Trees for urban environments in northern partsof Central Europe – a dendroecological studyin north-east Romania and Republic of Moldavia

Henrik Sjöman & Anders Busse Nielsen & Adrian Oprea

# Springer Science+Business Media, LLC 2011

Abstract A limited number of species and genera currently dominate the tree stock instreets and urban sites. There has been considerable and persistent argumentation for thenecessity of using a more varied and stress-tolerant selection of tree species. This paperreports results from a dendroecological study of six steppe forest reserves in north-eastRomania and in the adjacent part of the Republic of Moldavia, where water stressregimes during the growing season and winter temperatures are comparable to those ofinner city environments in northern parts of Central Europe and adjoining milder partsof Northern Europe (CNE-region). In each forest reserve, tree growth patterns werestudied in five 20 m×20 m plots, resulting in a total of 30 plots with an allocated areaof 1.2 hectares. For all trees, height and diameter were measured and related to tree ageby coring in order to detect the species growth and performance in these sites. In total23 tree species were found, 13 of which were represented by 25 or more individualswith documented good growth in the study plots. The majority of these 13 species havevery limited use in urban greenery in the CNE-region today and thus have the potentialto increase the species diversity of the current urban tree population through furtherselection work.

Keywords Dendroecology. Habitat studies . Site-adapted species use . Tree selection . Urbansites

Urban EcosystDOI 10.1007/s11252-011-0187-2

H. Sjöman (*) :A. B. NielsenFaculty of Landscape Planning, Horticulture and Agricultural Science, Department of LandscapeManagement, Design and Construction, Swedish University of Agricultural Sciences, Box 66, SE-23053Alnarp, Swedene-mail: Henrik.Sjoman@slu.se

A. B. Nielsene-mail: Anders.Busse.Nielsen@slu.se

A. OpreaAnastasie Fătu Botanic Garden, Dumbrava Roşie Street, Iasi, Romania

A. Opreae-mail: aoprea@uaic.ro

Introduction

A limited number of species and genera currently dominate the tree stock in streets andurban sites, as shown by recent surveys in European and North American cities (Pauleit etal. 2002; Raupp et al. 2006; Bühler et al. 2007). This lack of diversity is particularlyapparent in Northern Europe (Pauleit et al. 2002). In Oslo, Norway, one clone of lime tree(Tilia × europaea L. ‘Pallida’) represents 70% of all newly planted street trees, while inCopenhagen, Denmark, London plane (Platanus × hispanica Münchh.) and Tilia spp.represent 61% of all street trees planted between 1990 and 2000 (Bühler et al. 2007). In acompilation by Sæbø et al. (2005) based on a survey by Pauleit et al. (2002), only eight treespecies, Acer platanoides L., A. Pseudoplatanus L., Aesculus hippocastanum L., Betulapendula Roth., Betula pubescens Ehrh., Populus trichocarpa Hook., Sorbus spp. and Tilia xeuropaea L., dominated in street environments in Northern Europe, compared with 24species in Southern Europe.

Over the past few decades, a growing proportion of these commonly used species haveexhibited increasing difficulties in coping with the conditions at urban sites. The complexstress situation in most urban sites has resulted in tree decline and an increase in diseases(Pauleit 2003). This negative trend, combined with the challenges of climate change and thethreat of future disease and pest attacks (e.g. Sun 1992; Tello et al. 2005; Raupp et al.2006), has led to considerable and persistent argumentation for the necessity of using amore varied and stress-tolerant selection of tree species in urban locations (Richards 1983;Duhme and Pauleit 2000; Pauleit 2003; Roloff et al. 2009).

Increased temperature (based on global climate change and additional urban heatisland effects) and increased water run-off caused by impermeable surfacing make waterstress a main constraint for tree growth and health in the urban environment (e.g. Craul1999; Roloff et al. 2009). Furthermore, urban soils, especially in street environments, aregenerally well-drained, leading to even harsher conditions for trees (Sieghardt et al.2005). Research on the drought tolerance of trees has classically focused on physiologicalreactions in water balance/water use in terms of transpiration rates, sap flow measure-ments and the hydraulic architecture of the tree (e.g. Kozlowski et al. 1991; Sperry et al.1998; Breda et al. 2006; David et al. 2007; West et al. 2007). These investigations givevaluable information about tree physiology at species level, but are limited in theirpractical everyday use for urban tree planners, arborists, etc. (Roloff et al. 2009).Dendroecological studies, such as that presented in this paper, can contribute ecologicalknowledge that helps evaluate the reaction, tolerance and performance of different treespecies to water stress, and have been suggested as a valuable first step in the selectionprocess for ‘new’ tree species or phenotypes for urban sites (Roloff et al. 2009).Nevertheless, as pointed out by Roloff et al. (2009), dendroecological descriptions areseldom or never available for most species.

