DOS f l e

D1¢ a a ~

DLc p01

D l c ICO

DSp CO~

O r y c a ~

Ep£ a n g

~ [ u s y l

FO8 o v l

F r 8 ve8

Got s y l

Goo t o p

Gym az~

HLe mu~

H I e S p p

HLo umb

HV1 8 p l

Hyp mac

L l n b o r

L u z p l l

Lyc a n n

L y e c l s

H~L b l f

Z n ~ e n m d £ s t o 8 ~ L o e I P ~ d l o t ~ d 1 4 ~ r o age

DCAI DCA2 DCAI DCA2 DCA1 DCA2

AcA t l l ea m l l l e ~ o l L m - 3 . 2 7 3 . 5 5

Rg~oe~s csp$ l la~£s 3 . 6 9 2 . 2 0 3 . 7 0 1 .88

• l r~elLcs sg l vee~ r l e 3 . 8 7 - 0 . 9 6 4 . 1 9 2 . 9 9 4 . 2 2 2 . 1 0

R n ~ e n n a r t s d l o t c a 0 . 8 1 1 . 1 0 1 . 1 5 4 . 2 1 1 .59 3 . 8 5

A~hvr~u~ ~12Le-£ealna 4 . 8 7 - 0 . 7 7 - 4 . 3 2 - 2 . 1 4

Ca /amagcos~ is a z ~ n d / n a c e a 2 . 3 4 1 . 5 4 2 . 2 2 2 . 2 7 2 . 3 9 1 .57

C. epLge~os 1 . 0 4 2 . 4 2 1 . 6 3 - 0 . 4 2 1 . 3 0 1 .30

Cs l luna v~Iga=Ls 0 . 5 5 1 . 5 8 - 0 . 5 8 1 . 9 5 - 0 . 1 2 2 .21

Carez d lgL ts¢s 3 . 4 7 1 . 8 2 3 . 3 4 2 . 9 1 3 . 5 9 2 .10

C l a d o n / s a r b u s c u l s - 0 . 2 2 0 . 7 3 - 1 . 2 9 1 . 4 5 - 1 . 0 8 1 .43

C. r a n g ~ ' * r ~ n a 0 . 1 ~ 0 . 7 3 - 1 . 1 4 1 . 5 4 - 1 . 0 0 1 .21

l n t o n ~ L t s t e s p e c i e s P r o 4 1 c t e d x s t u r e age

DCAI DCA2 DCA1 DCA2 DCA1 DCA2

P~I p r s ~ l a m J ~ n ~ p r s t e n J e 1 . 8 7 0 . 6 6 1 . 1 5 1 . 7 3 1 . 6 4 1 .19

Nel s~1 X . ~ l v 4 f : L c t a a 3 . 3 3 - 0 . 5 4 2 . 3 7 - 1 . 7 5 2 . 8 1 - 0 . 8 2

