Vegetation control by steam treatment in boreal

forests: a comparison with burning and soil

scarification

G. Norberg, A. Jäderlund, O. Zackrisson, T. Nordfjell, D.A. Wardle,M.-C. Nilsson, and A. Dolling

Abstract: The Vaccinium myrtillus L. – feather moss vegetation community immobilizes nutrients in surface organic layersand suppresses growth of coniferous seedlings in northern boreal forests. On a site dominated by this type of vegetation, anew site preparation technique, involving steam treatment to kill ground vegetation, was tested and compared withconventional site preparation techniques such as soil scarification and burning. Steam treatment was as efficient as burningand soil scarification in reducing competing vegetation. After 4 years, Scots pine (Pinus sylvestris L.) seedlings planted in theburned and scarified treatments had lower growth, needle dry weight, and nitrogen contents compared with seedlings in sitestreated with steam. Soil microflora recovered quickly after steaming, suggesting that steaming does not directly causelong-term soil sterilization. We interpret the superior growth of P. sylvestris seedlings in steamed plots as being due to bothstrongly reduced resource competition and enhanced release of nutrients in the remaining humus. We conclude that steamtreatment has the potential to be an efficient and environmentally acceptable method to reduce the negative influences thatericaceous ground vegetation has on the growth of planted coniferous seedlings.

Résumé: L’association végétale composée de Vaccinium myrtillus L. et d’une mousse hypnacée immobilise les élémentsnutritifs dans les horizons organiques de surface et retarde la croissance des semis de conifères dans les forêts boréalesnordiques. Une nouvelle méthode de préparation de terrain, impliquant un traitement à la vapeur conçu pour tuer la végétationdu parterre, a été mise à l’essai et comparée à des méthodes conventionnelles de préparation de terrain comme le scarifiage etle brûlage sur un site dominé par ce type de végétation. Le traitement à la vapeur a été aussi efficace que le brûlage ou lescarifiage pour réduire la végétation compétitrice. Après 4 ans, la croissance, la masse anhydre des aiguilles et le contenu enazote des semis de Pinus sylvestris L. plantés dans les parcelles brûlées ou scarifiées étaient plus faibles comparativement auxsemis plantés sur les stations traitées à la vapeur. La microflore du sol a récupéré rapidement après le traitement à la vapeur, cequi suggère que ce traitement n’entraîne pas directement une stérilisation à long terme du sol. Les auteurs attribuent lacroissance supérieure des semis de P. sylvestris dans les parcelles traitées à la vapeur à l’effet conjugué d’une forte réductionde la compétition pour les ressources et d’une libération accrue des éléments nutritifs de l’humus résiduel. Ils concluent que letraitement à la vapeur pourrait s’avérer une méthode efficace et acceptable du point de vue environnemental pour réduire leseffets négatifs de la végétation d’éricacées au sol sur la croissance des plants de conifères.[Traduit par la Rédaction]

Introduction

In the northern boreal forests of Fennoscandia, the Vacciniummyrtillus L. – feather moss community is the most commontype of understory vegetation (Cajander 1926; Påhlsson 1994).In late postfire successional stages dominated by Norwayspruce (Picea abies (L.) Karst.) the annual biomass production

of the ground vegetation can exceed that of the tree layer(Havas and Kubin 1983). As most of these natural stands arerelatively open, ericaceous dwarf shrubs and feather mossesnormally form a continuous mat of ground vegetation prior totree harvest. When such forest stands are cut, a large part of thetotal phytomass in the forest understory is left undisturbed.Most of these ground vegetation species appear to be physi-ologically well adapted to increased light levels after cutting,and only minor changes in the composition of the ground vege-tation takes place (Malmström 1949; Sjörs 1989). Grasses suchas Deschampsia flexuosa (L.) Trin. increase somewhat incover after cutting, while ericaceous dwarf shrubs and feathermosses often prevail until a new stand develops.

After harvesting, the high phytomass of the Vaccinium –feather moss community effectively sequesters nutrients re-leased from decomposing roots and slash, with a subsequentincrease in biomass. This enhanced production of the groundvegetation reduces availability of nutrients and light forplanted tree seedlings. Previous investigations have shown thatV. myrtillus reduces seedling growth mainly through effectiveroot resource competition (Jäderlund et al. 1997). A dense

Received April 7, 1997. Accepted October 21, 1997.

G. Norberg,1 A. Jäderlund, O. Zackrisson, M.-C. Nilsson,and A. Dolling. Department of Forest Vegetation Ecology,Faculty of Forestry, Swedish University of AgriculturalSciences, S-901 83 Umeå, Sweden.T. Nordfjell. Department of Operational Efficiency, Faculty ofForestry, Swedish University of Agricultural Sciences, S-90183 Umeå, Sweden.D.A. Wardle. Department of Forest Vegetation Ecology,Faculty of Forestry, Swedish University of AgriculturalSciences, S-901 83 Umeå, Sweden, and Landcare Research,P.O. Box 69, Lincoln 8152, New Zealand.

