Forest Ecology and Management 258 (2009) 794–803

The impact of former agriculture on habitat conditions and distribution patternsof ancient woodland plant species in recent black alder (Alnus glutinosa (L.)Gaertn.) woods in south-western Poland

Anna Orczewska *

University of Silesia, Faculty of Biology and Environmental Protection, Department of Ecology, ul. Bankowa 9, 40-007 Katowice, Poland

A R T I C L E I N F O

Article history:

Received 5 March 2009

Received in revised form 19 May 2009

Accepted 19 May 2009

Keywords:

Recruitment limitation

Competitive exclusion

Ancient forests

Soil conditions

Groundwater level

Illumination

Forest recovery

A B S T R A C T

Little is known about the influence of former agricultural use of soils on the forest recovery process in

post-agricultural black alder (Alnus glutinosa) woods – the most fertile and the wettest forest habitats

among the European temperate forest types. Thus, studies focusing on edaphic, hydrological and light

conditions responsible for colonization mechanisms present in such woodlands adjoining ancient ones

were undertaken in south-western Poland. In the 16 m2 quadrats of the 33 transects laid out

perpendicularly across the ancient-recent forest boundary, data were collected on herb layer

composition, chemical soil properties, as well as illumination level. Additionally, groundwater level

in the spring months was recorded in piezometers. The number and cover of true woodland herbs were

higher in ancient woods, regardless of forest type. Soils in ancient woodlands reached higher levels of

Al3+, K+, cation exchange capacity (CEC), available K, P, and organic C, whereas their pH was lower. The

illumination level of the forest floor was greater in recent woods. Linear regression showed that recent

wood age had a negative effect on pH and base cations, but positive on Fe2+, Ca2+, available P and Mg, CEC,

and on C and N contents. CCA results showed that woodland age, pH, humus type, groundwater level,

available Mg and K were always among those variables having the highest contribution in explaining the

distribution pattern of woodland species in recent woods. Total N and available P contents were always

higher in ancient woodland soils than in recent, and their content grew with time. Thus, they cannot be

treated as indicators of former agricultural use of recent alder wood soils. Urtica dioica, Poa trivialis and

Galium aparine, the three competitive herbs, avoided sites with a high level of groundwater, combined

with poor illumination level. In order to create the best possible conditions allowing for effective forest

recovery in habitats of such high fertility, it is essential to maintain a good water regime and shade in the

forest floor. This in turn reduces the competitive exclusion of woodland flora by the aggressive herbs and

facilitates the immigration of typical woodland herbs.

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1. Introduction

The impact of former agricultural land-use on the soils of forestscurrently existing in the landscape has become an object ofgrowing interest to ecologists, since it is widely known thathistorical land-use can noticeably influence present-day soilcharacteristics, and through that also the diversity patterns offorest plant species (Tyler, 1989; Dzwonko and Gawronski, 1994;Koerner et al., 1997; Honnay et al., 1999; Compton and Boone,2000; Kolb and Diekmann, 2004; Hipps et al., 2005; Falkengren-Grerup et al., 2006). Most studies indicate that soils in post-agricultural forests show long-lasting evidence of their previous

* Tel.: +48 32 359 15 48.

E-mail address: anna.orczewska@us.edu.pl.

0378-1127/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2009.05.021

cultivation. The upper layer in such soils is characterized by highervalues of pH and a lower content of carbon and organic matter(Koerner et al., 1997; Bossuyt et al., 1999). Besides, many authorsreport higher levels of phosphorus and nitrogen in recent forestsoils compared to ancient woodland sites, which is explained asthe consequence of former cultivation of post-agricultural forestsoils (Koerner et al., 1997; Honnay et al., 1999; Dzwonko, 2001; DeKeersmaeker et al., 2004).