He1 n u t N e l i c 4 n u t a n s 3 . 5 6 2 . 5 7 3 . 2 9 1 .69 3 . 9 9 1 .58

O f t sec Ort~hllLs secunds 2 . 7 5 - 0 . 2 5 2 . 4 8 1 . 5 9 2 . 5 5 1 .06

Oze see OzslLs SCOtOSellS 3 . 7 6 - 0 . 3 5 3 . 6 3 0 . 5 0 3 . 8 6 0 .0S

Pc1 aph Pe l~ ige rs sph~hosa 0 . 1 0 1 . 7 0 0 . 2 6 - 0 . 2 0

P t c a b l P~cea 4h ies ( s a p l i n g s ) - 0 . 1 9 0 . 7 6 1 .09 - 0 . 0 8

Plm s a x p t a p £ n e l l e s a x l £ r a g a - 2 . 7 0 4 . 0 5

PLn Sy1 P l n u s sglvesCrLs (map lLngm) - 0 . 2 9 2 . 0 9 0 . 8 7 2 . 9 6

P l a b i f Plstanr .hers b l £ o l t a 3 . 5 0 0 . 2 2 2 . 4 3 1 . 7 8

P lS s c h Pl*m~oslua schreber~ 1 . 4 2 0 . 3 7 0 . 5 9 0 . 8 9 0 . 7 0 1 .08

Convair-ar ia wsJa l l s 1 . 6 1 1 . 9 6 1 . 2 5 1 .94

DeschampsLs cespit;osa 4 . 4 3 - 0 . 3 5 4 . 4 4 0 . 1 4

D. £ / e x u o s a 1 . 5 8 0 . 2 7 2 . 0 3 0 . 5 2 2 . 2 6 0 . 6 3

DLcranum aaJus 2 . 9 3 - 1 . 2 7 1 . 5 7 - 2 . 2 9 1 . 7 0 - 1 . 0 0

D. p o l y s e t m a 1 .31 0 . 2 2 0 . 2 2 0 . 5 3 0 . 3 9 0 . 8 8

D. scopar:Ltm 1 . 2 3 0 . 4 3 1 . 0 6 0 .81

DLphaslssf:rua coaplanaL~ml 0 . 5 6 0 . 7 5 0 . 0 6 2 . 2 1 -

D r y o p t e r L s c a r ~ s ~ a n s 4 . 3 3 0 . 0 8 3 . 5 1 0 . 2 1 3 . 5 5 - 0 . 0 1

~p l lobL tm angustL£ol: l tm 1 . 6 8 2 . $ 0 -

J q u / s e t m l s y l v s t L c u l 3 . 8 3 - 1 . 0 4 4 . 1 7 - 1 . 6 3 3 . 9 6 - 2 . 2 1

FeSL~CS o v l n a 0 . 5 6 - 1 . 1 4 1 . 3 0 2 . 4 6 1 . 3 0 2 . 3 0

1~4gatLS VeSCS 3 . 4 9 1 . 1 5 3 . 1 9 2 . 4 8 3 . 4 0 2 .23

O e r a n / u a sV l vs t l c tm 3 . 4 7 1 . 0 7 3 . 4 2 2 . 5 1 3 . 7 8 0 . 6 4

Goo~hJers : s p e n s 2 . 2 0 - 1 . 4 5 0 . 9 7 1 . 6 9 0 . 4 8 1 .16

G~mocarp:lua drVopf :s r ls 3 . 7 2 0 . 1 8 3 . 0 8 - 1 . 0 4 3 . 5 1 - 1 . 4 3

R l e r s c l m s l u r o r u a 2 . 8 7 2 . 3 0

Psi vu l Polypodlurn tn~lgars 2 . 2 9 3 . 0 8 2 . 6 7 1 .46

Po l c c e Polyt t~chum co~ l~ le 2 . 2 0 - 1 . 6 6 2 . 0 3 - 0 . 7 6

Po l J u n P. J u n / p e r l n m s - 0 . 2 2 0 . 6 5 0 . 1 0 3 . 2 6 0 . 4 9 2 .94

Po t s r s P o t e n t / l / s e r e c t s 3 . 3 3 1 .61 ' 3 . 0 6 1 .22

Pru vu l PJrune22s vu lga rLs 3 . 8 3 2 .43

P t e aqu P t e r / d L m s aqu~2Lmm 2 . 9 8 2 . 3 5 2 . 5 0 2 . 4 3 2 . 5 1 2 . 5 3

P t l crL PtSlSua c r l s t a - c s s f : r e n s l s 2 . 1 9 - 0 . 9 5 - 0 , 0 2 - 0 . 7 2 0 . 4 6 - 1 . 3 9

Py r c h l P v r o / a c h 2 o r a n ~ h a 1 .19 1 . 6 1 0 . 4 3 2 . 3 0

P y r reed P. a e d l a 1 .68 4 . 0 5 2 . 0 5 2 . 0 5

Py r s i n P. a l n o r 3 . 1 7 3 . 0 5

P~T r o t P. ~ o ~ u ~ t f o 1 1 4 3 . 6 2 - 1 . 6 7 3 . 7 4 2 . 5 8 3 . 9 7 1 .17

R a n s c r Ramar~cu lus a c r L s - 3 . 9 3 3 .42

Rhy t r L P J ~ t L d L a d e I p h u s t r L q u e ( : r u s 3 . 1 8 0 . 0 4 3 . 0 6 - 0 . 2 6 3 , 2 8 - 0 . 0 6

