The Effect of Season of the Year on the Frequency and Degree of Damage during Commercial Thinning in Black Alder Stands in Poland

Abstract

The aim of this study was to investigate the effect of season of the year on the frequency and degree of damage to residual trees caused during winter and summer timber harvesting operations in young alder stands. Analyses were conducted in pure black alder (Alnus glutinosa Gaertn.) stands aged 38 and 40 years, located in north-eastern Poland. Chainsaw logging was performed in the cut-to-length (CTL) system, while timber was extracted using an agricultural tractor with a trailer with manual timber loading and unloading. Damage caused in the stand as a result of early thinning operations was evaluated in terms of: (1) damage location, (2) wound size, (3) wound depth, and (4) the distance of the damaged tree from the skid trail. Timber harvesting caused damage in 8.3% of trees remaining in the stand. Both the total number of damaged trees (p = 0.001) and the number of trees damaged during felling (p = 0.01) and extraction of timber (p < 0.001) were greater in summer than in winter. Irrespective of the season, two-thirds of all cases of damage were caused during timber extraction, with 67.7% of damage recorded on trunks or root collars and 32.3% on roots. Irrespective of the season and the technological operation, slightly over 50% of cases of damage were small wounds of max. 10 cm². The proportions of medium-sized wounds (11–100 cm²) and large wounds (over 100 cm²) were comparable. The majority of damaged trees (85.1%) were found in the vicinity (<1 m) of the skid trails. The frequency of tree damage near the skid trail was twice as large in summer as in winter (p < 0.001).

1. Introduction

Timber harvesting causes enormous disruption to the natural environment. Apart from tree damage, it also leads to degradation of the undergrowth, ground layer and soil [1,2,3,4,5,6,7,8,9]. In view of the increasing ecological and economic importance of damage occurring in stands during timber harvesting, many researchers have investigated the potential to reduce such damage [10,11,12].

The negative impact of timber harvesting operations on the forest environment was observed relatively early. For example, Meyer et al. [13] reported the adverse effect of various machines used in timber cutting on the forest environment (the stand, undergrowth and soil). Studies concerning such damage have focused primarily on pine stands [7,10,14,15]. The question of tree damage has also been analysed in other stands, both coniferous (e.g., [5,6,16,17,18,19,20]) and broadleaves [21,22,23,24,25]. However, there are no publications presenting the results of similar investigations conducted in alder stands. Only Vasiliauskas [26] included alder in his study; however, it was solely an admixture in a spruce stand. In turn, Tavankar et al. [27] analysed the occlusion of wounds caused by timber harvesting and its effect on the growth of Caucasian alder trees in the Iran Caspian forest.

Black alder (Alnus glutinosa Gaertn.) is one of the forest-forming species in northern and central Europe. Alder stands (black alder and grey alder Alnus incana (L.) Moench.) cover 5.9% of forests in Poland [28]. Alder wood not only has numerous applications, but is also reasonably priced. It is a material used in the manufacture of veneer, plywood and furniture panels as well as wood-based materials. Moreover, it is a valuable and desirable fuel wood species and charcoal material [29,30].

The utilisation of alder stands, which mostly grow on waterlogged sites, is hindered primarily due to the high groundwater table, periodic or permanent flooding of those sites, and the presence of peat soils of low load-bearing capacity. Under such natural conditions, when planning to harvest timber, special attention should be paid to the selection of appropriate technology and organisation of work, as well as to the proper choice of season, which can considerably reduce the damage done to the forest environment.

Studies mainly focus on the influence of various technologies on the occurrence of damage in residual trees [31], followed by the type of harvesting (thinning, final cutting). Very few papers analyse the impact of the season of the year on the formation of tree damage during timber harvesting. The small number of such publications include a study by Limbeck-Lilienau [32] conducted in summer and winter during harvesting of short timber in mountain spruce stands, as well as a paper by Chmielewski and Porter [33] concerning pine stands. In turn, Larson et al. [34] investigated the effect of timber harvesting in different seasons on the growth of epiphytic lichens in boreal forests.

