New Forests 11 : 93-123, 1996 .© 1996 Kluwer Academic Publishers . Printed in the Netherlands .

Initial seedling emergence of hairy birch and silverbirch on abandoned fields following different sitepreparation regimes

ANDERS KARLSSONSwedish University ofAgricultural Sciences, Department of Silviculture, S-901 83 Umea,Sweden

Received 5 October 1993 ; accepted 1 August 1995

Key words : afforestation, arable land, Betula pendula, Betula pubescens, direct seeding,natural regeneration

Application . For reasons of economy, natural regeneration and (or) direct seeding of birch areinteresting regeneration methods on abandoned fields . High and uniform seedling emergenceis promoted by mechanical soil preparation, but since soil, weather and ground vegetation mayexert great influence on seedling emergence, managers should consider how different methodsof site preparation will affect seedbed quality. Results from this study can be used for guidancein preparing appropriate seedbeds for germination and seedling emergence .

Abstract. Experiments were carried out at four sites in Sweden to investigate the possibilityof establishing hairy birch (Betula pubescens Ehrh.) and silver birch (Betula pendula Roth)on abandoned fields using natural regeneration and (or) direct seeding . The effects of six soilpreparation methods (no preparation, ordinary ploughing, rotary cultivation, deep ploughing,inverted ground, removal of top-soil) and five additional treatments (no treatment, herbicide,peat litter, wood-ashes, slaked lime) on seedling emergence percentages (SEP) and vege-tation cover percentages (VCP) were studied . SEP-values were estimated in June (SEP(J))and October (SEP(O)) . The experimental designs used were : 1) split-plot design with wholeplots in a randomized complete block design (RCBD) ; 2) RCBD. The SEP-values observedwith no preparation were close to 0% while the SEP-values obtained with mechanical soilpreparation methods mostly were much higher (p < 0 .014) . Seedbeds with top-soil in thesurface, created by ordinary ploughing or rotary cultivation, and seedbeds with mainly baremineral soil in the surface, created by deep ploughing, inverted ground or removal of top-soil,obtained SEP-values of equal merit on silty soils, reaching SEP(O)-values up to 15% . Thelatter seedbeds obtained the best results on sandy soil, with as high SEP(O)-values as 47% afterremoval of top-soil . Seedbeds with top-soil in the surface were quickly colonized by groundvegetation, reaching VCP-values between 70% and 100% . Herbicide spraying with glyphosateand application of peat litter to the seedbed surface promoted seedling emergence . However,herbicide spraying before soil preparation was of little effect when followed by mechanicalsoil preparation .

94

Introduction

Background

Large areas of arable land have been abandoned in Sweden . A substantialpart of these areas have been afforested (Kumm, 1989), mainly with Norwayspruce (Picea abies (L .) Karst.) (Johansson and Karlsson, 1988) . There is anincreasing interest in hardwood species for farmland afforestation (Elfving,1986). Hardwoods are preferred for reasons of natural beauty and biodiversity(Gustavsson, 1991) and there is an increasing industrial demand for hard-woods (Ekstrom, 1987) . Hairy birch (Betula pubescens Ehrh.) and silverbirch (Betula pendula Roth), the hardwood species most common in Sweden(Kempe, 1991), have become interesting as alternatives to Norway spruce .Birch planting has been tried but has, in most cases, proved unsuccessful sincecompeting ground vegetation and foraging mammals have caused consider-able damage (Backe,1991) . Successful regeneration of planted birch thereforerequires protection, which might reduce financial incentives . Instead, naturalregeneration and (or) direct seeding may be feasible low-cost regenerationmethods for creating dense stands of birch (cf. Kalela, 1961) . Such stands maycarry a sufficient number of established seedlings despite high damage levels .However, there is a lack of experience on how to perform successful naturalregeneration or direct seeding on abandoned fields (Karlsson, 1994) .

Establishment ecology

Hairy birch and silver birch are ecologically similar to the commerciallyimportant birches of North America, paper birch (Betula papyrifera Marsh .)and yellow birch (Betula alleghaniensis Britt.) (Perala and Alm, 1990a,1990b). High soil surface moisture is crucial for seedling emergence (Linteau,1948 ; Palo, 1986), though rainfall may mechanically disturb the seeds and thegermination process (Marquis et al ., 1964). High as well as low temperaturesmay hinder germination (Mork, 1944) and very high surface temperaturesmay cause seedling mortality (Linteau, 1948 ; Marquis et al ., 1964) . Someshading during the first growing season increases germination and germinantsurvival (Marquis et al ., 1964; Horsley and Abbott, 1970). However, shademay also inhibit root and shoot growth (Marquis, 1969 ; Kinnaird and Kemp,1970) .

Seedbed properties affect germination (Perala and Alm, 1990a, 1990b) .Birch seeds germinate and survive better on mineral soil than on litter ormosses (Marquis et al ., 1964; Kinnaird, 1974) . This has mainly been attrib-uted to the higher, more constant moisture of mineral soil . For establishment,humus is preferable to litter (Sarvas, 1948 ; Marquis et al ., 1964). The seedbed

95

for yellow and paper birch should contain mineral soil mixed with organicmatter (Godman and Krefting, 1960; Perala and Alm, 1989), alternatively athin layer of humus on top of mineral soil (Winget and Kozlowski, 1965) .Other favourable germination substrates include Sphagnum mosses (Kin-naird, 1974) and peat (Palo, 1986) . The seedbed surface should be texturedwith minute depressions where seeds can establish good seed-soil contact(Goodman and Krefting, 1960) and be stable enough to allow good root-contact (Linteau, 1948). Fine-grained soils may be hard for roots to penetrate(Palo, 1986), while humus is easily penetrated (Winget and Kozlowski, 1965) .Good seedbeds may be prepared through burning (Sarvas, 1948), mechanicalsoil preparation (Raulo and Malkbnen, 1976 ; Perala and Alm, 1989), andsometimes by logging activity (Safford, 1983) . Draining encourages birchregeneration on peatland (Seppala and Keltikangas, 1978) . Soil preparationand fertilization with PK or NPK further promote the recruitment and growthof birch seedlings on peatland (Moilanen and Issakainen, 1981) .

Competition from ground vegetation is a primary reason for recommen-dations against natural regeneration or direct seeding of birch on abandonedfields (Bjorkbom, 1969 ; Kaunisto and Paivanen, 1985). Birch seedlings aresensitive to sod competition (Nash et al ., 1951) and competition from groundvegetation may grow worse after mechanical soil preparation (Barring, 1962) .Vole damage in plantations on abandoned fields is reduced if ground vege-tation is removed (Barring, 1967) . Since seeds and vegetative reproductiveorgans are found in the top-soil, it may prove possible to reduce competingvegetation by soil preparation methods that remove or translocate the top-soil and bring bare subsoil to the seedbed surface (cf . Orlander et al ., 1990) .Herbicides can also be used to control competing vegetation (Barring, 1967 ;Perala and Alm, 1989) .

