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Urban Forestry & Urban Greening 12 (2013) 238– 245

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Absorptive root areas of large pedunculate oak trees differing in health statusalong a road in South Bohemia, Czech Republic

Cermák Jan ∗, Simon Jaroslav, Kánová Hana, Tichá SonaFaculty of Forestry and Wood Technology, Mendel University in Brno, Zemedelská 3, 61300 Brno, Czech Republic

a r t i c l e i n f o

Keywords:Healthy and rotten treesModified earth impedanceNon-destructive methodQuercus roburUrban greenery and safety

a b s t r a c t

The health status of 200- to 400-year-old trees growing in an area alongside a road was described indetail on the basis of evaluation of hazard trees in urban areas and analyzed using the acoustic scanningand modified earth impedance (MEI) methods. Acoustic analysis was used to characterize tree trunks andcoarse roots; MEI was used to characterize the active absorptive root surfaces (the electric impedanceof soils and sapwood were also included). Several groups of trees were distinguished according to theextent of damage (slight, serious or extreme), which was easily detectable from the MEI results. It wasconfirmed that both methods gave similar results, which also corresponds to the detailed observations.Instrumental measurements provided additional information about trunk and root injuries, which cannotbe directly observed. A combination of both methods is recommended to achieve the most reliable results.This unique approach is applicable for testing eventual threats by old ill trees for safety reasons over thelandscape.

© 2013 Elsevier GmbH. All rights reserved.

Introduction

Trees are plants that make the landscape more suitable forhuman use, especially by virtue of their transpiration and coolingproperties (Makarieva et al., 2006; Pokorny et al., 2010). The largestLondon hospital (Hajat et al., 2002) published a study represent-ing for Greater London, indicating that if the mean air temperaturerises by 1 ◦C above 21 ◦C, then human mortality increases by 3%. Incontrast, if 10% more urban woody greenery is introduced, temper-atures decrease by 3 ◦C (Gill et al., 2007). However, woody greenerymust be well cared for because it often grows under unsuitable con-ditions (limited space, soil volume, water access, etc.) that couldcause it to become physiologically or mechanically unstable (i.e.,vulnerable to unexpected falls, thereby jeopardizing its surround-ings). Therefore, evaluation of the trees’ health and mechanicalstatus is an issue that has become particularly important in recentyears, although similar studies have been attempted for centuriesalthough based on observations alone. Practically speaking, treediagnostics and treatments for trees, as described in most com-plex arborist’s textbooks, are usually based on visible componentssuch as trunk and crown characteristics. Typically, roots can onlybe assessed indirectly (Kolarík et al., 2008; Kolarík, 2010; Pejchal,2008; Smykal et al., 2008a,b; Zd’ársky et al., 2008). Methods basedon the evaluation of static stability have been employed rather

∗ Corresponding author. Tel.: +420 545 134 181.E-mail addresses: [email protected] (C. Jan), [email protected] (T. Sona).

widely (Vicena et al., 1979; Wesolly and Sinn, 1987; Mattheck,1991).

Different instrumental methods of analyses, especially of treeroot systems, have already been employed. It is generally acceptedthat quantitative measurements of whole tree root systems are dif-ficult, although several methods have been used with this intent(Nadezhdina and Cermák, 2003). The most traditional method isthe manual excavation of skeleton roots, termed the archeologi-cal method (Jenik, 1957; Vyskot, 1976; Válek, 1977; Kutschera andLichtenegger, 2002; Mauer and Palatova, 2002). The use of othermethods has also been described, e.g., ground-penetrating radar(Hruska et al., 1999; Cermák et al., 2000; Wielopolski et al., 2002;Stokes et al., 2002); microscopic and scanning technologies withinthe laboratory (Cudlín and Chmelíková, 1999); electric tomography(Hagrey et al., 2004; Amato et al., 2008) and acoustic tomography(Rinn, 1989, 1999; Lorra and Kathage, 2004; Simon and Cermák,2011; Simon et al., 2011; Simon, 2004).

