Alternative conifer release treatments affect microclimateand soil nitrogen mineralization

Phillip E. Reynoldsa,*, Naresh V. Thevathasanb, James A. Simpsonb,Andrew M. Gordonb, R.A. Lautenschlagerc, Wayne F. Bellc,

Donald A. Greschb, Donald A. Buckleyb

aNatural Resources Canada, Canadian Forest Service, 1219 Queen St. East,

Sault Ste. Marie, Ont., Canada P6A 5M7bDepartment of Environmental Biology, University of Guelph, Guelph, Ont., Canada N1G 2W1

cOntario Ministry of Natural Resources, Ontario Forest Research Institute, 1235 Queen St. East,

Sault Ste. Marie, Ont., Canada P6A 5N5

Accepted 6 October 1999

Abstract

In 1993, the Fallingsnow Ecosystem Project was initiated in northwestern Ontario to assess the effects of alternative conifer

release practices on ecosystem processes, wildlife populations, and spruce production. Conifer release treatments: two

herbicide (glyphosate and triclopyr), two cutting treatments (brushsaw and Silvana selective mower), and controls were

established on four 30±60 ha clearcut and planted (spruce) blocks. Unharvested controls adjacent to each block constituted a

sixth treatment. Objectives of this study were: (1) to quantify soil nitri®cation rates for the control, glyphosate, brushsaw, and

unharvested forest treatments and (2) to relate these rates to soil temperature and moisture. Weather stations and buried

®berglass/resistance soil cells were established in 1994 to monitor soil temperatures and moisture. During the second

posttreatment growing season (1995), soil samples were collected at 5, 15, and 30 cm depth and incubated in polyethylene

bags at the same depth from which they were collected for 30 days prior to exhumation. The above procedure was repeated for

the months of June, July, August, and September. In the third posttreatment growing season (1996), bags were buried (mid-

June, mid-July, mid-August) at 5 cm only and exhumed 30 days after burial. Higher levels of nitrate (NO3ÿ) were observed for

the glyphosate and brushsaw treatments in August 1995 compared with the control and unharvested forest treatments. Rates

(mg 100 gÿ1 dry soil per day) of ammonium (NH4�) and nitrate production decreased with soil depth and exhibited a

distinctive seasonal trend, decreasing as soil temperatures declined. Ammonium production was signi®cantly correlated with

soil temperature and moisture, increasing with increasing temperature, and decreasing at higher moisture levels. By the third

(1996) posttreatment growing season, no treatment-related differences were observed, and ammonium production was less

correlated with soil temperature than during the second (1995) posttreatment growing season. These results af®rm that

application of glyphosate is the best option evaluated for effective weed control and optimal nutrient release. # 2000 Elsevier

Science B.V. All rights reserved.

Keywords: Fallingsnow Ecosystem project; Soil nitrogen mineralization; Alternative conifer release treatments; Glyphosate; Brushsaw

cutting; Vegetation management; Harvesting; Forestry practices; Ecosystem processes; Microclimate

Forest Ecology and Management 133 (2000) 115±125

* Corresponding author. Tel.: �1-705-949-9461.

E-mail address: preynold@nrcan.gc.ca (P.E. Reynolds)

0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 3 0 2 - 3

1. Introduction

Below- and near-ground microclimate is affected

by the amount of vegetation that is present on a forest

site. Changes in light, temperature, relative humidity

and moisture produced by different conifer release

practices affect plant succession, habitat quality, and

microsite suitability for crop production (Reynolds

et al., 1997). Temporary removal of vegetation

increases solar radiation reaching the forest ¯oor,

results in soil warming, and raises soil moisture levels.

Increased soil moisture (Stanford and Epstein, 1974)

coupled with soil warming (Powers, 1990) produces

conditions favourable for nutrient turnover (N and C

mineralization), and improved soil moisture enhances

seedling nutrient uptake and growth (Radosevich and

Osteryoung, 1987; Nambiar and Sands, 1993). The

present study was initiated (1) to quantify changes in

soil nitrogen mineralization rates resulting from alter-

native release treatments, (2) to assess the duration of

these treatment-related changes, and (3) to relate soil

nitri®cation rates to carbon/nitrogen ratios and soil

temperature and moisture. In addition, this study

documents microclimatic changes associated with

several alternative conifer release treatments in the

boreal mixedwood forest type.