Rub Sex ~ s a x a l : L l L s 2 , 7 5 1 . 1 6 2 . 6 7 1 .89 2 . 8 8 1 .32

S o l v i r Sol idago vLrgsursa 2 . 5 4 1 .09 2 . 1 9 0 . 6 6 2 .21 - 0 . 0 2

S t e g r e S f : e l l a r l a g r a a / n e a - 3 . 8 7 2 .97

H. I p p . 2 . 0 9 3 . 3 5 2 . 4 1 2 .92

!1. v=ibellsCtm 1 . 2 5 1 . 7 8 1 . 4 5 3 . 3 0 2 . 3 1 3 , 3 0

RV loc0m/u~ s p / e n d e n s 1 . 9 4 0 . 1 9 1 . 1 8 0 . 3 8 1 . 4 6 0 . 3 2

~ j p e r L c ~ m a m c u i s t u a 3 . 0 2 3 . 3 3 3 . 9 2 2 .19

L l n n a s s b o r s a / l s 1 . 4 5 - 0 . 8 0 1 . 3 9 0 . 4 4 1 . 5 0 - 0 . 8 8

L u s u / 4 p / l o s s 2 . 7 5 - 0 . 4 5 2 . 0 5 0 . 5 3 2 . 4 0 0 .31

r -ycopod/um annoy/ r ims 2 . 4 6 0 . 0 1 2 . 7 2 - 0 . 3 8 3 . 1 6 - 0 . 9 2

L. c l s v s t l ~ 2 . 4 1 3 .32

N a l a n t h ~ a m b L f o / L m s 3 . 0 2 - 0 , 7 0 2 . 3 0 - 0 . 2 0 2 . 6 1 - 0 . 0 9

T : I e u r l "z ' lenta3.Ls muropaes 2 . 3 6 - 0 . 4 7 2 . 2 2 0 . 7 8 2 . 7 0 0 . 3 2

VaC m~r VscciJ~luz ~ r t l l l u s 1 . 9 4 0 . 4 0 1 . 2 5 0 . 3 9 1 . 8 2 0 . 6 5

Vac v l t V. v l t L s - l d s e s 1 . 3 3 1 . 2 6 0 . 8 9 1 . 1 1 1 . 1 2 1 .29

ve= cha Veronlcs chamaed-~js 3 . 9 5 2 . 4 0 3 . 8 0 2 .52

V s r o f f V. OI t lCI .~LI1s 3 . 4 9 2 . 3 8 3 . 1 4 2 .25

ViO can V102S canlna 2 . 5 0 1 . 5 2 2 . 0 8 3 . 4 6 2 . 5 4 3 .04

V l o 1~1 V. Jpa lml l t r fa l - 4 . 4 9 - 1 . 2 8

V l o f l y V. r L v l n l a n a 3 . 7 2 0 . 9 3 3 . 1 1 2 . 4 5 3 . 4 6 2 .39

u m or a stage close to it in 4 0 - 7 0 years). This gave us 207 samples o f mature stands.

Samples were ordinated by detrended correspon- dence analysis using the Cornel l Eco logy Program D E C O R A N A (Hill , 1979; Hil l & Gauch, 1980; Gauch, 1982b). In numerous studies D C A has been found to give easily interpretable ordinat ions in

compar i son with m o s t other ordination methods . The axes o f D C A are scaled in units (standard devi- at ions, or sd) that have a definite meaning: a full turnover o f the species c o m p o s i t i o n o f samples is expected to occur in about 4 sd. One half-change (a 50% change in sample const i tut ion) will occur in approximate ly 1.2 sd (Gauch, 1973; Hill , 1979; Hil l

150

& Gauch, 1980). A problem with DCA is that it eliminates the so-

called 'arch effect' from the ordination, no matter whether this 'arch' is real or merely an artefact (Williamson, 1983; see also Beals, 1984). However, since the most important use of the ordination is ecological interpretation of resulting axes, DCA appears well suited for our purposes.

Because rare species clearly distorted the ordina- tion on higher axes, we omitted all species occur- ring in less than 19 of the sites. The remaining ma- trix of 207 samples and 61 species was then ordinated with DCA by using log-transformed (In(10*% cover + 1)) cover values. Because rare species were eliminated before ordination, it was not necessary to remove any of the samples as out- liers. DCA on non-transformed data gave similar results, but we preferred the log scale because of slightly better interpretability of the axes (on the advantages of logarithmic transformation, see Van der Maarel, 1979).

Site scores of DCA axes were used for estimating the within- and among-group variance components of the forest site types from a one-way analysis of variance (Sokal & Rohlf, 1981: p. 216). Relative magnitudes of the variance components will be ex- pressed as a percentage of their sum.