The influence of an increased level of soil nutrients on thecolonization of recent woods by woodland flora depends on soiltype and its fertility. The colonization capacity of woodland speciesin poor soils on an acidic substrate is enhanced due to the surplusnutrient content (Koerner et al., 1997), whereas in fertile, rich soils,additional levels of plant-available nutrients lead to the increaseddevelopment of vigorous herbs with high competitive ability,which inhibit the immigration of woodland herbs (Honnay et al.,

A. Orczewska / Forest Ecology and Management 258 (2009) 794–803 795

1999; Verheyen et al., 1999; De Keersmaeker et al., 2004).Furthermore, high fertility may sometimes also facilitate thedynamic growth of trees, which in turn, through dense tree canopy,reduces the light intensity below 2% in the forest interior. Thus, insuch circumstances, despite the fact that soils are rich in nutrients,light becomes a limiting factor, reducing the number of shade-tolerant, woodland herbs (Dupre et al., 2002). Nevertheless, inmost recent woodland sites, the illumination level is higher than inancient woods. Thus, a competitive exclusion by vigorous herbs is amore probable mechanism responsible for the reduced number oftrue forest species successfully establishing their populations inthe herb layer of recent woods.

In general, results of studies indicate that the direction and paceof changes in the soil chemistry of recent woodland sites arediverse and dependent on many factors, including the duration ofthe former agricultural use (Verheyen et al., 1999), time sinceafforestation (Bossuyt et al., 1999), habitat type and the type offormer agricultural use (pastures, gardens, croplands) (Koerneret al., 1997, 1999).

In addition, secondary succession is not only driven by thecombined influence of the edaphic and light conditions, but alsodepends on the availability of water (Leuschner and Rode, 1999).Most specialist, true woodland herbs (ancient forest plant species)avoid habitats with extreme hydrological conditions, both too wetand too dry, but show a clear affinity for sites with a moderate levelof this abiotic factor (Hermy et al., 1999; Dzwonko and Loster,2001). This feature, together with other specific life-history traitsof true forest herbs, is responsible for their low colonizationcapacity in recent, post-agricultural woods.

Despite the growing interest in the influence of formeragricultural use of soils in recent woods on the forest recoveryprocess, little is known about the relations between habitatconditions and herbaceous layer composition in post-agriculturalblack alder (Alnus glutinosa) woods representing the most fertileand the wettest forest habitats among the European temperateforest types. A. glutinosa belongs to the actinorhizal species, thus,due to symbiosis with Frankia alnii, responsible for N2-fixationfrom the atmosphere, it contributes to the enrichment of the soilwith this nutrient by approximately 50 kg ha�1 yr�1 (Schaede,1967). According to other sources, the efficiency of this process isestimated at 50–130 kg ha�1 yr�1 (Pancer-Kotejowa and Zarzycki,1980), 10–300 kg ha�1 yr�1 (Sprent and Sprent, 1990), or evenseveral hundred kg ha�1 yr�1 (Binkley, 1986). Enrichment of soilswith nitrogen is also possible due to the ‘open’ management type ofnutrition typical for black alder. This means that it does notwithdraw its nutrients but retains much foliar nitrogen in theleaves until they fall in the autumn. Therefore, most mineralelements which either were absorbed from the soil during thevegetation season or from the nodules fixing N2 from theatmosphere, are returned to the soil (Zimka and Stachurski,1976). The rate of leaf decomposition is very quick for alder(k = �0.908 yr�1 which corresponds to a mass loss of 66% duringthe period of 16.5 months) (Pereira et al., 1998). Thus, habitats withstands composed of A. glutinosa are eutrophic. Such characteristicsmake black alder an excellent pioneer species, capable ofcolonizing soils poor in nutrients. Furthermore, in contrast toother early successional trees, A. glutinosa tolerates heavy, water-logged soils (McVean, 1953; Obidzinski, 2004). Due to thesefeatures, it is a common practice in Polish forestry to use blackalder in afforestation on former meadows where agriculture hasbeen abandoned because of the permanently wet soils. In manycases recent A. glutinosa plantations are located in the proximity ofancient forests with alder as a main canopy species. The latter ones,including typical wet alder woods, alder-ash carrs, and the wettesttypes of oak-hornbeam communities, are still common in forestedareas of many regions of Poland. Their large species richness can be