2. Methods

2.1. Research site

This study is a component of the Fallingsnow

Ecosystem Project, located near Thunder Bay,

Ontario, and has been reviewed elsewhere (Lautens-

chlager et al., 1997, 1998). The project was initiated in

1993 and uses a randomized complete block design,

with four 28±52 ha spruce blocks that were cut and

planted 4±7 years before the study began. Each block

contains ®ve postharvest treatment plots including two

herbicides (i.e., glyphosate, Tradename � Vision,

manufactured by Monsanto and triclopyr, Tradena-

me � Release, manufactured by Dow-Elanco), two

cutting alternatives (i.e., manual release with brush-

saws and motor-manual release with a tractor mounted

Silvana Selective cutting head), and a control (no

treatment). Unharvested forests adjacent to each block

constituted a sixth treatment. The various release

treatments were designed to control overstory trembling

aspen (Populus tremuloides Michx.) and other non-

coniferspecies (Belletal.,1997;Thompsonetal.,1997).

2.2. Soil nitrogen mineralization studies

During the second (1995) posttreatment growing

season, a soil nitrogen mineralization study was

initiated on two of the four treatment blocks using

the control, brushsaw, glyphosate, and unharvested

forest treatments. The two blocks chosen for study

differed in topography, elevation, and microclimate

(Reynolds et al., 1997), one being cooler and drier, and

the other warmer and wetter. Four experiments, one

each in June, July, August, and September, were

conducted in 1995. For each experiment, soil samples

were collected at 5, 15, and 30 cm depth and incubated

in 0.025 mm polyethylene bags (Gordon et al., 1987)

at the same depth from which they were collected for

30 days prior to exhumation. Samples were collected

and incubated at three sub-plot locations/treatment

which corresponded to previously existing lysimeter,

seedling, and soil cell locations (Reynolds et al., 1997,

2000a,b; Simpson et al., 1997). Three experiments,

one each in June, July, and August, were conducted in

1996. In 1996, soils were collected and buried at 5 cm

only (four sub-plot locations) and exhumed 30 days

after burial. On each burial date in 1995 and 1996,

samples at each depth and location were split into two

bags ± a pre-sample and a second sample for burial.

Pre-samples, which established N, C, NH4�, and

NO3ÿ levels prior to burial, were placed on dry ice

and kept frozen prior to chemical analysis. Post- or

exhumed samples established ammonium and nitrate

levels after incubation, and were processed the same

as pre-samples.

In the laboratory, 20 g pre- and post-samples were

extracted with 60 ml of 2 N KCl on a shaker at 3500

RPM for 1 h. After agitation, the extract was allowed to

settle prior to ®ltering. Frozen soil solutions were kept

frozen prior to analysis on a Technicon Autoanalyzer II

system. Daily rates of NH4� or NO3

ÿ (mg 100 gÿ1 dry

soil per day) were calculated. Mineralization rates

were determined by subtracting the un-incubated sam-

ple values from the incubated sample values, and

dividing by the number of days (i.e., 30) incubated.

Data for surface (5 cm) samples were examined

using analysis of covariance (ANCOVA) with soil

116 P.E. Reynolds et al. / Forest Ecology and Management 133 (2000) 115±125

temperature as a covariate. Data for samples at 15 or

30 cm depth were analyzed using multiple analysis of

variance (MANOVA) with soil temperature and moist-

ure (resistance cells) as covariates (Snedecor and

Cochran, 1967). Temperatures were means for the

30-day incubation period based upon temperatures

when the bags were buried, when they were exhumed,

and midway through the incubation period. Moisture

levels at times of burial (i.e., bags allow for gas

exchange, but not moisture exchange) were used.

Treatment means were compared by Tukey's Test at

10% level of signi®cance. Statistical analysis and

graphics were performed using CSS Statistica (Stat-

Soft, Tulsa, OK).

2.3. Carbon : nitrogen ratios

Since soil nitrogen mineralization is often inversely

correlated with C : N ratio (Stevenson, 1982; Gordon,

1986; Haynes, 1986; Thevathasan, 1998), total N (%)

and total carbon (TC%) were determined for pre-sam-

ples for major treatments using standard procedures.

