Silva Fennica

Fig. 1. Top-left and centre: damage by pine shoot beetle results in pine trees deprived of their shoots. Bottom-left: shoots fallen to the ground. Right: two vertical monogamous galleries created by pine shoot beetle females, with larval galleries breaking into adjacent directions. Images by Antti Pouttu and Markus Melin, Natural Resources Institute Finland (CC-BY SA 4.0).

Fig. 2. A common sight in Finland: pile of roundwood (here, Scots pine) harvested during the previous winter is waiting to be transported to the factory. Image by Markus Melin, Natural Resources Institute Finland (CC-BY SA 4.0).

Fig. 3. Location of the roundwood piles used in the study on pine shoot beetle damage in the surrounding forests, separated by the year of field measurements.

Fig. 4. The three categories of bark thickness used in the 2020 field survey when counting the number of beetle exit holes from the logs; the bark of Scots pine is changing from thick to thin when moving from the base of a log (left) into its top (right).

Table 1. Auxiliary information on the roundwood piles used in the study on pine shoot beetle damage in forests surrounding the piles. The coordinates are in the WGS84 system (EPSG: 4326).
Pile	Latitude	Longitude	Year	Volume (m³)	No. of measured field plots (shoot damage)	No. of measured samples (emerged insects)
1	23.9025844	62.3706153	2017	356	9	20
2	24.5415178	62.3168368	2017	331	7	30
3	24.3453392	62.108855	2017	34	7	20
4	27.1107396	61.2459632	2017	42	5	16
5	24.6045835	62.0375501	2017	42	8	16
6	23.3038068	61.8281155	2017	130	6	31
101	30.5765414	63.2180891	2020	145	13	29
102	30.5371486	63.2287643	2020	127	8	29
103	30.5314965	63.22911	2020	59	10	17
104	30.5243809	63.2310969	2020	149	9	32
105	29.9444647	63.6232792	2020	40	9	13
106	29.9426211	63.6244717	2020	39	8	10
107	29.9159022	63.6183939	2020	144	8	33
108	29.5883251	63.60701	2020	125	8	0
109	29.5854189	63.6088174	2020	64	9	0
110	29.5818343	63.6096894	2020	124	8	34
111	25.2972062	62.0822797	2020	173	12	36
112	24.9662172	62.4973184	2020	110	13	28
113	24.3008883	63.1313398	2020	131	9	34
114	25.0644776	63.4503408	2020	297	8	0
115	25.0680649	63.4565652	2020	138	8	27
116	26.1272094	63.2416311	2020	242	9	0
117	26.1974721	62.0754757	2020	55	9	21
118	26.1973003	62.0728962	2020	175	9	42
119	30.3350948	63.1643917	2020	151	13	34

Table 2. Candidate variables in the LME models on pine shoot beetle damage in forests close to roundwood piles and on the number of emerged insects from the roundwood piles. * indicates a variable that was retained in the final model.
Model	Variable	Description
Model 1: Emerged insects	Emerged insects (the response)	Number of pine shoot beetle emergence holes in the sampling area, holes per m².
	Log diameter*	Diameter of the log (in centimetres), measured from the middle of the sampling window.
	Bark thickness*	Thickness of the bark, three categories: thin, medium, thick.
	Bark damage*	Proportion of harvester-damaged bark, estimated from the sampling areas at 10% intervals on a 0–100% scale. In the modelling, the variables were tested also as a factorial variable with five levels (0–20% bark damage, 20–40% bark damage etc.)
	Basal area, pine	Basal area of the pine in the surrounding forests, measured from the field plots, m² ha^–1.
	Site type	Site type of the surrounding forest according to Cajander (1949), estimated from the field plots.
	Damaged pine	Occurrence of pine shoot beetle breeding material in the surroundings (binary): snow or wind damage, logging residues, etc.
Model 2: Fallen shoots	Shoot damage (the response)	Number of fallen shoots in a field plot, shoots per m².
	Distance*	Distance (in meters) between the plot to the roundwood pile.
	Volume*	Volume of the neighbouring roundwood pile.
	Basal area, pine	As described above.
	Site type	As described above.
	Damaged pine	As described above.

Table 3. The LME model on the number of emerged beetles from the forest-stored roundwood piles.
FIXED EFFECTS
Parameter	Value	Std. error	p-value
Intercept	–103.94	73.48	0.158
Bark, medium	–1.80	28.15	0.949
Bark, thick	–117.83	32.77	<0.001
Bark damage, 20–40%	–64.74	37.50	0.0851
Bark damage, 40–60%	–98.41	36.66	0.0076
Bark damage, 60–80%	–159.06	58.26	0.0066
Bark damage, 80–100%	–205.41	69.17	0.0032
Log diameter	23.92	3.41	<0.001
RANDOM EFFECTS
	Std. dev.	Residual
Year	29.4
Roundwood pile	80.79	223.25

Fig. 5. Visualisation of the relationship between the variable BarkDamage (percentage of bark damaged by the harvester) and the number of beetle exit holes counted from the logs of the forest-stored roundwood piles. The shading illustrates the 95% confidence interval and the different colours indicate different survey years.

Table 4. Summary statistics on the number of Tomicus piniperda exit holes from each roundwood pile.
Pile	No. of emergence holes/m²
Pile	Average	Maximum	Std. dev.
1	162	1000	274
2	238	1063	273
3	592	1300	412
4	90	367	100
5	392	900	233
6	121	740	216
101	152	633	181
102	80	533	145
103	137	700	222
104	178	933	254
105	326	900	316
106	337	1100	297
107	226	900	270
110	303	1267	371
111	230	800	221
112	104	1333	254
113	192	900	255
115	264	900	287
117	138	633	183
118	233	1233	323
119	189	733	217
By bark thickness*	Average	Maximum	Std. dev.
Thin bark	69	667	156
Medium bark	220	1233	287
Thick bark	296	1333	275
* Only for piles 101–119, year 2020 measurements

Table 5. The LME model on the number of fallen shoots per plot (the damage) in a study on pine shoot beetle damage in forests close to roundwood piles.
FIXED EFFECTS
Parameter	Value	Std. error	p-value
Intercept	12.76	10.75	0.24
Distance to pile	–0.69	0.11	<0.001
Volume of pile	0.21	0.06	0.003
Residues	15.60	6.10	0.01
RANDOM EFFECTS
	Std. dev.	Residual
Year	4.63
Roundwood pile	14.84	23.3

Fig. 6. Number of fallen shoots per plot at different distances from the roundwood piles. At each boxplot, the black horizontal line indicates the median and the white vertical bar the 25th and 75th quantiles. The dashed horizontal line represents the baseline level of fallen shoots that can be considered “normal” in boreal forests.

Fig. 7. Linear relationship between the distance from roundwood piles of varying volumes and the number of fallen shoots in a plot. For this illustration, the originally continuous variable Volume was classified in the depicted classes. The grey shading illustrates the 95% confidence intervals.

Table 6. Tree-level damage (fallen shoots) caused by pine shoot beetles in relation to the distance from the pile.
Distance from the pile (m)	Fallen shoots per tree
Distance from the pile (m)	Min	Max	Mean	Std. dev.
<10	9	197	47	50
10–20	1	517	63	100
20–30	2	247	40	45
30–40	0	171	23	25
40–50	0	92	23	25
50–60	0	74	19	22
60–70	0	46	18	20
>70	0	100	41	50