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Article

Effects of Drainage Maintenance on Tree Radial Increment in Hemiboreal Forests of Latvia

by
Kārlis Bičkovskis
*,
Guntars Šņepsts
,
Jānis Donis
,
Āris Jansons
,
Diāna Jansone
,
Ieva Jaunslaviete
and
Roberts Matisons
Latvian State Forest Research Institute “Silava”, Riga Str. 111, LV-2169 Salaspils, Latvia
*
Author to whom correspondence should be addressed.
Forests 2025, 16(8), 1318; https://doi.org/10.3390/f16081318
Submission received: 10 July 2025 / Revised: 8 August 2025 / Accepted: 12 August 2025 / Published: 13 August 2025
(This article belongs to the Special Issue Effects of Climate Change on Tree-Ring Growth—2nd Edition)
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Abstract

Under cool and moist climates, timely implementation of ditch network maintenance (DNM) is crucial for sustaining productivity of drained forests, thus reducing operational costs, while mitigating environmental risks. This underscores the need to understand tree growth responses to DNM. This study evaluated the effects of DNM on tree radial increment in sites with both organic and mineral drained soils, focusing on regionally commercially important species: Scots pine (Pinus sylvestris), Norway spruce (Picea abies), and silver birch (Betula pendula). Responses of relative growth changes over eight years post-DNM to site and tree characteristics were assessed using a linear mixed-effects model. Species- and site-specific growth responses to DNM were indicated by significant interactions between tree species, site type, and distance from the drainage ditch. While growth responses were generally neutral (non-significant), variability among sites and species suggests that both organic and mineral soils might be prone to site-level moisture depletion near drainage infrastructure in the post-DNM period. The effect of stand age and density suggested higher responsiveness of older and less dense stands, hence positive effects of thinning to resilience of stands to DNM. These findings highlight the importance of adapting DNM strategies to local site conditions and stand characteristics to minimize drought-related growth limitations.