2.4. Treatment-related meteorological parameters

Treatment-related meteorological parameters (i.e.,

soil temperatures at 5 and 15 cm depth) were mon-

itored continuously throughout the incubation periods

by eight Li-Cor (Li-Cor, Lincoln, NB) weather sta-

tions (Reynolds et al., 2000b). Weather stations were

equipped to monitor photosynthetically-active radia-

tion (PAR) and air temperatures at 0.25 and 2 m above

the forest ¯oor and soil temperatures at 5 and 15 cm

depth (Reynolds et al., 1997). Li-Cor LI-1000 data-

loggers were programmed to monitor sensors con-

tinuously and to record integrated (PAR), mean,

maximum, and minimum values on a 3 h basis starting

at midnight. Li-Cor quantum and temperature sensors

were used.

Soil moisture and temperatures at 15 and 30 cm

depth were measured at the beginning, mid-point, and

conclusion of each incubation period for each burial

location from buried ®berglass/resistance soil cells

(ELE International, Lake Bluff, IL). In addition, soil

moisture and temperature were read bimonthly until

snowfall each year. The cells were installed on each

block and four treatments at 15 and 30 cm depth (5 per

treatment at each depth). Locations corresponded to

previously existing lysimeter locations, seedling sub-

plot locations, and Li-Cor weather stations (Simpson

et al., 1997; Reynolds et al., 2000a,b). Cell resistance

data were converted to soil moisture data (%) using

calibration curves developed for the Fallingsnow

research site.

Mean values (Li-Cor stations) calculated using

daily mean values for each incubation period were

examined using analysis of variance (ANOVA) pro-

cedures where each block was treated as a replicate

(Steel and Torrie, 1980). Treatment means were com-

pared by Tukey's HSD Test at 10% level of signi®-

cance. Soil cell data for each measurement date or the

means for each incubation period were also analyzed

using a replicated block design. Statistical analysis

and graphics were performed using CSS Statistica

software as described above.

2.5. Correlation of soil mineralization data with

environmental parameters

Soil nitrogen mineralization rates were correlated

with soil temperature and/or moisture by means of

linear/multiple regression. Analysis and graphics were

performed using CSS Statistica software.

3. Results

3.1. Nitrogen mineralization rates

Rates (mg 100 gÿ1 dry soil per day) of NH4� and

NO3ÿ production varied with soil depth and sampling

time (Fig. 1). Production rates were greater in 1996

(Fig. 2) than in 1995; however, treatment-related

differences were restricted to 1995 at the 15 cm depth.

Even these were few, but included higher NH4�

production for the unharvested forest in July and

August than for all other treatments (Fig. 1). Nitrate

production for glyphosate plots was higher in June

than for controls or for brushsaw plots, and also higher

for plots released with glyphosate than for brushsaw or

unharvested forest plots in August (Fig. 1).

3.2. Carbon : nitrogen ratios

No treatment-related differences in ratios were

observed (Fig. 3), but ratios varied with soil depth

P.E. Reynolds et al. / Forest Ecology and Management 133 (2000) 115±125 117

Fig. 1. Daily (mg 100 gÿ1 dry soil per day) rates of ammonium (NH4�) and nitrate (NO3

ÿ) production during the second (1995) posttreatment

growing season after conifer release with brushsaws and glyphosate. Legend: control: solid circle, brushsaw: open circle, Vision: square,

asterisk: unharvested forest. Mean values followed by the same letter are not significantly different at the 10% level according to Tukey's test.

Fig. 2. Daily (mg 100 gÿ1 dry soil per day) rates of ammonium

(NH4�) and nitrate (NO3

ÿ) production during the third (1996)

posttreatment growing season after conifer release with brushsaws

and glyphosate. Legend: control: solid circle, brushsaw: open

circle, Vision: square, asterisk: unharvested forest. No significant

treatment differences were observed.

Fig. 3. Carbon : Nitrogen (C : N) ratios during the second (1995)

posttreatment growing season after conifer release with brushsaws

and glyphosate. Legend: 5 cm depth: dark bar, 15 cm depth:

diagonal bar, 30 cm depth: clear bar. No significant treatment

differences were observed.

118 P.E. Reynolds et al. / Forest Ecology and Management 133 (2000) 115±125

and tended to be somewhat lower for the glyphosate

(Vision) treatment. Ratios generally were above 20 at

15 and 30 cm depth and tended to be below 15 at 5 cm

depth.

