Communication Root-Plate Characteristics of Common in Hemiboreal Forests of : A Case Study

1 2 2 1 1, Valters Samariks , Dace Brizga , Jel, ena Ruba¯ , Andris Seipulis and Aris¯ Jansons *

1 Latvian State Forest Research Institute “Silava”, R¯ıgas St. 111, LV-2169 Salaspils, Latvia; [email protected] (V.S.); [email protected] (A.S.) 2 Forest Faculty, Latvia University of Life Sciences and Technologies, Akademijas St. 11, LV-3001 Jelgava, Latvia; [email protected] (D.B.); [email protected] (J.R.) * Correspondence: [email protected]

Abstract: change will cause winds to strengthen and storms to become more frequent in Northern Europe. Windstorms reduce the financial value of forests by bending, breaking, or uprooting trees, and wind-thrown trees cause additional economic losses. The resistance of trees to wind damage depends on tree species, tree- and stand-scale parameters, and root-soil plate characteristics such as root-plate size, weight, and rooting depth. The root-soil plate is a complex structure whose mechanical strength is dependent on root-plate width and depth, as the root system provides root attachment with soil and structural support. In Latvia, the common aspen (Populus tremula L.) root system has been studied to develop a belowground biomass model, because information about root system characteristics in relation to tree wind resistance is scarce. The aim of this study was to assess the root-plate dimensions of common aspen stands on fertile mineral soil (luvisol). Study material was collected in the central region of Latvia, where pure mature (41–60 years old) common aspen stands were randomly selected, and dominant trees within the stand were chosen. In total, ten sample trees from ten stands were uprooted. The diameter at breast height (DBH) and tree height (H) were measured for each sample tree, and their roots were excavated, divided into groups,   washed, measured, and weighed. The highest naturally moist biomass values were observed for coarse roots, and fine root biomass was significantly lower compared to other root groups. All root Citation: Samariks, V.; Brizga, D.; group biomass values had a strong correlation with the tree DBH. The obtained results show that Ruba,¯ J.; Seipulis, A.; Jansons, A.¯ there is a close, negative relationship between the relative distance from the stem and the relative Root-Plate Characteristics of root-plate depth distribution. Common Aspen in Hemiboreal Forests of Latvia: A Case Study. Forests 2021, 12, 32. https:// Keywords: belowground biomass; coarse roots; Populus tremula L.; root biomass; root distribution; doi.org/10.3390/f12010032 wind resistance

Received: 6 November 2020 Accepted: 25 December 2020 Published: 29 December 2020 1. Introduction The world’s forests face a rapidly changing climate [1], and a better understanding of Publisher’s Note: MDPI stays neu- how changing conditions may affect them is needed [2]. It is projected that in the future, tral with regard to jurisdictional claims stronger winds and storms will become more frequent in Northern Europe [3]. Wind is a in published maps and institutional natural hazard that poses a significant threat to forests, specifically wind-throw of trees. affiliations. In the last 30 years, the number of storms and their intensity has increased in European forests [4]. Windstorms cause heavy losses in forestry by bending, breaking, or uprooting trees, thus considerably reducing their monetary value. Also, storms create large openings

Copyright: © 2020 by the authors. Li- in the forest stand [5,6] and increase the probability of secondary damages due to newly censee MDPI, Basel, Switzerland. This formed stand edges [7]. The effects of climate change on forests manifest as the replacement article is an open access article distributed of less adapted trees species with the ones more suitable for the new conditions, including under the terms and conditions of the new wind climate [8]. This has already occurred in Switzerland, where Pinus sylvestris L. Creative Commons Attribution (CC BY) has been replaced by Quercus pubescens Willd., and in Spain, where Fagus sylvatica L. has license (https://creativecommons.org/ been replaced by Quercus ilex L. [9]. Additionally, a drop in the financial value of stands, licenses/by/4.0/). due to increased frequency of disturbances and other socio-economic reasons, increase