1 Author to whom all correspondence should be addressed.e-mail: Gisela.Norberg@svek.slu.se

Can. J. For. Res. 27: 2026–2033 (1997)

2026

© 1997 NRC Canada

Can

. J. F

or. R

es. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y SU

NY

AT

ST

ON

Y B

RO

OK

on

11/1

0/14

For

pers

onal

use

onl

y.

cover of undisturbed ground vegetation can also potentiallyreduce soil temperatures during the growing season by causingless solar radiation to reach the soil surface (Margolis andBrand 1990; Weber et al. 1995). Disturbance of ground vege-tation by fire or other external factors is generally a prereq-uisite for natural tree seedling cohorts to establish in borealsites that are dominated by Ericaceae – feather moss commu-nities (Sirén 1955; Titus et al. 1995; Zackrisson et al. 1997a).Vegetation control strategies such as scarification and burningare currently used to reduce the negative effects of competingvegetation on tree seedling growth. However, the large-scaleuse of soil scarification and herbicides for reducing the com-petitive effects of ground vegetation is frequently criticizedbecause of the environmental side effects of such approaches(Gamlin 1988; Wagner 1993; National Board of Forestry1995). Prescribed fire is an alternative to mechanical sitepreparation but cannot be applied in all situations (Weber andTaylor 1992; National Board of Forestry 1995). Therefore,there is clearly a need to find alternative site preparation meth-ods that are environmentally acceptable, maintain site produc-tivity, and result in efficient growth of planted tree seedlings.Vegetation control by steam treatment has been suggested asa management strategy to meet these requirements, but so far,this approach has only been used in tree seeding experimentsin Empetrum hermaphroditum Hagerup dominated sites(Zackrisson et al. 1997b). Steam treatment techniques havealso been used as an alternative method to reduce weeds onrailroad banks (Ascard 1988) and in gardens (Belker 1990).

To develop a more comprehensive understanding of thesteam treatment technique and its suitability as a tool for vege-tation control from both a management and an environmentalperspective, we investigated the relative effects of steam treat-ment in comparison with traditional methods such as burningand scarification on a site dominated by V. myrtillus. This in-cluded determining the consequences of these approaches for(i) ground vegetation cover and (ii) growth and nutrient acqui-sition of planted Scots pine (Pinus sylvestris L.) seedlings aswell as evaluating environmental side effects on (iii) soil tem-perature and (iv) the biomass and activity of the soil microflora.Assessment of energy expenditure required for steaming rela-tive to soil scarification was also done. In addition, we inves-tigated the consequences of decreasing duration of steamtreatment and possible ways to improve the steam treatmenttechnique and reduce energy consumption.

Materials and methods

Study areaThe experiments were conducted in a 50-ha clearcut in northern Swe-den (Skavliden, Arvidsjaur, 65º35′N, 18º38′E, 450 m above sealevel). The site is located within the Northern Boreal Zone (sensu Ahtiet al. 1968). The mean annual precipitation and temperature for theperiod from 1961 to 1990 were 626 mm and –0.7ºC, respectively(data from the Storberget Meteorological Station located 10 kmsoutheast of the study area at 453 m above sea level). The bedrockconsists of archaean granites and gneisses, and soils are mostly com-posed of fine-textured bottom moraines.

Before harvest, the forest on the experimental site represented alate postfire succession dominated by Norway spruce with scatteredindividuals of Scots pine and downy birch (Betula pubescens Ehrh.).The forest floor vegetation was dominated by bilberry (V. myrtillus)and the pleurocarpous feather mosses Pleurozium schreberi (Brid)

Mitt. and Hylocomium splendens (Hedw.) B.S.G. The site is a classi-cal Myrtillus type (Cajander 1949), representing the most commonforest floor community within the boreal zone in Fennoscandia (Sjörs1989; Walter and Breckle 1989; Påhlsson 1994).

The site index H100 (height of the dominant trees at the age of100 years) (Hägglund and Lundmark 1977) of the forest stand was19 m for spruce (S19). Stand basal area was 21.4 m2⋅ha–1 and meanhumus depth was 78 mm. The stand was clear-cut in the winter seasonof 1991–1992 while the ground was frozen and covered in snow, andlogging slash was left in place. Timber and pulpwood with top diame-ters larger than 50 mm were taken out of the forest, meaning thatrelatively little residual wood was left on the ground.

Comparison of steam treatment, burning, and scarificationIn August 1992, 800 plots, each 0.6 × 0.6 m, were established in anarea with homogeneous ground vegetation cover and humus condi-tions in a randomized block design. Forty blocks were set up, eachconsisting of 20 plots, i.e., five replicates of each of the following fourtreatments: untreated vegetation, steamed vegetation, burned vegeta-tion, and soil scarification.

All treatments were imposed at the beginning of August 1992.Steam was applied at 100ºC using a standard steam nozzle on a 20 mlong rubber steam hose connected to a steam boiler with tube coils(BINI 610, O. Malmkvist AB, Alvesta, Sweden), which had a netpower of 50–57 kW. An open aluminum box, 0.6 × 0.6 m and 0.4 mhigh, was placed on the plot to reduce loss of steam laterally and toclearly define the treated area. An amount of steam equivalent to 13 Lof water was evenly sprayed over each plot for 2 min. A propaneburner was used to combust the ground vegetation and litter inside thesame aluminum box used for the steam treatment. The scarificationtreatment was performed by removing the humus layer from the min-eral soil in the entire plot.

In early June 1993 a 1-year-old rooted Scots pine seedling (com-mercial stock, Kilåmon, 67°30′N, 200 m above sea level) was plantedin the center of each plot. The total height and annual growth of eachseedling were recorded every autumn until 1996, and seedling mor-tality was recorded every spring and autumn until 1996. In October1996, the diameter of each seedling at the stem base was recorded(with a slide-caliper) and two randomly selected needles from theleading shoot of each living seedling were collected for subsequentnutrient analysis. Needles from all seedlings were randomly pooledinto 10 samples per treatment. Needles were then oven-dried (70ºC,72 h) and five of the 10 samples were selected at random for nitrogenanalysis (Carlo Erba NA 1500, Biospectron AB, Sweden).

