trench depth as a critical growth factor during the ......the three general microsites that are...

Trench Depth as a Critical Growth Factor During the Silvicultural Planning of

Mechanical Site Preparation Treatments

Written By:

Samantha O’Donnell

A capstone paper submitted in conformity with the requirements for the

degree of Masters of Forest Conservation

Graduate Department of Forestry

Daniels Faculty of Architecture, Landscape and Design Forestry

University of Toronto

Acknowledgements

I would like to express my gratitude to my internal supervisor, Dr. Ben Kuttner, whose constant

patience, helpfulness and guidance were crucial to bringing this project to completion. I would also like to

thank my external supervisor Josh Sherrill from J.D. Irving who helped me organize and conduct the

study, as well as Peter Davison, Clara Schortemeyer, and Roland Gagnon who assisted me with maps,

data collection, and with some of the analysis.

University of Toronto - O’Donnell 1

Trench Depth as a Critical Growth Factor During the Silvicultural Planning of Mechanical Site Preparation Treatments Samantha O’Donnell Daniels Faculty of Architecture, Landscape, and Design Forestry, University of Toronto, Toronto ON J.D. Irving Ltd., Sussex NB

Abstract: Growth development of planted white spruce (Picea glauca (Moench) Voss) was studied on 6

sites located in northern Nova Scotia and southern New Brunswick that had been prepared by

disc-trenching. The plantations were surveyed in their fourth year of growth. Growth response variables

measured included tree height, root collar diameter, and stem volume index, calculated from those data.

Analysis of Covariates (ANCOVAs) were used to analyse growth indicators in response to trench depth,

measured planted position, and forest floor thickness using site as a categorical variable to eliminate site

level bias. Non-parametric 1-way Analysis of Variates (ANOVAs) were used to analyse the effects of the

categorical planted position (hinge or top of the berm), trencher (2-row or 3-row), and trench type (middle

versus outside trenches) on the growth variables. A non-parametric 1-way ANOVA was also used to look

at the effects of the trencher and trench type on trench depth. Results indicate that both trench depth and

the measured planted position may have a significant effect on tree height, while the forest floor thickness

had a positive relationship with the root collar diameter in both provinces that were sampled. Results from

the analysis also indicated a positive relationship between the trench type and the depth of the trench,

meaning that the middle trenches in a 3-row trenched site are generally significantly shallower than the

outside trenches. This study indicates that trench depth has a positive effect on tree growth variables

meaning generally the deeper the trench, the larger the tree, and that the middle trench in a site prepared

by a 3-row trencher is more shallow than the outside trenches. Based on this study, recommendations

were made to enhance tree growth with the continued use of disc-trenching, with the addition of more

quality control monitoring and further research on the 3-row disc-trencher in regards to its efficiency.

Further research should be done to fill the gap in knowledge that exists surrounding the effects of trench

depth on tree growth.


Table of Contents

1. Introduction……………………………………………………………………..….4

1.1. Critical Growth Factors………………………………………….…...4

1.2. Disc-trenchers……………………………………………………..….5

2. Research Objectives………………………………………………………….…….6

3. Methods…………………………………………………………………….……....6

3.1. Site Descriptions……………………………………………………..………...6

3.1.1. Nova Scotia………………………………………………………….6

3.1.2. New Brunswick………………………………………………………7

3.2. Sampling Design……………………………………………………………….8

3.3. Data Collection………………………………………………………………...8

3.4. Statistical Analysis………………………………………………………..…….9

4. Results……………………………………………………………………………...10

4.1. Effects of trench depth, measured planted position, and forest floor

thickness ……………………………………………………….…………..10

4.1.1. Nova Scotia…………………………………………………..10

4.1.2. New Brunswick……………………………………………....13

4.2. Planting position effects…...…………………………………………...15

4.3. Trencher and trench type effects ..……………………………………...15

5. Discussion…………………………………………………………………………....17

5.1. Effects of trench depth, measured planted position, and forest floor

thickness………...…………………………………………………………...17

5.2. Planting position effects………………………………………………...19

5.2. Trencher and trench type effects………....……………………………...19

6. Conclusion and Practical Implications……………….………………………….…...20

Appendices……………………………………………………………...………………….….21


1. Introduction

Disc trenching is the most common type of mechanical site preparation that is used during forest

management and reforestation processes in Canada (Ersson et al. 2017). Disc trenching has been proven

to be an effective tool aiding in the growth and survival rates of planted seedlings. Mechanical site

preparation is a crucial step in a silvicultural prescription during the regeneration process of a harvested

stand. Studies have indicated that mechanical site preparation can increase the seedling survival rates by