In addition to acoustic tomography on tree trunks and largecoarse roots (Simon et al., 2011), in this study we also appliedthe modified earth impedance (MEI) method, which is usefulfor indirect measurements of the active absorptive root sur-face (Stanek, 1997; Aubrecht et al., 2006). The MEI method wasfirst tested on the basis of allometric relationships in severalhundred trees of varying sizes, from small seedlings to largetrees (Cermák et al., 2006). The results were compared withthe results of combinations of classical methods (Cermák andNadezhdina, 2011). Here we focused first of all on the appli-cation of this method for practical purposes, namely for theevaluation of root size (the actual active absorptive root area)

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C. Jan et al. / Urban Forestry & Urban Greening 12 (2013) 238– 245 239

coordinated with the acoustic study (Simon and Cermák, 2011;Simon et al., 2011) on the same set of trees with differing healthstatuses and functional stabilities.

Materials and methods

Site characteristics

An alley of pedunculate oak (Quercus robur L.) trees is situatedclose to the castle Ohrada (lat 49◦10′12′′, long 14◦05′42′′) in thegeomorphologic district of the Zliv basin, which is in the easternpart of the geomorphologic region of Ceské Budejovice and neigh-bors that of Bechyne, Czech Republic. The Ceské Budejovice basin ispredominantly composed of pseudogley to hydromorphic soil. Themost frequent soil type (occurring also at the experimental site)is modal pseudogley, followed by pelic pseudogley, stagnogley andgley (occupying almost 20% of the area). The soil components in thepedochor are distributed in a non-regular-to-asymmetric manner.This combination of soil forms is slightly complex, contrasted (espe-cially due to contrast hydromorphism), and has a medium-to-highheterogeneity. From the viewpoint of geobotany, the area is classi-fied as an acidophyllic oak forest (Querceto robori – petrae) that isalso characterized by numerous wetlands and water sites (Málek,1984). Presently, there is a balance of ponds, wet meadows, artificialScots pine (Pinus sylvestris L.) forests and arable land.

Climatically, the area is warm (Quitt, 1971) with long sum-mers (40–50 days) and a mean annual temperature of 15–16 ◦C.The region is also relatively moist, with an annual precipitation of200–400 mm (100–140 days with precipitation over 1 mm). Thewinter is moderately long, with 50–60 cold days (2–3 ◦C) and pre-cipitation over 400 mm (both snow and rain). The typical durationof snow cover (if any) is also 50–60 days.

Stand characteristics

The pedunculate oak stand included 46 trees growing in the areaalongside of an asphalt road situated parallel to a brick wall of thezoological garden at the Ohrada castle. The stand comprised twoage classes of trees: trees over 300 (up to 400) years old, with diam-eters of approximately 150 cm (growing mostly in the central partof the alley), and trees approximately 200 years old (in the east-ern and western edges of the alley), with diameters approximately100 cm or more. The younger trees had been planted in areas for-merly occupied by an older generation of oaks. The trees grew on an8 m wide grass-covered lawn strip including 2 m wide parking lot(made of hollow concrete bricks close to the road) located approx-imately between the brick wall of zoological garden and the edgeof the road. Their growth and biometric parameters were typicalfor the given region. The physiological state of the trees was largelydetermined by the soil conditions around stems at the site. Rootswere reduced at the road edge (caused by bulldozers about 20 yearsago and by manual work long time ago during its construction),partial soil compression in the parking lot alongside of the roadand especially under the bare loamy soil pavement (not covered byany protecting materials) along the brick wall of zoological garden,which was situated below a slight slope from the road. Soil salinity(from de-icing road treatment) slightly increasing after snow hadmelted estimated by measurement of electric conductivity (Marákand Kás, 1948) reached 103.9 and 117.1 �S along the transect inthe area between trees and at the loamy pavement, respectivelyon lighter soil). Soil water reached 14.2 and 21.9%DW in the sameplaces as mentioned above. Generally, high and unstable levels ofunderground water (associated with water level in the pond, whichperiodically changed due to precipitation and fishing operations)limited root growth in the deep soil layers.

Wood-decomposing fungi (e.g., Fistulina hepatica (Schaffer) Fr.and Phellinus robustus (Karst.) Bourd de Galz identified by the spe-cialist represented the largest threat for oak trees. Younger treeswere in the early phases of damage (and at low risk) whereas oldertrees suffered more, sometimes severely (such as when the heart-wood, and sometimes sapwood, was completely decomposed sothat there were open holes in the lower parts of the tree trunks).Breakages of large branches (20–30 cm in diameter) and fruitingbodies of fungi occurred on trees, indicating a high risk of unpre-dictable falls on these frequently used roads and paths along thiscorridor of oaks.