Species composition and percent cover of the ground vegetation(divided into field- and bottom-layer vegetation: Arnborg 1990) werevisually determined in August 1996 for all the plots of three randomlyselected blocks. Species with less than 1% cover in the plots were notrecorded. Humus depth was measured at two positions in the centerof each plot for six randomly chosen blocks. On September 11, 1996,humus and mineral soil samples were randomly taken from each treat-ment for 10 blocks to determine soil basal respiration (BR) and substrate-induced respiration (SIR, a relative measure of microbial activity) asdescribed by Wardle (1993) and based on the approaches by Ander-son and Domsch (1978) and West and Sparling (1986). Briefly, BRwas determined for a 1.5-g (dry weight) subsample, with moisturecontent adjusted to 150% (dry-weight basis). This material wasplaced in a sealed 169-mL container and incubated at 22ºC. BR wasdetermined as the total CO2-C released between 1 and 3 h of incuba-tion, measured using an infrared gas analyzer. Determinations of SIRwere performed in exactly the same way, but with the sample beingamended with 10 000 µg glucose⋅g–1 immediately prior to incubation.The ratio of BR to SIR (BR/SIR) was used as a relative measure ofthe microbial metabolic quotient (Anderson and Domsch 1985),which is a measure of microbial efficiency.

Soil temperatures were measured by placing one temperature log-ger with bead sensor (Tinytalk, Orion Components (UK) Ltd.) in the

Norberg et al. 2027

© 1997 NRC Canada

Can

. J. F

or. R

es. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y SU

NY

AT

ST

ON

Y B

RO

OK

on

11/1

0/14

For

pers

onal

use

onl

y.

center of each of 12 plots (i.e., four treatments in each of three ran-domly selected blocks) on August 15, 1996. Temperature measure-ments were performed at 1-h intervals at 5 cm soil depth untilAugust 4, 1997. Results are presented as means of the three sensorlocations and are averaged on a daily basis.

Duration of steam exposureA separate experiment was established in August 1992 to determinehow the duration of steam exposure affected vegetation recovery.Three blocks were established, each consisting of 15 plots (0.6 ×0.6 m), i.e., three replicates of each of five treatments: untreated andsteam treatment for 15, 30, 60, or 90 s, with steam treatment beingperformed exactly as described above. Vegetation recovery, speciescomposition, and percent cover of ground vegetation present in eachplot were determined annually until 1996.

In August 1996, we conducted another experiment designed toexamine the effects of steam duration on soil temperature and soilmicrobial activity and biomass after steam treatment. Six blocks wereset up, each consisting of six plots (each 0.6 × 0.6 m), i.e., one plot ofeach of the following six treatments: untreated and steam treatmentfor 15, 30, 60, 90, or 120 s, with steaming performed as describedabove. The temperature during steam treatment was measured at twolevels in the middle of each plot: 1 cm below the top of the humuslayer (posA) and 1 cm above top of the mineral soil (posB). Tempera-ture was also measured 5 cm outside each plot 1 cm below the top ofthe humus (posC). Measurements were made at 4-s intervals withbead probes and temperature loggers (Tinytag PT 100 IP-68, GeminiData Loggers (UK) Ltd.) for 1 h. The mean humus depth of the plotswas 72 ± 20 mm and the moisture content of the humus before treat-ment was 176 ± 36% (dry-weight basis). The average soil temperature

prior to treatment was 11°C for all positions. Soil humus sampleswere taken from all the plots and microbial BR and SIR were deter-mined as described above.

Energy consumptionBecause energy expenditure is an important environmental and man-agement consideration, the energy utilized during vegetation controlby steaming was evaluated and compared with forest soil scarifica-tion, with regard to both net and gross energy consumption.

The net energy consumption, or the energy required for the actualsteaming procedure, was calculated assuming a pressure of500–600 kPa, a water temperature of 124 ± 6°C, and a water flow of0.11 kg⋅s–1 in the steam boiler. The temperature of water entering theboiler was 10°C. The water pump in the boiler was run on a powersupply of 65 W. We then compared energy consumption for the steamtechnique with that of conventional forest soil scarification. The com-parison was performed on the basis of 2000 evenly distributed plant-ing plots per hectare.

The gross energy consumption for steaming, which also includesall the energy needed for transportation to and on the site and powerutilization by the equipment, was calculated from the amount of dieselconsumed, assuming a diesel energy content of 35 900 kJ⋅L–1

(Anonymous 1987). This was compared with the data from Hallon-borg and Landström (1993) who calculated the gross energy con-sumption for six different combinations of forwarder and scarifier tobe in the range 550 000 – 860 000 kJ⋅ha–1 (assuming that 10% ofconsumption is required for extra driving to avoid obstacles and forturning around).

For the steam treatment the following assumptions were made:(1) The maximum load of water on the vehicle is 10 000 L.(2) To treat 1 ha (2000 plots), 3600 L of water was required for a

treatment of 15 s/plot, 7200 L for 30 s, and 14 400 L for 60 s. Foreach treatment, 10% was added to cover time and energy con-sumption to warm up the steam boiler.

(3) The degree of efficiency for the steam boiler is 72.5%.(4) To load water, the vehicle has to drive 500 m to a water supply,

and the diesel consumption for this trip is 5.2 L (Sondell 1979;Hallonborg and Landström 1993). The diesel consumption to fillup the vehicle with water was 0.74 L.

(5) A power amount of 8.7 kW was used to run the water pump inthe boiler.