used as a potential reservoir of rare plants for the colonization ofrecent woods planted in their proximity. Thus, the studies on therecovery of the herbaceous layer in recent black alder woods,resulting in calculating migration rates of 51 forest herbs, wereundertaken. These studies have shown that the process ofcolonization of the herb layer by woodland flora in such forestsproceeds faster compared to less fertile and drier habitats.Nevertheless, herb layer recovery is limited by the abundantgrowth of high nutrient, and light-demanding competitor Urtica

dioica (Orczewska, in press-b).It is known that species composition in woods on former

agricultural sites is a result of the combined influence of dispersaland recruitment limitation, and that the pace of secondarysuccession is strongly influenced by habitat type. Thus, the studiesfocusing on abiotic conditions responsible for colonizationmechanisms present in alder woodlands were undertaken. Morespecifically, edaphic, hydrological and light conditions present inadjoining ancient and recent alder woods were compared andrelated to differences in species composition observed in thecourse of detailed floristic research.

2. Study area description

The studies on vegetation composition and habitat conditions inblack alder forests differing in their management history werecarried out in two neighbouring geographical regions of south-western Poland, namely the Olesnica Plain (518040N; 178430E), a partof the Opole Silesia, and the Zmigrod Valley (518280N; 168540E),belonging to Lower Silesia. In both regions ancient forests with ahigh level of naturalness reach a distinctive proportion. Among themthe stands with A. glutinosa are well represented. As a result ofcartographic studies, based on the first old map available for this partof Europe (the Schmettausche map of 1765–1780) (Orczewska, inpress-a) the status of ancient forests, where the ecological researchwas carried out, was confirmed. These woodlands have existedcontinuously in the landscape for the last 230 yr. Among recentwoods of post-agricultural origin, reaching ca. 15% of the totalforested area, black alder woodlands, planted on former dampmeadows, represent a high proportion as well. The existence of suchspecific situations in nature offered suitable sites for research on therate of herbaceous layer recovery in post-agricultural alder woods,and on environmental conditions present in these woods.

3. Data collection

The data on herb layer composition were compiled from samplestaken from the 33 transects, of approximately 80 m in length by 4 min width, laid out perpendicularly across the boundary between theancient and recent woodlands. Post-agricultural black alderplantations, representing the following age classes: up to 10,10–20, 20–30, 30–40, and 40–50 yr (older alder stands wereavailable in only two cases), were situated in direct proximity ofthe following types of ancient forests: wet types of oak-hornbeamwood (either Tilio-Carpinetum Tracz. 1962 or Galio-Carpinetum

Oberd. 1957 community), alder-ash carrs (Fraxino-Alnetum W. Mat.1952) or typical wet alder woods = alder swamp forests(Ribeso nigri-Alnetum Sol.-Gorn. (1975) 1987). In total 11 transectswere set up in the habitat of oak-hornbeam forest, 12 in alder-ashcarr, and 10 in wet alder wood. The quadrat plots of 16 m2 were laidout at intervals of 4 m. Transects consisted of four plots located inancient forest and six in recent woodland, which gave 28 m of eachtransect located in the ancient forest, and 44 m into the recent wood.Such a layout of the plots permitted a calculation of the colonizationrates of woodland species migrating into recent alder woods (formore details on transect design and calculations of migration ratessee Orczewska, in press-b).