Results

Ordination: species

Using ecological indicator values for the species (based on field knowledge), the approximate direc- tions of three distinct ecological gradients, fertility, moisture and grazing intensity, were derived from the species ordination in Fig. 2. DCA1 reflects the fertility gradient. However, there is also some addi- tional variation caused by moisture and grazing in- tensity on DCA1, and these two gradients are fur- ther confounded on the second and higher axes. Actually, it is not surprising to find the fertility and moisture gradierfts to be partially intercorrelated, because one of the theoretically possible combina- tions, namely wet sites with low fertility, does not occur in the forests on upland soils.

The upper right corner of the ordination space is characterized by species occurring typically in forests under more or less intensive cultural in-

fluence, especially cattle grazing. The grazing gra- dient runs from the center of the diagram with spe- cies intolerant of cattle grazing (such as Linnaea borealis, Solidago virgaurea, Trientalis europaea, and all moss species) to indicators of grazing pres- sure (e.g. Agrostis capillaris, Fragaria vesca, Angel- ica sylvestris, Veronica chamaedrys, V. officinalis, Festuca ovina) in the upper right section of the or- dination space (on the indicator value of the spe- cies to grazing see, e.g., Linkola, 1916).

Ordination of the intermediate-aged samples (273 stands, 66 species; log-transformation) produced a similar result, although slight differ- ences occur in the species sequences along the gra- dients (Table 1). Several indicators of grazing pres- sure that were not included in the ordination of mature forests are found in the upper right corner of the DCA1 by DCA2 plane (e.g., Achillea millefo- lium, Pimpinella saxifraga, Prunella vulgaris, Ranunculus acris and Stellaria graminea).

There are 56 species common in our ordinations of mature and intermediate-aged forest sites, of which 42 were also included in our a priori model based on Kujala's (1979) tables. Linear correlation coefficients between the first two DCA axes of our data and the a priori ordination model are given in Table 2. In each case, the species scores of the fir.st DCA axes correlate strongly, having 78-95°7o of the variance (100 r 2) in common. Additionally, it may be noted that the a priori model still explains 30°7o of the variance in intermediate-aged forests on DCA2, despite the robustness and generalized nature of the original data in Kujala (1979). It is also statistically significantly correlated with the DCA2 of mature forests.

Ordination: samples

All samples were projected into the ordination space of mature forests and the scores of the first two DCA axes were calculated (see legend of Ta- ble 1). Subsequent univariate statistical tests are based on these two new quantitative variables.

We expected the strong fertility gradient along DCA1 in the species ordination to be reflected in the sequence of forest site types in the sample ordi- nation. This is indeed the case: mean score values of successive forest site types differ from each other on DCA1 (t-test, P < 0.001 in all cases). These ob- served mean scores are highly correlated with the

151

Table 2. Correlations between the species scores o f the first two axes from three separate D C A ordinations: (1) predicted values calcu- lated from the ideal data o f Kujaia (1979), (2) mature (_>70 yr old) and (3) intermediate-age ( 4 0 - 6 9 yr old) forest samples. Signs of statistical significance are based on 4 3 - 5 6 species from Table 1 ( **=P<0 .01 , ***=P<0.001) .

(1) (2) (3)

DC A 1 DCA2 DCA 1 DCA2 DCA 1 DCA2

(1) DCAI 1.00 DCA2 - 0.20 1.00

(2) DCA1 0.90*** - 0.13 1.00 DCA2 - 0.24 0.44** - 0.05 1.00

(3) DCAI 0.89*** - 0.00 0.98*** 0.00 DCA2 - 0.41"* 0.57*** - 0.10 0.88***

1.00 - 0.03 1.00

predicted scores of the forest site types (Fig. 3). DCA1 of the mature forests was t hus found to

effectively separate the average scores of the forest site types into a sequence of increasing fertility. Of special interest is, however, how much the score val- ue distributions of neighbouring forest site types overlap. Figure 4 shows the frequency histograms of different forest site types in young and mature forests on DCA1. There is remarkable overlap be- tween the neighbouring forest site types, which in

D C A l ( o b s e r v e d )

OMT 2

• VT

1

I I I I

0 1 2 3 4

DCA1 (predicted)

Fig. 3. Regression between the observed and predicted sample scores on DCAI (y = 0.54 + 0.53x; r = 0.993, P>0.001) . Values o f the abscissa have been computed from the matrix of 107 species with robust weight values on the nine forest site types, and those o f the ordinate f rom the data o f 207 field layer samples representing forests o f at least 70 yr o f age. For the lowest point, 12 C1T and 2 CT samples were available for the y- axis; thus, using the ratio o f 12:2, a weighted mean was calculat- ed for the x-axis from the predicted scores of C1T and CT.

general have some 30 to 40070 of their variance in common. Within each forest site type group the variance tends to be larger in young than in mature forests.