15-20% and can allow for survival rates of 80-90% on planted conifer stands 10 years after the

mechanical site preparation treatment (Sikstrom et al. 2020). It has also been found that mechanical site

preparation can increase tree height 10-15 years after planting by 10-25%. This study done by Sikstrom et

al. (2020), also indicates that the increase in growth rate that is associated with mechanical site

preparation might be temporary but that the height enhancement likely persists.

The objective of this study was to determine whether trench depth should be considered a critical

growth factor during silvicultural planning. Mechanical site preparation is a key aspect of silviculture

planning cycles, with the main objective being to create a favourable environment allowing for better

light, nutrient, and moisture availability for crop tree performance, including successful seedling survival,

and establishment along with rapid growth rates (Sutherland & Foreman, 1995). There are several factors

that must be considered when attempting to promote early growth, including, soil temperature, soil

moisture, hylobius control, forest floor thickness, debris management, nutrition, weed control, spacing

management, drainage, and frost heaving; each of these having either a large or small role in the

successful growth and survival of planted trees. A secondary objective was to be able to provide

recommendations based on the research that has been done that will be able to help guide silvicultural

planning methods through appropriate site preparation choices to enhance seedling survival and growth

rates.

1.1. Critical Growth Factors

There are many critical factors that determine the success of newly established seedlings. Some of

these include soil moisture levels and drainage, soil temperature and frost heaving, soil composition,

nutrient availability, as well as competition and pest management. Mechanical site preparation is used to

help create the ideal microsites with these factors in mind. A central objective is to make suitable, well

spaced growing sites that will increase growth and survival rates among newly planted seedlings, without

causing excessive soil disturbances while being cost effective. Disc trenching can help to facilitate planter

access, control competing vegetation, control pests, create ideal microsites with appropriate soil


temperature, and moisture levels, as well as reduce soil compaction. Disc trenching can help to reduce or

eliminate limiting factors on seedling growth and establishment (Sutherland and Foreman, 1995).

1.2. Disc-trenchers

When pulled through a reforestation area, a 2-row disc-trencher’s discs rotate outward forming

two parallel furrows, while the 3-row trencher has 3 discs, 2 of them flipping the berm the same way and

then the third berm is flipped in the opposite direction. To the outside of each furrow, side cast materials

form a loose berm containing organic matter, mineral soil and slash. Disc trenchers can disturb about

25-50% of the ground surface (Von Der Gönna, 1995). This creates several varying microsites or planting

positions available for the planter. The three general microsites that are created with the disc trencher are

the trench, the hinge, and the berm (depicted in Figure 1), choice of planting site usually depends on

specific site conditions, and the result of the disc trenching (Sikström et al., 2020).

The berm or the top of the trench allows for a raised planting spot composed of mineral soil

above a double humus layer, in which seedlings can be planted. There are certain site conditions, like a

wet site, where the top of the trench is the microsite of choice, or if the berm has a high component of

well-decomposed organic material and the site is reasonably wet, the risk of drought on the berm is

minimal and it decreases the risk of flooding by planting on the top (Von Der Gönna, 1995 and

Sutherland and Foreman, 1995). Planting on the berm is not always the appropriate planting spot, in drier

sites when there is coarse woody debris incorporated into the berm, or when it consists of loose

undecomposed humus, there is a high likelihood of drought. Another downfall of planting on the berm is

that the trees will be slightly more exposed than the trees planted on a lower profile, this can affect the

growth of young seedlings, especially during severe winter conditions (Von Der Gönna, 1995).

The hinge position provides a microsite that is between the berm and the trench, and is often the

planting spot of choice on a disc trenched site. If the seedlings are planted high enough on the hinge, their

root system should be surrounded by organic matter, decreasing the likelihood of frost heaving and

increasing nutrient availability (Sikström et al., 2020).

The final planting spot in a disc-trenched site is the trench; this is not usually the microsite of

choice, except in some cases on drier sites. Although planting in the trench is not common, there are some

advantages including the increased availability of water in areas that are prone to drought, and there is

also less exposure to winter damage (Sutherland and Foreman, 1995).