1. Introduction

In Northern Europe, particularly during the second half of the 20th century, vast areas of boreal peatlands and wet mineral soils were drained for forestry purposes [1]. Currently, approximately 20–25% of the total forest area in Finland, Sweden, and the Baltic States is drained [2,3]. Considerable drained forest areas also exist in the British Isles, Poland, Germany, European Russia, Belarus, and Ukraine [2]. Due to the evolution of environmental regulations [4,5,6], the construction of new drainage networks has ceased in most regions, shifting the focus to the maintenance [2,7] and modernization of existing drainage networks [8,9]. Properly maintained drainage networks help sustain soil aeration and thus metabolic activity, which typically declines when air-filled porosity falls below 20% and becomes inhibited below 10% [10]. Maintaining groundwater levels at 35–40 cm below the surface is considered optimal for sustaining suitable soil moisture conditions in drained sites managed for commercial forestry in Northern European regions [8,11].
Drained forests have gained renewed attention due to the implementation of the EU Nature Restoration Law (Regulation (EU) 2024/1991) [12]. This legislation prioritizes the restoration of degraded ecosystems with high potential for carbon sequestration, including peatlands and wet forests. Rewetting has been proposed as a climate mitigation measure to reduce greenhouse gas emissions (GHG) from previously drained areas [13]. However, this approach remains controversial, as rewetting may decrease forest productivity and increase tree mortality under prolonged inundation [14,15], potentially leading to carbon losses. Moreover, rewetting may increase anaerobic microbial activity, which can offset climate benefits through elevated methane emissions [16], presenting a challenge in balancing nature restoration and climate mitigation goals.
Over time, drainage ditches may lose their functionality due to collapse of ditch walls, sediment accumulation, vegetation overgrowth, and peat subsidence, all of which reduce drainage efficiency and contribute to rising groundwater tables [2,17]. The severity and rate of drainage network deterioration depend on site properties, including peat characteristics, local climate [18], topography [2] and time elapsed since drainage network establishment [17]. In Northern Europe especially, deterioration is accelerated in flat lowland areas, and is often exacerbated by beaver (Castor sp.) activity [19,20,21,22], which can obstruct water flow, increase flooding, and ultimately lead to suppressed tree growth and dieback [14,23]. Maintaining effective water regulation requires periodic ditch network maintenance (DNM), which includes ditch cleaning, deepening, or complementary ditching [2,7,24]. In Latvian state-owned forests, ditch depths used during DNM typically range from 0.9 to 1.3 m in mineral soils and from 1.2 to 1.5 m in peat soils [25].
While DNM can improve soil moisture conditions and support tree growth, it also can present risks [26]. With rising temperatures and more frequent droughts, DNM may increase the risk of soil over-drying, particularly on well-draining mineral soils [24]. Additionally, maintenance activities can lead to increased nutrient leaching, transport of suspended solids, and the export of dissolved organic carbon into adjacent aquatic systems [26,27,28]. In contrast, by reducing water stagnation, DNM may also lower the potential for mercury (Hg) methylation [29], thereby decreasing the risk of bioavailable Hg leaching in downstream ecosystems [26,30,31].
The response of trees to DNM is highly site-specific and strongly influenced by the pre-maintenance water table depth [2,18]. In Scots pine–dominated peatlands in Finland, the greatest increases in mean annual volume growth were observed at shallow pre-maintenance water table depths (<25–30 cm), whereas sites with deeper water tables (>35–40 cm) often showed little or no response [18]. However, considerable variation in growth outcomes at intermediate water levels (20–35 cm) suggested that relying solely on water table depth is insufficient [18], and additional site-specific indicators are needed to guide DNM decisions. Additional factors include both stand characteristics and local site conditions [32,33].