Objective

The purpose of this study was to investigate different methods of establishinghairy birch and silver birch on abandoned fields using natural regeneration and(or) direct seeding . Effects of different site treatments on seedling emergenceand competing vegetation were investigated . The major questions addressedwere: (1) Is mechanical soil preparation a prerequisite for obtaining highand uniform seedling emergence? (2) What kinds of seedbeds, created bymechanical soil preparation, will encourage high and uniform seedling emer-gence? (3) What kinds of seedbeds, created by mechanical soil preparation,will suppress competing vegetation? (4) Will herbicide spraying before soilpreparation reduce competing vegetation and increase seedling emergence inthe first growing season after soil preparation? (5) Will application of peatlitter, wood-ashes and slaked lime to the seedbed surface promote seedling

96

emergence? This report is limited to the study of seedling emergence duringthe first growing season.

Materials and methods

General procedures

Four field experiments were carried out (Table 1) . Experiment 1 (Exp . 1)was established at Stocke in 1988, Experiment 2 (Exp . 2), in Savar in 1989,Experiment 3 (Exp. 3) at Asa in 1989, and finally Experiment 4 (Exp . 4)at Asa in 1989 and 1990 . All experimental sites were abandoned fields,uncultivated for at least five years before establishment . Where it occurred,woody vegetation was removed . To describe weather and soil conditions,minimum temperature 0.25 m above ground was measured weekly throughoutthe growing season (minimum thermometers, Geraberg Termometerwerk,Turingen, Germany) as was groundwater level (plastic groundwater tubes) .Precipitation was recorded at the nearest climatic station (maximum distancewas 7 km) for all sites, except for Exp . 2 where a rain-gauge (Pronamic,Them,Denmark) was used on the location itself .

To study the effect of top-soil or subsoil in the seedbed surface and theinfluence of different soil profiles, different seedbeds were created using sixsoil preparation methods (S 1-S6) . No preparation (S 1) was used as a control .Ordinary ploughing to a depth of 20-30 cm (S2), and rotary cultivation toa depth of 10-15 cm (S3), left top-soil in the seedbed surface, the lattermethod creating a looser seedbed . Deep ploughing to a depth of 40-50 cm(S4) brought subsoil to the surface and transported top-soil (including sods)downwards in the soil profile. Inverting the ground using an excavator (S5)

Table 1 . Location and site description according to the definitions of Hagglund andLundmark (1977)

Experiment1 2 3 4

Latitude (°N)Longitude( °E)Altitude (m a .s .l .)Soil-textureSoil-moistureField layer

63°44'20° 15'10silty tillmoistgrass types

63°55'20 ° 33'25sand- fine sandmoistgrass types

57° 10'14 ° 46'180siltmesic - moistgrass types

57° 10'14°46'180siltmesic - moistgrass types

97

made top-soil (including sods) and subsoil change places, putting about 15 cmof subsoil on top of the top-soil . Removal of top-soil (S6) left subsoil in thesurface and a profile without top-soil .

In addition, five additional treatments (Al-A5) were used to modify theseedbeds. No treatment (Al) was used as a control . Herbicide spraying,before soil preparation with Roundup® (glyphosate 360 g .litre), 0 .015 litresof Roundup® per litre of water in a dosage of 0 .06 litres/m2 (A2), wasperformed to control the weeds . Application of peat litter, 0 .3 kg.(dw)/m2to the seedbed surface (A3) was done to obtain a favourable germinationsubstrate. To simulate a burned site, application of wood-ashes incl . smallpieces of charcoal, 1 .1 kg.(dw)/m2 , to the seedbed surface (A4) was done .Since wood-ashes to a large extent consist of calcium, application of slakedlime, 0.2 kgJm2 to the seedbed surface (A5) was also tested .

Seed dispersal, from birches standing at the edges of the fields, was esti-mated for all experimental sites except that for Exp . 4. Since the bulk ofthe seed was dispersed from late July or early August to mid-November, theseedfall was already completed at the time of site preparation in Exp . 4 . Seedtraps of 0.25 m2 covered with meshed wire netting as a protection againstseed-eating birds were put up 0 .7 m above ground . As soil preparation wascarried out, these traps were aligned systematically over the sites (Fig . 1 and2). Every two weeks the traps were emptied and seeds found were air-dried,to a moisture content of about 7%, cleaned and recorded before storage insealed packages at -5 °C. After seedfall in late autumn, all seeds found inthe traps were tested for germination rate and percentage at 20 °C/21 days(ISTA, 1985 ; Table 2) . For traps where the seed-number was deemed unman-ageable a random sample was selected for testing. The number of germinableseeds falling between traps was estimated using the number of seeds andtheir germination percentage for the surrounding four seed traps . The seed-fall was assumed to increase or decrease in a linear way between the traps .Germinable seeds were required to make sure that site treatment effects weremeasurable. Therefore direct seeding was carried out in Exp . 1, 2 and 4, butnot in Exp . 3 due to lack of seeds . The seedlots used were tested the sameway as the naturally dispersed seeds (Table 2) and the seeds were countedbefore sowing .

The first growing season two inventories were carried out, one in mid-June and the other in late September or early October. The number of livingseedlings on the experimental sites was tallied . In June, germinating seedswere recorded as seedlings when the hypocotyl was longer than the seedcoat, regardless of how successful the germinant had been in establishingroot-soil contact. In the second inventory the seedlings were recorded only

98

Table 2 . Seed material of Betula pubescens and Betula pendula . The Jacobsen apparatus(20 °C/21 days) was used in the germination test of 150-2 000 seeds (typical number = 400)

' The proportion of germination that occurred within 7 days .2 Seed from Finnish seed orchard "Loppi" . Origin of clones ranges from about 62 °N toabout 65°N .3 The local seedfall .° Seed from seed-production stand .5 The corresponding values for seeds used in spring sowing were 80% and 78%, respectively .6 Seed from Swedish seed orchard "Asarum" . Origin of clones ranges from about 56 °N toabout60 °N .7 The corresponding values for seeds used in spring sowing were 45% and 21 %, respectively .

when root-soil contact had been successfully established . Seedling emergencepercentage (SEP) was calculated by :

SEP = (number of seedlings emerged/number of estimated or seededgerminable seeds) . 100

(1)

Since the definition of SEP differed for the inventories, SEP(J) representsthe June-inventory while SEP(O) represents the October-inventory . Livingvegetation coverage (VCP) was subjectively estimated to the nearest tenpercent on rectangular spots systematically aligned on the experimental sites .The exact procedure for this varied between experiments, and is elaboratedon below .