Table1: Summary table of Disc Trenching Benefits with Varying Site Factors (Sutherland and Foreman, 1995 & Von Der Gönna, 1995)

2. Research Objectives

This study was designed to determine how common indicators of tree growth are affected based

on varying trench depths, as well as to determine what other factors may interfere or confound the results.

Thus we compared several variables and their effects on tree growth indicators including tree height, root

collar diameter (RCD) and the stem volume index (SVI).

In cases where trench depth had a significant influence on growth indicators, further analyses

were conducted to test for significant differences in growth indicators between stands that had a 2-row as

opposed to 3-row trencher treatment, which translates to comparing growth indicators for seedlings

planted on the outside versus the middle trenches. To guard against site-level differences affecting results,

sites, also referred to as stands, were included as categorical variables in the associated models.

3. Methods

3.1. Site Descriptions

3.1.1. Nova Scotia

The data collected for this study were from groups of sites located in Nova Scotia (n=3) and New

Brunswick (n=3), respectively. Sites in Nova Scotia were sampled relatively early in the growing season,

whereas sites in New Brunswick were sampled relatively late due to travel restrictions imposed by the

2020 COVID19 pandemic.

In Nova Scotia, data were collected in May 2020 from three planted white spruce (Picea glauca

(Moench) Voss) stands located on private owned land in northern Nova Scotia, owned and managed by


Soil Temperature

Moisture Regime

Soil Composition

Nutrient Availability

Non-crop Competition

Disc Trenching

-Increases soil temperature

- Provides varying microsites for desired moisture regime - Use for dry sites

- Versatile, can be used with varying soil composition characteristics

- Helps with nutrient availability

- Limits competition alongside a vegetation management plan

J.D. Irving, selected based on specific predetermined criteria. The stands were plantations of white spruce

that were planted in 2016 that had been disc-trenched prior to planting, allowing 4 years of growth. This

age was determined to represent the optimal amount of tree growth, while still being able to clearly see

and measure the trenches. The three stands in Nova Scotia had all had the mechanical site preparation

treatment using the 2-row disc-trencher, since the 3-row trencher was newer equipment and not used as

readily in the smaller scale forestry that takes place in Nova Scotia.

3.1.2. New Brunswick

Data were collected from three stands in southern New Brunswick in August of 2020. Two of

these stands were disc-trenched with a 3-row disc-trencher, while the third stand was disc-trenched with a

2-row disc trencher. All three stands consisted of white spruce planted in 2016. Table 2 summarizes the

key differences between stands. These stands were located on private owned forest land, owned and

managed by J.D. Irving.

Table 2: Descriptions of the sites that were sampled looking at trench depth and its potential effects on tree height and other growth indicators.


Stand Number

Province Disc-Trencher type

Month of Data

Collection

Number of locations

Year Planted

GPS

1 NS 2-row May 10 2016 WS 45° 45’ 22” N 63° 58’ 46” W

2 NS 2-row May 10 2016 WS 45° 32’ 58” N 64° 8’ 20” W

3 NS 2-row May 10 2016 WS 45° 26’ 47” N 64° 30’ 48” W

4 NB 3-row August 10 2016 WS 45° 53’ 7” N 65° 2’ 33” W



3.2. Sampling Design

A total of 600 samples were collected from 6 stands (Table 1). 3 of the stands were located in

northern Nova Scotia, and 3 stands were in southern New Brunswick. The 6 stands were chosen based on

the year that they were planted, the type of mechanical site preparation treatment, as well as the amount of

weed control that had been used. All of the sampled stands were planted in 2016, having 4 years of

growth. Choosing to keep all of the stands the same age eliminated age as a factor. For the purpose of this

study stands were only selected if they had been disc-trenched prior to planting. 3-row disc-trenched was

preferred, however it was more difficult to find in Nova Scotia since it was newer machinery at that time.

Stands were chosen based on how successful weed control had been as well, since heavy competition can

limit the growth rates of the crop trees, and would also make the crop trees and trenches much harder to

find and measure. Within each stand 10 separate locations were chosen, avoiding areas with poor

drainage, heavy competition, or stocking issues since these factors might limit growth rates causing

interference. Ten trees were measured at each of the ten locations within the stand, for a total of 100 trees

per stand (See Appendix A). A GPS coordinate was taken in a central part of each of the different

locations. The stands that were sampled that had the 3-row disc-trencher treatment had 5 trees measured

from a middle trench and 5 trees sampled from an outside trench at each location.