Classification system for the visual damage

Visual tree damage was evaluated according to the classicalmethod of Evaluation of Hazard Trees in Urban Areas, whichincludes a description of the general view of the trees and classifi-cation data based on a series of biometric measurements collectedaccording to Pokorny (2003). In the present study, we focused onthe tree crowns and stems and on a classification of random defects,which may seriously impact the mechanical stability of the treesand their components (Simon et al., 2010). All trees were charac-terized according to their biometric data and their risk of survivalor falling based on the acoustic method (Simon et al., 2011; seeTable 1), which was: (1) low, when tree trunks and visible partsof roots were slightly damaged by fungi, so that small holes andfruiting bodies occurred in branches, (2) intermediate, when largerholes in branches appeared, no holes detected in trunks by acoustic,(3) high, when fruiting bodies and holes occurred in most branchesand some also on main stems (such visible on the acoustic records),but trees still have functional green crowns and (3e) Extremelyhigh, when large holes occurred on tree trunks (detected by acous-tics, but also visually), remnants of crowns were small and theirheartwood was completely rotten and hollow as shown in Fig. 1B.

Evaluation of tree trunks and large roots by the acoustic method

Cross-sections of the trunk bases of the sampled trees werevisualized using the acoustic method, specifically with pulsetomography. This method is based on the detection of sound pulses,generated by a small hammer at the trunk surface, and recordedby a series of a dozen of microphones mounted at equal dis-tances around tree trunks. This method was also applied to largeroot detection, in which microphones remained on the trunks andpulses were generated by a large hammer hitting a metal desk thatwas gradually moved over the soil surface around the trunk circum-ference at a distance of 2–2.5 m. The results from this measurement,which characterized the risk of root destruction, were published inother papers (Simon and Cermák, 2011; Simon et al., 2011).

Measurement and evaluation of the active absorptive root area

Active absorptive root area was estimated on the basis ofmeasurement of tree and soil electric impedance as described in fol-lowing equations. The general electric current continuity equation,which is suitable for describing the environment at the interface ofthe soil and the root (resp. its active absorptive area, or absorptivezone AZ), was applied as the basis for quantification (Stanek, 1997;Aubrecht et al., 2006):

S�j · d�S = S1

�j · d�S + S2�j · d�S = −I + I = 0 (1)

where S is the total surface area of the conductor, d�S is the vec-tor element of the surface, S1 and S2 are the areas through whichthe current passes in and out of the conductor, �j is the vector of


240 C. Jan et al. / Urban Forestry & Urban Greening 12 (2013) 238– 245

Table 1Biometric parameters of pedunculate oak sample trees and general classification of their damage based on acoustic analysis.

Sampletree no.

Diameter atbreast height(cm)

Diameter atground level(cm)

Topheight(m)

Height of max.crown width(m)

Crown baseheight (m)

Crown widthE–W/N–S (m)

Projected crownarea (m2)

Reduced crownvolume (%)

Generalclassification oftree damage(1 = min, 3 = max,ex = rotten trunk)

23 51 57 11 7 4 (8/10) 64 10 222 73 89 22 14 6 (5/9) 38 30 214 75 103 22 11 5 (6/6) 28 10 2Opena 76 83 20 13 8 (12/10) 95 5 26 83 102 21 16 7 (6/4) 20 20 1–2

24 118 118 22 20 9 (8/8) 50 40 324a 131 143 18 16 8 (12/7) 71 30 317 150 169 18 14 8 (10/8) 64 30 3

20 131 143 18 16 8 (12/7) 71 40 3ex18 166 191 22 14 11 (8/11) 71 35 3ex

19 121 131 23 14 6 (8/8) 50 50 3ex21 134 153 23 16 10 (4/6) 20 70 3ex25 140 153 24 19 10 (8/4) 28 50 3ex

a The tree in the fourth row was growing in the open meadow close to the alley.

current density a +I, and −I is the current entering (or leaving) theconductor.