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the share of natural regeneration and unmanaged areas [6]. It, in turn, will increase the proportion of broadleaved trees over forests, including common aspen (Populus tremula L.). Therefore, it is important to evaluate the traits related to aspen wind resistance. Tree susceptibility to wind is affected by several factors, including stand parameters, tree species, tree dimensions and shape, root-soil plate size, and changes caused by forestry operations (i.e., thinning and tending) [2,10]. Trees have the capacity to adapt to wind influence. In storms, relatively young and middle-aged trees are damaged more often because of insufficient mechanical strength [11,12]. Also, trees in areas with lower wind speeds or trees after thinning are more damaged in storms because of a lack of adaptation to the influence of wind [12]. Additionally, poor root development reduces tree stability [13]. The common aspen is a fast-growing tree species that is widely distributed across European boreal forests [14,15] and grows on fertile and well-drained soils alongside other tree species [16]. In Latvia, common aspen stands cover 7% of the total forest area [17]. The aspen has a plate root system characterized by wide but shallow rooting. The main elements of the plate root system are the large lateral roots, which elongate horizontally out from the stump before tapering and branching into narrower absorption roots. Another important element is the vertical sinker roots, which emerge from the laterals and branch downwards into the subsoil [18]. The interaction between root architecture and soil type is important for the robustness of the root system, such that roots resist bending more effectively as rooting depth, soil density, and compaction increase [19]. In addition, the mechanical strength of the root system is dependent on root-plate radius (width) and depth [19]; hence, the root-plate in complex with coarse and small roots determines tree stability in the soil [20]. Fine roots with a diameter smaller than 2 mm are often excluded from analysis because they are not related to wind resistance [21]. Aspen achieves tight root contact with the soil by forming an abundance of small roots that significantly increase the root surface area [22]. Additionally, the symmetry of the root system is important to ensure the stability of the tree, especially in conditions, where root depth is limited by soil conditions [23]. Responses of the aboveground parts of trees to different stress factors, including wind, has been well studied in controlled and forest environments, but until recently, relatively fewer studies have been conducted on root response to different stresses due to technical complications [8,24–27]. Models had been developed to characterize the effects of root architecture on tree anchorage [28,29], and root traits, including root-soil plate volume, have been linked to parameters of the aboveground part of trees [30,31] for economically important tree species. However, studies of the root system of common aspen are very scarce [32] and, to the best of our knowledge, none address wind resistance. Based on studies of other species, we hypothesize that there is a close relationship between tree diameter at breast height (DBH) and soil-root plate volume. The aim of this study was to assess the root-plate dimensions of common aspen stands on fertile mineral soil (luvisol).

2. Materials and Methods The study was conducted in the central region of Latvia (56◦520 N, 24◦210 E) in pure, mature (41–60 years old) common aspen stands, growing on freely drained mineral soil [33], corresponding to luvisol [34]. Because a thick layer of sediment covers the territory of Latvia, bedrock cannot limit rooting depth. The soil in such a forest type is characterized by rich, sandy loam, loamy soil, typical podzolic or sod podzolic, and fine sand soil [26,35]. Study data were obtained in ten randomly selected common aspen stands from ten dominant trees that corresponded to the average DBH2 in the stand. DBH and height (H) of each tree were measured (Table1). Sample trees were pulled with steel cable and manual winch, fixed at the base of opposite trees until uprooting was achieved. The cable (pulling line) was fixed at 50% of the tree height [36]. Forests 2021, 12, 32 3 of 9

Table 1. Sample tree above- and belowground dimensions.