3.3. Data Collection

Digital maps were created that included polygons outlining land ownership, mechanical site

preparation, and the year that the stands had been planted. The maps were uploaded into Avenza, a

navigation and mapping application that allowed for easy navigation to the stands as well as a means to

note and keep track of coordinates at each of the locations within each stand. A GPS coordinate was taken

from a central point of each of the 10 locations per stand.

For each tree there were several measurements that were taken. Tree measurements included tree

height (cm), and root collar diameter (cm), measured using manual calipers. Stem volume index (SVI)

was subsequently calculated according to the following formula, where d = diameter and hV I d h ,S = 2

= height.

To find the depth of the trenches in a consistent manner, a level was used across the top of the

trench and a meter stick was used to measure the height (cm) from the base of the trench to where it

intersected with the level. A similar method was used to determine the height of the base of the tree from

the trench, the difference being that the level was in line with the base of the tree instead of the top of the

trench. The measurement for the forest floor thickness was found by measuring a distance of 20 cm with


the meter stick from the tree off the side of the trench and measuring the sample found in that location

with the meter stick in centimeters. The planted position was determined by comparing the actual planting

spot against the best practice guide for planting disc-trenched land (Figure 1). To determine whether the

stand had been disc-trenched by a 2-row or a 3-row trencher an initial analysis was done to look for the

applicable trenching pattern; with a 2-row trencher the berms will turn away from each other in two rows,

while the 3-row trencher will have two berms facing the same direction and one flipped to the opposing

side in a repetitive pattern. Once disc-trencher type was determined, outside versus inside trenches were

identified for 3-row trencher sites. All trenches in 2-row sites were considered to be outside trenches for

the purposes of our analysis. At each of the 10 locations within a stand that had been 3-row disc-trenched

5 samples were taken from an inside trench and 5 were taken from an outside trench.

Figure 1: shows the possible planting spots when planting a disc-trenched site, best planting practice is

usually planting on the hinge and sometimes the berm (top), rarely the trench unless there are very dry

soil conditions. (Image: Sutherland and Foreman, 1995)

3.4. Statistical Analysis

For the first deliverable we were interested in determining if any of the measured growth

indicators (response variables) were affected by the varying trench depths. Due to unforeseen

circumstances data collection took place in Nova Scotia at the beginning of the growing season while the

data collection in New Brunswick did not take place until the end of the season, which most likely biased

height and root collar diameter growth measurements between the two provinces. To account for the bias

between provinces in regards to the time of sample collection the data for each province was looked at

separately for each model. Key assumptions for parametric tests, for example, normality and homogeneity


of variance were assessed, and were deemed to have been met for tree height and root collar diameter.

Once two outliers in the SVI data were removed from the dataset, it too met the assumptions for

parametric statistics. Since there were more than one continuous independent variable that could explain

the variance in the dependent variable, a multiple linear regression model was used to test for significant

correlations. Regressions were followed by Analyses of Covariance (ANCOVAs) with site as a

categorical variable and trench depth, height to the base of the tree, and forest floor thickness as

covariates, respectively, in pairwise ANCOVAs in order to account for any site-level effects. In other

words ANCOVAs were undertaken to test effects of trench depth, measured planted position, and the

forest floor thickness on the response variables including tree height, root collar diameter (RCD), and the

stem volume index (SVI) while accounting for the potential unknown site factors.

Because planting position is a categorical variable and the underlying data were unbalanced,

non-parametric 1-way Analysis of Variance (ANOVA; also known as Kruskal Wallis tests) were

undertaken to test planting position effects on the three growth variables. Similarly, Kruskal Wallis tests

were used to assess differences in growth response variables attributable to both trencher type (2-row

versus 3-row) and trench type (outside versus middle), respectively. Interaction terms were included in

exploratory ANOVA tests; none were deemed to be significant so they were dropped from the models and

relationships were re-assessed without interaction terms.