(6) During steam treatment, the vehicle was driving in the same pat-tern as a forwarder used for forest soil scarification, which meansit travels 2237 m for treatment of 1 ha. On average, the load was5000 L, and the diesel consumption was the average of the twovalues in (4).

Results

Comparison of steam treatment, burning, andscarification

Four growing seasons after planting, steam treatment had thegreatest positive effect on pine seedling height and basal areaincrement (Figs. 1 and 2). Seedlings grown in burned plotswere smaller than those grown in scarified plots but were sig-nificantly larger than seedlings grown in untreated plots. Vege-tation control by steam treatment and soil scarification also hada significant positive effect on current needle mass of pineseedlings (Fig. 2). The nitrogen content in the current-yearneedles of seedlings grown in steam-treated plots was signifi-cantly higher than for seedlings grown in burned and untreatedplots (Fig. 2). Nitrogen concentration in needles did not differsignificantly between treatments (data not shown). Seedlingsurvival (1996) was highest in the scarification treatment(84%) followed by steam treatment (70%), burned treatment

0

50

100

150

200

250

300

350

See

dlin

g he

ight

(mm

)

intact vegetationsteamed plots

burned plots

soil scarification

a a a b

a a bb

ba

b b

c

a

bcb

A.

0

20

40

60

80

100

120

1993 1994 1995 1996

Ann

uals

hoot

heig

ht(m

m)

a a a aa

a a

b

c

a

bab

c

a

b

a

B.

Fig. 1. (A) Total height and (B) annual shoot growth of Scots pineseedlings during the first 4 years after planting in three vegetationcontrol treatments and intact V. myrtillus vegetation. Within eachyear, bars topped with different letters are significantly different atP ≤ 0.05 (Tukey’s test following one-way ANOVA).

Can. J. For. Res. Vol. 27, 19972028

© 1997 NRC Canada

Can

. J. F

or. R

es. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y SU

NY

AT

ST

ON

Y B

RO

OK

on

11/1

0/14

For

pers

onal

use

onl

y.

(69%), and untreated (66%). Most of the mortality occurred inthe first year after planting (data not shown).

The mean humus depth (4 years after treatment) was61 mm in untreated plots, 53 mm in steam-treated plots, and48 mm in burned plots, but the differences between these treat-ments were not statistically significant. In the scarificationtreatment, no humus had developed. Total cover of both thefield- and bottom-layer vegetation was significantly lower intreated plots compared with untreated intact plots (Table 1).However, there were no differences in total field-layer coverbetween the three vegetation control treatments; although theburned plots had more vegetation than the steamed and scari-fied plots, these effects were not significant. In the bottom-layer,recolonization was fastest in the scarification treatment, pri-marily due to establishment of mosses including Polytrichumjuniperinum (Hedw.) and Polytrichum piliferum (Hedw.). Incontrast, P. schreberi and H. splendens dominated in the un-treated plots.

There was significant lower BR and SIR in the scarificationtreatment (because the substrate present represents mainlymineral soil) compared with the other treatments (where theuppermost substrate is humus) (Table 2). Steamed and burnedplots did not differ significantly from each other, but both hadlower SIR rates than the untreated plots. BR did not differamong the burned, steamed, and untreated plots. The BR/SIRratio was reduced in the scarification treatment relative to theothers, suggesting that microbial efficiency was greater in theother treatments.

Soil temperatures were highest in the scarified plots in thespring and summer but were lower than for the other treat-ments in the autumn (Fig. 3). Also, the scarification treatmenthad a greater diurnal range of temperatures than the other treat-ments (data not shown).

Duration of steam exposureIn the first year after treatment, the field-layer vegetation wasstrongly reduced even by the shortest time period of steamexposure (Table 3). The cover of bottom-layer species wasalso strongly reduced, especially for P. schreberi and H. splen-dens. Over time, there was an increasing colonization of pio-neer mosses in the treated plots whereas these mosses werevirtually absent in the untreated plots. The effects of steamtreatment may be long-term. Four years after steam treatmentfor 15 s, the total cover of field-layer species was still reducedto 51% of that in the untreated plots whereas for 90 s of treat-ment, this cover was reduced to 26%.

There were significant effects of steam treatment on bothmicrobial BR and SIR for samples collected 2 weeks aftertreatment (September 1996); increased steam duration reducedboth of these variables (Table 4). However, the BR/SIR ratiowas unaffected by steam treatment. No effects of steam treat-ment on any of the microbial variables were observed for sam-ples collected 6 weeks after treatment (Table 4).

Temperatures exceeding 55°C in the top of the humus layer(posA) continued for several minutes for all treatment times(Table 5). An increased duration of steam treatment resultedin a higher temperature in the humus layer 1 cm above themineral soil (posB), but there was a large variation in maximumtemperatures (Table 5). Five centimetres outside the treated plot(posC) the temperature only increased 3°C ± 3°C, and therewere no differences between treatments (data not shown).

Energy consumption for preparation of steamThe steam process required large amounts of energy, e.g., 15 sof steaming required at least 14 times higher net energy con-sumption than traditional forest soil scarification (Table 6).When all energy-consuming variables were considered, theshortest period of steaming required four to six times moreenergy than traditional mechanical soil scarification.

Discussion

In this study, steaming and soil scarification were the mosteffective methods of controlling vegetation. Temperatures above55–60°C are lethal for the majority of boreal ericaceous spe-cies, grasses, and herbs (Flinn and Pringle 1983; Granströmand Schimmel 1993). During steaming, temperatures in theupper part of the humus layer ranged between 62 and 68°C,which explains the effectiveness of steam as a vegetation control

0

4

8

12

Nee

dle

dry

wei

ght(

mg)

b

a

b

a

A.