Tree Age Root-Plate Root-Plate Variable H (m) DBH (cm) (Years) Width (m) Depth (m) Min 23.3 21.0 1.5 0.2 Max41–60 26.7 36.0 2.5 1.2 Average (±CI *) 25.0 ± 0.7 27.5 ± 3.3 1.2 ± 0.2 0.8 ± 0.1 * CI: ± 95% confidence interval. DBH: diameter at breast height; H: tree height. For every uprooted tree, we measured root-plate dimensions and parameters. Com- mon aspen root-plate width was measured parallel to the ground surface and perpendicular to the stem in all cardinal directions (0◦, 45◦, 90◦, 135◦, 180◦, 225◦, 270◦, and 315◦) as the dis- tance from the centre to the edge of the root-plate. Root-plate width was used as the radius of the root-plate for volume and shape estimation. The rooting depth from the root-plate ground surface to the bottom of the root-plate was measured on the northern and southern direction of the root-plate to assess root depth distribution. The first depth measurement was conducted as close as possible to the stem, while the others were conducted after every 50 cm. At each depth measurement point in every 20 cm-thick layer, sample tree roots were manually dug out, removed, divided into four biomass groups, and weighed in these groups. Root group classification was based on root diameter: F—fine roots (di- ameter < 2 mm), S—small roots (diameter 2–20 mm), C—coarse roots (diameter > 20 mm), and TS—tree stump [37]. The number of depth observations and observed layers was dependent on root-plate width and thickness, respectively. F biomass was assessed with a 100 cm3 cylinder by taking soil samples (20 repetitions) in the root-plate area at six depths measurement points at every 10 cm-thick layer. Later these samples were delivered to the laboratory, washed, separated from the soil or other small fragments, and weighed. TS, C, and S biomass were assessed on the spot by manually excavating the roots, washing them, separating them from the soil, and weighing them on a scale. TS was defined as the non-root fraction, leftover after root excavation and removal, and not exceeding 30 cm in height from the ground surface. Every sample tree root fraction was assessed and dried at 105 ◦C until they reached a constant weight (ISO 11465, 1993) [38] to determine the relative moisture content [37]. Common aspen total root biomass was calculated by summing the TS, C, and S root group weight of naturally moist samples. ANOVA was used to determine the differences in statistical parameters between root groups. Most studies determine tree wind resistance with maximum base bending moment [28,30,36]; however, scientists have found that H multiplied by DBH2 yielded the best prediction of maximum base bending moment for uprooting for two conifers (Scots pine (Pinus sylvestris L.), spruce (Picea abies (L.) Karst.)) and silver (Betula pendula Roth.) in mineral soils (unfrozen) [30]. We based tree wind resistance indication modelling on aboveground parameters, where H was multiplied by DBH2 to determine stem susceptibility to uprooting [30]. Pearson’s correlation analysis was used to assess the relationship between tree size, measured variables, and calculated variables (H, DBH, H × DBH2, depth, and biomass). Root-plate depth and width at measurement points were expressed in relative values. Relative root depth and relative distance from the stem were used as model predictors to calculate root depth distribution using a generalized additive model. For each model, the 95% confidence interval (CI) was calculated. Data processing and analysis were performed using the statistical software R 4.0.0. [39].

3. Results 3.1. Root-Plate Biomass Average biomass values for the root groups expressed as naturally moist weight (Figure1), were 177.5 ± 94.5 kg, 101.2 ± 45.2 kg, and 84.2 ± 30.7 kg, for C, S, and TS, respectively, and 6.7 ± 2.4 kg for F, which was significantly lower than the other root groups. Biomass of the C, S, and TS root groups did not differ significantly, but all root ForestsForests 2021 2021, ,12 12, ,x x FOR FOR PEER PEER REVIEW REVIEW 44 of of 9 9

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andand 6.7 6.7 ± ± 2.4 2.4 kg kg for for F, F, which which was was significantly significantly lower lower than than the the other other root root groups. groups. Biomass Biomass ofof thethe C,C, S,S, andand TSTS rootroot groupsgroups diddid notnot differdiffer significantly,significantly, butbut allall rootroot groupgroup biomassbiomass group biomass values had a strong correlation (0.86 < r < 0.96) with DBH. Relative moisture valuesvalues had had a a strong strong correlation correlation ( (0.860.86 < < r r < < 0.96 0.96)) with with DBH. DBH. Relative Relative moisture moisture content content varied varied content varied between root groups: 43%, 55%, and 61% for TS, C, and S, respectively. betweenbetween root root groups: groups: 43%, 43%, 55%, 55%, and and 61% 61% for for TS, TS, C, C, and and S, S, respectively. respectively.