3.3. Treatment-related meteorological parameters

Noteworthy variations in microclimate occurred

during the ®rst three posttreatment growing seasons

(1994, 1995, 1996). Cumulative precipitation

(�186.5 mm) was approximately 25% less in 1995

than in 1994 for the period 1 June through 5 September

(Fig. 4). In 1995, no rainfall occurred between 7 June

and 24 June and again between 24 July and 8 August.

The 1996 growing season was wetter than 1994, and

nearly 150% wetter than 1995. More light (PAR)

was measured near the forest ¯oor (0.25 m) for

all treatments in 1995 than in 1994, but decreased

in 1996 (Reynolds et al., 2000b). Daily mean air

temperatures were highest in 1995, but higher in

1996 than in 1994 (Fig. 4). Soil temperatures also

rose in 1995 (Fig. 4).

Treatment-related microclimatic differences were

more pronounced in 1996 than in 1995 (Reynolds

et al., 2000b). In 1996, these included PAR (2 m) and

air temperatures (2 and 0.25 m). The PAR was higher

for the brushsaw and glyphosate treatments than for

the clearcut control treatment in both years. By 1996,

nearly all other differences were restricted to differ-

ences between these two treatments and the unhar-

vested forest (Reynolds et al., 2000b). This trend had

already begun to emerge in 1995 (Table 1). Similarly,

for soil cells, generally most treatment differences

observed in 1995 (Fig. 5, Table 1) or in 1996, involved

the control and both release treatments versus the

unharvested forest treatment. In both years, soil tem-

peratures for the release treatments were higher and

soil moisture levels were lower than for the unhar-

vested forest. In 1995, soil temperatures for the release

treatments were higher (8 and 24 June, 7 and 25 July,

10 and 24 August, and 7 September) and moisture

levels were lower (24 June and 7 July) than for the

unharvested forest (Fig. 5). Control soil temperatures

were higher than for the unharvested forest on 8 and 24

Fig. 4. Yearly changes in cumulative rainfall, air, and soil temperatures for clearcut treatments during the first (1994), second (1995), and third

(1996) posttreatment growing seasons after alternative conifer release treatments applied in 1993. Rainfall is cumulative precipitation (mm).

Other values are means of daily mean values for the period 11 August±12 September. All data are for Block 3. Legend: (A) 1994: black line,

1995: grey line, 1996: dashed line; (B, C, D) control: solid circle, brushsaw: open circle, Vision: square.

P.E. Reynolds et al. / Forest Ecology and Management 133 (2000) 115±125 119

June, 25 July, 10 and 24 August. Brushsaw soil

temperatures were higher than for the control treat-

ment on 24 June and 10 August. Glyphosate and

control soil temperatures did not differ on any mea-

sured date. On 24 June, soil temperatures for the

brushsaw treatment were higher than for the glypho-

sate treatment. On 10 August, soil moisture (30 cm)

was higher for glyphosate plots than for control plots.

3.4. Effects of environmental parameters on soil

nitrogen mineralization rates

In 1995, NH4� production (mg 100 gÿ1 dry soil per

day) for surface soils (5 cm) was correlated

(r � 0.53, P � 0.011, y � ÿ11.68 � 1.2043x) with

duff temperature, and increased as temperature

increased (Fig. 6); in 1996, NH4� production for

surface soils was less correlated (r � 0.35,

P � 0.041, y � ÿ186.2 � 13.472x) with duff tem-

perature (Fig. 6). Unlike surface soils, linear miner-

alization rates tended to decrease with increasing

temperature and moisture for soils at 15 or 30 cm

depth. For example, NH4� production at 30 cm depth

(r � ÿ0.32, P � 0.082, y � 9.3808 ÿ 0.2429x),

decreased as soil moisture increased (Fig. 7).

Optimal production for both NH4� (R � 0.451,

P � 0.065, z � 31.72 ÿ 1.358x ÿ 0.4688y) and

NO3ÿ (R � 0.420, P � 0.097, z � 29.11 ÿ 2.696x �

0.2489y) occurred at approximately 158C and 35%

moisture at 30 cm depth (Figs. 8 and 9), whereas

surface production for NH4� had not peaked at

188C (Fig. 10).