The equation of continuity for a single fine root at the root–soilinterface is as follows:

I = U1

R1= U2

R2= U1S1

�L1= U2S2

�L2(2)

where R is the electric impedance (in ohms, �, which is practicallythe same as resistance under low frequency as applied), U is thevoltage (in volts, V) � is the electric impedance of the root tip (orAZ), which is likely to be the same as the soil closely surrounding theroot surface, AZ. We are interested in the area of S1 (or AZ surfacearea). Therefore, we applied Eq. (2) because the current continu-ity between the root surface AZ and the conducting elements oftracheids or tracheas evidently fit the following equation:

S1 = S2 = 1U2

�L2. (3)

We applied the potential electrode P2 to the soil at approxi-mately the area in which the roots end (where section “0” contactsSection I of potential characteristics (Fig. 2). The potential electrodeP1 was placed into the wood at the ground level (the stem–groundinterface). The voltage U∗∗

2 between P1 and P2 (which are at dis-tance L∗∗

2 ) determines the S2 or AZ of the single root, which can bedescribed as follows:

S1 = S2 = 1U∗

2�L∗

2 = 1U∗∗

2�L∗∗

2 . (4)

where � represents stem tissue resistivity obtained by independentmeasurements.

We moved the potential electrode P2m in defined steps (lengthgradually increasing from 0.3 to 1.0 m) from the sample tree to thecurrent electrode C2 when measuring AZ segments with discreteinstruments. Logometer data (FLUKE 1625, Fluke Corporation, USA,operating under voltage of 48 V and frequency of 94–128 Hz) were

Fig. 1. General view of the sampled trees along the road and an example of an extremely damaged tree with completely rotted heartwood and sapwood, to the north (towardthe brick wall of the fence).



Fig. 2. Schematic of the electric field in the soil around a sample tree.

recorded at each step. The critical distance Lcrit and correspond-ing impedance �crit were reached, when the data plateaued (i.e.,at the interface between sections I and II of the potential charac-teristics). These measurements were repeated for each measuredsegment given by an angle of 60◦. Four zones characterizing theabsorptive root area were situated in the lawn strip along the road(right and left), one in direction to the road (South) and one to thefence (North). We also measured stem and soil impedance fromfour directions and included measured data in the rearranged Eq.(5):

SAZ = �critLcrit I

U��ϑ[m2, ˝m, m, V, A] (5)

The first dimensionless coefficient � considers the effect ofmutual electric shading of roots. It causes small negative errors inthe estimates of AZ. The second dimensionless coefficient � char-acterizes mechanical damage to the roots (if any). It causes small,positive errors in the AZ measurements. The third dimensionlesscoefficient ϑ represents also a small negative error in the AZ esti-mates that is caused by electric currents that have been directedbeyond the measured segment (e.g., through the neighboring seg-ments).

The measured values are located at the curve of the surfacehyperbole of the impedance conus. We can estimate the interfacesbetween sections I and II, and therefore, the distance of Lcrit. We

introduced this value along with the measured stem impedance Rresulting in the following equation:

SAZ = �critLcrit

Rlog o��ϑ[m2, ˝m, m, ˝] (6)

Four measured segments with corresponding derived criticaldistances and the measured stem and soil impedances character-ized the root system of each tree. Two of these segments wereoriented along the road (to the east and west or the right and leftsides of each tree), allowing the characterization of roots along theroad in a grass strip. Of the other two segments, one was orientedto the south (in the perpendicular direction of the road), and theother was oriented toward the north in the direction of the loamypavement and fence brick wall.

The measured values of the active root surfaces described thesample trees of given diameters at breast height and basal areas.However, an evaluation of the basal areas was ineffective becausemost of the stems were rotten in the center. Therefore, we appliedthe cross-section area of the sapwood at breast height (Ssapw),derived from its approximate thickness and stem xylem circum-ference. The results can be better compared when we calculate themean active root area from the SAZ of individual sample trees usingthe following equation, where indexes t and i for Ssapw characterizedwhole tree and measured section size, respectively:

SAZ ti = SAZ i

(Ssapw t

Ssapw i

)(7)



We expressed the results of the measurements using pointsarranged along an opened stem circumference (azimuth) and con-nected by the corresponding curve.