In natural habitats, trees have been stress-tested and selected over evolutionary periodsof time. Some species have developed an extensive plasticity and tolerance to a range ofenvironmental conditions, while others have specialised in certain habitat types (Rabinowitz1981; Gurevitch et al. 2002). For instance, steep south-facing mountain slopes with thin soillayers or warm and dry steppe environments represent distinct habitat types where theenvironmental parameters that define the particular habitat and separate it from otherhabitats have shaped the evolution of plants. Such environmental parameters also screen outmany potential colonising species not adapted to the particular habitat. Investigating theecological background and performance of species growing in habitats with drought duringthe growing season and winter temperatures similar to those of inner city environments in a

Urban Ecosyst

particular area can assist in identifying alternative tree species and phenotypes for furtherstudies regarding use in urban sites (Flint 1985; Ware 1994; Ducatillion and Dubois 1997;Sæbø et al. 2005; Roloff et al. 2009). Moreover, the position of a species in the foreststructure of its natural habitat provides additional information. Trees in the canopy layermodify the wind, humidity and temperature microclimate for species in the understoreylayer (Oliver and Larson 1996). Canopy species, on the other hand, experience much moreeffective transpiration due to their exposure to wind and sun, which leads to warmertemperatures. However, in order to determine these site-related properties a suitable ecotypehas to be defined, since population differentiation within species regarding tolerance to aspecific habitat and climate is well documented (e.g. Jones et al. 2001; Mijnsbrugge et al.2010). This is especially important among species with a large natural distribution.

From the perspective of northern parts of Central Europe and adjoining milder parts ofNorthern Europe (in the following abbreviated to the CNE-region), it is unlikely that thespecies-poor native dendroflora can provide a large range of tree species with extendedtolerance of the environmental stresses characterising e.g. paved sites within urban areas ofthe region (Ellenberg 1988; Duhme and Pauleit 2000). However, other regions with acomparable climate but with a rich dendroflora may have the potential to provide new treespecies and genera for this purpose (Takhtajan 1986; Breckle 2002; Roloff et al. 2009). Innorth-east Romania and the Republic of Moldavia, on the eastern side of the CarpathianMountains, there are large areas of steppe and steppe forest vegetation with a richdendroflora of many rare and untested tree species (Duhme and Pauleit 2000). Here treesare exposed to a warm, dry summer climate and winter temperatures similar to those ininner city environments in the CNE-region (Breckle 2002; Ursu 2005). This paper reportsfindings from extensive dendroecological fieldwork in steppe forest reserves in north-eastRomania and in the adjacent part of the Republic of Moldavia and discusses the usepotential of the studied tree species phenotypes for inner city environments in the CNE-region. In particular, the study focused on:

– Characterising growth patterns and performance of tree species identified– Presenting and discussing promising tree species for further studies regarding use in

urban sites in northern Europe.

The study forms part of a four-year research programme initiated by the SwedishUniversity of Agricultural Sciences to examine selection of site-adapted species for urbansites in the CNE-region. Other case study areas are located in the Qinling Mountains(China) and the Caucasus (Georgia). The underlying hypothesis in this selectionprogramme is that ‘new’ tree species for urban use can be identified through studies ofnatural habitats where trees are exposed to stresses similar to those in the urban pavedenvironment.

Materials and methods

Study area

North-east Romania and the Republic of Moldavia have a temperate continental climate,with hot summers, long, cold winters and distinct seasons. Due to the higher summertemperatures and lower rainfall compared with Western Europe, large forest steppe systemshave developed in this area (Breckle 2002).

Urban Ecosyst

Due to the long history of civilisation and related agriculture, with extensive browsingcombined with the warm and dry climate, forest has been on the retreat for centuries, andtoday the landscape is characterised by agriculture and steppe meadow vegetation. Most ofthe forests have been converted to Robinia pseudoacacia L. plantations, while semi-naturalforest remnants are few. During the 1960s and 1970s, many of these remnants weredesignated nature reserves and were thereafter spared from logging. During September-November 2009, field studies were carried out in six such semi-natural forest reserves:Roşcani Forest Reserve (53°21′47′′ N, 23°74′26′′ E), Repedea Hill Reserve (52°82′96′′ N,24°00′27′′ E), Codri Forest Reserve (52°95′86′′ N, 24°51′86′′ E), Bădeana Forest Reserve(51°01′51′′ N, 24°08′67′′ E), Bursucani Forest (51°67′88′′ N, 24°26′23′′ E) and RădeştiForest (51°71′92 N, 24°29′35 E) (Fig. 1).