4. Discussion

Three years after alternative conifer release treat-

ments were applied, some treatment-related microcli-

mate differences continue to persist, but have greatly

diminished compared with the ®rst posttreatment

Table 1

Treatment-related differences in microclimate during the second (1995) posttreatment growing season after alternative conifer release

treatments applied in 1993. Temperature data are mean values based upon daily mean observations (Li-Cor stations, soil temperatures at 5 cm)

or means based upon the beginning, mid-point, and end values (soil cells, 15 and 30 cm) for each incubation period. Moisture (%) means are

for the start of each incubation period. Valuesa are based upon a replicated block design

Variable Location Incubation period Control Brushsaw Vision Forest

N 2 2 2 2

Soil temperature (8C) 5 cm June 15.27 15.65 14.79 ±

July 16.06 16.42 17.19 ±

August 15.94 ab 16.59 a 17.16 a 15.03 b

September 9.91 a 10.57 a 10.54 a 10.01 a

15 cm June 11.12 a 12.36 a 11.70 a ±

July 14.04 a 15.50 a 14.54 a 11.73 a

August 14.37 a 15.18 a 14.61 a 12.90 b

September 9.54 ab 9.80 a 9.72 a 9.12 b

30 cm June 10.17 a 11.08 a 10.32 a ±

July 13.16 ab 14.30 a 13.51 ab 10.85 b

August 13.79 a 14.57 a 14.10 a 12.48 b

September 10.06 a 10.11 a 10.15 a 9.69 a

Soil moisture (%) 15 cm June 37.46 a 36.57 a 34.98 a ±

July 18.28 a 18.38 a 15.28 a 22.59 a

August 30.36 a 29.86 a 28.83 a 32.54 a

September 23.80 a 25.05 a 23.52 a 29.92 a

30 cm June 40.16 a 39.86 a 37.85 a ±

July 19.26 a 19.30 a 19.22 a 26.69 a

August 29.74 a 34.13 a 33.00 a 34.72 a

September 22.29 a 23.31 a 26.54 a 33.21 a

a Values are means. Numbers within rows followed by the same letter are not significantly different at the 10% level according to Tukey's

Test.

120 P.E. Reynolds et al. / Forest Ecology and Management 133 (2000) 115±125

growing season (Reynolds et al., 1997). The ®rst

treatment-related differences to disappear were those

associated with RH and soil temperature. Treatment

differences related to light (PAR) remained the stron-

gest. Differences among release treatments for soil

moisture disappeared in 1995 (Reynolds et al., 2000b).

In 1994, soil moisture levels for all release treatments

were greater than for controls (Reynolds et al., 1997).

By 1995, soil moisture levels for release treatments

were generally less than for controls (Fig. 5).

The reduction in differences in microclimatic vari-

ables among alternative release treatments corre-

sponds with a rapid and progressive revegetation of

treated plots. Leaf area index (LAI) of new and

resprouting vegetation was lowest in 1994, but was

highest in 1996 (Reynolds et al., 2000a). This vegeta-

Fig. 5. Seasonal variation in soil temperatures (8C) and moisture (%) at 15 cm depth in 1995 after alternative conifer release treatments

applied in 1993. Temperature differences at 15 cm depth (A) occurred on 8 June (P � 0.006), 24 June (P � 0.024), 7 July (P � 0.078), 25 July

(P � 0.009), 10 August (P � 0.040), 24 August (P � 0.004), and 7 September (P � 0.055). Temperature differences at 30 cm depth (B)

occurred on 8 June (P � 0.014), 24 June (P � (0.002), 7 July (P � 0.072), 25 July (P � 0.060), 10 August (P � 0.003), and 24 August

(P � 0.023). Moisture differences at 15 cm depth (C) occurred on 24 June (P � 0.032) and 7 July (P � 0.036). Moisture differences at 30 cm

depth (D) occurred on 10 August (P � 0.031). Generally, treatment differences were restricted to all release treatments versus the unharvested

forest, except on 24 June and 10 August (B), when brushsaw soil temperatures at 30 cm were higher than control temperatures (P � 0.035 and

0.064, respectively). On 24 June, soil temperatures at 30 cm depth were higher (P � 0.051) for brushsaw plots than for glyphosate (Vision)

plots. Glyphosate and control soil temperatures did not differ on any measured date. On 10 August, soil moisture at 30 cm depth was higher

(P � 0.023) for glyphosate plots than for control plots. Legend: control: solid circle, brushsaw: open circle, Silvana: diamond, Vision: square,

release: triangle, unharvested forest: asterisk.