0

0.06

0.12

0.18

Nee

dle

N c

onte

nt(m

g)

c

a

bc

ab

B.

0

20

40

60

80

intactvegetation

steamedplots

burned plots soilscarification

Ste

mba

sal a

rea

(mm

)2

d

a

cb

C.

Fig. 2. (A) Needle dry weight, (B) needle nitrogen content, and(C) stem basal area of Scots pine seedlings 4 years after planting inthree vegetation control treatments and in intact V. myrtillus

vegetation. Bars topped with different letters are significantlydifferent at P ≤ 0.05 (Tukey’s test following one-way ANOVA).

Norberg et al. 2029

© 1997 NRC Canada

Can

. J. F

or. R

es. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y SU

NY

AT

ST

ON

Y B

RO

OK

on

11/1

0/14

For

pers

onal

use

onl

y.

method. The less effective vegetation control that we observedin the burned plots may be attributable to the low fire intensityassociated with burning small plots. It is known that rhizomesof Vaccinium species survive if fire intensity and related soiltemperature are low (Schimmel and Granström 1996). After4 years following initiation of the experiment, the untreatedvegetation plots had changed little. The minor increase incover of D. flexuosa and the corresponding decrease in coverof V. myrtillus (data not shown) in the untreated control plotsare a function of a reduced assart effect (the period of increasednutrient availability in the soil following a disturbance resultsin increased site fertility and changes in the rate of growth andin species composition of the plant community while it lasts:Kimmins 1997) that is typical for northern boreal sites aftercutting (Malmström 1949; Eriksson et al. 1979).

Four years after planting, pine seedlings grown in steam-treated plots had greater height and stem basal area comparedwith the other treatments. Needle dry weight and nitrogen con-tent data showed that seedlings in the steaming and scarifica-tion treatments had higher nitrogen contents than those inburned and untreated plots. However, the nitrogen concentra-tions in needles of the seedlings in all treatments(i.e., 1.5–1.6%) were above those levels that are critical for

plant growth (Timmer 1991; Braekke 1994). The improvedseedling growth in steam-treated versus scarified plots waslikely due to the loss of the humus layer in the scarified plots,as the humus layer contains a large part of the easily mineral-ized nutrients. The higher soil temperatures in the scarifiedplots contributed less to improved seedling growth than pre-vious reports have suggested (e.g., Ritari and Lähde 1978; Sut-ton 1993).

The seedling mortality in steamed, burned, and untreatedplots that occurred during the first summer after planting wasmostly caused by pine weevil (Hylobius abietis L.) feeding onthe seedlings and resulted in typical bark injuries. Soil scarifi-cation is well known to reduce seedling mortality by pine weevil(Örlander et al. 1990; Eidmann et al. 1996). However, whencomparing steam treatment and prescribed burning, we suggestthe problems with pine weevil to be of the same magnitude.

Experiments in which the duration of steam treatment wasvaried also provided evidence of strong vegetation control.Steam treatment for 15 s killed vegetation effectively, andeven after 4 years, vegetation cover was still only half of thatin untreated plots. This indicates that the duration of steamtreatment could be shortened to a few seconds and still workeffectively as a vegetation management technique.

We found that the 60- and 120-s steam treatments had sig-nificant effects on microbial properties, but these effects endedwithin 6 weeks. Further, steaming did not affect the BR/SIRratio in either experiment. This suggests that steaming does nothave important sterilization effects in soil and no destabilizingeffects on the soil microflora (Anderson and Domsch 1985).

The superior growth of seedlings in the steamed plots indi-cates that nutrient mineralization rates were maintained at alevel sufficient for adequate plant nutrition. Long-term loss ofsite productivity and nutrient leaching from steam-treatedplots may also be reduced because of the lower degree of dis-turbance of the soil relative to scarification. There has beenconcern that treatments that disturb forest soils, especially soilscarification, increase mineralization and leaching of nutrientsand that this contributes to a long-term loss of site productivity

Intactvegetation Steamed Burned

Soilscarification

Field layerVaccinium myrtillus 30.4a 3.3b 8.3b 3.7b

Vaccinium vitis-idaea 2.9a 1.4bc 2.4ab 0.9c

Deschampsia flexuosa 10.5a 2.3b 4.3b 1.5b

Empetrum hermaphroditum 1.3a 0.1a 0.1a 0.2a

Betula spp. 0b 0b 0b 1.1a

Other species 2.5a 1.9a 1.4a 1.3a

Total 47.5a 9.1b 16.5b 8.7b

Bottom layerPleurozium schreberi 31.7a 0.5b 0.3b 0.9b

Hylocomium splendens 15.7a 0.4b 0b 0.1b

Polytrichum spp. 1.5b 3.4b 1.4b 25.6a

Dicranum spp. 9.9a 0.1b 0.7b 1.5b

Pohlia spp. and Bryum spp. 0b 0.9b 5.9a 2.3b

Total 58.9a 5.3c 8.3c 30.4b

Note: Within a row, numbers followed by different letters are significantly different at P ≤ 0.05(Tukey’s test following Wilk’s Λ multivariate test of significance).

Table 1.Mean vegetation cover (%) in the experimental plots 4 years after treatment.