Calculation of potential water stress

Since water stress is the main concern for urban trees (e.g. Craul 1999; Roloff et al. 2009),the potential water stress (net water difference) in the study plots was calculated. Conditionsat the study sites in Romania and Moldavia were compared with the present climate in twodifferent site situations in the inner city environment of Copenhagen (Denmark) in order toexamine the potential usefulness of the studied tree species for two types of urbanenvironments in the CNE-region: 1) urban paved sites; and 2) urban park environments.Copenhagen was used as an example to illustrate growth conditions in a large city in thenorthern CNE-region (Fig. 2).

In calculating potential evapotranspiration the regression presented by Thornthwaite (1948)was used, with monthly potential evapotranspiration based on the values of temperature(Table 1), number of sunshine hours per day and cloudiness. Sunshine hours per day wereestimated on a monthly basis by combining them with information about day length (Meeus1991), while days with rainfall were used as an indicator of cloudiness (Ursu 2005).

Climate data for the field study areas (Table 1) were obtained from the nearbymeteorological stations of Iaşi (for Repedea Hill and Roşcani Forest), Galaţi (for Rădeşti

Fig. 1 Position of the six study sites (marked in red) in north-east Romania and the Republic of Moldavia

Urban Ecosyst

Copen

hage

n (p

arkn

viron

men

t)

Ceorn

esti (

Codri)

Bârlad

(Bad

eana

and

Bur

suca

ni)

Galati (

Rades

ti)

Copen

hage

n (p

aved

env

ironm

ent)

Iasi

(Rep

edea

Hill

and

Rosca

ni)

47,72

65,14

82,57

100,00

Sim

ilari

ty

Dendrogram

Jan Feb March April May June July Aug Sept Oct Nov Dec Ia i (Repedea Hill and Ro cani) 24.3 48.9 63.2 44.4 -26.9 -103.5 -189.6 -287.0 -331.8 -336.0 -317.0 -285.5 Gala i (R de ti) 27.6 51.6 60.0 47.5 -7.4 -76.5 -109.7 -323.6 -396.5 -421.4 -401.9 -372.8 Corne ti (Codri) 34.9 66.2 89.9 93.6 52.5 24.6 -44.0 -145.9 -188.4 -213.3 -179.8 -143.7 Bârlad (B deana and Bursucani) 12.2 27.5 37.5 35.9 -1.7 -78.2 -186.5 -317.1 -386.8 -405.2 -389.7 -366.9 Copenhagen (park environment) 32.4 54.0 67.5 65.7 33.8 -8.6 -41.5 -83.2 -92.9 -87.1 -55.2 -18.0 Copenhagen (paved environment) 10.8 18.0 11.1 -11.7 -67.6 -137.0 -204.1 -278.8 -320.3 -342.7 -342.0 -333.0

Fig. 2 Calculated potential water stress in the study plots and at urban paved sites and park environments inCopenhagen, Denmark, and a dendrogram illustrating the site match between the natural and urban sites

Table 1 Mean monthly temperature (°C) and precipitation (mm) in the field study areas

Jan Feb Mar April May June July Aug Sep Oct Nov Dec Annualmeana

Iaşi (Repedea Hill and Roşcani)Mean monthlytemp. (°C)

0 0.4 3.6 11.2 16.6 20.8 22 20.9 15.7 9.9 4 0 M=10.4

Precipitation (mm) 27 28 29 36 34 64 74 70 56 48 31 35 S=532

Galaţi (Rădeşti)Mean monthlytemp. (°C)

0 0 4.1 10.6 16.5 20.3 22.6 22 17.6 11.5 5.2 0 M=10.5

Precipitation (mm) 30.7 26.6 23.6 37.4 49.2 66.3 47.3 40.5 38.7 33.4 34.5 32.3 S=460.5

Corneşti (Codri)Mean monthlytemp. (°C)

0 0 1.9 9 15 18.1 20.6 20 15.5 9.6 3.1 0 M=8.7

Precipitation (mm) 38.8 34.8 33.8 52 59.4 99.1 83.8 59.8 61.8 27.8 46 40.1 S=637.2

Bârlad (Bădeana and Bursucani)

Mean monthlytemp. (°C)

3.6 1.7 3.6 10.1 16.0 18.7 21.2 21.1 16.6 10.4 4.4 0.8 M=9.8

Precipitation (mm) 23.7 21 24.7 48.9 67.2 46.6 42.4 35.6 35.7 36.2 28.7 26.6 S=448

aMean annual temperature (M) and cumulative precipitation (S) in the respective area

Urban Ecosyst

Forest), Corneşti (for Codri Forest) and Bârlad (for Bădeana and Bursucani Forests) (Sîrbu2003; Ursu 2005).