4. Results

4.1 Effects of trench depth, measured planted position, and forest floor thickness on the three growth

indicators

4.1.1. Nova Scotia

The ANCOVA tests that were done on the Nova Scotia data set indicated trench depth, and the

measured planted position from the trench shows a highly significant positive relationship with the

measured tree height (see Table 3). The results also show that the forest floor thickness is highly

significant with the root collar diameter, and that trench depth, forest floor thickness, and the measured

planted position all show a significant relationship with the stem volume index calculation. The

ANCOVA model accounted for variations between sites by factoring it into the model, showing that site

had a significant effect on all of the tests that were done as indicated in Table 3. Scatter plots representing

significant results are shown in Figure 2.


Table 3: ANCOVA results on growth variables taking site factors into consideration in Nova Scotia

* Indicates significant relationship


Independent Variable Growth Variable Significance (p-value) Significance of Site Influence

Trench Depth Tree Height <0.0001* Significant

FFT Tree Height 0.3156 Significant

Tree base from trench Tree Height <0.0001* Significant

Trench Depth RCD 0.1264 Significant

FFT RCD <0.0001* Significant

Tree base from trench RCD 0.1135 Significant

Trench Depth SVI 0.0013* Significant

FFT SVI 0.0009* Significant

Tree base from trench SVI 0.0099* Significant

Figure 2: ANCOVA models showing significant relationships with growth variables by site in Nova Scotia. The top two scatterplots show positive relationships between trench depth (cm) and tree height (cm), as well as measured planting position (cm) and tree height (cm), with a line of best fit representing each sampled site in Nova Scotia. The middle two plots, and the bottom left plot all represent a positive relationship between the SVI and measured planting position (cm), FFT (cm) and trench depth (cm), respectively with a line of best fit to represent each site. The bottom right scatterplot shows a positive relationship between the RCD (cm) and the FFT (cm), with a line of best fit indicating each sampled site in Nova Scotia.


4.1.2. New Brunswick

In New Brunswick, trench depth, forest floor thickness, and the measured planted position all

indicated a significant relationship with the tree height (see Table 4). Again the forest floor thickness was

shown to be highly significant for RCD and for SVI through using the ANCOVA model. Site variations

were taken into account through the use of the ANCOVA model allowing them to be factored in; the

results are outlined in Table 4. Scatter plots showing significant results are shown in Figure 3.

Table 4: ANCOVA results for growth variables in New Brunswick taking site factors into consideration

* Indicates a significant relationship


Independent Variable Growth Variable Significance (p-value) Significance of Site Influence

Trench Depth Tree Height <0.0001* Significant

FFT Tree Height 0.0106* Significant

Tree base from trench Tree Height 0.0227* Significant

Trench Depth RCD 0.9767 Significant

FFT RCD <0.0001* Significant

Tree base from trench RCD 0.4495 Significant

Trench Depth SVI 0.0812 Significant

FFT SVI <0.0001* Significant

Tree base from trench SVI 0.8010 Significant

Figure 3: ANCOVA scatter plots showing significant relationships with growth variables by site in New Brunswick. The top two scatterplots represent a positive significant relationship between the FFT (cm), and the RCD (cm) and SVI, respectively, with a line of best fit to represent each of the sampled sites in New Brunswick. The bottom three scatterplots represent a positive correlation between the measured planted position (cm), FFT (cm), and trench depth (cm), respectively on tree height (cm) with a line of best fit representing each of the sites sampled in New Brunswick. The green line of best fit on all of the scatterplots, shows significantly less growth than the other two sites and is explained in the discussion below.


4.2 Planting position effects

Planted position (hinge versus berm) had no significant effect on tree growth indicators in either

province. The results from the non-parametric Kruskal Wallis 1-way ANOVA are described in Table 5

and 6.

Table 5: Kruskal Wallis one way ANOVA results for planted position on growth variables in New Brunswick

Table 6: Kruskal Wallis nonparametric one way ANOVA results for planted position on growth variables in Nova Scotia

4.3 Trencher and trench type effects

High significance was found for all of the growth variables when looking at whether the site had

2-row or 3-row disc-trenching treatment done prior to planting. The p-values are outlined in Table 7 and

shown visually in a series of boxplots in Figure 4. No significance was indicated when looking at the

trench type (middle or outside) and its effects on the growth variables.