To analyse the effects of the different treatments, t-tests and analyses ofvariance were carried out using SAS Procedures TTEST and GLM (SAS,1988a). The following variables were used to estimate treatment response :SEP, VCP and the percentage of spots with no seedlings (PZS). PZS-valueswere recorded only for the October inventory . Residuals were studied bymeans of plots (Sabin and Stafford, 1990), and the Shapiro-Wilk test fornormality in SAS Procedure UNIVARIATE (SAS, 1988b) . When the assump-tions of normality and constant variance for the residuals were met by thepercentage data, analyses of variance were carried out . In some cases viola-tions of the assumptions necessitated transforming of the variables before

Experiment Species Latitude(N)

1000-seedweight (g)

Germination(%)

Germination energy7/21(%)'

1 Betula pubescens 2 0.364 77 931 Betula pubescens 63 °44" 0.151 14 722 Betula pubescens 2 0.366 79 912 Betula pubescens 63°55i3 0.167 39 623 Betula pubescens 57 ° 10'3 0.186 30 704 Betula pubescens 57°04r4 0.196 87 5 82 54 Betula pendula 6 0.260 55 7 22 7

C = z=' YiEn I yi

99

analysis of variance could be carried out . The transformations tested were : 1)square root ; 2) logarithmic ; 3) arc sine square root ; 4) logit; 5) rank. Thesetransformations gave similar results . The logarithmic (P + 1) transformation,recommended by Steel and Torrie (1980) when some values of the responsevariable are small, showed good results according to the residual analyses andwas selected . Correction for logarithmic bias was performed when retrans-formation was done, calculating a constant (C) so :

(2)

yi is the antilogarithmic transformation of the predicted logarithmic valueand yi is the original value . Finally, the new predicted means, in terms of theoriginal units are :

Y = C y

(3)

Experiment 1

A split-plot design with whole plots in a randomized complete block design(RCBD) was used (Table 3) . The area was divided into blocks on the basis ofexpected seedfall . The blocks were divided into whole plots and the wholeplots split into subplots . The soil preparation methods "no preparation", "ordi-nary ploughing" and "deep ploughing" were randomly assigned to whole plotsinside each block and the additional treatments "no treatment", "peat litter","wood-ashes" and "slaked lime" were randomly assigned to the subplots

within the whole plots (Fig . 1). On each subplot six pairs of rectangular spotswere systematically aligned (Table 3 and Fig. 1) . Each pair of rectangularspots consisted of one spot with and one spot without direct seeding . Thesespots were located before seeding could take place . Direct seeding of hairybirch was done in September (Table 3) . Seedlings originating from directseeding were estimated as the difference between the two spots in everypair of rectangular spots . The analyses of variance were performed with themodel :

Yijk = Si + Bj + (SB)ij + Ak + (BA)jk + (SA)ik + Eijk

(4)

Y i j k is the response variable and the model is a mixed model . The soilpreparation effects Si, the additional treatment effects Ak and their interac-tions (SA)ik were regarded as fixed effects . The block effects Bj and theinteractions, (SB)ij and (BA)jk were regarded as random effects . Eijk isthe remaining error, which is at random. Appropriate error variances weredetermined by the set of expected mean squares . The following mean squares

1 00

Table 3 . Description of experimental design and dates of treatments

a On whole plots where no herbicide application was done, there was only one subplot withthe size of 4 x 20 m .•

Divided into 8 parts of 4 x 1 .25 m (Exp . 2 and 3) or into 6 parts of 4 x 1 .67 m (Exp . 4).•

According to the standard germination test . See Table 2 .•

When spring sowing was done, the numbers of germinable seeds were 45 of silver birch(SB) and 48 of hairy birch (HB) .• The following soil preparation methods were tested : S 1 = No preparation; S2 = Ploughingto a depth of 20-30 cm ; S3 = Rotary cultivation to a depth of 10-15 cm ; S4 = Ploughing toa depth of 40-50 cm ; S5 - Inverting the ground, putting 15 cm of subsoil on the top-soil ; S6= Removal of top-soil .f The following additional treatments were tested : A 1 = No treatment ; A2 = Herbicide ; A3= Peat litter, A4 = Wood-ashes ; A5 = Slaked lime .g The soil preparation method "Inverted ground" was carried out on 10 04 90.h Spring sowing was done 23-27 04 90.

(MS) were used as denominators for the fixed effects and their interaction :1) MS (SB) for the S-effect ; 2) MS (BA) for the A-effect ; 3) MS E for the(SA) interaction effect (Table 4) . To determine differences between treat-ments, Tukey's studentized range test (HSD) was used with the above MS'sas error terms .

Experiment 2

A split-plot design similar to that of Exp. 1 was used (Table 3) . The soilpreparation methods "no preparation", "rotary cultivation", "deep ploughing"and "removal of top-soil" were randomly assigned to the whole plots and theadditional treatments "no treatment" and "herbicide spraying" were randomly

Experiment1 2 3 4

No . of blocks 3 5 5 6No . of whole plots/block 3 4 4 4

(size, m.) 2.5 x 20m 4 x 20 m 4 x 20 m 4 x 20mNo . of subplots/whole plot 4 2a 2 a 2a

(size, m.) 2 .5 x 5 m 4 x 10 mb 4 x 10 mb 4 x 10 mbNo . (of pairs) of rectangular 6 8 8 6spots/subplot (size, m.) 0 .25 x I m 0 .2 x 0 .5 m 0 .2 x 0 .5 m 0 .2 x 0 .5 mNo . of germinable° seeds sown 62 79 - 55 of SBd, 52per rectangular spot of HBdSoil preparation methodse S1, S2, S4 Sl, S3, S4, S6 S1, S2, S3, S6 S1, S3, S5, S6Additional treatmentsf A1, A3, A4, A5 Al, A2 Al, A2 A1, A2Soil preparation 22-2608 88 07-11 08 89 10-1407 89 13-17 11899Additional treatments 29-31 08 88 26-300689 12-160689 07-11 08 89Seed-sowing time 19-2309 88 09-13 10 89 - 20-24 1189 h

Experiment 1

20 m

B1

B1

S4

S2

S1

B2 B3

10 1

II

ION

= Al -subplot

= A3 - subplot

= A4 - subplot

A5 - subplot

Awhole plot

® = seed trap

II = pair of rectangular spots

~O = mature birches

F--- 7 .5 m

Fig.] . Experimental layout for Exp . ],showing three blocks (B 1-B3) with the following soilpreparation methods and additional treatments : S1, no preparation; S2, ordinary ploughing ;S4, deep ploughing ; A1, no treatment; A3, peat litter A4, wood-ashes ; A5, slaked lime .