BR(µg CO2-C⋅g–1⋅h–1)

SIR(µg CO2⋅g–1⋅h–1) BR/SIR

Intact vegetation 19.98a 72.2a 0.288ab

Steamed 18.21a 48.6b 0.367a

Burned 14.16a 52.4b 0.270b

Soil scarification 0.71b 5.1c 0.174c

Note: Data are for mineral soil (soil scarification treatment) or humus(other treatments). Within a column, numbers followed by different lettersare significantly different at P ≤ 0.05 (Tukey’s test following one-wayANOVA).

Table 2.Microbial basal respiration (BR), substrate-inducedrespiration (SIR), and metabolic quotient (BR/SIR) in response tovegetation management 4 years after treatment.

Can. J. For. Res. Vol. 27, 19972030

© 1997 NRC Canada

Can

. J. F

or. R

es. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y SU

NY

AT

ST

ON

Y B

RO

OK

on

11/1

0/14

For

pers

onal

use

onl

y.

(Rosén and Lundmark-Thelin 1986; Johansson 1994; Lundmark-Thelin and Johansson 1997).

Although steam treatment appears to be an effectivemethod for vegetation control, there are several energy-relatedquestions that need to be addressed before this technique canbe recommended for practical use in forestry. Production ofsteam is a very energy-consuming process and the calculationswe performed indicate that 15 s of steam treatment had a grossenergy consumption that was four to six times greater thantraditional soil scarification. However, water heated to just be-

low the boiling point may also kill vegetation (Hansson et al. 1995)which could reduce the energy consumption to a level closerto that of traditional soil scarification. It may also be possibleto reduce the steam quantity and energy consumption by mix-ing hot water with hot air and adding a surfactant foam that wouldinsulate from loss of radiant heat. Insulation material has pre-viously been used in greenhouses to lower energy consumptionduring weed control by steaming (Belker 1990; Labowsky 1990).Further improvements of steam nozzles that evenly distributethe steam over the plots may also reduce energy consumption.

Fig. 3. Mean daily soil temperature 5 cm below ground surface in three vegetation control treatments and in intact V. myrtillus vegetation4 years after initiation of treatments. The horizontal line indicates period with snow cover (November 12 to May 5). During the period betweenDecember 30 and April 15 the mean temperature was –0.1 ± 0.3°C for all treatments. Other short breaks in the measurements were fortechnical reasons.

1993 1996

15 s 30 s 60 s 90 s 15 s 30 s 60 s 90 s

Field layerDeschampsia flexuosa 67.6 57.4 20.6 8.8 54.3 57.0 37.7 13.2Vaccinium myrtillus,Vaccinium vitis-idaea

25.6 11.4 6.6 5.7 57.4 42.6 31.6 25.0

Total 35.5 23.0 11.2 9.6 51.0 49.6 36.8 25.5Bottom layer

Dicranum spp.,Hylocomium splendens,Pleurozium schreberi

7.9 6.7 4.6 4.8 10.7 2.7 0 0

Bryum spp.,Pohlia spp.,Polytrichum spp.

7.7 23.1 61.5 76.9 220 700 1160 2040

Total 7.6 7.6 9.1 10.8 16.5 17.8 25.7 44.3No. of plots 6 6 6 3 6 6 6 3

Table 3.Mean vegetation cover in the steamed plots (% of that in the control plots) for different timesof treatment (15, 30, 60, and 90 s) in 1993 and 1996 (1 and 4 years after treatment, respectively).

Norberg et al. 2031

© 1997 NRC Canada

Can

. J. F

or. R

es. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y SU

NY

AT

ST

ON

Y B

RO

OK

on

11/1

0/14

For

pers

onal

use

onl

y.

Conclusions

We conclude that steam treatment has potential as an effectivealternative to traditional soil scarification and prescribed burn-ing approaches for reducing resource competition from groundvegetation against planted tree seedlings. Although short-termreductions of microbial activity occurred after steam treat-ment, this did not lead to reduction of seedling growth or nu-trient uptake. The duration of steaming time while vegetationcontrol is being performed may potentially be limited to a fewseconds without reducing efficiency of vegetation control; thiscould allow for the development of a continuous driving unitfor steam application. The technique may also be a potentialfuture vegetation management alternative in sites where tradi-tional soil scarification or burning is less appropriate throughpossible risks of nutrient leaching, soil erosion, soil cryotur-bancy, or general public environmental concern. However,

further technical developments to reduce energy and time con-sumption are crucial to make this technique appropriate for usein forestry operations.

Acknowledgments

We thank J. From, P. Sunesson, G. Oleskog, M. Tobiaeson,A. Svedskog, and A. Sundberg for assistance in the field. Thestudy was financially supported by the Swedish Council forForestry and Agricultural Research, the Kempe Foundations,and the SLO Foundation.

References

Ahti, T., Hämet-Ahti, L., and Jalas, J. 1968. Vegetation zones andtheir sections in Northwestern Europe. Ann. Bot. Fenn.5: 169–211.

September 1996 sampling October 1996 sampling

Time of steamtreatment (s)

BR(µg CO2-C⋅g–1⋅h–1)

SIR(µg CO2⋅g–1⋅h–1) BR/SIR

BR(µg CO2-C⋅g–1⋅h–1)

SIR(µg CO2⋅g–1⋅h–1) BR/SIR

0 27.8a 61.1a 0.502a 26.3a 82.3a 0.341a

15 30.8a 56.4ab 0.609a 25.0a 92.6a 0.274a

60 21.8b 47.3bc 0.513a 25.0a 80.7a 0.313a

120 20.1b 40.6c 0.541a 26.3a 86.5a 0.304a

Note: The steam treatment was performed on August 28, 1996. Within a column, numbers followed by different letters are significantlydifferent at P ≤ 0.05 (Tukey’s test following one-way ANOVA).