Figure 1. FigureFigure 1. 1. Average AverageAverage naturally naturally naturally moist moist moist root root root biomass biomass biomass per per per tree tree tree by by root root group group ( defined (defined(defined by by diameter: diameter:diameter: F F—fineF—— finefineroots roots roots (<2 ( <( mm),<22 mm mm S—small),), S S——smallsmall roots roots roots (2–20 ( 2(2––20 mm),20 mm mm C—coarse),), C C——coarsecoarse roots roots roots (>20 ( >(>20 mm),20 mm mm and),), and and TS—tree TS TS——treetree stump). stump) stump). . Accordingly, total naturally moist root biomass values ranged from 147.7 kg to 895.3 kg, Accordingly,Accordingly, total total naturally naturally moist moist root root biomass biomass values values ranged ranged from from 147.7 147.7 kg kg to to 895.3 895.3 and the average belowground biomass per tree was 362.9 ± 166.3 kg. DBH and total root kgkg, ,and and the the average average belowground belowground biomass biomass per per tree tree was was 362.9 362.9 ± ± 166.3 166.3 kg. kg. DBH DBH and and total total biomass of naturally moist samples had a strong correlation (r = 0.96) (Figure2). rootroot biomass biomass of of naturally naturally moist moist samples samples had had a a strong strong correlation correlation ( (rr = = 0.96 0.96)) ( (FigureFigure 2) 2). .

FigureFigureFigure 2. 2. 2.Total TotalTotal naturally naturally naturally moist moist moist root root root biomass biomass biomass per per per tree tree tree against against against diameter diameter diameter at at atbreast breast breast he he heightightight (DBH). (DBH). (DBH). Grey Grey Grey areaareaarea denotes denotes denotes a a 95% a95% 95% confidence confidence confidence interval. interval. interval.

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3.2. Structural Root-Plate Depth Distribution 3.2. Structural Root-Plate Depth Distribution In the structural root distribution analysis, F roots were excluded because their low In the structural root distribution analysis, F roots were excluded because their low biomass could not affect wind resistance (Figure1). biomass could not affect wind resistance (Figure 1). Relative root depth and relative distance from the stem were used as model predictors Relative root depth and relative distance from the stem were used as model predic- to assess the structural root-plate depth distribution of common aspen (Figure3). An anal- tors to assess the structural root-plate depth distribution of common aspen (Figure 3). An ysis of the data obtained in the study confirmed a close, negative relationship (r = −0.96) analysis of the data obtained in the study confirmed a close, negative relationship (r = between the relative distance from the stem and the relative root-plate depth. As the −0.96) between the relative distance from the stem and the relative root-plate depth. As measurement point distance from the stem increased, the root depth decreased. At the edgethe measurement of the root plate, point relative distance rooting from depththe stem decreased increased to, 31%the root of the depth total decreased root depth.. At the edge of the root plate, relative rooting depth decreased to 31% of the total root depth.

FigureFigure 3.3. RelativeRelative root-plateroot-plate depth depth distribution distribution of of measurement measurement points points at at a relativea relative distance distance from from the the stem. Grey area denotes a 95% confidence interval. stem. Grey area denotes a 95% confidence interval.

VerticalVertical rooting rooting is is an an important important factor factor that that ensures ensures tree tree resistance; resistance; trees trees with with larger larger and deeperand deepe rootr systemsroot systems are less are prone less prone to wind-throw, to wind-throw, as roots as roots resist resist bending bending more more effectively effec- astively rooting as rooting depth, depth, soil density, soil density, and compactness and compactness increase increase [19]. The[19]. averageThe average depth depth of the of aspenthe aspen root root plate plate was was 0.8 ±0.80.1 ± 0.1 m. m. The The average average depth depth in in the the center center of of the the root root plate plate was was 1.11.1± ± 0.2 m. Maximum depth values (1.2 m) were concentratedconcentrated inin thethe firstfirst 5050 cmcm fromfrom thethe stem;stem; thus,thus, thethe deepestdeepest rootsroots werewere locatedlocated closeclose to to the the centre centre of of the the root root plate. plate.