Stand properties—such as tree species composition, age, and density—affect water use through evapotranspiration [2,17,32,34]. For example, birch with a high transpiration capacity [35], can reduce groundwater levels and serve as a form of biological drainage [9,36,37]. Younger stands, due to their greater growth potential, may show stronger responses to improved soil moisture conditions following DNM [38]. Meanwhile, local site conditions such as soil type, peat depth, and topography affect water retention and drainage effectiveness [2,8,9]. Fine-textured soils and flat terrain may limit drainage efficacy, thereby constraining the impact of DNM [9]. Increased soil aeration in peat promotes humification and compaction, raising bulk density and reducing hydraulic conductivity, which gradually impairs drainage efficiency [2,39]. Hence the estimation of DNM efficiency is likely complex.
To maintain optimal soil moisture for tree growth, DNM should be performed timely; however, determining the optimal timing is not always straightforward [36,40]. Optimal timing of DNM depends on stand characteristics and drainage network condition, with limited effects observed in mature stands where groundwater levels are already low due to high evapotranspiration [2,34]. Seasonal waterlogging and the dominance of moisture-tolerant understory species can indicate the need for DNM [3]. On mineral soils, drainage ditches can remain functional throughout the entire rotation period [3,9]. In contrast, organic soils often require DNM every 20–30 years due to peat’s weak mechanical stability [41,42]. Additionally, organic soils are prone to gradual nutrient depletion over time [3,43], adversely affecting tree growth and productivity [33].
The long-term impact of DNM on forest growth remains a topic of active research. Studies from Finland and Sweden have demonstrated that DNM can significantly enhance tree growth, particularly in drained peatlands [44,45]. However, results vary depending on the length of the post-maintenance evaluation period, as growth responses to DNM are often delayed and may not be fully captured by short-term studies [43]. In Scots pine–dominated peatlands in Finland, reported increases in mean annual volume growth range from 0.5 to 1.8 m3 ha−1 yr−1 over 15–20 years [2]. Despite these findings, most available data are limited to Scots pine-dominated boreal peatlands, with relatively few studies addressing growth responses of Norway spruce, silver birch, or stands on mineral soils.
Quantifying the growth response to DNM is challenging, mainly due to difficulties in defining an appropriate reference growth rate. As groundwater levels gradually rise, growth declines slowly, making the effect of DNM dependent on the degree of prior growth limitation [2]. Experimentally treated stands are often compared to non-treated controls [18], but this approach may be biased by incomplete isolation of treatments in the experimental layout. The alternative approach—comparing growth before and after DNM using analysis of increment allows for a retrospective assessment of stand development without the need for long-term experimental setups and can reveal subtle changes in growth trends that may not be apparent from inventory data [46].
In Latvia, forests with excessive soil moisture are more closely linked to groundwater discharge zones shaped by topography and soil conditions, rather than regional precipitation regimes [3]. These areas, characterized by persistently high soil moisture, often exhibit limited tree growth if unmanaged [37]. Consequently, large-scale drainage projects were implemented between the 1960 s and 1990 s to increase timber yields, which proved to be an effective strategy [3]. Drained forests in Latvia span both organic (12%) and mineral (17%) soils [47]. Scots pine, Norway spruce, and silver birch are among the most economically important tree species in Latvia, with reported post-drainage growth increments increasing two- to threefold [37]. Drained forests dominated by these species represent a substantial share of the national forest area, and maintaining their productivity through targeted drainage practices is essential for the long-term sustainability of commercial forestry and forest management.
This study aimed to evaluate the effect of DNM on tree stem radial increment in hemiboreal forests of Latvia. We hypothesize that DNM has a positive effect on increment, particularly for conifers, but interactions among species, site type, and tree age can influence these responses. We presume that DNM might cause over drainage, particularly facing increasing drought effects.