Estimates of water run-off for the studied woodlands were based on P90 (2004), with anassumed 10% run-off.

Urban sites in Copenhagen currently have a mean annual temperature of 8–12°C whenthe urban heat island effect is included (+1–3°C) (DMI 2009; US EPA 2009) and meanannual precipitation of 525 mm (DMI 2009). In calculating potential water stress (net waterdifference) for the sites in Copenhagen, park environments were assumed to have 10%water runoff and paved areas 70% water runoff (P90 2004).

Calculations of potential water stress in the study areas showed that the sites at Iaşi hadan early negative net water balance of 26.9 mm in May, while Galaţi and Bârlad had alower negative net water balance during the same period of 7.4 and 1.7 mm, respectively.This difference continued throughout June. In Iaşi and Bârlad the negative net waterbalance in July increased much more rapidly, while the trend was less dramatic at Galaţi.However, in August Galaţi experienced a significant alteration, with a peak negative netwater balance (Fig. 2) due to its southerly location with a warmer summer climate and lowlevels of precipitation (Table 1). In early autumn Galaţi experienced the highest waterdeficit, followed by Bârlad and Iaşi (Fig. 2). With its lowest net water balance in Julyonwards and with no abrupt changes, Corneşti deviated from the other study areas (Fig. 2)due to higher precipitation (637.2 mm) and lower mean annual temperature (8.7°C)(Table 1). Overall, Codri differed from the other five study areas due to higher precipitationand lower mean annual temperature, whereas the other sites had rather similar water stressstatus over the season. Rădeşti had a somewhat slower negative net water balance early inthe season compared with Bădeana, Bursucani, Repedea Hill and Roşcani, but had a muchmore rapid negative trend in August (Fig. 2). The net water balance at paved sites inCopenhagen showed negative values from as early as April, with a continuously ratherdramatic negative trend throughout the season, while park environments developed anegative net water balance in June, followed by a less dramatic negative trend (Fig. 2).

In order to detect site matches between the study sites and inner-city environments ofCopenhagen, cluster observations were made in Minitab 16 Statistical Software, creating adendrogram (Fig. 2). Codri showed a closer match with park environments in Copenhagen,while the other study sites showed a closer match with paved sites in Copenhagen.

Field measurements

In each forest reserve, tree growth patterns were studied in five 20 m×20 m plots, resultingin a total of 30 plots with an allocated area of 1.2 hectares. Plots were strategically placedwithin recognised forest stands, with particular attention paid to areas with mature forestand homogeneous site conditions.

In all plots, soil texture, humus content and pH value were determined. Soil sampleswere collected at three different depths (0–20, 20–30, 30–50 cm) from 2 pits randomlydistributed in each field plot, resulting in 10 pits in each forest reserve (Klute 1986; FAO2006). The replicate samples for each depth and plot were pooled before analysis (FAO2006). Soil texture was analysed using the soil grain analyser method (Ehrlich andWeinberg 1970), organic matter using the K2Cr2O4 method (Sims and Haby 1971), and pHusing the potentiometric determination method (soil/water = 1:2) (Tan 2005). Since thedifferences between the three soil depths were very small, mean values for all samples arepresented in Fig. 2. The soil analysis revealed that the sites could be divided into twoseparate groups (Fig. 3). Bădeana, Bursucani and Rădeşti had comparable lower levels of

Urban Ecosyst

6050403020

35

30

25

20

15

10

Silt (%)

Cla

y (%

)BadeanaBursucaniCodriRadestiRepedea HillRoscani

Site

Scatterplot of Clay vs Silt

7,06,56,05,55,04,5

9

8

7

6

5

4

3

2

1

0

pH

Hu

mu

s (%

)

BadeanaBursucaniCodriRadestiRepedea HillRoscani

Site

Scatterplot of Humus vs pH

Area Mean clay (%) Mean silt (%) Mean humus (%) Mean pH

1.64.41.453.72Cirdo

R de 4.50.48.535.61it

7.63.20.858.62lliHaedepeR

Ro 0.57.21.850.33inac

B 9.54.27.828.71anaed

8.48.18.334.61inacusruB

Fig. 3 Selected properties of the soil in the field plots at the six different sites. Values shown are the mean ofthree different sampling depths

Urban Ecosyst

clay and silt and had therefore lower water-holding capacity compared with Codri, RepedeaHill and Roşcani (Fig. 3). Furthermore, the humus content differed between the plots, withCodri and Rădeşti having mean values over 4%, while the remaining sites had less than2.7% (Fig. 3).