1 02

Table 4 . ANOVA for a split-plot design with whole plots in a randomized completeblock design

assigned to the subplots . Herbicide spraying was not performed where thesoil preparation was to be removal of top-soil (Fig . 2) . Each subplot consistedof eight replicates, with pairs of rectangular spots similar to Exp . 1 (Table 3and Fig. 2). Direct seeding of hairy birch was done in October (Table 3) .Analyses of variance were carried out with two models . They were the samemodel (4) as in Exp . 1, and the following model :

Yij = Si + Bj + Eij

(5)

The terms have the same meanings and effects as in Exp. 1, i .e . this is amixed model, comparing soil preparation methods using only subplots withno treatment. In this model, MS E was used as denominator for the S-effect(Table 5) . Model (4) was used and treated as in Exp . 1 . For examining strong(p < 0.05) (SA)ik-interactions, found with model (4), a modified model(5) was used. Each soil preparation method was separately analysed andthe Si-effect was replaced by the Ak-effect, with MS E as denominator.To determine differences between treatments, Tukey's studentized range test(HSD) was used with MS E as error term . When only two treatments werecompared, the t-test was used .

Experiment 3

The same experimental design as in Exp . 2 was used (Table 3) . The soilpreparation methods "no preparation", "ordinary ploughing", "rotary cultiva-tion" and "removal of top-soil" and the additional treatments "no treatment"and "herbicide spraying" were tested. Field establishment was carried outfollowing the same procedures as for Exp . 2 . Since only natural seeding wasstudied, there was only one rectangular spot per replicate within subplots .

Source of variation d .f. MS F-value

Soil Preparation (S) $ - 1 MSS MSS/MSE(1)Blocks (B) b-1Error(1) (S x B) (s - 1)(b - 1) MSE(1)Additional Treatments (A) a - 1 MSA MSA/MSE(2)(S x A) (s - 1)(a - 1) MSSA MSSA/MSE(3)Error(2) (A x B) (a - 1)(b - 1) MSE(2)Remaining En-or(3)Total

(s - 1)(a - 1)(b - 1)sab - 1

MSE(3)

Experiment2

%%mm mom m~mm mUnnuffunn~~m~m%mm~~m~~ mm~-m

mm%!mmm mom:iii

11 = Al - subplot

= A2 - subplot

® = seed trap

11 = pair of rectangular spots

= whole plot

0 = birches, mature treeso and saplings

Source of variation

d.f.

MS

F-value

Soil Preparation (S)

s - 1

MSS

MSS/MSEBlocks (B)

b - I

Error (E)

(s - 1)(b - 1)

MSETotal

sb - 1

103

Fig. 2 . Experimental layout for Exp . 2, showing five blocks (BI-B5) with the following soilpreparation methods and additional treatments : S 1, no preparation; S3, rotary cultivation; S4,deep ploughing ; S6, removal of top-soil ; A1, no treatment ; A2, herbicide spraying .

Table 5. ANOVA for a randomized complete block design (RCBD)

0 o 'S1m 0 S6

(oS3

o0 131 fog m o B5

)S40

0 S4 m o 0®0 B2~OS

0 0 S3mo

( :

o® 0O

B3{o~(o: 0 0 0B4

m 0 ot0

:

0O Q1 OS1

o,, 0m o

0

(::i w00l

1 04

Model (4) and (5) were used, and analyses followed the procedures of Exp .2. Differences between treatments were determined as for Exp . 2 .

Experiment 4

The design of Exp . 2 was used (Table 3) . Only direct seeding was studied .The soil preparation methods "no preparation", "rotary cultivation", "invertedground" and "removal of top-soil" and the additional treatments "no treat-ment" and "herbicide spraying" were used . Herbicide spraying was not usedwhere soil preparation were to be inverted ground or removal of top-soil(Fig. 3) . However, since the effect of herbicide spraying proved questionable,due to rainy weather shortly after spraying, only subplots with no treatmentwere used in the analyses, implying that this experiment was treated as aRCBD-experiment . In addition, since the inverted ground-preparation wasdelayed, the experiment had to be divided into three parts : 4a) Autumn sow-ing of hairy and silver birch was done on plots with no preparation, rotarycultivation or removal of top-soil ; 4b) Spring sowing of hairy and silver birchon plots with inverted ground or removal of topsoil ; 4c) The effect of seed-sowing time (Tm) was tested on plots with removal of top-soil . These plotswere split into an autumn subplot and a spring subplot (Fig . 3). Before springsowing, the seedbed surfaces were loosened by means of a rake . Model (5)was used and treated as in Exp . 2 and 3 . Effect of seed-sowing time was testedwith a modified model (5) where the Si-effect was replaced by the Tm-effect .Differences between treatments were determined as in Exp .2 .

Results

Environmental conditions

All sites experienced frosts during the growing season, especially duringspring and autumn (Table 6) . The groundwater level was occasionally belowone meter at the sites for Exp . 3 and 4. Exp. 1 had high precipitation inMay and August but low in July and September, compared with averageprecipitation. For Exp . 3 and 4, the rainfall was low in May, and high inJune .

Experiment 1

The estimated seedfall in 1988 was uniformly poor over the area, an averageof 19 seeds per m2 . Since germination averaged 14%, the contribution fromnatural seeding was only about 0 .7 germinable seeds per rectangular spot, i .e .

B2

B3

B4

B5

B6

1

1

1

S1S6S5S3S1S5S3S6S1S6S5S3S3S5S6S1S6S3S1S5

S3S5S1S6

S3

S5

S1

S6

Experiment 4

% II II

II

II

0// II

II IIIIIflII

= Al -subplot (autumn)

=A2 - subplot (autumn)

=A1 -subplot (spring)

= whole plot

II = pair of rectangular spots

1l16 B1

1

1 05

Fig . 3 . Experimental layout for Exp . 4, showing six blocks (BI-B6) with the following soilpreparation methods and additional treatments : S 1, no preparation ; S3, rotary cultivation ; S5,inverted ground ; S6, removal of top-soil: A1, no treatment; A2, herbicide spraying .

negligible . As a confirmation, only a few seedlings originating from naturalseeding were identified . Soil preparation exerted a strong effect on the SEP(J)-values (p = 0.003) and the SEP(O)-values (p = 0.014). The two ploughingmethods were of equal merit, showing a positive effect on the SEP-values(Fig. 4a). The SEP(O)-values were half of the SEP(J)-values . The effect ofadditional treatment on SEP(J)-values was weak (p = 0 .108) while the effecton SEP(O)-values was strong (p = 0.019) (Fig. 4b). The highest SEP-valueswere obtained with application of peat litter while wood-ashes and slaked lime

I

106

Table 6. Climatic data for the experimental sites

1 The following abbreviations are used : Groundwater levels (GL), recorded in cm belowground surface ; Minimum temperatures (MT), recorded in °C; Precipitation (PR), recordedin mm .2 Average precipitation for the years 1961-1990, in mm at SMHI's (Swedish Meteorolog-ical and Hydrological Institute) nearest stations in Umed (Latitude 63 °48' N, Longitude20 ° 17' E) and Vdxjo (Latitude 56 ° 52' N, Longitude 14°48' E) .3 Minimum (min) and maximum (max) values .4 There was only one registration in May.

showed, if anything, a negative effect, on the SEP(O)-values. Soil preparationalso showed a strong effect (p = 0.005) on the PZS-values (Fig. 4c), with anotable decrease when the ground had been ploughed . There was no effectof additional treatments (p = 0.555) on the PZS-values (not shown in Fig . 4) .Soil preparation exerted a strong effect on the VCP(J)-values (p = 0.006) andthe VCP(O)-values (p = 0 .012) (Fig. 4d). The lowest values were obtainedwith deep ploughing. Additional treatments exerted no effect (p > 0 .741) onVCP-values (not shown in Fig . 4) .