The electric impedance of soil and sapwood was measured bythe Wenner’s method using four stainless steel electrodes insertedat different distances (2, 4 and 6 cm in sapwood, 0.5, 1.0 and 1.5 min soil), which eliminated the impact of surface resistances on theresults. The electric field under this configuration reached differentdepths with increasing electrode distances (approximating thesedistances) and this was expressed in terms of approximate relativedepths in corresponding materials (%max).

Results and discussion

Electric impedance of soil and oak sapwood

The electric impedance of the soil around the oak sample trees(Fig. 3) indicates high data variability, with peak values obtainedclose to the soil surface that dramatically decreased as the soil lay-ers became deeper. It is important to bear in mind that the appliedmeasurement technique gradually integrates values (i.e., the upperlayer characterizes only itself; the middle upper layer, middle lay-ers and lower layers characterize one or both of the layers above it).The situation was similar on different sides of the trunk althoughindividual values can differ several-fold. The results indicate thatthe soil was highly electrically conductive, especially in the lowerlayers, which indicated a high level of underground water and apossible accumulation of ions. These ions are most likely de-icingsalts, from snow sometimes melted in the winter months thatflowed along the slight slope from the road to the brick wall ofthe fence. The electric impedance of the soil surrounding the oaktrees was very different when comparing the shallow, medium anddeep rooting layers. The highest variations were in the southernstem sides (i.e., close to the road; the smallest values were observedin old, seriously damaged trees characterizing wet rot xylem) andwestern stem sides (in this case, the old, seriously damaged treeshad the highest values, characterizing dry rot xylem). Variationcaused by observed wet or dry rot tissue in sapwood, occurred dueto different fungi development phases and eventual open holes tobranches (rotten heartwood), which allowed penetration of rain-water into hollow trunks. Other reasons for variation than xylemwetness are unknown but occurred perhaps due to accidental accu-mulation of de-icing salt along the road.

The electric properties of soils are dependent on water con-tent, ion concentration and compaction (Gupta and Hanks, 1972;McCarter, 1984). The soil water content was close to the saturationpoint in our case, in which the trees were growing on a flat areaabout 50 m aerial distance from the nearest pond. The soil was uni-formly, but not strongly, compacted along the lawn strip; however,but it was highly compacted at the surface of the frequently usedpavement situated along the brick wall located north of the trees(this was also reflected in significantly lower electric impedancethere, especially in shallow soil layers; see Fig. 1). This evidentlyworsened the growing conditions for the roots, which had startedto decay especially in such places. It was not surprising that mostseverely damaged trees had completely rotten and hollow stemson this side.

The electric impedance of sapwood from different cardinalpoints (Fig. 4) was similar in younger, nearly healthy trees butappeared very different in the north-facing trunk side of all seri-ously damaged trees. The variation between different sapwooddepths (which was difficult to specify due to the rot) was relativelysmall and insignificant. The larger values occurred in shallow andintermediate sapwood depths in seriously damaged trees but lessso, and in the opposite order, in the extremely damaged trees. This

Fig. 3. Soil electric impedance across different soil depths (%max) in young, relativelyhealthy trees (upper panel), old trees that are seriously but not yet critically damaged(middle panel) and extremely damaged trees (lower panel), according to the side ofthe trunk. The south faces an asphalt road, the north faces a brick wall of the fencealong which is situated the bare loamy soil pavement beneath and the east and westare along the asphalt road.

most likely indicates significantly lower tissue water content onthis side, which could be the result of partial but advanced trunkdecomposition after the trunk passed a wet phase of this process.

The distribution of stem resistivity at different depths of Quer-cus robur is known to be highly heterogeneous, having layers ofcontrasting resistivity from pith to cambium (Gocke et al., 2007).The degree of resistivity depends on many factors, of which watercontent and the concentration of chemical elements are the mostimportant. Low resistivity can indicate increased moisture con-tent whereas hollowed structures can cause increased resistivity.Close to the heartwood, there is less-conductive storage sapwood;



Fig. 4. Sapwood electric impedance at different depths below cambium (%max) inyoung, relatively healthy trees (upper panel), old trees that are seriously but notyet critically damaged (middle panel) and extremely damaged trees (lower panel)according to the side of the trunk. The south faces an asphalt road, the north faces abrick wall with bare loamy soil pavement beneath and the east and west are alongthe road.

healthy trees have a region of low resistivity in the center of thestem, but have very high resistivity if the stem is rotten and there-fore hollow. It was not possible to assess the sapwood in rotten treesprecisely because the transition zone between the inner edges ofthe remaining sapwood was unclear and highly variable.