Trunk diameter at breast height (DBH, 1.3 m above the ground), total tree height andtree age were determined for each individual tree in the plots. Tree height was measuredwith a clinometer (Haglöf Vertex IV). To establish age, all trees were subjected to drillingfor growth ring counts as close to the ground as possible (Grissino-Mayer 2003). Moreover,the position of the trees within the vegetation structure was surveyed to distinguish canopyfrom understorey.

Field data were entered into Minitab 16 Statistical Software and for species representedby 25 individuals or more in the study plots, the height and DBH were divided by age,allowing mean annual tree growth to be calculated (Table 4). Moreover, in order to detectany differences in growth of the species between the different study sites, one-way ANOVAfollowed by a Fisher’s post-hoc test were performed for all species found in more than onestudy area (Table 3).

Results

A total number of 1159 trees representing 23 species were found in the 30 plots throughoutthe six different areas (Table 2). The following 13 species had 25 individuals or moreoccurring in the studied plots: Acer campestre L., Acer tataricum L., Carpinus betulus L.,Carpinus orientalis Mill., Cornus mas L., Crataegus monogyna Jacq., Fraxinus excelsiorL., Quercus dalechampii Ten., Q. frainetto Ten., Q. pubescens Wild., Q. robur L., Sorbustorminalis (L.) Crantz. and Tilia tomentosa Moench. (Table 2).

Among these 13 species, the four oak species Quercus dalechampii Ten., Q. frainettoTen., Q. pubescens Wild. and Q. robur L., together with Fraxinus excelsior L., were mainlypresent in the canopy layer, while Acer tataricum L., Cornus mas L., Crataegus monogynaJacq. and Sorbus torminalis (L.) Crantz. were mainly present in the understorey layer (Table4). The remaining species were present in high numbers of individuals in both structurallayers of the stands (Fig. 4).

In the comparison of mean yearly height increment of the species in different studyareas, significant differences were only observed for four of the 10 tree species that existedin more than one study area (Table 3). The remaining species did not show any significantgrowth differences between the sites. The largest difference in mean yearly heightincrement between the sites was for Tilia tomentosa Moench., with an difference of 14 cmbetween the trees in Bursucani (27 cm) and Repedea Hill (41 cm) (Table 3). Furthermore,among the species found in Codri and in other study areas, none had greater growth inCodri, although the growth conditions were moister and cooler there compared with at theother sites. For example, Tilia tomentosa Moench., had 9 cm greater yearly heightincrement in Repedea Hill compared with Codri (Table 3).

In the comparison of the mean yearly trunk diameter increment (DBH) between thespecies in different study sites, six of the 10 species which existed in more than one studyarea showed a significant difference (Table 3). However, the differences were rathermarginally, with the largest difference being 0.29 cm for Tilia tomentosa Moench., betweenCodri and Repedea Hill (Table 3).

The soil analysis of the plots (Fig. 3) revealed two separate groups, with plots inBădeana, Bursucani and Rădeşti having less clay and silt content, indicating less water-

Urban Ecosyst

Species

Tilia to

men

tosa

Sorbu

s tor

mina

lis

Querc

us ro

bur

Querc

us p

ubes

cens

Querc

us fr

ainet

to

Querc

us d

alech

ampii

Fraxin

us e

xcels

ior

Crata

egus

mon

ogyn

a

Cornu

s mas

Carpin

us o

rient

alis

Carpin

us b

etulu

s

Acer t

atar

icum

Acer c

ampe

stre

180

160

140

120

100

80

60

40

20

0

Co

unt

UtLt

distributionVertical

Vertical distribution

Fig. 4 Structural distribution of species represented by 25 or more individuals in study plots, distinguishedbetween trees in lower tree layer (Lt) and upper tree layer (Ut)

Table 2 Total number of species and their occurrence in the study plots at the six different field sites