Table 7: 2-row versus 3-row trencher effects on growth variables on sites in New Brunswick


Growth Variable Independent Variable Significance (p-value)

Tree Height Planted Position 0.2134

RCD Planted Position 0.4193

SVI Planted Position 0.9598

Growth Variable Independent Variable Significance (p-value)

Tree Height Planted Position 0.8062

RCD Planted Position 0.1536

SVI Planted Position 0.1916

Growth Variable Significance (p-value)

Tree Height <0.0001

RCD <0.0001

SVI <0.0001

Figure 4: Boxplots depicting the results of the non-parametric Kruskal Wallis test, outlining the median, quartile, and range as the middle, box, and whiskers respectively. These plots each represent the trencher type having a significant effect on SVI, RCD (cm), and tree height (cm) respectively.

When looking at the effects that the disc-trencher type (2-row versus 3-row) and the trench type

(middle versus outside) had on the depth of the trenches, the results from the Kruskal Wallis test indicate

that there is significance with the outside trenches generally being deeper, but no significance was found

looking at the type of disc-trencher on the depth of the trenches. The p-values are outlined in Table 8 and

shown in Figure 5.


Table 8: The effects of the disc-trencher and trench type on trench depth in New Brunswick

Figure 5: Boxplots depicting the results of the non-parametric Kruskal Wallis test, outlining the median, quartile, and range as the middle, box, and whiskers respectively. The first plot looks at the effects of the trencher type on trench depth, and the second boxplot signifies deeper trenches in the outside trenches when compared to the middle trenches on sites that were treated with a 3-row trencher.

5. Discussion

5.1. Effects of trench depth, measured planted position, and forest floor thickness on the growth

indicators

The results determined that the effects of trench depth, measured planted position and the forest

floor thickness on the three growth indicators suggest that trench depth and the measured planted position

have a highly significant effect on tree height, while the forest floor thickness had a highly significant

relationship with the root collar diameter in the sites sampled in Nova Scotia. These results indicate that

there is a positive relationship with the depth of the trench as well as the measured planted position on the

height of the tree suggesting that the deeper that the trench is, as well as, the higher that the tree is planted

on the berm result in taller tree growth. This would be influenced by other site factors which are taken


Independent Variable Significance (p-value)

Trencher (2-row/3-row) 0.07094

Trench Type (middle/outside) 0.0003291

into consideration in the chosen ANCOVA model, however the results for the planted positions

significance could possibly vary depending on site conditions including specific soil moisture levels and

temperatures.

The forest floor thickness was found to have a positive significant relationship with the root collar

diameter growth variable in Nova Scotia. This result is likely due to the higher nutrient profile and better

accessibility in the thicker forest floor layers. Evidence that the soil disturbance caused by the

disc-trencher flipping the forest floor over onto itself also suggests that that flip acts as a preservation of

the nutrients in the organic matter layer and doubles up when flipped onto itself leaving more nutrients

available for the seedling upon planting for a longer period of time (Henneb et al., 2019). Trenching is

also found to enhance nutrient mineralization by warming the soil. Studies from Sweden and Finland

found that the mineralization of nitrogen, potassium, and phosphorus was increased for a minimum of

three years after site preparation was done (Sutherland and Foreman 1995).

The third growth variable that was looked at was the stem volume index, which is a calculation

combining the tree height with the root collar diameter as described by the equation above. The results

indicated that trench depth, measured planted position, and the forest floor thickness all had significant

effects on the SVI for the data collected in Nova Scotia. These results are expected since those factors

individually were highly significant on either the tree height or the RCD, and therefore combined are still

significant but slightly less as shown in table 3.

Results for the New Brunswick sites were slightly different. Similar to the results for Nova

Scotia, trench depth had a highly significant positive relationship with tree height. But for New

Brunswick sites, both FFT and the measured planted position were also highly significant predictors of

tree height. Although we controlled for age during site selection, differences in timing of sampling may

have influenced these results. New Brunswick sites were sampled towards the end of the growing season,

whereas Nova Scotia sites were sampled before it began.

The sites sampled in New Brunswick also suggested a highly significant relationship between the

forest floor thickness and the root collar diameter. This again is likely due to ecological reasons including

more accessible organic matter, nutrient availability and preservation (Henneb et al., 2019).