The aim of this study was to investigate the impact of season of the year on the frequency and degree of damage to residual trees during commercial thinning (felling and timber extraction) in young black alder stands. The study is intended to fill a gap in knowledge concerning the conditions and impact of timber harvesting on residual alder stands. The aim was to provide quantitative and qualitative characterisation of such damage and of the distribution of damaged trees in the residual alder stands formed as a result of timber harvesting conducted in winter and in summer.

2. Materials and Methods

2.1. Study Area

Investigations concerning timber harvesting during early thinning in alder stands were conducted in the Płaska Forest District (the Regional Directorate of the State Forests in Białystok, Poland), in the Okop Forest Division, in two neighbouring compartments (101h and 132c). Two pure black alder stands aged 38 and 40 years, originating from planting, were selected. They were stands with moderate canopy closure, growing on a flat alder swamp forest site, on lowmoor peat soil (Figure 1). The mean tree height was 18.0 m in compartment 101h and 17.0 m in compartment 132c; the mean diameter at breast height in both stands was 15 cm [35]. The mean volume of growing stock was 249 m³·ha⁻¹ in compartment 101h and 241 m³·ha⁻¹ in compartment 132c. The studied operation was the first commercial thinning in these stands. The thinning intensity rate amounted to 13.12% of stand volume.

A total of 24 sample plots were selected. Each plot was a square with side 50 m (area 0.25 ha). Experimental plots were numbered according to the order of performed operations, and the boundaries of each were marked on external trees located outside the individual plots. A skid trail approx. 3 m in width was established in each plot running through its centre and along its boundary. Neighbouring skid trails were separated by a distance of 25 m (Figure 2).

2.2. Characteristics of the Timber Harvesting Process

Timber harvesting performed in early thinning operations consisted of tree felling, delimbing and cutting of logs, as well as extraction of timber to a storage yard. The process was executed in the cut-to-length system. Harvested logs were 1.2 and 2.5 m in length. The sample plots were utilised alternately—in half of them (12 plots) operations were performed in winter (January), while in the remainder operations were performed in summer (August) (Figure 1 and Figure 2). In the winter season timber harvesting was performed when the ground was frozen (the air temperature ranged from −17.6 °C to −5.6 °C) and the mean snow cover was approximately 32 cm. Operations in the summer season were conducted in August when the groundwater level had subsided sufficiently to facilitate timber harvesting.

Both tree felling and timber extraction were carried out by two teams of two members, working simultaneously. Felling and cutting of logs were performed by two qualified, experienced lumberjacks and their assistants, who then worked on timber extraction. Timber extraction was carried out using a Zetor 7045 agricultural tractor (65 H.P.) with a trailer, with manual timber loading and unloading (Figure 1). This was the only possible extraction method under such forest site conditions. The distance of wood extraction in both seasons ranged from 270 to 820 m. The entire timber harvesting process (felling and extraction) was performed by the same four workers in both seasons of the study.

2.3. Assessment of Tree Damage

To analyse the damage caused as a result of early thinning operations in alder stands in the summer and winter seasons, the damage to residual trees was assessed after the completion of each stage of the work (tree felling, timber extraction). The type of damage was determined based on visual inspection, and other wound parameters were recorded based on measurements using a tape measure and a Nikon Forestry Pro laser rangefinder.

In the sample plots, the recorded data included the number of all damaged trees (total number of damaged trees) and the number of damaged trees depending on the type of operations (total number of trees wounded during felling and cutting to length/timber extraction) in the season of the year in which operations were performed.

Damage occurring in the stand as a result of early thinning operations was assessed depending on: (1) damage location (roots, root collar or trunk), (2) wound size (max. 10 cm², 11–100 cm², over 100 cm²), (3) wound depth (exposed or damaged phloem, damaged xylem), and (4) distance of the damaged tree from the skid trail (below 1 m, 1–7 m, over 7 m).

2.4. Statistical Analysis

All statistical analyses were performed using the Statistica 10 software package (StatSoft Inc., Tulsa, OK, USA). To characterise individual analysed types of damage, descriptive statistics were applied. The numbers of trees damaged in summer and in winter were compared using Student’s t-test for independent samples. The type and size of wounds and the distance of damaged trees from the skid trail were characterised based on the Mann–Whitney non-parametric U-test.