3.3.3.3. TreeTree WindWind ResistanceResistance Indicator Indicator AA strongstrong correlationcorrelation ( r(r= = 0.96)0.96) indicatedindicated aa goodgood modelmodel fitfit (Figure(Figure2 )2) between between DBH DBH and and totaltotal rootroot biomass. biomass. Therefore, Therefore, we we used used H H× × DBHDBH22 as an indicator ofof treetree windwind resistanceresistance toto uprootinguprooting inin mineralmineral soilssoils [30[30].]. AverageAverage H H× × DBHDBH22 valuevalue waswas 2.02.0± ± 0.5,0.5, andand thethe valuesvalues rangedranged from from 1.1 1.1 to to 3.3 3.3 (Figure (Figure4). The4). The presented presented data data also also showed showed a good a good model model fit (r = fit 0.97) (r = 2 between0.97) between H × DBH H × DBHand2 totaland total root biomass.root biomass.

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Figure 4. Tree wind resistance (H × DBHDBH22) )in in relation relation to to total total naturally naturally moist moist roo roott biomass biomass per per tree. tree. Grey area denotes 95% confidenceconfidence interval.

4. Discussion Discussion VVerticalertical rooting is an important factor that ensures tree resistance: the larger a tree’s root system, thethe lessless proneprone it it is is to to wind-throw wind-throw [19 [19].]. In In many many cases, cases, trees trees are are able able to to adapt adapt to togrowing growing conditions conditions and and form form different different types types of roots of roots to increase to increase their their resistance resistance to wind to wind [40]. [40].In our In study, our study, the aspen the aspen root system root system was divided was divided into four into groups, four groups, and average and average root biomass root biomassvalues differed values betweendiffered between these groups. these Thegroups. largest The naturally largest naturally moist root mo biomassist root biomass was for wasC roots, for C and roots, F root and biomass F root biomass was significantly was significantly lower than lower biomass than biomass in any otherin any root other group root group(Figure (Figure1). Naturally 1). Naturally moist rootmoist biomass root biomass values values were lower were forlower TS for than TS for than C orfor S C roots. or S roots.Furthermore, Furthermore, the biomass the biomass of all root of groupsall root hadgroups a strong had correlationa strong correlation with DBH. with Therefore, DBH. Therefore,with increasing with DBH,increasing an increase DBH, inan rootincrease biomass in root can bebiomass observed can inbe aspen observed stands. in aspen stands.Several authors [25,28,41] have studied the relationship between the aboveground partsSeveral of the tree authors and the [25,28,41] root system. have Directstudied relationships the relationship between between the root-plate the aboveground width and partsdepth of had the been tree foundand the [12 root], determining system. Direct root platerelationsh volume.ips between Root plate the volume root-plate has awidth close andrelationship depth had with been tree found DBH, [12], as established determining for root European plate volume. andRoot Norway plate volume spruce has [31 ],a closewhich relationship was later confirmed with tree in DBH, studies as established by other authors for European [30,42,43 ].beech In our and study, Norway aspen spruce DBH [31],and totalwhich root was biomass later confirmed had a close in studies relationship, by other as authors the correlation [30,42,43]. coefficient In our study, (r = aspen 0.96) DBHindicated and total a good root model biomass fit. Thus,had a ourclose study relationship, hypothesis as the was correlation confirmed coef (Figureficient2). (r = 0.96) In a study on the structural root-plate characteristics of Norway spruce [43], a strong indicated a good model fit. Thus, our study hypothesis was confirmed (Figure 2). correlation (r = 0.92) between H × DBH2 and root-plate volume was observed in mineral In a study on the structural root-plate characteristics of Norway spruce [43], a strong soils. Our results are in accordance with the aforementioned study: the correlation we correlation (r = 0.92) between H × DBH2 and root-plate volume was observed in mineral found between ’ H × DBH2 and total root biomass was strong (r = 0.97). The model soils. Our results are in accordance with the aforementioned study: the correlation we showed a good fit (r = 0.97) for common aspen, and the trend indicated that with an found between aspens’ H × DBH2 and total root biomass was strong (r = 0.97). The model increase in total root biomass, an increase in H × DBH2 could be observed (Figure4). showed a good fit (r = 0.97) for common aspen, and the trend indicated that with an in- The capacity of a tree to resist wind-throw and windbreak is determined not only crease in total root biomass, an increase in H × DBH2 could be observed (Figure 4). by the tree’s height and relative crown height but also by a well-established root system, The capacity of a tree to resist wind-throw and windbreak is determined not only by determined by its width and depth, as the root system provides soil attachment and the tree’s height and relative crown height but also by a well-established root system, de- structural support [10]. Establishing a strong linkage between the root system and the terminedsoil significantly by its width increases and rootdepth, surface as the area root [22 system]. To evaluate provides the soil depth attachment distribution and struc- of the turalcommon support aspen [10]. structural Establishing roots, a TS,strong C, andlinkage S roots between were the used root because system they andaffect the soil aspen sig- nificantlytree resistance increases in wind-throw root surface (Figure area [22].1). C To roots evaluat are locatede the depth closer distribution to the stem of and the bond com- monmore aspen effectively structural with roots, the soil TS, [ 44C,], and but S the roots formation were used of Cbecause roots maythey beaffect influenced aspen tree by resistancevarious factors, in wind such-throw as soil (Figure pressure, 1). C soil roots aeration, are located light, closer temperature, to the stem and and humidity bond more [45]. effectivelyHowever, thewith data the soil on the[44], spatial but the arrangement formation of of C the roots root may system be influenced still remains by various scarce. factors,Several such studies as havesoil pressure, been conducted soil aeration, on the light, persistence temperature, of pine and and humidity spruce roots [45]. during How- ever, the data on the spatial arrangement of the root system still remains scarce. Several