Experiment

GL'1

PR' GL'2 1&2 3

PRA GL'4 3&4

MT PR' APMT MT' PR' AP GL' MT

MAY 68 27 41 22 22 48mina 13 -3 .6 38 -7.4 -10 .1 -4.3max i 39 -1 .4 50 -2 .9 101 4 -5.4 734 -1 .4

JUNE 49 51 44 90 90 55mina 36 -1 .6 42 -5.4 83 -3.6 54 -1 .4max i 59 2 .6 52 -1 .5 121 5 .2 119 7.0

JULY 6 73 53 70 70 75mina 61 -0.7 42 -1 .6 85 -0.5 63 0.3maxi 94 2 .5 57 4.1 101 1 .8 82 3.3

AUG 111 67 78 71 71

57mina 54 -2 .4 46 -2.9 110 -1 .9 106 -1 .0max i 70 3 .0 71 1 .2 119 2 .5 122 3 .5

SEPT 29 95 73 84 84 71min a 54 -6.4 36 -8.3 70 -7.9 49 -7 .1max i 58 -3.9 61 -1 .1 116 -1 .0 99 -1 .3

0

0

30

20

10

100

a) SEP - SOIL PREPARATION

b) SEP - ADDITIONAL TREATMENTS

c) PZS -SOIL PREPARATION

A

S1

S2

0_"I /// A,S1

S2

S4

Al

A3

A4

A5

B

B

AAB

S4

S1 S2

B

S4

l

1 07

Fig. 4 . Seedling emergence percentages (SEP) of hairy birch (Betula pubescens), percentagesof zero spots (PZS) and vegetation cover percentages (VCP) for different soil preparationmethods (S1, no preparation; S2, ordinary ploughing ; S4, deep ploughing) and additionaltreatments (Al, no treatment ; A3, peat litter, A4, wood-ashes ; A5, slaked lime) in Exp . 1 .White bars show the June inventory and striped bars show the October-inventory . Differentcapital letters above bars from the same inventory indicate differences (p < 0 .05) accordingto Tukey's studentized range test.

Experiment 2

Estimated seedfall averaged 1,000 germinable seeds per m 2, with an averagegermination percentage of 39% . Seedfall ranged from 90-2,935 germinableseeds per m2 (Fig. 5), and the corresponding range for the germination percent-ages was 32%-44%. In all analyses, soil preparation showed a strong effect(p < 0.00 1) on the SEP-values (Fig . 6a, d, g, j) . The SEP-values obtained withno preparation were close to 0% while the SEP-values observed for removal

1 08

of top-soil (Fig . 6g, j) were more than twice the values obtained with othermethods. Soil preparation also exerted a strong effect (p < 0 .001) on thePZS-values (Fig. 6c, f, h, k) . The PZS-values obtained with mechanical soilpreparation methods were below 10%, but close to 100% for no preparation .Additional treatments showed a strong effect on SEP(J)-values (p< 0 .039) buta weak effect on SEP(O)-values (p _> 0 .153) (Fig . 6b, e) . SEP-values obtainedafter herbicide spraying were slightly higher than the SEP-values observedfor no treatment. Additional treatments showed a weak effect (p > 0 .052) onthe PZS-values (not shown in Fig 6). The PZS-values observed for herbicidespraying were lower (25-27%) than for no treatment (39-40%) . The interac-tion between soil preparation and additional treatments was strong (P< 0 .001)for the VCP-values . Only together with no preparation, herbicide sprayingmarkedly reduced (p < 0.00 1) the VCP-value (Fig . 7) . When only soil prepa-ration methods were analysed, using model (5), soil preparation exerted astrong effect (p < 0.001) on the VCP-values . The VCP-values obtained afterremoval of top-soil were below 20% while the VCP-values observed for nopreparation and rotary cultivation were close to 100% (Fig . 6i, 1) .

Experiment 3

Seedfall averaged 380 germinable seeds per m 2 , with an average germinationpercentage of 30% . The range in seedfall was 70-1,320 germinable seeds perm2 (Fig. 5), and the corresponding range for the germination percentage was20%-41 %. The interaction between soil preparation and additional treatmentswas strong (p < 0 .00 1) for SEP(J)-values, SEP(O)-values and PZS-values . Inaddition, the interaction was also strong for the VCP(J)-values (p < 0 .001)and the VCP(O)-values (p = 0 .030) . Herbicide spraying raised the SEP(J)-value (p = 0.003, Fig . 8a) and the SEP(O)-value (p = 0.013, Fig . 8b) onlytogether with no preparation . Only together with no preparation did herbicidespraying reduce the PZS-value (p = 0.004, Fig . 8c), the VCP(J)-value (p <0.001, Fig . 8d) and the VCP(O)-value (p = 0 .009, Fig . 8e). When only soilpreparation was analysed, using model (5), soil preparation exerted strongeffects (p < 0.001) on SEP-values (Fig . 9a), PZS-values (Fig . 9b) and VCP-values (Fig . 9c) . The SEP-values and PZS-value received with no preparationwere close to 0% and 100%, respectively, while the corresponding valuesobtained with the other methods were much more favourable for an uniformseedling emergence . Notably low VCP-values (< 5%) were observed forremoval of top-soil .

0 = birches0

Fig . 5 . Dispersal of germinable seeds (%) . Index 100 is given to the seed trap that caughtmost germinable seeds (2,935 and 1,320 seeds, respectively, per m 2 at the sites for Exp. 2 and3) during the seedfall .

Experiment 4

In Exp. 4a, soil preparation had a strong effect (p < 0 .001) on SEP-values,PZS-values and VCP-values for both hairy birch and silver birch . Low SEP-values (Fig. 10a, b) and high PZS-values (Fig . 10c, d) were obtained for nopreparation . The VCP(O)-values observed for removal of top-soil were lowwhile the VCP(O)-values obtained for no preparation and rotary cultivationwere high, close to 100% (Fig . 10e, f) .