3. Results

3.1. Number of Damaged Trees

Upon completion of the early thinning operation in the 24 experimental plots a total of 4718 trees were left in the stands, with an average of 196 per plot. 390 trees were damaged, which accounted for 8.3% all trees. The percentages of damaged trees in individual plots in winter ranged from 2.9% to 9.6% (mean 5.8%), while in summer the range was from 3.8% to 21.0% (mean 10.6%). Both the total number of damaged trees (p = 0.001) and the number of trees damaged during felling (p = 0.01) and timber extraction (p < 0.001) were greater in summer than in winter (

Table 1. Number of damaged trees.

Variable	Season	N	%	Mean	Standard Deviation	t-test	p
Total number of damaged trees	Winter	134	5.8	11.17	3.90	4.21	0.001
	Summer	256	10.6	21.33	7.39
Number of damaged trees during felling	Winter	52	38.8	4.33	1.61	2.96	0.010
	Summer	92	36.0	7.67	3.55
Number of damaged trees during extraction	Winter	82	61.2	6.83	2.41	4.67	<0.001
	Summer	164	64.0	13.67	4.46

).

3.2. Type of Operation

Timber extraction caused markedly more damage than felling, with an almost twofold difference in the level of damage (62.6% versus 37.4%). The proportions of damage were independent of the season of the year in which the operations were performed, as both in winter and in summer the number of trees wounded during timber extraction was almost twice as great (61.2% and 64.0%) as during felling (38.8% and 36.0% respectively).

3.3. Location of Damage and Wound Depth

A total of 67.7% of cases of damage were found on the trunk or root collar, while only 32.3% represented root damage (wounds of exposed roots). A similar distribution of damage was recorded in winter and in summer, with a slightly greater incidence of root damage in the summer season (

Table 2. Share of damaged trees with respect to place and depth of damage, and type of forest harvesting operation.

Place of Damage	Winter		Summer		Total
	N	%	N	%	N	%
Trunk and root collar	96	71.6	168	65.6	264	67.7
Roots	38	28.4	88	34.4	126	32.3
Depth of Wound	Winter		Summer		Total
Phloem	107	79.9	199	77.7	306	78.5
Wood	27	20.1	57	22.3	84	21.5
Size of Wound	Winter		Summer		Total
≤10 cm2	72	53.7	139	54.3	211	54.1
11–100 cm2	33	24.6	60	23.4	93	23.8
>100 cm2	29	21.7	57	22.3	86	22.1
Depth of Wound	Felling		Extraction		Total
Phloem	112	28.7	194	49.8	306	78.5
Wood	32	8.2	52	13.3	84	21.5
Size of Wound	Felling		Extraction		Total
≤10 cm2	85	59.0	126	51.2	211	54.1
11–100 cm2	29	20.0	64	26.0	93	23.8
>100 cm2	30	21.0	56	22.8	86	22.1

).

In terms of the viability of the residual stand, wound depth is of greater importance than wound size. In view of the limited role of bark abrasion and superficial bark damage (without phloem exposure) these were not included in the study. In the case of damage to deeper tissues, almost 80% of cases affected the phloem (bark removal and phloem exposure), while only slightly over 20% constituted xylem damage (Table 2). Timber extraction caused 5% more deep wounds damaging the wood fibres and almost twice as many cases of phloem damage as were attributed to felling and the cutting of logs (Table 2).

3.4. Wound Area

Irrespective of the season of the year and the technological operation, slightly over 50% of all damage was accounted for by small wounds, with a surface area of max. 10 cm². The proportions of medium-sized wounds (11–100 cm²) and large wounds (over 100 cm²) were comparable (Table 2). A statistical dependence was shown for timber extraction between the season of the year and the frequency of small wounds of max. 10 cm² (p < 0.01) and large wounds with a surface area over 100 cm² (p < 0.05). Both types of wounds were formed twice as often in summer as in winter (

Table 3. Number of damaged trees in particular wound size categories.