To study the importance of rare species that were eliminated from the analysis above, it was repeated by using ordination information of 115 species, which occurred in at least four samples of over 70 years old forests. The overlap values of Fig. 4 changed 0-2070 units, except those between OMT and FT, in which the decrease was from 56 to 45 units in young, and from 32 to 26 in mature forests. Thus the results of our basic analysis, which was based on the 61 most common species, did not suf- fer from exclusion of about 270 species.

The strong forest site type gradient on DCA1 was not reflected on higher axes, thus calling for addi- tional explanations. On the average, Picea- dominated stands had lowest, Pinus-dominated forests had highest, and Betula-dominated forests had intermediate scores on DCA2. The scores of Picea, Betula and Pinus-dominated forests differ significantly (P<0.001 in VT, MT, OMT and FT, one-way ANOVA) from each other on DCA2, probably reflecting the influence of tree layer com- position on undergrowth vegetation.

A summary of our results on the first two DCA axes is given in Fig. 5. The following trends are seen in the mean scores of forest site types:

a) Fertility axis DCA1 separates the forest site types in Pinus, Picea, and mature Betula- dominated forests.

b) Tree species axis DCA2 separates Pinus, Picea and Betula forests, independently of the age- class of forests.

c) The DCA1 scores of Betula-dominated forests

0.38

1.31

1.94

2.50

2.94 0.41

(15)

S D (N)

0.32 (5)

0.47

(49)

0.34

(117)

0.33

(122)

1 5 - 6 9 years

! ' . ' , ',', CiT

3 4 %

P ,

42 %

, ~ ~ 1 , OMT

q i i

1 2 3

DCA1 SCORE

152

70 years

, ,

i i

32 %,r'~l,

i

i 1

0 2 3

DCA1 SCORE

.~(~ ~ S D (N)

10 0.73 0.21 (12)

-30

10

f 30

10 i

.30

10

30

10

1.29

1.81

2.28

2.94

0.28

(66)

0.29

(82)

0.31

(34)

0.36 (7)

Fig. 4. Distribution histograms of the DCA1 scores of young and mature forests. Values between the histograms give the percentages of the within-group variance component, i.e. they indicate how much the distributions of the forest site types overlap each other. Means, standard deviations and the number of samples for the scores of the forest site types are also given.

DCA2

1.5

0,5

R i c h f o r e s t s

VT M..T.T

o " o

0

• u 0

I I I I I I

0.5 1 1.5 2 2.5 3

DCA1

Fig. 5. Sequence of the mean scores of the first two DCA axes from young (circles; age 15-69 yr) to old forests (dots; age >70 yr) in relation to the dominant tree species. The lower field of the figure represents Picea, the central field Betula, and the upper field Pinus forests. Lines connect the points of the three tree species within each forest site type and age class. Only cases with at least three samples are enclosed. The data of herb-rich forests are rather heterogeneous, because the samples of OPyT, OMaT and FT have been combined.

are significantly higher than those of Pinus and Picea-dominated forests within each forests site type. This trend is emphasized in young Betula forests (shifts from MT towards OMT and from OMT towards the nutrient-rich forests being most spectacular).

d) Vegetation succession along with tree stand maturation is associated with the following changes in understory vegetation:

Pinus forests. The fertility score of the most bar- ren type (CIT) is increased and that of the lush type (OMT) decreased. On DCA2, Pinus-dominance is most spectacular in young and barren forests.

Picea forests. On DCA2 Picea-dominance in- creases in MT and OMT as the canopy cover closes through succession.

Betula forests. Fertility scores decrease through succession to MT and OMT.

e) Rich forests. The number of our samples is very small. Thus, the only conclusion we can draw is that the heterogeneity of these samples is very high, especially on the tree species axis DCA2.

