Keeping mixtures of Norway spruce and birch in production forests: insights from survey data

Figures & data

Figure 1. Location of NFI sample plots and the stand survey, framed within the shaded study area in southern Sweden.

Table 1. Stand mean and standard deviation of stem density and standing volume for age categories and stand types (n = 5).

Age class	Stand type	Stems ha^−1	Volume m^3 ha^−1	Site index
20–29	Birch dominated	1427 ± 801	78 ± 16	38 ± 2
	Mixed stands	1504 ± 237	136 ± 45	36 ± 2
	N. spruce dominated	1593 ± 598	146 ± 21	36 ± 1
30–39	Birch dominated	707 ± 208	97 ± 22	34 ± 4
	Mixed stands	1471 ± 851	172 ± 62	34 ± 1
	N. spruce dominated	917 ± 193	182 ± 39	35 ± 1
40–49	Birch dominated	820 ± 398	131 ± 41	35 ± 5
	Mixed stands	893 ± 303	195 ± 30	36 ± 1
	N. spruce dominated	838 ± 385	327 ± 70	36 ± 2
50–59	Birch dominated	796 ± 238	161 ± 61	31 ± 2
	Mixed stands	994 ± 430	263 ± 88	34 ± 2
	N. spruce dominated	691 ± 192	354 ± 56	35 ± 1

Table 2. Summary statistics of the NFI sample plots between revisions. Basal area (m² ha⁻¹) corresponds to the total sample plot basal area and birch proportion is the percentage birch basal area in the sample plot.

Revision	Inventory years	Stand age	Basal area	Birch proportion
	Interval	Mean	Mean, St. dev	Mean, St. dev
1	1983–1987	51	23 ± 10	17 ± 17
2	1988–1992	50	24 ± 10	17 ± 17
3	1993–2002	50	26 ± 12	17 ± 17
4	2003–2007	49	29 ± 13	17 ± 17
5	2008–2012	49	28 ± 15	18 ± 17
6	2013–2017	49	29 ± 17	19 ± 17

Figure 2. Box and whisker plot of the birch proportion in basal area (%) in each sample plot, grouped by stand age at the time of the revision. The number of observations is stated (with n = j) in red above the stand age presented in black, in age classes of 10 years where “20” corresponds to age 16–25, etc.

Figure 3. Box and whisker plot of the ratio of birch vs Norway spruce quadratic mean diameter in each sample plot, grouped by stand age at the time of the revision. The number of observations is stated (with n = j) in red above the stand age presented in black, in age classes of 10 years where “20” corresponds to age 16–25, etc.

Table 3. Statistical characteristics of Equation (1).

Fixed effect variable	Fixed Effect Parameter	Estimate	Std. Error	ρ
Intercept	µ	9.999e-01	2.337e-05	<2e-16
Intercept TSB	µ	−2.550e-04	4.451e-05	<2e-16
DBH	β1	−8.174e-06	1.954e-07	<2e-16
DBH: TSB	β1	5.072e-06	4.745e-07	<2e-16
DBH2	β2	9.821e-09	4.477e-10	<2e-16
DBH2: TS	β2	−7.986e-06	1.121e-09	1.044e-12
G	β2	1.126e-05	4.607e-07	<2e-16
Age	β3	6.792e-05	3.290e-07	<2e-16
Age:TSB	β3	−2.872e-06	2.193e-07	<2e-16
	Random Effect	Std.Dev
	b	0.0002299
	Residuals	Std.Dev
	ϵ	0.0003343

Figure 4. Visualization of the composition and the canopy structure in plots (left panel) and as mean values over stands (right panel). The combination of colour and symbol is representing a unique stand.

Figure 5. Top panel: Sample plot basal area vs the mean basal area for the five sample plots. Each line corresponds to the min and max value from each stand. Lower panel: Coefficient of variance of basal area between sample plots in the same stand. Dark grey and light grey symbols correspond to Norway spruce monoculture and birch stand, respectivley.

Figure 6. The basal area proportion (%) of Norway spruce in sample plots vs the stand mean value. Lines correspond to the stand max and min value. Dark grey and light grey symbols correspond to Norway spruce monoculture and birch stand, respectively.

Data availability

Data from the national forest inventory (NFI) are open source and available from the SLU webpage: https://www.slu.se/en/Collaborative-Centres-and-Projects/the-swedish-national-forest-inventory/. Detailed information regarding exact coordinates of sample plots from both stand survey and NFI is not open source but may be retrieved after written agreements have been made with the original data supplier. The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request (excluding exact location of the plots).