Comparison of ground disturbance of frozen peatland during stump harvesting using a stump drill and rake

Figures & data

Figure 1. Measured stumps (left) marked before harvest (photo by Robert Prinz, Luke), and map (right) showing the locations of measured stumps (map by Mikko Nivala, Luke).

Figure 2. The New Holland Kobelco E200SR excavator (left) equipped with an Ellettari stump drill, and the Terosa KK-900 conventional stump rake (right). Photos by Raul Fernandez Lacruz, SLU, and Jaakko Mietinen, Luke, respectively.

Figure 3. The net (left) used for estimating the ground disturbed area placed in position over an area disturbed by the stump rake. The rulers (right) placed over the disturbed area and hole created by the stump drill.

Table 1. Definitions of the work elements (Element) in the time consumption study of the stump drill.

Element	Start point	End point	Priority
Moving machine	Tracks start turning	Tracks stop turning	2
Crane movement	Crane starts to move towards the stump	Drilling starts or crane stops moving	1
Drilling	The head is spinning and touching the stump	Stump extraction starts	1
Stump extraction	The stump head is pulled out from the stump hole and is above ground	Stump is pushed out from the head	1
other	Other time (mostly delays related to unproductive work)		3

Table 2. Summary of ANOVA to examine differences in performance between the stump rake and stump drill.

Variable	ANOVA		Normality test		T-test
	p-Value	R^2 adj	S–W	K–W	p-Value	t-Value
Y0 Ground dist. (ln(m^2))	<0.001	93.5	0.001	<0.001	–	–
Y0 Depth (cm)	0.090	2.7	0.092	–	–	–
Y1 Ground dist. (ln(m^2))	<0.001	93.7	0.678	–	–	–
Y1 Depth (cm)	0.057	3.8	0.472	–	–	–
Y0 vs. Y1 Ground dist. Rake (m^2)	–	–	–	–	0.008	2.8439
Y0 vs. Y1 Depth Rake (cm)	–	–	–	–	<0.001	5.1959
Y0 vs. Y1 Ground dist. Drill (m^2)	–	–	–	–	0.416	−0.8221
Y0 vs. Y1 Depth Drill (cm)	–	–	–	–	<0.001	8.9794
Y0 vs. Y1 Hole Drill (m^2)	–	–	–	–	<0.001	4.9832

Notes: – indicates that the value was not calculated. P-values obtained from the Shapiro–Wilk’s normality test (S–W) of residual distributions and Kruskal–Wallis (K–W) rank sum tests when the residuals were not normally distributed for the response variables and adjusted R² values (R² adj) are shown. The response variables were the area of disturbed ground (Ground dist.) and depth of the holes (Depth) created by harvesting stumps using the stump rake (Rake) and stump drill (Drill), respectively, in the harvest year (Y0) and following year (Y1). T-tests were applied to test the significance of differences between the two years (Y0 vs. Y1) in the response variables. The areas of stump holes created by the stump drill (Hole) in the two years were also compared using a t-test (areas of holes created by the rake were not measured).

Table 3. Areas of disturbed ground and depths of holes created by the stump rake and stump drill, respectively, and sizes of the holes created by the stump drill.

Extraction head	Measuring time	Ground disturbance (m^2)	Hole size (m^2)	Hole depth (cm)
Stump rake	Year 0	9.04 (4.01)	–	36.4 (7.6)
Stump drill	Year 0	0.90 (0.17)	0.31 (0.03)	39.3 (7.1)
Stump rake	Year 1	7.60 (2.62)	–	29.3 (5.5)
Stump drill	Year 1	0.93 (0.20)	0.27 (0.04)	31.9 (6.0)

Notes: – indicates that the value was not measured. Standard deviations indicated in parentheses. Year 0 refers to the harvest year and Year 1 to the following year.

Figure 4. Left: observed time consumption (s/stump) and regression line. Top right: ground disturbance in Year 0 and Year 1 created by the conventional stump rake. Bottom right: ground disturbance in Year 0 and Year 1 created by the stump drill.

Table 4. Least squares regression functions for time consumption in seconds per stump (Time), for the stump drill (Drill), and the ground area disturbed in m²/stump (Area), in the harvest year (Year 0) and the year after harvest (Year 1) for both the stump drill and stump rake (Rake).

Response	Machine	Year	Corr.	Parameter				Modeĺs
				Independent variable	Parameter estimate	Standard error	p-Value	RMSE	Adj R^2 (%)
Time	Drill	–	1.022	Intercept	−2.8705	1.0641	0.0098	0.4407	42.83
				LN(DSH)	1.1456	0.1924	3.65e ^− 07
Area	Rake	0	1.246	Intercept	2.014	9.568e ^− 02	<2e ^− 16	0.3473	9.30
				DSH^5	1.118e^− 13	5.468e^− 14	0.0498
Area	Rake	1		Intercept	7.329371	2.134710	0.00176	2.64	−3.17
				DSH	0.001824	0.008440	0.83035
Area	Drill	0		Intercept	0.3208	2.713e^− 02	1.35e^− 13	0.02789	−2.54
				DSH	−3.878e ^− 05	1.059e ^− 04	0.716
Area	Drill	1		Intercept	2.926e ^− 01	4.260e ^− 02	6.56e^− 08	0.04379	2.25
				DSH	−7.995e ^− 05	1.663e^− 04	0.634

Notes: The root mean square error (RMSE), adjusted coefficient of determination (Adj R²) and correction for logarithmical bias (Corr.) of the functions are also shown. Response and Machine in bold indicate functions with significant p-values for all parameters. Abbreviation: DSH = diameter at stump height.

Figure 5. Estimated time consumption (s) per kg dry substance (DS) when harvesting Scots pine stumps. The DS for harvesting Scots pine stumps in the studies by Athanassiadis et al. (2011), von Hofsten et al. (2012), and Laitila et al. (2019) was calculated according to Hakkila (1972), assuming that 53% of the stump and root system was harvested by the head studied by von Hofsten et al. (2012), Laitila et al. (2019), and the stump drill in our study. The time consumption was assumed to be equal when harvesting Scots pine as when harvesting Norway spruce.

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