Operation	Size of Wound	Season	Mean	Standard Deviation	Z-test	p
Felling	≤10 cm2	Winter	2.50	1.68	1.69	0.101
		Summer	4.58	3.03
	11–100 cm2	Winter	0.92	1.16	0.81	0.478
		Summer	1.50	1.73
	>100 cm2	Winter	0.92	1.44	1.39	0.198
		Summer	1.58	1.44
Extraction	≤10 cm2	Winter	3.50	1.73	2.97	0.002
		Summer	7.00	3.16
	11–100 cm2	Winter	1.83	1.85	1.76	0.089
		Summer	3.50	2.58
	>100 cm2	Winter	1.50	1.98	2.52	0.012
		Summer	3.17	1.64

).

3.5. Distance of Damaged Trees from the Skid Trail

The largest number of cases of damage (85.1%) was recorded in the vicinity (<1 m) of the skid trails. With an increase in the distance from the skid trail the number of wounded trees decreased rapidly. Only 8.5% of damaged trees were found between 1 and 7 m from the skid trail, and 6.4% at distances over 7 m. A similar distribution of wounding was observed both in summer and in winter, and also for felling and for timber extraction operations.

In the case of felling, the season had no significant effect on the number of damaged trees depending on the distance from the skid trail. In the case of timber extraction a statistically significant difference was found only for the frequency of wounding adjacent to the skid trail, which was twice as great in summer as in winter (p < 0.001) (

Table 4. Number of damaged trees with respect to distance from skid track.

Operation	Distance from Skid Track	Season	Mean	Standard Deviation	Z-test	p
Felling	Near track (<1 m)	Winter	3.00	1.21	1.29	0.219
		Summer	4.33	2.46
	1–7 m	Winter	0.67	0.89	1.93	0.068
		Summer	2.08	1.78
	>7 m	Winter	0.67	0.65	1.05	0.347
		Summer	1.25	1.29
Extraction	Near track (<1 m)	Winter	6.67	2.31	3.34	<0.001
		Summer	13.50	4.34
	1–7 m	Winter	0.00	0.00	1.00	0.755
		Summer	0.08	0.29
	>7 m	Winter	0.17	0.58	0.06	0.977
		Summer	0.08	0.29

).

4. Discussion

Early thinning in alder stands using chainsaw harvesting caused damage to 8.3% of trees. The level of damage exceeds that recorded in coniferous stands. Karaszewski et al. [7] reported 4.7% damaged trees in pine stands caused by early thinning, while Bobik [36] reported a figure of 5.8%. Following an analogous operation performed in lowland spruce stands Bembenek et al. [5] showed that trees with various types of damage accounted for 4.5% of the total, while in mountain spruce stands 4.4% of trees suffered damage during thinning [37]. A comparable level of damage was recorded by Maciak and Popczyński [38] in a study of stands subjected to thinning performed with the use of chainsaw harvesting and mixed skidding technology.

A markedly higher level of damage (16.4–31%) was recorded during timber harvesting in broadleaved stands [39,40,41]. A greater level of damage to residual trees was also found in fully mechanised timber harvesting. Lopes et al. [42] showed that after early thinning operations using a harvester and a forwarder in stands of Pinus taeda the total level of damage amounted to 36.1% of all trees. Heitzman and Grell [43] reported from 25% up to 46% damaged trees along forwarder trails in thinning operations in spruce stands. In turn, Câmpu and Borz [44], who conducted investigations in the Carpathian mountains in spruce stands with sporadic beeches and larches, in which thinning operations were performed with a harvester and a forwarder in the CTL system, found an exceptionally low level of damage for machine timber harvesting, amounting to as little as 7.5%.

Following timber harvesting in young alder stands in individual plots the number of damaged trees ranged from 2.9% up to 21.0%. The percentages of damaged trees in the winter season ranged from 2.9% to 10.0%, while in summer they were greater: from 3.8% to 21.0%. A similar variation of results during short timber harvesting in mountain spruce stands in summer and winter was recorded by Limbeck-Lilienau [32]. In that study, in winter the percentage of damaged trees ranged from 3% to 21%, whereas in summer the values were much greater, ranging from 10% up to 42%.

The greater scatter of results in the cited study may result from the fact that timber harvesting was performed in a mountainous area, where the slope greatly hinders the process of timber extraction, thus potentially contributing to a greater level of damage to the remaining trees. The differences may also be affected by the method of operations. Limbeck-Lilienau [32] analysed machine timber harvesting, while in this study the felling and cutting of logs were performed using chainsaws, while extraction was performed using an agricultural tractor with a trailer and with manual timber loading and unloading.