A. Orczewska / Forest Ecology and Management 258 (2009) 794–803796

Twice in the vegetation season, that is in April and June of 2002–2006, in the plots of each transect, a list of the herbaceous specieswith, in each case, an estimate of their percentage cover (1%, 5%,10%, and then at 10% intervals), was recorded. A similar scale wasapplied to estimate the total cover of the shrub layer, whereas thecover of the stand canopy was assessed using the sphericaldensiometer, with four readings per plot taken and their meanvalue used in further analyses.

Floristic research was followed by studies on the chemicalproperties of soils, on groundwater level, and on light conditions inalder woods. To describe edaphic conditions in each plot wherevegetation was studied, five soil samples from the topsoil, withoutthe litter layer, representing the main rooting area of most herblayer plants (approximately upper 10–15 cm) were taken andbulked. After air-drying and sieving over 2 mm, they were analysedfor: pH, measured potentiometrically in H2O and in 1N KCl, totalorganic C (%), according to the Tiurin method, total N content (%)with the application of the Kjeldahl method, exchangeable Ca2+,Mg2+, Na+, K+ in 1 M ammonium acetate, and cation exchangecapacity (CEC), exchangeable Al3+ according to Sokołow method(me/100 g = cmol/kg), iron content, following Tamm methods(ppm = mg/kg), and P, K and Mg available (P2O5, and K2O) –Egner–Riehm method, and MgO by Schachschabel method (mg/kg)(Litynski et al., 1976; Ostrowska et al., 1991). Besides, soil profileswere dug to describe the soil types, one in each of the adjacentforests. Soil samples from each horizon of the profiles were takenand analysed for the same set of chemical parameters as the bulksamples from the transects’ plots. The humus form, following theclassification given by Prusinkiewicz (1988), was also estimated.Humus form is one of the components reflecting soil nutrient levelsand the availability of soil nutrients to plants, since it plays a role innutrient storage and cycling. That is why it is one of the mostimportant determinants of the assessment of forest soil quality. Forthese reasons, plants with growth strictly dependent on humusform may work as indicators of humus form quality (Klinka et al.,1990).

In order to characterize the groundwater level in recent forestsand their surroundings, piezometers were installed (ø = 5 cm,h = 50–200 cm, one piezometer per transect), where the oscilla-tions of the water level (cm) were recorded at monthly intervalsover the whole hydrological year. However, for the purpose of thestatistical analyses, the mean values of groundwater level duringthree months in the spring were used. The symbol ‘+’ indicateswater stagnating on the surface, whereas ‘�’ refers to the level ofwater below the ground.

To characterize the light conditions inside the woods, at groundlevel, the intensity of the photosynthetically active radiation (PAR)was measured. Readings were taken in five points of each quadrat,using photometer LI-250 and Quantum Sensor LI-190A(Gesellschaft fur Daten-, Mess- und Pruftechnik – DMP, Fehraltofr,Switzerland), and expressed as a percentage of the PAR levelmeasured in the open area at the same time. Besides, in statisticalanalyses, canopy (A_cover) and shrub cover (B_cover) estimateswere also used as environmental variables reflecting the lightavailability on the forest floor.

Table 1Mean and standard deviation values of the number and total cover of ancient woodlan

Oak-hornbeam habitat

Ancient Recent

Number of ancient woodland species (mean � SD) 14.1**** (�5.4) 7.5 (�Total cover of ancient woodland species (mean � SD) 96.4**** (�48.8) 53.9 (�

** Significance level according to the t-test, 0.001 < p < 0.01.**** Significance level according to the t-test, p < 0.0001.