P.E. Reynolds et al. / Forest Ecology and Management 133 (2000) 115±125 121

tion developed rapidly on release treatments and along

with released seedlings capitalized on increased soil

moisture that resulted from the treatments. Despite a

decrease in LAI in 1995, induced by drought condi-

tions, this new vegetation and enhanced seedling

growth were presumably suf®cient to reduce soil

moisture levels below those observed on controls.

Of the four release treatments, glyphosate caused

the greatest reduction in vegetation cover and resulted

Fig. 6. Seasonal ammonium (NH4�) production for surface soils

(5 cm) versus soils temperatures during the second (1995) and third

(1996) posttreatment growing seasons after alternative conifer

release treatments applied in 1993. Regressions are based upon all

treatments and incubation periods. Predictive equations are (A)

y � ÿ11.681� 1.204x, r� 0.53, P� 0.011 and (B) y�ÿ186.231

� 13.471x, r � 0.35, P � 0.041.

Fig. 7. Seasonal ammonium production (NH4�) for deeper soils

(30 cm) versus moisture levels during the second (1995) posttreat-

ment growing season after alternative conifer release treatments

applied in 1993. Regresssion (y � 9.381 ÿ 0.243x, r � ÿ0.32,

P � 0.082) is based upon all treatments and incubation periods.

Fig. 8. Ammonium (NH4�) production at 30 cm depth versus soil

temperatures and moisture levels during the second (1995)

posttreatment growing season after alternative conifer release

treatments applied in 1993. Regression (z � 31.72 ÿ 1.358x ±

0.4688y, R � 0.45, P � 0.065) is based upon all treatments during

the third incubation period (August).

Fig. 9. Nitrate (NO3ÿ) production at 30 cm depth versus soil

temperatures and moisture levels during the second (1995)

posttreatment growing season after alternative conifer release

treatments applied in 1993. Regression (z � 29.11 ÿ 2.696x

� 0.2489y, R � 0.42, P � 0.097) is based upon all treatments

during the third incubation period (August).

122 P.E. Reynolds et al. / Forest Ecology and Management 133 (2000) 115±125

in successional vegetation and microclimate that is the

most dissimilar to the control. Since woody vegetation

recovery in the glyphosate treatments is not expected

to occur (Bell et al., 1997; Reynolds et al., 2000b),

treatment differences will persist for an indeterminate

amount of time.

The various microclimatic changes produced con-

ditions favourable for increased soil nitrogen miner-

alization, and released white spruce [Picea glauca

(Moench) Voss] seedlings exhibited higher photosyn-

thetic rates and growth than controls (Reynolds et al.,

2000a). Both conifer release treatments increased soil

nitri®cation rates, but only during the second (1995)

posttreatment growing season. Increased soil nitrogen

mineralization was presumably caused by treatment-

related vegetation reductions and ensuing microcli-

matic changes (Reynolds et al., 2000b). Increased

NO3ÿ production rates in 1995 for glyphosate plots

can be compared to those reported by Gordon and Van

Cleve (1983), who conducted similar research in

clearcut areas in interior Alaska (formerly supporting

white spruce) shortly after harvesting. Those authors

reported much higher values on average, although

their study area had recently been harvested. The

values reported here come from a post-harvest situa-

tion almost 10-years-old where successional processes

are heavily at work. Given this key difference between

successional stages of the two sites, the values

reported here are reasonable. Treatment-related or soil

depth differences for C : N ratios probably played a

lesser role since signi®cant differences were not

observed. Nonetheless, C : N ratios tended to be lower

for the glyphosate (Vision) treatment and near the soil

surface, and may have favoured increased mineraliza-

tion for this treatment and at shallower soil depths

(Stevenson, 1982; Gordon, 1986; Haynes, 1986;

Brady, 1990; Thevathasan, 1998). Conversely, C : N

ratios which tended to be higher with greater soil depth

or for treatments with greater vegetation (i.e., all

except glyphosate) may have resulted in more immo-

bilization than mineralization of soil nitrogen (Brady,

1990).