Active absorptive root area of oaks

The applied variant of the modified earth impedance methodavoided some substantive errors that have previously been prob-lematic (Urban et al., 2011) and confirmed the validity of our own

Fig. 5. The mean active absorptive root area in three groups of oak trees: young,healthy and only slightly damaged trees, old, seriously but not yet critically damagedtrees and extremely damaged trees, according to the side of the trunk. The south(lower values) faces an asphalt road, the north faces a brick wall with bare loamy soilpavement beneath (lower values) and the east and west are along the road (highervalues).

earlier results as well as those of other researchers (Cermák et al.,2006; Cermák and Nadezhdina, 2011; Butler et al., 2010; Cao et al.,2010, 2011). This method yields results that differ from other tech-niques, such as multi-electrode resistivity imaging (Hagrey, 2007;Amato et al., 2008, giving a 3D picture of resistivity), when the out-put represents m2 per stem section or the whole tree. However, thegeneral conclusions about the presence and number of roots aroundstems, based on the results obtained by both different methods, aresimilar.

The active absorptive root areas (mean values of all sample trees– Fig. 5) were relatively high on both sides (in both the east andwest) along the road (below a grass-covered strip) but significantlylower in the direction of the road (where the root systems had beendamaged by bulldozers during road construction to the south andbrick wall construction to the north). High soil water content (andlow soil impedance – see Fig. 1) occurring along the slight slopeover the lawn strip down to the bare loamy-soil pavement, char-acterizes excess of water, which played a role in worsening theconditions for root growth and survival. Roots cannot grow downto a high and stagnant (hypoxic) water table, and they also had beendamaged from two sides (from the south and also from the north)by human interference and the presence of stones and remnantsof construction materials in the soil (concrete, bricks and stones).Very low impedance in deep soil layers occurring near all sam-ple trees reflects the presence of a high water table that reachedapproximately 60–70 cm during data collection.

The differences between relatively healthy and damaged rootsystems of trees are even more visible when considering theirrelationships to tree diameter at breast height, DBH (Fig. 6). Thevalues of approximately healthy and relatively young (100–200years) oaks occurred almost at the same line when the effectiveabsorptive root area increased with DBH. However, this relation-ship only holds true until trees reach a certain diameter. Largerand older trees started to deviate from this line around the singlehinge point. Old trees with seriously but not yet critically damagedtrunks and remaining large crowns deviated only slightly (by about18◦) from the relationship shown in healthy trees. When consider-ing the old trees that were seriously but not yet critically damagedand have small crowns, their pattern is even more deviant (by 55◦)from healthy trees and the absorptive root area decreases as thetrunks grow. The largest deviations (by 75◦) occurred in extremely



Fig. 6. The relationships between active absorptive root area and trunk diameterat breast height in a series of oak sample trees in relatively young, healthy or onlyslightly damaged trees (straight dashed line full square marked points, classifica-tion of 1–2), old trees that are seriously but not yet critically damaged and haveremaining large crowns (“+” marked points, line deviating by about 18◦), old treesthat are seriously but not yet critically damaged and have remaining small crowns(“x” marked points and line deviating by 55◦ , classification of 3) and extremely dam-aged trees, roots, stems and crowns (by full triangle marked points and line deviatingby 75 deg, classification of 3EX). Interestingly, lines representing all groups rotatearound the single hinge point, corresponding to DBH = 110 cm.

damaged trees in which fungi had split the trunks and theremaining crowns were small and damaged (the absorptive rootarea decreases even more as the trunks grow). The hinge point atthe given figure corresponds to DBH of approximately 110 cm andage approximately 250–300 years.