2. Materials and Methods

2.1. Study Sites

The research was conducted in Latvia, located within the hemiboreal forest zone of the Eastern Baltic region [48]. The climate is characterized as humid continental [49], with a mean annual air temperature of 6.8 °C and an average annual precipitation of 685.6 mm [50]. Study sites were selected across the country at four locations, with nine forest stands chosen at each location (Figure 1). The selection was stratified based on the presence of DNM, stand age (representing the rotation interval of commercial stands), and composition (stands formed by the main economically important species). Accordingly, the dominant species of the stands were Scots pine, Norway spruce, and silver birch, where they comprised at least 60% of the canopy layer (by basal area). DNM was conducted primarily in 2012 (29 stands), while in the remaining seven stands, DNM was carried out in either 2006 or 2010. The study sites were selected to ensure that no commercial thinning or other forestry operations had occurred during the ten years preceding DNM.
The stands of each species were represented in three age groups: for pine: (1) 21–50 years, (2) 51–80 years, (3) 81–100 years; for spruce and birch: (1) 21–40 years, (2) 41–60 years, (3) 61–100 years.

2.2. Data Collection

In each forest stand, three circular sample plots, each with an area of 500 m2 (R = 12.62 m), were established along a transect perpendicular to the drainage ditch at distances of 15 m, 45 m, and 75 m (center of sample plot) from the edge of drainage ditch. These distances were selected considering the typical spacing between drainage ditches at the study sites, which ranged from 150 to 200 m. The distance from the ditch to the edge of the stand, as well as the width and direction of the ditch, were measured.
Diameter at breast height (DBH) was measured for all living and dead standing trees that were at least 6.1 cm in DBH. Additional subplots with a radius of 5.64 m (area = 100 m2) were established as a sector (0–90°) at the center of the sample plot, to measure growth parameters for trees with a DBH of 2.1–6.1 cm. Tree height was measured for 10 individuals of the dominant species in the upper canopy layer and for three individuals of each additional species in each canopy layer. For the same trees increment cores were taken using Pressler’s increment corer. The heights of all standing dead trees were measured. Forest site type for each sample plot was noted according to the local classification developed by Bušs [51]. In total, 6143 living trees were measured, and from those, 1036 increment cores were collected (346 for pine, 385 for spruce, and 305 for birch). The studied forest stands varied in age, ranging from young to mature (Table 1). The age of the studied trees ranged from 6 to 120 years, with the stands on organic oligotrophic soils being slightly older than others.
Tree-ring width (TRW) was measured with an accuracy of 0.01 mm using a LINTAB6 measuring system (RinnTech, Heidelberg, Germany). TRW data were cross-dated both visually and statistically using the COFECHA (version 6.06P) software to ensure accuracy [52]. TRW was used as the proxy of growth. To evaluate responses of trees to DNM, relative radial increment was calculated as the ratio between the mean TRW 8 years before vs. 8 years after DNM. The 8-year post-treatment period was selected as it represented the longest consistent timeframe available across all study sites, allowing for standardized comparison of growth responses. A linear mixed-effects model (LMM) was used to analyze the factors influencing relative radial increment. Both tree-level and stand-level variables were tested, including numeric and categorical variables. The model included three tree species (pine, spruce, birch) across four forest site types, representing two soil types (organic and mineral) and two fertility levels (oligotrophic and mesotrophic). Tree slenderness index, calculated as the ratio of tree height (H) to DBH, was tested as an implicit proxy of stand density [34]. The effect of DNM was represented in the model by the distance of each individual tree from the ditch. Model selection was performed using backward elimination, starting with all variables and sequentially removing non-significant variables and interactions. Interaction terms between species, site type, and distance from the ditch were also evaluated to assess species and site-specific responses to DNM. A stand and sample plot identifiers were included as random effects.
The model was fitted using the ‘lmer’ function from the lme4 [53] package in R version 4.4.1. [54]. Model fitting was performed using restricted maximum likelihood (REML). Model performance was evaluated using Type II Wald chi-square tests for fixed effects, marginal and conditional R-squared values for explained variance, and variance inflation factors (VIFs) to assess multicollinearity. Diagnostic checks included residual analysis using QQ plots and histograms to confirm assumptions of normality and homoscedasticity. The significance of random effects was evaluated using likelihood ratio tests with the ‘ranova’ function from the ‘LmerTest’ package (version 3.1-3) [55].