4. Data analysis

To estimate whether there are differences between ancient andrecent forests in their richness and total cover of ancient woodlandindicator species (species were selected on the basis of a list givenby Dzwonko and Loster, 2001), the t-test was applied. Differencesbetween environmental conditions (soil chemical features andillumination level on the forest floor) in ancient and recent forestswere also compared with the t-test. ANOVA and the Tukeysignificance test were used to compare these environmentalvariables among post-agricultural woods representing differenthabitats, whereas for a comparison of variables in woods ofdifferent age classes of their stand, the Kruskal–Wallis test wasapplied. For the last analysis recent woods were grouped in threeage classes, with 20 yr intervals, with the first, including stands upto 20 yr old, the second – 21–40, and the third – 41–60 yrs.Furthermore, the linear regression analysis was used to assess theinfluence of a recent woodland age on the chemical parameters ofsoils. However, this statistical procedure was limited only to thosevariables which showed a linear relationship with woodland age(without or after square root or logarithmic transformation). Thesestatistical procedures were applied separately for each type ofhabitat.

In order to study the vegetation gradients in recent alder woodsand their relation with the habitat conditions, canonical corre-spondence analysis – CCA was implemented (the length of thegradient of the first axis in the detrended correspondence analysis– DCA allowed for the selection of a unimodal CCA technique). Thedata from the plots of the recent woods, again separately for eachhabitat type, were analysed. Among the CCA explanatory variablesthose which correlated with each other, such as Ca2+, Mg2+, Na+, K+

with cation exchange capacity, were excluded from the ordinationanalysis. Thus, the following quantitative environmental variableswere included in the CCA: age of a recent wood, canopy (A) andshrub (B) cover, PAR level, mean water level in spring months, pH(in KCl), Al3+, Fe2+, P2O5, K2O, MgO, CEC, organic C, total N, whereashumus type represented a qualitative variable (0, 1), and included:moist mull, wet mull, moist moder-mull, and wet moder-mull. Anautomatic forward selection procedure was applied to select theenvironmental variables contributing to the species compositionin recent woods. The Monte Carlo permutation test under thereduced model was used to examine the statistical significance ofthe explanatory variables. The CCA ordination technique was runwith the application of Canoco 4.5, whereas other statisticalanalyses were done using the Statistica 8 package.

5. Results

The number and total cover (calculated as a sum of covers of allthe individual species) of true woodland herbs were higher inancient woods than in adjoining recent ones, regardless of foresttype (Table 1). However, individual differences among the habitatswere observed. In alder-ash carrs, the most fertile forest typesstudied, ancient woodland species in recent woods reached a muchhigher abundance than in the other two remaining habitats. When

d indicator species present in ancient and recent woodlands.

Alder-ash carr habitat Typical wet alder wood habitat

Ancient Recent Ancient Recent

4.4) 13.4**** (�5.1) 8.8 (�4.0) 11.1**** (�3.7) 8.1 (�3.0)

47.5) 153.1**** (�51.0) 100.7 (�61.7) 71.3** (�45.8) 47.4 (�32.9)

Table 2Relations between soil types and habitat types, both for ancient and post-agricultural alder woods (nomenclature after WRB, 1998).

Soil type Forest type

O-H ancient O-H recent A-A ancient A-A recent W-A ancient W-A recent

Saprihistic gleysols GLhis x

Luvic gleysols GLIv x

Mollic gleysols GLmo x x x

Stagni-haplic gleysols GLha-st x x

Stagni-eutric gleysols GLeu-st x x

Haplic gleysols GLha x

Ferri-umbric gleysols Glum-fr x x x x x

Umbric gleysols GLum x

Sapric histosols HSsa x x

Humic gleysols GLhu x x x x x x

O-H - oak-hornbeam forest; A-A - alder-ash carr; W-A - typical wet alder wood.

A. Orczewska / Forest Ecology and Management 258 (2009) 794–803 797

only ancient woodlands are compared, the lowest number of trueforest species was observed in wet alder woods. On the other hand,if the woodland species number in recent woods is expressed as apercentage of their richness in ancient woodland sites, recent wetalder woods have the highest proportion of ancient woodlandspecies present in the neighbouring ancient forests (73%), whereastheir proportion in recent alder-ash carrs reaches 65.7%, and inoak-hornbeam 53.2% of that present in migration source ancientwoods. Similar relations and proportions are observed in the totalcover of true woodland species, with ancient wet alder woodshaving the lowest (71.3%), and alder-ash carrs the highest values(153.1%). An intermediate value is achieved in oak-hornbeamhabitats (96.5%). The total cover of ancient woodland speciespresent in recent oak-hornbeam forest reaches 56% of thatrecorded in ancient woods, whereas it is 66% in the case of wetalder wood and alder-ash carrs.