Unlike the results for Nova Scotia, SVI in New Brunswick sites was only significantly influenced

by FFT; measured height to position on slope was not significant. We could not control for differences in

planting practices in site selection, so it may be that site level differences in planting position for NB sites


confounded those results. Site-level effects were significant in all ANCOVA models, which supports this

conclusion.

5.2. Planting position effects

No significance was found for either Nova Scotia or New Brunswick for the effects of the

categorical planted position on any of the growth variables. This is of interest since the last deliverable

determined that the measured planted position was significant with tree height in both Nova Scotia and

New Brunswick, as well as on the SVI in Nova Scotia. This could be a result of the more general terms of

whether the seedling is planted on the top of the berm or the hinge in place of an actual measurement. The

measured planting position also partially coincides with the depth of the trench , since a shallower trench

that had been planted on either the hinge or the top would have a much smaller measurement than a

significantly deeper trench. This further indicates that the depth of the trenches could potentially be more

influential on seedling growth than the best planting practices.

5.3. Trencher and trench type effects

The results for the third deliverable are solely representing the sites sampled in New Brunswick

since the 3-row disc-trencher was not used on any of the sites that were sampled in Nova Scotia. The

expected results for this deliverable were that the 2-row trencher would have higher growth rates due to

the expectation that the middle trench created by a 3-row disc-trencher would generally be more shallow.

The results determined that there was a highly significant relationship between the disc-trencher that was

used (2-row versus 3-row) and all of the growth variables, however it indicated that it was the 3-row

disc-trencher that had the higher growth rates.

Upon further consideration of the site descriptions and locations, it was noted that this specific

stand in New Brunswick that was sampled as having the 2-row disc trenching treatment had been exposed

to a more intensive herbicide regime and had had visible damage to some of the trees within the stand as a

result. This is likely the reason that the 3-row disc-trenching treatment appears to have a much more

positive effect on the growth variables.

There was no significance found when comparing the middle to the outside trenches against any

of the growth variables. This may have also been a result of the previously mentioned herbicide treatment.

The outside trenches are thought to be deeper than the middle trenches generating the expectation that

they should have had a significant effect on the growth variables, including tree height. This would have

been confounded by the fact that stand 6 had all outside trenches since it had a 2-row disc-trencher


treatment, and resulting from the herbicide regime all of these trees were significantly shorter than the

other two stands indicated in the plots generated from the ANCOVA models for New Brunswick in

Figure 3.

This conclusion is backed in the final part of the analysis looking at the effects of the trencher and

the trench type on the depth of the trenches. We determined that there is a significant effect on trench

depth in relation to the trench type. The outside trench was found to generally have deeper trenches than

the middle trenches at a site that had been mechanically site prepared by the 3-row disc-trencher as was

assumed to initiate this study.

6. Conclusion and Practical Implications

Overall, results showed that trench depth has a strong positive influence on tree height, and that

the relationship is highly significant. They also indicated that forest floor thickness is important for the

growth of the root collar diameter, and that the middle trenches of the 3-row disc-trencher are generally

more shallow than the outside trenches. The results from this study indicate that there may need to be a

change in forest management and silvicultural planning regimes. Some of the potential changes that could

be beneficial to forest management include looking at more frequent and widespread use of a

disc-trencher during site preparation and planning, also implementing a more closely monitored quality

control process of the resulting trenches after a treatment has occurred to ensure consistent quality.

Further research, data collection, and analysis need to take place looking at other factors including cost,

availability of operators, and overall efficiency of the 2-row trencher versus the 3-row disc trencher, to

determine if there should be a reduction in their use or perhaps just a recommendation of a double pass by

the 3-row disc-trencher. There has been little research done looking at the topic of trench depth and how it

relates to varying growth factors and therefore there are many gaps in knowledge surrounding this issue.

More research and analysis need to take place to form a solid conclusion of how differences in

mechanical site preparation methods, specifically disc-trenching may affect seedling growth.


Appendices

Appendix A:

Figure 6: Map outlining the 6 sites that were sampled, 3 in northern Nova Scotia (red), and three in

southern New Brunswick (green).


Figure 7: Detailed map of the 3 sites that were sampled in Nova Scotia, showing the ten sampling

locations in each stand.


Figure 8: Detailed map of the 3 sites that were sampled in New Brunswick, showing the ten sampling

locations at each stand.


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trench depth as a critical growth factor during the ......the three general microsites that are...

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