3. Results and Discussion

The DNM had a significant, yet complex effect on the relative radial increment of tress, as indicated by a significant interaction between species-site type and distance to the drainage ditch (Table 2, Supplementary Material Table S1). The conditional R2 of the model was 0.36, while the marginal R2 was only 0.09, indicating that most of the variation in relative radial increment after DNM was explained by random effects, specifically—forest stand (p < 0.001) and sample plot (p = 0.044). This suggests that local site-specific conditions have a greater influence on tree response than individual tree characteristics, likely due to soil properties [33] and microrelief [32,56].
In most cases, tree stem radial increment did not respond to DNM, as indicated by the confidence intervals of the slope coefficients, which contained zero (Figure 2A). Nevertheless the radial increment was not suppressed (Supplementary Material Table S2) This indicated that overall soil moisture conditions prior to DNM were not a major constraint on tree growth [18]. However, in two specific cases—pine on mineral oligotrophic sites and spruce on organic oligotrophic sites—the relative radial increment increased significantly with increasing distance from the ditch [57]. This indicates a growth limitation for trees growing closer to the ditch, likely due to water shortage after DNM [56,58,59]. Conifer responses to DNM are likely influenced by species-specific adaptations to environmental changes [60].
Responses of conifers may be rooted in differing drought tolerance strategies, with spruce and pine both showing sensitivity to post-DNM soil drying, yet on different site types (Figure 2A). While spruce is more vulnerable in peat-based environments that become too dry, pine may be more affected in nutrient-poor mineral soils that offer limited buffering against drought once drainage is enhanced.
Peat humification and the resulting subsidence are typically more pronounced adjacent to drainage ditches, where the depression of the groundwater table is greatest [61], potentially leading to increased soil drying following DNM [34]. However, the absence of data on drainage network condition prior to DNM constrained the ability to assess pre-maintenance soil moisture condition and determine whether the drainage network was functioning poorly or sub-optimally. The maintenance was carried out at the discretion of forest planners, likely based on observed ditch degradation or declining tree growth, although specific justifications were not recorded. Presumably, under high soil moisture conditions prior to DNM, trees may have developed shallower root systems [62]. Following DNM, the resulting drop in ground water level could expose tree roots to increased drought stress, especially during dry periods [2,63]. The pronounced response of conifers at greater distances from the ditch may reflect more favorable post-DNM soil moisture regimes, where soil water conditions tend to be more stable [56].
A contrasting response was observed for spruce in mineral oligotrophic sites (Figure 2A), where the relative radial increment showed marginally significant declined with increasing distance from the ditch. The proximity to the drainage ditch likely provided better soil aeration, which is favorable for spruce, as it prefers well-aerated soil conditions [64]. This suggests that soil moisture prior to DNM may have been suboptimal for spruce, which is particularly sensitive to excessive moisture [64]. This dual pattern observed in spruce highlights its sensitivity to both water excess and water deficit, emphasizing the importance of local/site conditions. This supports the relevance of considering both species traits and site hydrology when evaluating possible impacts of DNM.
Birch response to DNM showed no significant increase or decrease in relation to the interaction between site type and proximity to the drainage ditch (Figure 2A), indicating greater plasticity to altered edaphic conditions. Additionally, birch has high evapotranspiration potential, thus, it is likely that sub-optimal drainage conditions before DNM were compensated by evapotranspiration [18,44]. This suggests that birch is more tolerant of hydrological changes and is a more suitable species in drained sites [65].
Tree age had a uniform effect on relative radial increment across all ages, as indicated by the non-interacted effect, despite the high variation between stand ages (Table 2, Figure 2B). Even though the studied species differed by biological age, the estimated uniformed response suggests similar growth response under drained conditions. Age showed a positive influence on relative radial increment [44], indicating that older trees tend to exhibit higher relative growth following DNM, likely due to their larger canopy and greater ability to assimilate carbon [66]. While younger stands, those without yet undergone non-commercial thinning, may still be competing for light, which can limit their ability to exploit improved edaphic conditions fully [67].
The effect of stand density on tree responses to DNM [18] was implicitly supported by the significant effect of slenderness index (Table 2, Figure 2C), as greater relative increment was observed in trees with smaller slenderness index values. Dense stands can lower the water table more effectively; however, after DNM, improved drainage conditions in combination with pronounced evapotranspiration may lead to excessive soil drying as suggested by observed negative relationship between increment and distance from the ditch (Figure 2A). Therefore, forest management strategies in drained stands should carefully consider stand density [34,36] when evaluating the necessity and timing for DNM in conjunction with thinning to optimize tree soil moisture conditions and mitigate drought stress following DNM.
The non-interacted effect of site type indicated that relative radial increment was significant only in organic oligotrophic soils, whereas other site types showed only minor or negligible increases following DNM (Table 2, Figure 2D). This pattern likely reflects the fact that organic oligotrophic soils—characterized by higher water content due to their greater retention capacity—exhibit more favorable moisture conditions following DNM than mineral soils. DNM likely improved the aeration of the root zone, leading to enhanced topsoil metabolic rates and, consequently, increased nutrient availability, which had a notable effect on tree growth. In contrast, mineral soils—characterized by lower water retention and faster drying—may have already been sufficiently drained or responded less noticeably to DNM [18].
The independent effect of tree species showed that all species tended to have a positive relative radial increment following DNM (Table 2, Figure 2E), although this may be underestimated due to the increment comparison method, which does not account for the natural decline in growth expected in untreated stands [68]. Spruce and birch showed similar responses, with significantly greater relative growth change (p < 0.05) compared to pine. Birch, in particular, showed a stable response across different site conditions, highlighting its ecological plasticity in adapting to altered soil moisture regimes [69]. Suitability of birch in drained stands is further supported by its high wind resistance compared to conifers, making it a more reliable species in peatland forests prone to windthrow [70]. In contrast, pine’s relatively lower growth response may reflect its adaptation to drier conditions and a more conservative growth strategy [63] under improved soil aeration, which limits its responsiveness to changes induced by DNM Similar results have been reported from Finland [59], where Scots pine in drained peatlands grew better under shallow water tables, contradicting the view that a deep water table is necessary to support growth, thus supporting the idea that DNM may not always have positive effects on pine growth.
The hypothesis was partly supported as the anticipated positive effect of DNM, particularly for conifers, was not observed. However, interactions among species, site type, and tree age had a significant effect on relative radial increment. The assumption that DNM can lead to over-drainage was supported in two cases—pine on mineral oligotrophic sites and spruce on organic oligotrophic sites—where relative radial increment was significantly greater at increasing distances from the ditch, indicating reduced growth in trees located closer to the ditch.