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windstorms [23,37,42,43]. The analysis of the data obtained in our study confirms findings of these studies: there is a close, negative relationship (r = −0.96) between the relative distance from the stem and the relative root-plate depth (Figure3). Comparison of our results with those obtained for other tree species in similar condi- tions [4,30,42,43,46–48] revealed only slight differences in root plate width between aspen and spruce, pine, silver birch. However, root-plate depth of aspen was notably larger than that of mentioned other tree species [30,43,48]. Further research in details of root system development, e.g., in relation to various stand density and microclimatic as well as soil conditions could inform recommendations for foresters and forest owners to improve the resistance of stands in a period of increasing frequency of windstorms and droughts [49,50].

5. Conclusions The increasing frequency of storms and the consequent probability of wind damage in the future might affect forest productivity. The aspen root system is deeper compared to those of other economically important tree species in hemiboreal forests, and aspen also grows faster. Therefore, it can be used to improve wind stability at forest edges, protecting the stands. The practical implication of this approach might be limited by high browsing pressure, but aspen regenerates abundantly by root sprouts and also can be preserved as retention trees of previous generations during the final harvest.

Author Contributions: Conceptualization, A.J.;¯ methodology, A.J.¯ and A.S.; formal analysis, A.S. and V.S.; data curation, A.S.; writing—original draft preparation, A.S., V.S., and D.B.; writing—review and editing, J.R. and A.J.;¯ project administration, A.J.¯ All authors have read and agreed to the published version of the manuscript. Funding: The study was funded by the European Regional Development Fund Project “Birch and as- pen stand management decision support tool for reduction of wind damages” (No. 1.1.1.1/18/A/134). Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to policy of the institute. Acknowledgments: We acknowledge the help of Martins Zeps in data acquisition. Conflicts of Interest: The authors declare no conflict of interest.

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