Species Bădeana Bursucani Codri Rădeşti Repedea Hill Roşcani Total

Acer campestre – 5 38 13 10 26 92

Acer platanoides – – – – – 3 3

Acer tataricum 36 38 – – – 18 92

Carpinus betulus – – 57 3 35 – 95

Carpinus orientalis – – – – – 112 112

Cornus mas – – 71 4 7 1 83

Crataegus monogyna 17 13 – – – 18 48

Fagus taurica – – – – 7 – 7

Fraxinus excelsior – – 17 7 – 8 32

Malus sylvestris – 1 – – – – 1

Prunus avium 4 4 2 – – – 10

Pyrus pyraster – – – – – 1 1

Quercus dalechampii – 19 – 23 3 60 105

Quercus frainetto – 95 – – – – 95

Quercus pedunculiflora 7 – – – – 7

Quercus pubescens 152 – – – – – 152

Quercus robur – 14 16 – – 1 31

Robinia pseudacacia 4 – – – – – 4

Sorbus domestica – 1 – – – – 1

Sorbus torminalis – – 10 15 2 – 27

Tilia tomentosa – 3 66 63 33 – 165

Ulmus glabra 4 – – – – – 4

Ulmus minor – – – – – 4 4

Urban Ecosyst

Table3

Yearly

meanincrem

entinheight

(m)andDBH(cm)fortreesrepresentedby

25or

moreindividualsin

studyplots,with

standard

deviationin

brackets.D

ifferent

capital

letters

indicate

significantdifferencesin

height

increm

ent,while

differentlower-caseletters

indicate

significantdifferencesin

DBH

increm

entof

thespeciesbetweenthestudy

sites

Species

Bădeana

Bursucani

Codri

Rădeşti

Repedea

Hill

Roşcani

P-value

Mean

Height

DBH

Height

DBH

Height

DBH

Height

DBH

Height

DBH

Height

DBH

Height

DBH

Acercampestre

––

0.17 (0.04)

A0.17 (0.06)

a0.29 (0.06)

B0.29 (0.06)

b0.29 (0.08)

B0.29 (0.09)

b0.27 (0.13)

B0.31 (0.07)

b0.20 (0.06)

A0.27 (0.07)

b0.000/

0.004

0.26 (0.08)

0.28 (0.08)

Acertataricum

0.20 (0.05)

A0.22 (0.05)

0.18 (0.05)

B0.22 (0.06)

––

–-

––

0.17 (0.04)

B0.25 (0.04)

0.010/

0.072

0.18 (0.05)

0.22 (0.05)

Carpinus

betulus

––

––

0.29 (0.06)

0.28 (0.06)

a0.34 (0.06)

0.28 (0.05)

a0.29 (0.08)

0.34 (0.07)

b–

–0.445/

0.000

0.29 (0.07)

0.30 (0.07)

Carpinus

orientalis

––

––

––

––

––

0.19 (0.06)

0.22 (0.06)

–0.19 (0.06)

0.22 (0.06)

Cornusmas

––

––

0.18 (0.07)

0.16 (0.04)

a0.12 (0.04)

0.20 (0.05)

a0.15 (0.03)

0.25 (0.06)

b0.15

(−)

0.20

(−)

a0.358/

0.000

0.17 (0.07)

0.17 (0.04)

Crataegus

monogyna

0.18 (0.06)

0.21 (0.04)

0.15 (0.05)

0.22 (0.05)

––

––

––

0.15 (0.05)

0.22 (0.04)

0.235/

0.266

0.16 (0.05)

0.22 (0.05)

Fraxinus

excelsior

––

––

0.29 (0.08)

0.33 (0.11)

a0.36 (0.06)

0.42 (0.07)

a–

–0.31 (0.05)

0.49 (0.10)

b0.065/

0.005

0.31 (0.07)

0.38 (0.11)

Quercus

dalecham

pii

––

0.25 (0.04)

A0.36 (0.06)

a–

–0.34 (0.08)

B0.44 (0.10)

b0.21 (0.07)

A0.46 (0.04)

b0.24 (0.05)

A0.39 (0.07)

b0.000/

0.000

0.26 (0.06)

0.38 (0.08)

Quercus

frainetto

––

0.26 (0.05)

0.36 (0.08)

––

––

––

––

–0.26 (0.05)

0.36 (0.08)

Quercus

pubescens

0.21 (0.05)

0.33 (0.09)

––

––

––

––

––

–0.21 (0.05)

0.33 (0.09)

Quercus

robur

0.26 (0.05)

0.41 (0.07)

0.26 (0.08)

0.46 (0.12)

––

0.16

(−)

0.28

(−)

0.354/

0.094

0.25 (0.06)

0.43 (0.10)

Sorbus

torm

inalis

––

––

0.25 (0.17)

0.28 (0.15)

0.19 (0.03)

0.25 (0.07)

0.16 (0.03)

0.27 (0.19)

––

0.368/

0.809

0.21 (0.11)

0.26 (0.11)

Tilia

tomentosa

––

0.27 (0.08)

A0.45 (0.14)

a0.32 (0.08)

A0.36 (0.09)

b0.39 (0.07)

B0.49 (0.10)

a0.41 (0.11)

B0.60 (0.18)

c–

–0.000/

0.000

0.36 (0.06)

0.46 (0.15)

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holding capacity. However, none of the tree species studied had significantly slower growthat these sites compared with the other study sites with better water-holding soils (Table 3).