Vegetation reductions led to increased soil moisture

and solar radiation reaching the forest ¯oor, which

warmed the soil. Vegetation reductions were greatest

on the glyphosate treatment, where succession to

nonwoody vegetation occurred. The glyphosate treat-

ment exhibited the highest daily rates of mineraliza-

tion, differing from the control and brushsaw

treatments on two occasions. Fewer signi®cant treat-

ment-induced mineralization increases were observed

than expected two and three growing seasons after

treatment, suggesting that changes probably occurred

during the ®rst (1994) posttreatment growing season,

and/or higher rates might have also been observed in

May prior to leaf-out. By the second growing season,

posttreatment vegetation recovery was well underway,

and recovery increased even more in 1996. Daily rates

for NH4� production for surface soils may have also

been restricted by moisture levels in 1995 because of

prevailing drought conditions (Fig. 4). Powers (1990)

reported that nitri®cation rates for surface soils were

decreased along an elevational gradient as soil moist-

ure or temperature declined. This decrease in NH4�

production for surface soils in 1996 was heralded by a

lower correlation with temperature than that observed

in 1995. This lower correlation was probably related to

greater amounts of shading vegetation. In this study,

distinctive differences observed in optimal tempera-

tures for mineralization for surface versus deeper

soils, suggests that differing species of microorgan-

isms may be involved in the breakdown of nitrogen

throughout the soil pro®le.

Fig. 10. Ammonium (NH4�) production at 5 cm depth versus soil

temperatures and moisture levels during the second (1995)

posttreatment growing season after alternative conifer release

treatments applied in 1993. Regression (z � ÿ9.759 � 1.234x

ÿ 0.08336y, R � 0.54, P � 0.040) is based upon all treatments

and all incubation periods.

P.E. Reynolds et al. / Forest Ecology and Management 133 (2000) 115±125 123

Increased soil nitrates likely bene®ted both spruce

seedlings and early successional vegetation (Reynolds

et al., 2000a,b). Increased availability of nitrates

probably contributed signi®cantly to the germination

of new species (e.g., herbs and grasses) derived from

seedbanks and also contributed to the growth of other

species (e.g., other grasses and ferns) released by the

treatments. The greatest change in species composi-

tion, accompanied by the greatest increase in nitri®ca-

tion rates, occurred for the glyphosate treatment where

the pre-treatment woody plant community was shifted

to a posttreatment plant community dominated by

herbs and grasses. Ferns and grasses released by

cutting treatments and chemical release with triclopyr

increased in cover through 1996. White spruce seed-

lings, released by the treatments, were characterized

by both enhanced photosynthesis and growth (Rey-

nolds et al., 2000a). Although these seedling gains

were also due in part to increased light and moisture

availability, increased soil moisture, coupled with

improved nitrogen turnover, probably improved seed-

ing nutrition (Nambiar and Sands, 1993). Seedling

gains after conifer release were greatest for herbicide

treatments, especially glyphosate.

5. Conclusions

After the ®rst three posttreatment growing seasons

1. All conifer release treatments signi®cantly in-

creased available light (PAR), air and soil

temperatures, and soil moisture, and lowered

relative humidity during the ®rst posttreatment

growing season, but these changes were greatest

and lasted longest after herbicide release.

2. All conifer release treatments produced microcli-

matic conditions favourable for enhanced seedling

growth.

3. Except for PAR, microclimatic changes induced

by conifer release were short-lived, generally

fading or disappearing after the first posttreatment

growing season, and as progressive revegetation of

treated plots occurred.

4. Microclimatic differences between release treat-

ments and controls were greatest for the glypho-

sate treatment where succession to nonwoody

vegetation had occurred.

5. By 1996, most treatment-related differences in

microclimate were associated with the glyphosate

and the unharvested forest treatments.

6. Compared with controls, both brushsaw cutting

and herbicide release with glyphosate resulted in

posttreatment increases in soil mineralization

rates, but increases were greatest after release

with glyphosate.

7. Increases in soil nitrification rates during the 1995

growing season likely affected early plant succes-

sion trends and improved seedling physiological

and growth performance, especially following

glyphosate treatment, where changes in species

composition and seedling growth were greatest.

8. Soil nitrification increases were short-lived, with

no significant treatment-related differences ob-

served after two growing seasons, probably

because of rapid regrowth of vegetation and the

gradual loss of favourable posttreatment increases

in soil temperature and moisture.

Acknowledgements

Funding for this research was received from Ontar-

io's Vegetation Management Alternatives Program

(VMAP) via the Agriculture Research Institute of

Ontario (ARIO), the federal Government of Canada

Green Plan Forestry Practices initiative, the Canadian

Forest Service, and the University of Guelph. We

thank Bob Lailey for technical computer support

and for writing the macros needed to process the

meteorological data. Finally, we thank Drs. Bob

Wagner, Paul Addison, and Bill Meades for their

continued support for this project.

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