Table 4.Microbial basal respiration (BR), substrate-induced respiration (SIR), and metabolic quotient (BR/SIR) in humusin response to duration of steam treatment.

Time of steamtreatment (s)

posA

posB: maximumtemperature during

steaming (°C)

Energyconsumption (kJ)

Meantemperature (°C)*

Time exceeding55°C (s) Mean Range

15 62† 285† 24a 14–43 750–86030 64a 444a 29a 16–53 1500–172060 67ab 536ab 31a 25–39 3010–343090 68b 807b 54b 38–70 4510–5150

120 68b 644ab 61b 36–90 6020–6860

Note: Measurements were made at two positions: posA (1 cm below the top of the humus layer) and posB (1 cm above themineral soil). Within a column, numbers followed by different letters are significantly different at P ≤ 0.05 (Student’s t-test).

*Mean temperature during the time that the temperature exceeded 55°C.†Calculated from only three out of the five replicates that reached 55°C during steaming.

Table 5.Humus temperature, time that temperature remained above 55°C, and energy consumption required forsteam treatment.

Gross energy consumption (kJ⋅ha–1)

Time of steamtreatment (s)

Net energyconsumption (kJ⋅ha–1)

Quotient (net energyconsumption)*

Collect and fillwater

Driving onthe site

Productionof steam Total

Quotient (gross energyconsumption)*

15 1.50×106–1.72×106 13.5–50.3 0.08×106 0.42×106 2.75×106 3.24×106 3.77–5.8730 3.01×106–3.43×106 27.1–100.6 0.15×106 0.42×106 5.50×106 6.07×106 7.07–11.0060 6.02×106–6.86×106 54.1–201.3 0.31×106 0.42×106 10.99×106 11.70×106 13.64–21.2090 9.03×106–10.30×106 81.2–302.0 0.42×106 0.42×106 16.49×106 17.33×106 20.20–31.40

120 12.04×106–13.73×106 108.3–402.6 0.61×106 0.42×106 21.98×106 23.01×106 26.82–41.70

*Quotient of energy consumption by steaming and consumption by scarification.

Table 6.Energy consumption for steam treatment and the ratio of energy consumption for steam treatment to the energy consumption forforest soil scarification.

Can. J. For. Res. Vol. 27, 19972032

© 1997 NRC Canada

Can

. J. F

or. R

es. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y SU

NY

AT

ST

ON

Y B

RO

OK

on

11/1

0/14

For

pers

onal

use

onl

y.

Anderson, J.P.E., and Domsch, K.H. 1978. A physiologically activemethod for the quantification of microbial biomass in soil. SoilBiol. Biochem. 10: 215–221.

Anderson, T.-H., and Domsch, K.H. 1985. Determination of eco-physiological maintenance requirements of soil microorganismsin a dormant state. Biol. Fertil. Soils, 1: 81–89.

Anonymous. 1987. Energifakta. Information från Svensk Ener-giförsörjning, Stockholm, Sweden.

Arnborg, T. 1990. Forest types of northern Sweden. Vegetatio, 90: 1–13.Ascard, J. 1988. Thermal weed control. Flaming for weed control and

crop defoliation. [In Swedish with English summary.] Rep. 130.Department of Agricultural Engineering, Swedish University ofAgricultural Sciences, Uppsala, Sweden.

Belker, N. 1990. Umweltfreundliche Bodenentseuchung mit wasser-dampf. Deutscher Gartenbau, 44: 672–677.

Braekke, F.H. 1994. Diagnostiske grensverdier for naeringselementeri gran-og furunåler. Aktuellt fra skogforsk Norsk institutt for skog-forskning, og institutt for skogsfag. Nor. Landbrukshogsk. 15: 1–11.

Cajander, A.K. 1926. The theory of forest types. Acta For. Fenn.29: 1–108.

Cajander, A.K. 1949. Forest types and their significance. Acta For.Fenn. 56: 1–71.

Eidmann, H.H., Nordenhem, H., and Weslien, J. 1996. Physical pro-tection of conifer seedlings against pine weevil feeding. Scand.J. For. Res. 11: 68–75.

Eriksson, L., Kardell, L., and Ingelög, T. 1979. Bilberry, lingonberry,raspberry. Occurrence and production in Sweden 1974–1977. [InSwedish with English summary.] Rep. 16. Section of Environ-mental Forestry, Swedish University of Agricultural Sciences,Uppsala, Sweden.

Flinn, M.A., and Pringle, J.K. 1983. Heat tolerance of rhizomes ofseveral understory species. Can. J. Bot. 61: 452–457.

Gamlin, L. 1988. Sweden’s factory forests. New Sci. 118: 41–47.Granström, A., and Schimmel, J. 1993. Heat effects on seeds and

rhizomes of a selection of boreal forest plants and potential reac-tion to fire. Oecologia, 94: 307–313.

Hägglund, B., and Lundmark, J.-E. 1977. Site index estimations bymeans of site properties. Scots pine and Norway spruce in Swe-den. Stud. For. Suec. 138.

Hallonborg, U., and Landström, M. 1993. Measuring the tractiveforce and power requirement in scarification. [In Swedish withEnglish summary.] Rep. 6. Forestry Research Institute of Sweden,Uppsala.

Hansson, D., Mattsson, B., and Schroeder, H. 1995. Vegetation con-trol on railway embankment — a review of preventive measureson nonchemical methods. [In Swedish with English summary.]Rep. 191. Department of Agricultural Engineering, Swedish Uni-versity of Agricultural Sciences, Alnarp, Sweden.