In exp. 4b, soil preparation showed no effect (p > 0 .303) on SEP(J) andSEP(O)-values for silver birch (Fig . 11 a), but a strong effect on SEP(J)-values (p = 0.002) and SEP(O)-values (p = 0.038) for hairy birch (Fig . 1 lb) .The SEP-values obtained for removal of top-soil or inverted ground wereof equal merit for silver birch, while the highest SEP-values for hairy birch

109

Site 2 Site 3

1- 20 -1

00D00O

metres

100 68 O 7 12 1 2O :8 . . .

0 90 66 37 0 8 . 1 :

0 67 3: :5 O0 72 43 23 O 7 11 31

O 85 43 21 0 1 . 3.

0 0 10 18 522880

;8

28metres

:5 16 :1 0 10 18 58

N D 13 28 7811

12

ii9

6

8

18 0 16 a0 72

5

14 O 1. 46 75

i i O ~~23 63 100

1 - 20-Imetres

110

0

0

100

75

50

25

100-

75-

50

25-B

00

100-

75-

50

25-

a) SEP-SOIL PREPARATION

BA

- B

AC C~/lII,.=S1

S3

S4

Al

d) SEP SOIL PREPARATION

C C

S1

A

crF-6B A

r-,,A

S1

S3

S4

Al

B

j) SEP-SOIL PREPARATION

S3 S4

b)

c) PZS-SOIL PREPARATIONSEP-ADDITIONAL TREATMENTS

A

0)SEP-ADDITIONAL TREATMENTS

A

A

B

Y//AI/n

B

S1

S3

S4

S1

A

A

S6

S1

A

PZS-SOIL PREPARATION

C

k)

PZS-SOIL PREPARATION I) VCP-SOIL PREPARATION

A

A

B

B

r7-MS4~

S

Fig. 6. Seedling emergence percentages (SEP) of hairy birch (Betula pubescens), percentagesof zero spots (PZS) and vegetation cover percentages (VCP) for different soil preparationmethods (S1, no preparation ; S3, rotary cultivation ; S4, deep ploughing ; S6, removal oftop-soil) and additional treatments (Al, no treatment ; A2, herbicide) in Exp . 2 . White barsshow the June inventory and striped bars show the October inventory . Different capital lettersabove bars from the same inventory indicate differences (p < 0 .05) according to t-test orTukey's studentized range test . First and second rows show the split-plot analyses for nat-ural regeneration and direct seeding, respectively, and the third and fourth rows show therandomized complete block analyses for natural regeneration and direct seeding, respectively .

100

00-`r 700

0 50

A

A

A

AlA2

S1

S3

S4Soil preparation method

Fig. 7. Vegetation cover (%) for different combinations of soil preparation methods (Sl,no preparation ; S3, rotary cultivation; S4, deep ploughing) and additional treatments (Al,no treatment; A2, herbicide) in Exp . 2, the October inventory. Each soil preparation methodis separately analysed regarding differences between additional treatments . Different capitalletters above bars, within the same soil preparation method, show differences (p G 0 .05)according to t-test .

were observed for removal of top-soil . Soil preparation had no effect (p >0.363) on the PZS-values (not shown in Fig . 11), which all were below 4% .Soil preparation showed no effect (p > 0 .363) on VCP(J)-values but exerteda strong effect (p < 0 .003) on VCP(O)-values (Fig. 11c, d). The highestVCP(O)-values were observed for inverted ground .

1 1 1

1 1 2

42

100

750

50

25

0

100-

75-0

50-

25-

0

a) SEP(J)

B

A

A

S1

S1

S2

S3

S1

S2

Soil preparation method

c) PZS

S1

S2

S3

Soil preparation method

d) VCP(J)

A

Flin

A

b) SEP(O)

B

A

AlA2

Additional treatment

e) VCP(O)

A

S2

S3

S1

S2

Soil preparation method

A

A

S3

S3

Fig . 8. Seedling emergence percentages in June (SEP(J)) and October (SEP(O)) of hairy birch(Betula pubescens), percentages of zero spots in October (PZS) and vegetation cover percent-ages in June (VCP(J)) and October (VCP(O)) for different combinations of soil preparationmethods (S1, no preparation; S2, ordinary ploughing ; S3, rotary cultivation) and additionaltreatments (A1, no treatment ; A2, herbicide) in Exp . 3 . Each soil preparation method is separ-ately analysed regarding differences between additional treatments . Different capital lettersabove bars, within the same soil preparation method, show differences (p < 0 .05) accordingto t-test .

15

10

100

75

0-0-

50

25

a) SEP

C B

B

A

B A -

S1 S2 S3 S6Soil preparation method

S1

A

b) PZS

BB

JuneOct

C) VCPA

A

B

C

S2 S3 S6 S1 S2 S3 S6

Soil preparation method

1 1 3

Fig . 9. Seedling emergence percentages (SEP) of hairy birch (Betula pubescens), percentagesof zero spots (PZS) and vegetation cover percentages (VCP) for different soil preparationmethods (S1, no preparation ; S2, ordinary ploughing ; S3, rotary cultivation ; S6, removalof top-soil) in Exp. 3 . Different capital letters above bars from the same inventory indicatedifferences (p < 0 .05) according to Tukey's studentized range test .

In Exp. 4c, seed-sowing time showed a strong effect (p< 0 .008) on SEP-values (Fig . I Ie, f), but a weak effect (p _> 0 .075) on PZS-values (not shownin Fig. 11). Higher SEP-values were obtained with spring sowing than withautumn sowing. Seed-sowing time exerted no effect on the VCP(J)-values,

1 14

which all were 0%, but showed a varying effect (p = 0 .286, silver birch ;p = 0.042, hairy birch) on the VCP(O)-values, which all were below 5% (notshown in Fig . 11) .

Discussion

Mechanical soil preparation versus no preparation

Mechanical soil preparation has proved to be a prerequisite for obtaining anabundant and uniform seedling emergence in these experiments . The SEP-values and PZS-values observed for no preparation in this study were closeto 0% and 100%, respectively (Fig . 4, 6, 9 and 10). These SEP-values andPZS-values may of course be somewhat underestimated and overestimatedsince small seedlings are hard to find in dense vegetation. Successful seedlingestablishment on wet and moist untreated ground has been reported by Sarvas(1948) and Karlsson (1994) . However, seed germination and seedling estab-lishment has been reported to be at its highest on disturbed ground (Miles,1973 ; Raulo and Malkonen, 1976; Perala and Alm, 1989; Holm, 1994; Karls-son, 1994) .