For Carpinus orientalis Mill., Quercus frainetto Ten. and Q. Pubescens Wild., growthcomparisons between different sites were not possible, since they were only found in one ofthe six study areas.

Discussion

It has been suggested by a number of authors that investigating the ecological backgroundand performance of species growing in habitats experiencing drought during the growingseason and winter temperatures similar to those of inner city environments in a particulararea can assist in identifying alternative trees for urban use (Flint 1985; Ware 1994;Ducatillion and Dubois 1997; Sæbø et al. 2005; Roloff et al. 2009). In order to evaluate thesite match between the study plots and urban environments in the CNE-region, acomparison was made of the cumulative water deficit calculated for the study sites andfor paved and park environments of Copenhagen. Based on the meteorological data andsubsequent observations, the trees at all study sites except Codri experienced warmer anddrier conditions than those in current park environments of Copenhagen, where a negativenet water balance had a considerably less dramatic negative trend.

However, this comparison present a somewhat skewed picture due to the favourable siteconditions in some of the study plots with regard to high levels of clay and silt in the soil,and the more impoverished soils at paved sites in the CNE-region. While there was a fairlyclose match in net water balance between the study sites at Repedea Hill, Roşcani, Rădeşti,Bădeana and Bursucani and paved sites in Copenhagen, the sites at Romania and Moldaviahad superior soil water-holding capacity. Soil structure and soil content contributed in thiscase to counteracting the cumulative water deficit in the study plots. Evaluation of the siteconditions in this study revealed that the sites could be divided into two separate groupsconcerning soil texture. Bădeana, Bursucani and Rădeşti had comparable lower levels ofclay and silt and therefore lower water-holding capacity than Codri, Repedea Hill andRoşcani. For example, although there was a 26.9 mm deficit in Repedea Hill and Roşcani inMay, water was probably accessible to the trees for a longer period due to the good water-holding capacity of the soil (Brady and Weil 2002). Moreover, since silt has very goodcapillary water transporting capacity, groundwater can be transported upwards from depthand become accessible to the trees (Brady and Weil 2002). This said, the division of thestudy sites into two groups based on soil texture was not visible in the growth pattern of thespecies present at all sites.

Comparisons of the development of different species in the study areas indicated that nospecies had significantly greater mean annual height increment at Codri, despite the moisterand cooler growing conditions at this site. However, this conclusion should be taken withsome caution due to uneven species occurrence between the study areas. For example, inthe case of Cornus mas L., 71 individuals of the species were found in Codri, while a totalof 12 were found in plots in the other study areas. On the other hand, Acer campestre L.,Carpinus betulus L., Fraxinus excelsior L., Quercus robur L. and Tilia tomentosa Moench.showed a trend for greater height increment in Repedea Hill and Rădeşti than in Codri,indicating that they benefited from a warmer climate. Quercus dalechampii Ten., Acertataricum L. and Crataegus monogyna Jacq. showed a good height increment at sites suchas Roşcani and Bursucani, indicating that these species tolerate and even benefit from awarm climate, at least in the late stage of the growing season. These latter species were not

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recorded at Codri. Carpinus orientalis Mill., Quercus frainetto Ten. and Q. pubescens Wild.were recorded at only one location each, making it difficult to draw conclusions on anygrowth pattern concerning different climate or water availability between the study plots.

There were only marginal differences as regards the diameter increment (DBH) of thespecies between the different study sites.

In identifying suitable tree species for urban plantations, tolerance to warm andperiodically dry situations is not enough (Roloff et al. 2009). It is equally important topredict the growth and development rate of the species despite these tough growingconditions. Of the 13 tree species studied, a total of seven (Acer campestre L., Carpinusbetulus L., Fraxinus excelsior L., Quercus dalechampii Ten., Q. frainetto Ten., Q. robur L.and Tilia tomentosa Moench.) had an annual height increment of at least 25 cm. Of theseseven species, five were found in Codri and showed a stronger or equal growth rate at otherwarmer, and thereby, drier study sites. Of the 13 species, four (Acer tataricum L., Carpinusorientalis Mill., Cornus mas L. and Crataegus monogyna Jacq.), could be considered smallto medium-sized trees, making it unfair to compare them with the other high growingspecies. These four species displayed uniform height growth, with the mean annual heightincrement being 22 cm for Carpinus orientalis Mill., compared with 18 cm for Acertataricum L., 14 cm for Cornus mas L. and 15 cm for Crataegus monogyna Jacq.