Havas, P., and Kubin, E. 1983. Structure, growth and organic mattercontent in the vegetation cover of an old spruce forest in NorthernFinland. Ann. Bot. Fenn. 20: 115–149.

Jäderlund, A., Zackrisson, O., Dahlberg, A., and Nilsson, M.-C.1997. Interference of Vaccinium myrtillus on establishment,growth, and nutrition of Picea abies seedlings in a northern borealsite. Can. J. For. Res. 27: 2017–2025.

Johansson, M.-B. 1994. The influence of soil scarification on turn-over rate of slash needles and nutrient release. Scand. J. For. Res.9: 170–179.

Kimmins, J.P. 1997. Forest ecology. Macmillan Publishing Co.,New York.

Labowsky, H.J. 1990. Dämpftechniken im Gewächshaus und imFreiland. Landtechnik, 45: 270–271.

Lundmark-Thelin, A., and Johansson, M.-B. 1997. Influence of me-chanical site preparation on decomposition and nutrient dynamicsof Norway spruce (Picea abies (L.) Karst.) needle litter and slashneedles. For. Ecol. Manag. 96: 101–110.

Malmström, C. 1949. Studien uber Waldtypen und Baumartenvertei-lung in län Västerbotten. Medd. Statens Skogsforskningsinst.37: 1–231.

Margolis, H.A., and Brand, D.G. 1990. An ecophysiological basis forunderstanding plantation establishment. Can. J. For. Res.20: 375–390.

National Board of Forestry. 1995. Aktionsplan för biologisk mång-fald och uthålligt skogsbruk. National Board of Forestry,Jönköping.

Örlander, G., Gemmel, P., and Hunt, J. 1990. Site preparation: aSwedish overview. FRDA Rep. 105, B.C. Ministry of Forests,Research Branch, Victoria, B.C.

Påhlsson, L. 1994. Vegetationstyper i Norden. Nordiska Minister-rådet, Köpenhamn.

Ritari, A., and Lähde, E. 1978. Effects of site preparation on physicalproperties of the soil in a thick humus spruce stand. Comm. Inst.For. Fenn. 92: 1–36.

Rosén, K., and Lundmark-Thelin, A. 1986. Hyggesbruket ochmarkvården. In Skogsfakta. Konferens 9. Skogen som natur ochresurs—mark–flora–fauna. Swedish University of AgriculturalSciences, Uppsala, Sweden. pp. 42–49.

Schimmel, J., and Granström, A. 1996. Fire severity and vegetationresponse in the boreal Swedish forest. Ecology, 77: 1436–1450.

Sirén, G. 1955. The development of spruce forest on raw humus sitesin northern Finland and its ecology. Acta For. Fenn. 62: 1–363.

Sjörs, H. 1989. Vaccinium myrtillus, a plant portrait. [In Swedish withEnglish summary]. Sven. Bot. Tidskr. 86: 411–428.

Sondell, J. 1979. The performance of “Forwarder trials-78” and areport of the speeds and fuel consumption of forwarders operatingon- and off-road. [In Swedish with English summary.] Rep. 8.Logging Research Foundation Sweden, Stockholm.

Sutton, R.F. 1993. Mounding site preparation: a review of Europeanand North American experience. New For. 7: 151–192.

Timmer, V.R. 1991. Interpretation of seedling analysis and visualsymptoms. In Mineral nutrition of conifer seedlings. Edited byR. van den Driessche. CRC Press, Inc., Boca Raton, Fla.pp. 113–134.

Titus, B.D., Sidhu, S.S., and Mallik, A.U. 1995. A summary of somestudies on Kalmia angustifolia L.: a problem species in New-foundland forestry. Can. For. Serv. Nfld. Labr. Reg. Inf. Rep.N-X-296.

Wagner, R.G. 1993. Research directions to advance forest vegetationmanagement in North America. Can. J. For. Res. 23: 2317–2327.

Walter, H., and Breckle, S.W. 1989. Temperate and polar zonobiomesof northern Eurasia. Springer-Verlag, Berlin.

Wardle, D.A. 1993. Changes in the microbial biomass and metabolicquotient during leaf litter succession in some New Zealand forestand scrubland ecosystems. Funct. Ecol. 7: 346–355.

Weber, M.G., and Taylor, S.W. 1992. The use of prescribed fire in themanagement of Canada’s forested lands. For. Chron.68: 324–334.

Weber, M.G., McAlpine, R.S., Wotton, B.M., Donnelly, J.G., andHobbs, M.W. 1995. Prescribed burning and disk trenching effectson early plantation performance in eastern Ontario, Canada. For.Ecol. Manag. 78: 159–171.

West, A.W., and Sparling, G.P. 1986. Modifications to the substrate-induced respiration method to permit measurement of microbialbiomass in soils of different water content. J. Microb. Methods,5: 177–189.

Zackrisson, O., Nilsson, M.-C., Dahlberg, A., and Jäderlund, A.1997a. Interference mechanisms in conifer–Ericaceae–feather-moss communities. Oikos, 78: 209–220.

Zackrisson, O., Norberg, G., Dolling, A., Nilsson, M.-C., and Jäder-lund, A. 1997b. Site preparation by steam treatment: effects onforest vegetation control and establishment, nutrition, and growthof seeded Scots pine. Can. J. For. Res. 27: 315–322.

Norberg et al. 2033

© 1997 NRC Canada

Can

. J. F

or. R

es. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y SU

NY

AT

ST

ON

Y B

RO

OK

on

11/1

0/14

For

pers

onal

use

onl

y.