Seedbeds encouraging seedling emergence

In Exp. 1 and 2, in the north of Sweden, the SEP-values obtained after mechan-ical soil preparation are about the same as those reported by Palo (1986) forforest sites, i .e. up to 30% of the germinable seeds produce seedlings . Anexception to this is removal of top-soil, which showed SEP(O)-values up to47% (Fig. 6). Bare mineral soil, a moist site and a sandy soil texture areobviously good qualities for a seedbed . Ordinary ploughing (Exp . 1) anddeep ploughing (Exp . 1 and 2) also showed good results (Fig . 4 and 6). TheSEP-values obtained with mechanical soil preparation in Exp . 3 and 4a, in thesouth of Sweden, were much lower than the corresponding values in Exp . 1and 2. In addition, there were small differences between the mechanical soilpreparation methods (Fig . 9 and 10) . These results might be indications ofthe impact of site and soil conditions . The SEP(O)-values may be somewhatoverestimated, especially in Exp . 3 and 4 in the south of Sweden, since theremight have been some autumn germination of seeds dispersed in the sameautumn (cf. Miles and Kinnaird, 1979) . On sandy soil, higher SEP-valueswere obtained with mainly bare mineral soil in the seedbed surface than withtop-soil in the surface, while the corresponding values on silty soils proved tobe of equal merit. Creating seedbeds of bare mineral soil has its drawbacks,e.g. the risk of frost heaving (Ledgard, 1976) and splash and flooding effects

00

00

10

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4

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a) SEP-SILVER BIRCH

b) SEP-HAIRY BIRCH

AA

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/j B C

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S6

S1Soil preparation method

S1

e) VCP-SILVER BIRCH100-

A

A

75

50

25

A

A

C) PZS-SILVER BIRCH

d) PZS-HAIRY BIRCH

B

B

,

V772jS6

S1

S3

Soil preparation method

Soil preparation method

Fig. 10. Seedling emergence percentages (SEP) of silver birch (Betula pendula) and hairybirch (Betula pubescens), percentages of zero spots (PZS) and vegetation cover percentages(VCP) for different soil preparation methods (S1, no preparation ; S3, rotary cultivation ; S6,removal of top-soil) in Exp . 4a. White bars show the June inventory and striped bars showthe October inventory. Different capital letters above bars from the same inventory indicatedifferences (p < 0 .05) according to Tukey's studentized range test.

0 VCP-HAIRY BIRCHA

B

1 1 5

1 1 6

30

25

20

15-

0 10-

0050

25

30

5

a) SEP-SILVER BIRCH

A

B

j

A

S5

S6

S5

S6

Soil preparation method

100-

c) VCP-SILVER BIRCH

d) VCP -HAIRY BIRCH

75 -

A

A

B

A

A

B

S5

S6

S5

S6

Soil preparation method

F,

b SEP-HAIRY BIRCH

B

j B

e SEP-SILVER BIRCH

f) SEP-HAIRY BIRCH A

T1

T2

T1

T2

Seed-sowing time

Fig. 11 . Seedling emergence percentages (SEP) of silver birch (Betula pendula) and hairybirch (Betula pubescens)and vegetation cover percentages (VCP) for different soil preparationmethods (S5, inverted ground ; S6, removal of top-soil) in Exp. 4b . Also SEP-values of silverbirch and hairy birch for different seed-sowing times (TI, November ; T2, April) in Exp . 4care shown . White bars show the June inventory and striped bars show the October inventory .Different capital letters above bars from the same inventory indicate differences (p < 0 .05)according to t-test .

A

due to rainfall (Sharma and Gupta, 1989) . The SEP-values obtained withspring sowing, after removal of top-soil or inverted ground in Exp . 4b, weremany times higher than the SEP-values obtained with autumn sowing in Exp .4a, indicating that seed-sowing time and environmental conditions may alsobe factors to be regarded .

The seedbed created by inverting the ground was similar to the seedbedcreated by deep ploughing, with a more level surface but approximatelythe same qualities. Seedbeds created by these two methods also retainedpatches of top-soil in the surface . There is probably a difference in soilmoisture conditions between a seedbed created by inverting the ground (ordeep ploughing) and a seedbed created by removal of top-soil since theinverted top-soil layer (incl . sods) will probably break the capillarity of thesoil (cf. Pohtila, 1977) while removal of top-soil preserves the capillarityand shortens the distance to the groundwater. Removal of top-soil will mostlikely result in a higher and more constant soil moisture, which probably is anadvantage for germination . Since hairy birch is better adapted to moist sitesthan silver birch (Sarvas, 1948 ; Gimingham, 1984), a high and constant soilmoisture might be of greater importance to hairy birch . This may explain thelow SEP-values obtained for hairy birch on inverted ground (Fig . 11) .

Seedbeds suppressing competing ground vegetation

The lowest VCP-values were obtained on seedbeds created by removal oftop-soil, inverted ground or deep ploughing, while seedbeds created by rotarycultivation or ordinary ploughing were quickly colonized by ground vege-tation, obtaining high VCP-values (Fig . 4, 6, 9, 10 and 11) . It seems likemost of the top-soil, incl . weed seeds and vegetative reproductive organs, hasto be removed or translocated to suppress competing ground vegetation . Ifseedling mortality is defined as the difference between the SEP(J)-values andthe SEP(O)-values, competition for water and light from the ground vegeta-tion is probably the major cause of the high seedling mortality after ordinaryploughing and, especially, rotary cultivation . Vegetation cover will to someextent, as that after deep ploughing or inverting the ground, cause competi-tion and seedling mortality, but may also create some shade and shelter fromthe sun and the rain . Abiotic factors, e .g. mechanical damage caused by rain,are probably primary causes of first year seedling mortality after removal oftop-soil .

Effects of herbicide spraying

In Exp. 2, higher SEP-values were obtained with herbicide spraying thanwith no treatment (Fig . 6). This may be a result of a reduced competition

1 1 7

1 1 8

from weeds . However, in Exp . 3, herbicide spraying showed positive effectson SEP-values only together with no preparation (Fig . 8), and the herbicideeffect on the VCP-values in both Exp . 2 and 3 was positive mainly whenno preparation was done (Fig . 7 and 8) . It is obvious that the herbicidetreatment was of little effect when followed by mechanical soil preparation,which probably released a lot of seeds and vegetative reproductive organs .Two herbicide treatments would probably be more effective, one before soilpreparation killing the rhizomatous weeds and the other afterwards killingthe seed-propagated weeds .