4. Conclusions

In most cases, DNM had a generally neutral effect on tree growth, as the drainage network remained functional and maintained optimal soil moisture conditions. Although delayed DNM may lead to growth decline, high stand density (implicitly suggested by the effect of slenderness index) could partially compensate for elevated moisture levels. Although drought risk following DNM was not pronounced, some early warning signs were observed. Overly intensive DNM may reduce soil water availability, increasing the risk of drought stress in both drained peat and mineral sites. Although the applied DNM was largely balanced, to mitigate drought risk more effectively, future DNM planning should be more targeted, considering both tree species, stand density and site type. Furthermore, controlled experiments on drainage intensity are needed to provide valuable insights into optimal soil moisture conditions for sustaining tree growth.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f16081318/s1. Table S1: Fixed-effect parameter estimates from the model assessing the response of relative radial increment to distance from the ditch, tree age, slenderness index, tree species, site type, and their interactions. Table S2: Estimated marginal mean tree-ring width (±95% of confidence interval) of trees before and after drainage ditch maintenance in stands differing by soil type (mineral and organic) in Latvia. The estimates are made accounting for the spatial distribution (spatial dependence of sites), dominant species and stand age.

Author Contributions

Conceptualization, R.M. and K.B.; methodology, J.D., G.Š., Ā.J. and R.M.; software, R.M.; validation, K.B., Ā.J., J.D. and G.Š.; formal analysis, K.B. and R.M.; investigation, R.M. and K.B.; resources, Ā.J. and R.M.; data curation, K.B., I.J., D.J., J.D. and G.Š.; writing—original draft preparation, K.B.; writing—review and editing, R.M., J.D., Ā.J., G.Š. and D.J.; visualization, R.M.; supervision, R.M.; project administration, R.M. and Ā.J.; funding acquisition, Ā.J. All authors have read and agreed to the published version of the manuscript.

Funding

The study was funded by Latvia Council of Science national research program project: “Forest4LV—Innovation in Forest Management and Value Chain for Latvia’s Growth: New Forest Services, Products and Technologies” (No.: VPP-ZM-VRIIILA-2024/2-0002) and Research program supported by the JSC Latvian State Forests “Effect of climate change on forestry and associated risks” (Agreement No. 5-5.9.1_007p_101_21_78).

Data Availability Statement

Data available on request from the corresponding author (The data set contains potentially sensitive commercial information, e.g., location of monitoring sampling plots).

Acknowledgments

We thank the technical staff, especially Andis Adamovičs, for assistance with fieldwork, and Endijs Bāders for creating the graphical illustration of the research map. During the preparation of this manuscript, the authors used ChatGPT (GPT-4o) and Grammarly Pro to assist with language editing, paraphrasing, and grammar improvement. The authors have reviewed and edited the output and take full responsibility for the content of this publication.

Conflicts of Interest

The authors declare no conflicts of interest. Funding sources are listed above.