In view of the seasonal character of the operations performed, the quantity of damage was almost twice as great in summer (10.6%) as in winter (5.8%). This may be explained by the fact that in winter, when tree growth is stopped, the water content in the circumferential part of the trees is lower and the adherence of the bark to deeper tissues is much stronger, making the bark less susceptible to damage [2]. Bobik [36] showed a statistically significantly smaller number of damaged trees during timber harvesting in the winter compared with studies conducted in other seasons of the year. Opposite results were reported by Chmielewski and Porter [33], who analysed timber harvesting in pine stands in summer and in winter—also applying the cut-to-length method—and recorded a higher level of damage in the winter months (6.3%) than in the summer (2.4%).

Two-thirds of all cases of damage (63.1%) were caused by extraction of timber. This was not significantly affected by the season in which operations were performed. Both in winter and in summer timber extraction operations were responsible for over 60% of damage. A higher level of damage caused during timber extraction has been confirmed by many studies. For example, during felling of trees in a mixed broadleaved forest, 4.8% of trees were destroyed or damaged, while during timber extraction (skidding) the figure was 16.3% [41]. For this reason, some researchers have investigated damage only in the case of this part of the timber harvesting process.

Analysis of damage type indicates different locations of wounding than those reported by other researchers. Many authors have reported that over 50% of wounds occur in the root zone [8,24,45]. In contrast, in this study root damage affected 32.3% of trees. No effect of season of the year on damage location was observed. The lower percentage of root damage probably results from the fact that—in contrast to the cited studies—here investigations were conducted also in winter in the presence of snow cover, which acted as a buffer, reducing the level of damage to roots. This was also manifested in a lower total level of damage.

Similarly, the season of the year had no significant effect on the surface area of wounds (damage size). Both in winter and in summer over 50% of cases were small wounds with a maximum area of 10 cm². The other 50% were almost equally distributed between medium-sized wounds (11–100 cm²) and large wounds with an area exceeding 100 cm². Most studies indicate a greater share of larger wounds. Danilović et al. [24] reported that in pure beech stands wounds with a surface area exceeding 200 cm² accounted for 52.6% of the total, while in mixed stands the figure was 43.4%. In turn, in a study by Picchio et al. [46] wounds of 25–100 cm² predominated, and the share of larger wounds was also considerable.

Similarly as in the case of wound size, the season of the year had no major effect on their depth. Both in summer and in winter a greater number of cases of damage were recorded in the phloem zone, with damaged bark and exposed phloem (80%), while the remainder were deep wounds reaching the xylem. A comparable percentage of bark removal (70%) in pine stands and a lower percentage in beech stands (30–40%) were reported by Picchio et al. [46]. Those authors also recorded at least a 30% incidence of xylem damage in both types of stands.

The frequency of damage depending on the distance of wounded trees from the skid trails was not correlated with the season of the year. Both in winter and in summer the largest number of wounded trees was found at the distance less than 1 m from the skid trail (overall this zone accounted for up to 85% of cases of damage). The further from the skid trail, the more rapidly the frequency of damage decreased. The correlation between the vicinity of skid trails and the percentage share of damage has been confirmed by many studies (e.g., [19,43,47]).

5. Conclusions

• As a result of timber harvesting in early thinning operations in alder stands, 8.3% of trees were damaged. The total level of damage in the residual stand was twice as great in summer as in winter.
• Timber extraction led to almost twice as many damaged trees in the residual stand than felling, irrespective of the season of the year in which operations were performed.
• The trunk and root collar were damaged to the greatest extent. The season of the year had no significant effect on wound location.
• Felling in summer caused more damage to the xylem of the trunk or the root collar. In turn, timber extraction conducted in summer caused more damage to the phloem in all parts of trees.
• A majority of cases of damage were small wounds. The proportions of medium-sized and large wounds were equally distributed, and they were formed more frequently during timber extraction in summer than in winter.
• The largest number of damaged trees was recorded adjacent to the skid trail, resulting from timber extraction operations. With an increase in the distance from the skid trail the number of damaged trees decreased rapidly. Trees growing alongside the skid trail were damaged significantly more frequently by timber extraction in summer than in winter.