Discussion

Ecological gradients

1~vo of the three gradients discerned, namely fer- tility and moisture, are commonly mentioned in studies of boreal forest vegetation (e.g., Carleton & Maycock, 1980; Bergeron & Bouchard, 1983; Kuusipalo, 1983). A strong trend in the data need not imply one single controlling factor (e.g., Hill & Gauch, 1980). In our results the partial interdepen- dence of fertility and moisture gradients is likely due to an absence from our data of certain combi- nations of these variables, especially wet, poor sites.

Soil fertility and moisture are perpetual ecologi- cal factors in boreal forests, whereas our third gra- dient, intensity of cattle grazing, is not. During this century cattle grazing in the forests of Finland has decreased drastically, and the impact of this cultur- al practice is gradually disappearing from the forests (see Linkola, 1916; Lampimiiki, 1939). The same holds for the even more distant effects of slash and burn cultivation, which were probably rather similar to those of cattle grazing. Compari- son of our data with more recent material (collect-

153

ed during the 8th National Forest Inventory, now in progress) might be used to quantify this gradual change.

Forest site types and the vegetation continuum

Our ideal model based on Kujala's (1979) materi- al showed that the average scores of the Finnish forest site types form an evenly spaced sequence along the dominant ordination axis. Mean differ- ences between the forest site types, calculated from empirical vegetation data, closely approximate the model. In this sense our ordination test may be seen to corroborate Cajander's forest site type theory.

However, the continuum nature of vegetation is revealed in the amount of overlap between the for- est site types on the first DCA axis. The overlap be- tween the neighbouring types was pronounced, about 30-40%, which would mean a high proba- bility of misclassifying a stand, if our result is generalizable in the field.

Part of the overlap can be explained by differ- ences in the methodology of determining the site types in comparison to our ordination approach. The description of site types emphasizes constant and abundant plant species, and the relative quan- tity of certain plant groups like lichens, mosses, herbs, grasses, dwarf-shrubs and shrubs (Cajander, 1926). Further, the relatively narrowly defined site types have also been delimited using criteria (such as the vitality of the species) that are not easily in- cluded in a numerical analysis (Oksanen, 1983a). This may increase the chance for differences in the classification of the site types by field observers varying in ability and experience.

In the a priori forest site type model we showed that mean DCA1 scores are highly correlated with mean values of pH, calcium, and nitrogen. There are no chemical data available in our material, but measurements have been made in the 1980-83 in- ventory of Finnish forest soil fertility. They show that the overlaps of pH, exchangeable calcium and total nitrogen between the neighbouring forest site types are notably larger than those of our DCA1 scores (Fig. 6; also here the DCA1 means for the site types were correlated with the mean values of pH, Ca and N, r = 0.93- 0.94, P(one-tailed) < 0.01, N=5). The heterogeneity within the forest site types is thus apparent both in the understory vege- tation and in the nutrient status of the humus. A

154

significant part of the nutrient heterogeneity within the forest site types is apparently explained by ef- fects of tree species; see Vuokila's (1956) reanalysis of the data of Valmari (1921).

In Fig. 6 we draw parallels between the constitu- tion of forest vegetation in the 1950s and the chem- ical properties of the humus in the 1980s, assuming that the structure and environmental background of the forest ecosystem has remained constant. Due to the radical changes caused by modern forestry during this period, this assumption may be untena- ble. The composit ion of the understory vegetation may have changed, and because the humus layer it- self is a product of plants, the vegetation change may have affected the nutrient status of the humus.

Forest site types, tree species and feather-mosses

One of the controversial issues in the Finnish for- est site type theory has been the effect of tree layer on undergrowth vegetation (see Introduction). Cajander himself thought this influence so small that it could be largely neglected in the determina- tion of a forest type in the field. However, since the first forest inventory the proport ion of Picea- dominated forests increased in the productive forest land in southern Finland f rom 2707o to 35070, and the proport ion of Pinus-dominated forests

decreased from 51% to 43°70 (Ilvessalo, 1956: Ta- ble 21). At the same time shifts occurred in the frequencies of the forest site types from CT and VT to MT and OMT.

We suggest that field observers may have been in- fluenced by the increase of Picea and related changes in the understory vegetation from the first to the second inventory. Thus, they tended to classi- fy stands into MT in the latter inventory.