It was possible to make a peer ranking of the relative use potential of the 13 treespecies represented by 25 or more individuals in the study plots by distinguishingbetween species with their main position in the canopy layer and those in theunderstorey layer. In this study, tree species with their main position in the canopy layerwere Fraxinus excelsior L., Quercus dalechampii Ten., Q. frainetto Ten., Q. robur L. andQ. pubescens Wild. Thus, these species experienced a more stressful environment, withmuch higher evapotranspiration compared with the species in the structure below (Oliverand Larson 1996). An understanding of different levels of tolerance to shady conditions,as well as warm, exposed and periodically dry environments, can prove important in fullyidentifying the potential of new species for use in various urban situations, e.g. cold andshady spots or warm and sun-exposed locations. As Fig. 4 shows, Acer campestre L.,Carpinus betulus L., Carpinus orientalis Mill., Fraxinus excelsior L. and Tilia tomentosaMoench. had individuals both in the canopy layer and in the understorey layer. Incontrast, the oak species (Quercus dalechampii Ten., Q. frainetto Ten., Q. pubescens Wild.and Q. robur L.), mainly existed in the canopy layer. Species capable of establishingsuccessfully in both the canopy and understorey layer may be useful in e.g. streetplantations, where the light conditions can differ between the two sides of the street(Sieghardt et al. 2005). For warm and sun-exposed areas, species of oak have goodpotential for use. Moreover, Acer tataricum L., Cornus mas L., Crataegus monogynaJacq. and Sorbus torminalis (L.) Crantz., all existing primarily in the understorey layer,can be useful either in warm courtyards and squares that are shaded for most of the day, oras understorey vegetation underneath existing trees, e.g. at paved sites.

Many of the tree species identified in this study are adapted for a warm, dry climate andhave developed an extensive plasticity and tolerance for this habitat type (Rabinowitz 1981;Gurevitch et al. 2002). This becomes apparent on studying the natural distribution of someof the species, which is mainly in warm, dry summer climates, whereas other western andcentral European tree species have been screened out due to the warmer and drierconditions (Ellenberg 1988; Breckle 2002). However, for species such as Acer campestre L.,Carpinus betulus L., Crataegus monogyna Jacq., Fraxinus excelsior L. and Quercus roburL., which have a large natural distribution in moist and cool areas as well as dry and warmareas, it is important to use the right phenotype for the specific use. Since population

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differentiation within the species tolerance for specific habitat and climate is welldocumented (e.g. Jones et al. 2001; Mijnsbrugge et al. 2010), tree phenotypes from thestudy areas in Romania and Moldavia have a greater use potential for inner cityenvironments, e.g. street sites, in the CNE-region compared with phenotypes andprovenances originating from more westerly or northerly locations. Before any furtherrecommendations can be made, these phenotypes have to be collected and tested in full-scale urban plantations in the CNE-region, where complementary evaluations can be carriedout on the tolerance and performance of the species/phenotype in e.g. the heat and droughtof another geographical region. Furthermore, even though the study region represents amuch more continental climate than the CNE-region, earlier experiences of provenances forthis type of climate have been used successfully. For example, for southern Sweden therecommended provenances of spruce (Picea abies (L.) H. Karst.) originate from east Polandand Belarus, which have a much more continental climate than southern Sweden (SwedishForestry Agency 2010).

Conclusions

This study takes a different approach to traditional selection of urban trees and can beseen as the first step in the selection process to identify and introduce promising speciesfor inner city environments in the CNE-region. Since the majority of the 13 tree speciesfound in the field studies are currently little used in urban applications in northern partsof Central Europe, they can increase the diversity of the current urban tree population.However, all 13 species can be found in botanical gardens and arboretums in the CNE-region, so experiences from these collections regarding health status, wood stability,allergy risks and propagation issues can further speed up the introduction process.Moreover, aspects such as the invasiveness of introduced species must also be examinedwith regard to present and future climate variables and conditions before furtherrecommendations can be made. In general, this kind of study can speed up the selectionprocess for new city trees, since promising species for a particular application can beidentified in an early phase compared with random testing without this dendroecologicalbackground information. Dendroecological studies such as this contribute ecologicalknowledge that gives a much wider knowledge base in the selection process and can helpevaluate the reaction, tolerance and performance of different tree species to differentstressors. Since water stress is often the main constraint for tree growth and health, treegrowth and performance in warm and periodically dry conditions is an obvious startingpoint for a targeted selection process.

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