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Forests 16 01318 g001
Figure 1. The geographic locations of the 36 studied forest stands distributed across four study locations (red circles).
Figure 1. The geographic locations of the 36 studied forest stands distributed across four study locations (red circles).
Forests 16 01318 g001
Forests 16 01318 g002
Figure 2. Tree species and age, site type, distance from the drainage ditch, and slenderness index effects (gray bars) on relative radial increment after DNM (Bars sharing the same lowercase letter are not significantly different at α = 0.05).
Figure 2. Tree species and age, site type, distance from the drainage ditch, and slenderness index effects (gray bars) on relative radial increment after DNM (Bars sharing the same lowercase letter are not significantly different at α = 0.05).
Forests 16 01318 g002
Table 1. Mean values (± SE) of stand age, diameter at breast height (DBH), tree height (H) and number of trees for categories (N) across site types and species.
Table 1. Mean values (± SE) of stand age, diameter at breast height (DBH), tree height (H) and number of trees for categories (N) across site types and species.
Site TypeSpeciesStand Age, YearsDBH, cmH, mN
Mineral,
mesotrophic		Birch33.5 ± 1.818.8 ± 0.620.8 ± 0.4211
Spruce28 ± 0.918.4 ± 0.517.3 ± 0.377
Pine58 ± 1.728.7 ± 0.928.1 ± 0.6330
Mineral,
oligotrophic		Birch34.6 ± 3.020.8 ± 1.420.8 ± 0.840
Spruce34.3 ± 2.420.8 ± 0.918.2 ± 0.7109
Pine29.5 ± 1.922.7 ± 0.720.1 ± 0.6113
Organic,
mesotrophic		Birch37.6 ± 1.619.6 ± 0.721.8 ± 0.577
Spruce33.9 ± 1.222.2 ± 0.819.2 ± 0.557
Pine59.2 ± 2.332.7 ± 1.028.3 ± 0.6171
Organic,
oligotrophic		Birch34.2 ± 3.911.7 ± 0.912.3 ± 1.38
Spruce39.4 ± 5.616.6 ± 1.515.9 ± 1.379
Pine82.6 ± 5.221.7 ± 1.017.9 ± 0.821
Data analysis.
Table 2. Type-III analysis-of-variance table for the model assessing the response of relative radial increment to the distance from the ditch, tree age, slenderness index, tree species, site type, and their interactions (set of the fixed effects) and variance and significance of the random effects (sample plot, forest stand, year of maintenance) (Χ2—chi-square statistic, Df—degrees of freedom).
Table 2. Type-III analysis-of-variance table for the model assessing the response of relative radial increment to the distance from the ditch, tree age, slenderness index, tree species, site type, and their interactions (set of the fixed effects) and variance and significance of the random effects (sample plot, forest stand, year of maintenance) (Χ2—chi-square statistic, Df—degrees of freedom).
Fixed EffectsΧ2Dfp-Value
Distance from the ditch0.01810.893
Tree age4.8510.028
Slenderness index18.241<0.001
Tree species10.4220.005
Site type9.6430.022
Distance from ditch by tree species by site type interaction48.5511<0.001
Random effectsVariancep-value
Sample plot0.0040.044
Forest stand0.044<0.001
Year of maintenance0.0020.818
Residual0.117
N obs.1268
Inter class correlation0.30
R2m0.09
R2c0.36
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Bičkovskis, K.; Šņepsts, G.; Donis, J.; Jansons, Ā.; Jansone, D.; Jaunslaviete, I.; Matisons, R. Effects of Drainage Maintenance on Tree Radial Increment in Hemiboreal Forests of Latvia. Forests 2025, 16, 1318. https://doi.org/10.3390/f16081318

AMA Style

Bičkovskis K, Šņepsts G, Donis J, Jansons Ā, Jansone D, Jaunslaviete I, Matisons R. Effects of Drainage Maintenance on Tree Radial Increment in Hemiboreal Forests of Latvia. Forests. 2025; 16(8):1318. https://doi.org/10.3390/f16081318

Chicago/Turabian Style

Bičkovskis, Kārlis, Guntars Šņepsts, Jānis Donis, Āris Jansons, Diāna Jansone, Ieva Jaunslaviete, and Roberts Matisons. 2025. "Effects of Drainage Maintenance on Tree Radial Increment in Hemiboreal Forests of Latvia" Forests 16, no. 8: 1318. https://doi.org/10.3390/f16081318

APA Style

Bičkovskis, K., Šņepsts, G., Donis, J., Jansons, Ā., Jansone, D., Jaunslaviete, I., & Matisons, R. (2025). Effects of Drainage Maintenance on Tree Radial Increment in Hemiboreal Forests of Latvia. Forests, 16(8), 1318. https://doi.org/10.3390/f16081318

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