Sarvas (1951) concluded that the heavy shade of Picea brings about noticeable changes compared with the more light requiring ground flora of Pinus forests. Tree species composit ion is clearly reflected on our second DCA axis, with Pinus and Picea having opposite and Betula intermediate effects. I f this is mainly taken as a light gradient, the location of Betula forests between Pinus and Picea forests may be associated with the abundant leaf litter produced by Betula.

We consider the dependence of feather-mosses on rain drip from the canopy for nutrients (Tamm, 1953, 1964; see also Johnson, 1981) to be another possible factor affecting the tree species gradient. The cover of the feather-mosses is very dense in mesic boreal forests, and being effective competi- tors, they may rapidly spread and weaken or elimi- nate other ground layer plants. In our study area the feather-mosses (especially Pleurozium schre-

DCA1 C a l c i u m % N i t r o g e n % pH

Rich fores ts 7 24

34 189

MT r/N i l l r l - G i l l 82 469

66 205

2 34 I I I I I I l l l l l i 0 2 4 1.5 2 2.5

I I I I I I I I I I 0.1 O~ G5 0.7 3.5 4

I 4.5

Fig. 6. Comparison of understory vegetation and chemical properties of the humus layer between the forest site types. DCA1 refers to the score values of the first DCA axis, computed of the forest inventory data from 1951-53. Data on exchangeable calcium and total nitrogen contents (% dry humus), and pH (H20) in the humus layer from the inventory of Finnish forest soil fertility 1980-83. Number of stands sampled in the two data sets appear by the bars. Black bars represent two standard errors (2 SE) at either side of the means, which are indicated by small triangles. Black plus white bars depict one standard deviation (SD) on either side of the means. If distribution is normal, mean + 1 SD contains 68.3°7o of the stands, and mean _+ 2 SE sets 95.4070 confidence limits for the mean.

beri, Hylocomium splendens and Dicranum poly- setum) generally cover 50-100% of the ground surface in the three most common site types VT, MT and OMT, i.e. in about 90% of the forest land. They are not 'rooted' in the substrate, but possess an efficient absorbing system for the nutrients released and washed out of the tree crowns. Conse- quently, the feather-moss carpet is almost indepen- dent of the nutrient and water supply of the soil substrate.

In the most barren Pinus forests the productivity values increased with age (Fig. 5). This conclusion was based on few data, which consisted of inter- mediate stages between the Cladina and Calluna site types (in this paper, however, referred to as Cl~, for difficulties met in the delimitation of the forest site types in heath-like Pinus forests, see Oksanen & Ahti, 1982, and Oksanen, 1983a, 1983b). The more extreme nature of the young barren forests is real, however, because the feather-mosses increase in late phases of the forest succession together with canopy cover (see Oksanen, 1983b: Table 3, for a similar result).

The age-related decrease of productivity grades in the Pinus forests of the OMT site type is proba- bly related to the decrease of Betula due to logging. The Pinus-dominated OMT forests generally have a rich mixture of Betula in younger stages. After the removal of full-grown Betula trees, the under- story vegetation reaches a structure typical of rather fertile coniferous forests (Fig. 5). Of probable im- portance in this process is the decrease in the amount of litter produced by deciduous trees. If the litter is abundant, the feather-mosses withdraw to the small rises in the ground, from where the litter slides away. From there they invade the ground, if the litter production of the canopy decreases.

The ecological importance of the feather-mosses has probably become more pronounced during the last 30 years, after the period when our field data were collected. At least three factors have potential- ly increased the cover of moss-carpets:

(1) The role of fire has become insignificant in Finnish forests. In the preceding centuries, the time interval between forest fires was 80-100 years. Nowadays, due to efficient fire control, it is in many areas 150-200 years ago since the last fire occurred. And more importantly, the use of fire in forest regeneration has strongly decreased.

(2) The proportion of Betula has decreased in

155

the forests. Consequently, the amount of leaf litter has diminished.

(3) Cattle-grazing in forests has ceased, and the feather-mosses probably have largely replaced the weedy flora that flourished in the forests 30 years ago.

Data presented in this paper on the nature of vegetation gradients in Finnish south boreal forests, as well as those of Sepponen (1985) on the structure of ground vegetation in relation to the properties of forest soils in northern Finland, demonstrate the need to find complementary alternatives to the for- est site type classification.

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Accepted 29.8.1986.