Root estimates by the acoustic pulse method and modifiedelectric impedance method are based on completely differentprinciples and are absolutely independent. The first method char-acterize sound speed in large coarse roots (diameter say around10 cm), which can conduct water and are visible on the records aspeaks, the second one flow of ions in water through the tiny fineroots (diameter around 1 mm), particularly the absorptive area inthe defined sections around trees. The active absorptive root areaof fine roots must be balanced with the corresponding conduct-ing area of coarse roots. If the coarse roots are rotten or missing insome directions (which is well visible on acoustic records), thereare certainly also missing the already decomposed absorptive rootsconnected with them. However some of fine roots can be at theedges of the wider (60◦) sections, which are connected to remainingneighboring coarse roots, therefore no zero area values were found.The variables estimated by the above mentioned methods can-not be directly plotted against each other, but general conclusions(i.e., large root presence, their high acoustic conductivity and highabsorptive area values or no presence of coarse roots or presence oflow acoustic conductivity in partially decomposed roots and mini-mal values of absorptive area) can confirm each other very well.

The above mentioned stem diameter is most likely the pointat which tree size or its structural balance becomes more andmore unfavorable. We understand by the structural balance theroot/shoot ratio (more precisely the ratio of the active absorptiveroot area to sunlit leaf area), trees living under conditions in whichthe active absorptive root area becomes increasingly small in rela-tion to leaf area are less and less resistant to a secondary attack byany biological pests, such as fungi and insects.

The information obtained regarding the absorptive root statusof the trees and other similar characteristics, such as the acousticscanning of tree trunks (Nicolloti et al., 2003; Gocke et al., 2007) andlarge, coarse roots, is needed to make decisions about whether anyindividual tree is functionally and mechanically stable and if it willnot jeopardize its surroundings by an unexpected fall. If so, the tree

should be removed as soon as possible to prevent any unfortunateevents; the goal of protecting heavily damaged and unstable treescannot play a role here. In contrast, if it is objectively proved thata tree is stable enough, it should be left alone or mildly treated byan arborist (e.g., shortening branches, etc.).

Fungal attack by Fistulina hepatica and Phellinus robustus seemsto be the most important, long-term process leading to the gradualdestruction of old trees near the Ohrada castle. Variations in the lev-els of underground water can cause a gradual decline of the sinkerand taproots if allowed to develop in relatively young trees. Theextent of fungal damage to younger trees was low (eventual firstoccurrence of fungi), indicating that the trees were low-risk. Fun-gal infections were very serious in older trees and quite extreme insome cases. These infections can grow larger with increasing age,causing more and more serious damage. The worst situation was inthe extremely damaged trees. Such trees typically have destroyedsapwood and root systems in direction to the brick fence and ahigh occurrence of fragments, all of which represent an unaccept-ably high risk of unexpected fall (see Fig. 1B). In such cases it willbecome a topic for forensic engineering practices (Alexandr, 2010).

Conclusions

The fine root characteristics of the old oak trees, particularlythe active absorptive root area as described above, correspond tothe characteristics of the skeleton root distribution and the healthstatus of the tree trunks obtained by the acoustic method (Simonand Cermák, 2011; Simon et al., 2011). Similar conclusions aboutthe status of the root systems (high sound speed in coarse rootsand large absorptive area in fine roots in healthy trees and viceversa in damaged or destroyed, i.e., decomposed roots) confirm thatboth methods are suitable for field applications to study whole-treeroot systems. The results indicate that the applied modified earthimpedance method could detect the functional state of root sys-tems critical for their survival in terms of actual active absorptiveroot surface. A combination of acoustic and impedance techniquesmakes the obtained results substantially more reliable and repre-sents the unique approach applicable for testing of eventual threatsby old ill trees for safety reasons near settlements as well as overthe landscape.

Acknowledgements

The study was performed as a part of the project VZ MSM6215648902; the purpose of this project is forest and water supportof functionally integrated forest husbandry and the application ofwood as a renewable material. The authors would like to expresstheir appreciation of the temporal technical help they received dur-ing the field season from V. Zidlík, K. Rebrosová, J. Kadlec, and R.Stepánek and to K. Rejsek for estimation of soil conductivity, allfrom the Faculty of Forestry and Wood Technology at the MendelUniversity in Brno. The authors are also very grateful for languagecorrections by American Journal Experts and valuable comments ofboth reviewers.

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Author's personal copy - otevrenazahrada.cz · Author's personal copy 240 C. Janet al. / Urban Forestry& Urban Greening12 (2013) 238 245 Table 1 Biometric parameters of pedunculate