Effects of application of peat litter, wood-ashes and slaked lime

The SEP-values achieved with application of peat litter was higher than theSEP-values achieved with the other additional treatments (Fig . 4). In rainyweather it was obvious to the eye that organic matter in the surface layercushioned the splash effects, leading to less micro-erosion than for a surfacewith bare mineral soil . Some organic material, e.g. humus in the surfacelayer is also considered to be a source of favourable soil moisture conditions(Vaartaja, 1954; Winget and Kozlowski, 1965 ; Houle, 1992) . It is not clearwhy application of wood-ashes and slaked lime resulted in low SEP-values .Some creeping species, e .g. creeping buttercup (Ranunculus repens L.) andwhite clover (Trifolium repens L.), which may be more competitive species,were apparently more abundant, where wood-ashes and slaked lime had beensupplied. Note that these results originate from only one experiment, and onesingle year.

Influence of environmental conditions and seed material

Birch seeds are small (Table 3) and probably very sensitive to environmentalconditions at the time of seedling emergence (Sarvas, 1948 ; Marquis, 1969) .Seedling mortality was visibly higher in e .g. Exp. 1 when precipitation waslow in July (Table 6). Drought and high seedbed temperatures may cause lowgermination and high mortality during the months after germination (Linteau,1948; Vaartaja, 1954 ; Miles and Kinnaird, 1979). Mortality caused by splashand flooding effects was subjectively assessed for all sites, especially on baremineral soil. Seeds and germinants may easily be disturbed by an ordinaryrainfall (cf. Sharma and Gupta, 1989). Where the top-soil was removed, theseedbeds in some periods were flooded, especially during winter at the sitesfor Exp . 3 and 4. At these sites the soil-textures were fine-grained (Table 1),implying slow percolation after heavy rains . It is not clear whether seeds werewashed away during these periods, but it is one possible explanation to the

1 19

low SEP-values and relatively high PZS-values after removal of top-soil andautumn sowing in Exp . 3 and 4a (Fig . 9 and 10) .

The low minimum temperatures in May and June (Table 6) may offeranother explanation of the low SEP-values and the high PZS-values in Exp . 3and 4a. Seeds wintering on the soil become stratified and germinate at lowertemperatures than seeds sown in the spring (Mork, 1951) . Frosts may killsmall seedlings (Linteau, 1948), and germinants are probably very sensitive .Moist silty soils are also susceptible to frost heaving (Penner, 1957) and it islikely that seeds, and perhaps also germinants, have been heaved or coveredduring the freezing-thawing cycles that occurred . Covering is negative, sinceeven a thin covering decreases germination percentage (Mork, 1944, 1951) .Since removal of top-soil, at the above sites, probably created seedbeds withhigh soil moisture and intact soil capillarity, frost heaving was likely to occurespecially after removal of top-soil . At the sites for Exp . 3 and 4 there wereprobably many freezing-thawing cycles during the winter . However, spring-sown seeds have also been exposed to some frost heaving cycles and thesame soil textures, but still show higher SEP-values (Fig . 11). Accordingto Ledgard (1976), spring sowings may be more successful than autumnsowings on sites not covered with snow throughout the winter. It is obviousthat the birches are rather frost hardy, new silver-birch shoots can tolerategrowing season temperatures of -3 °C--5 °C (Christersson et al ., 1982) .Although frosts were frequent in this study they caused only slight visibledamage. Predation on seeds and seedlings was considered a natural influenceon seedling emergence, and was therefore not separately studied .

Birches are prolific seeders (Sarvas, 1952), with great variation in annualseedfall (Koski and Tallqvist, 1978) . The dispersal patterns at the sites ofExp. 2 and 3 (Fig. 5) are in accordance with Heikenheimo's (1944, cited inSarvas, 1948) and Sarvas' (1948) statements that quantity dispersal is usuallylimited to a distance of about twice the height of the seed-producing trees .The high PZS-values in Exp . 3 (Fig. 8 and 9) are to some extent caused bya poor seedfall (Fig . 5). The number of germinable seeds falling on eachrectangular spot was sometimes less than 10 . Seedlots with a high proportionof fresh, ungerminated seeds after a standard test may obtain the highestSEP-values in the field experiments. This may explain the high SEP-valuesobtained with natural seeding, compared to direct seeding, in Exp . 2 (Fig . 6) .In Exp. 4, differences in seed properties between hairy birch and silver birchwere observed (Table 2). The interactions between these seed properties andthe seedbeds may explain the large difference in SEP-values obtained whenthe ground was inverted (Fig . 11) .

It is uncertain whether the seedfall declines with distance in a linear way,which has been assumed when estimating the number of germinable seeds

120

falling on the rectangular spots . Sarvas (1948) and Ford et al . (1983), the latterstudying Betula uber, reported an exponential dispersal profile . However,B jorkbom (1971) reported that the dispersal pattern for Betula papyriferainto a clearcut varied between years . It is also hard to say whether differencesin microtopography and microclimate between different treatments withinthe experimental sites have affected the results . Thus, for example, removalof top-soil created broad "drains" with the seedbed surface at a lower levelthan in the other methods studied . Such differences may influence, e .g. windvelocity, and thus also seedfall as well as the length of time a dispersed seedremains stationary (cf. Johnson and Fryer, 1992) . At site 2, where the numberof seedlings originating from direct seeding was estimated as a difference, afew spots showed negative numbers and some spots showed larger numbersthan the number of seeds that was direct-sown on the spot. However, it isassumed that the mean for the spots is an accurate estimate of the expectedvalue .

Conclusions

From the results achieved and the subsequent discussion it can be concludedthat:

1) mechanical soil preparation is a prerequisite for obtaining an abundantand uniform seedling emergence;

2) seedbeds with top-soil in the surface and seedbeds with mainly bare min-eral soil in the surface were of equal merit regarding seedling emergenceon silty soils ;

3) seedbeds with mainly bare mineral soil in the surface obtained the highestand most uniform seedling emergence on sandy soil ;

4) seedbeds with mainly top-soil in the surface were quickly colonized byground vegetation ;

5) seedling emergence might be promoted by herbicide spraying withglyphosate, but the glyphosate treatment is of little effect when followedby mechanical soil preparation ;

6) application of peat litter to the seedbed surface promoted seedling emer-gence ;

7) soil, ground vegetation, seed material, seed-sowing time and weatherare factors that may exert a great influence on seedling emergence .

Since conditions good for seedling emergence may be less so for growth(Marquis, 1969), the following growing seasons must also be studied beforeevaluating the methods tested and the possibility of establishing birch onabandoned fields using natural regeneration and (or) direct seeding .

Acknowledgements

This work was financed by the Swedish Council for Forestry and AgriculturalResearch and was carried out with moral and practical support from myfriends at the Department of Silviculture and at the experimental station inAsa. I thank Dr. Tord Johansson and two anonymous reviewers for usefulcomments on the manuscript and Gunnel Brannstrom for drawing some ofthe figures. I am also grateful to Prof. Lennart Bondesson and Soren Holmfor statistical advice and to Lill Bengtsson and Jonas Cedergren for checkingthe English .

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