Studies on soil types present in alder forests revealed thatmesotrophic and eutrophic soils, on moist, wet or periodicallywaterlogged sites predominate. They mainly include a widespectrum of gleysols and, to a far lesser extent, also histosols,

Table 3Mean and standard deviation values of the environmental variables in ancient and rec

Variable Mean

(SD) – O-H_ancient

(N = 43)

Mean

(SD) – O-H_recent

(N = 66)

Mean

(SD) – A-A_anci

(N = 48)

pH_H2O 4.81 � 0.86 5.31 � 0.58*** (b) 5.53 � 0.77

pH_KCl 3.97 � 0.84 4.42 � 0.57** (b) 4.79 � 0.84

Al+3 2.11 � 2.47**** 0.62 � 1.22 0.51 � 1.13*

Fe2+ 4669.11 � 3217.76 7066.35 � 5022.24** (b) 12604.38 � 1475

Mg2+ 1.167 � 0.77 1.19 � 0.78 (b) 2.80 � 1.91

Ca2+ 12.15 � 8.11 13.25 � 8.35 (b) 27.47 � 17.67

Na+ 0.07 � 0.08y 0.04 � 0.03 (a) 0.07 � 0.05

K+ 0.13 � 0.07* 0.10 � 0.07 (b) 0.25 � 0.27**

Base cations 13.51 � 8.73 14.58 � 8.93 (b) 30.59 � 19.44

Cation exchange

capacity (CEC)

28.79 � 9.67* 24.15 � 10.93 (b) 47.58 � 32.20y

P2O5 40.95 � 25.26 35.24 � 21.80 (a) 40.06 � 19.23

K2O 71.50 � 52.18**** 37.56 � 32.91y (a) 93.71 � 110.11**

MgO 155.70 � 91.53 161.18 � 75.94 (b) 251.04 � 135.05

Corg 8.49 � 4.76** 5.86 � 4.37 (b) 11.79 � 7.90*

Ntot 0.64 � 0.32 0.58 � 0.39 (b) 0.99 � 0.64

C/N 13.10 � 3.17**** 10.15 � 2.19 (a) 11.90 � 2.08

PAR 7.77 � 8.21 10.99 � 7.69* (a) 11.43 � 9.31

A_cover 84.96 � 10.54 82.81 � 11.25 (b) 84.86 � 8.07

B_cover 19.47 � 23.06** 9.47 � 16.60 (b) 18.83 � 20.20**

n.s. – not significant. O-H – aok-hornbeam forest; A-A – alder-ash carr; W-A – typical

significant difference according to the ANOVA, Tukey test.* Significance level according to the t-test, 0.01 < p < 0.05.** Significance level according to the t-test, 0.001 < p < 0.01.*** Significance level according to the t-test, 0.0001 < p < 0.001.**** Significance level according to the t-test, p < 0.0001.y Significance level according to the t-test, 0.05 < p < 0.1.

with humic gleysols and ferri-umbric gleysols (following the WRBnomenclature, 1998) being the most frequent, and present in allhabitats (Table 2).

Many properties of soils in ancient and recent woodlandsdiffered significantly, regardless of habitat type (Table 3). Suchdifferences were recorded for: K+, CEC, available K, and organic C,for which the values in ancient woods were notably higher.Furthermore, significantly higher levels of Al3+ were observed inancient woods in the habitats of oak-hornbeam forests and inalder-ash carr, whereas pH level in those cases reached lowervalues compared to recent woods. Besides, soils in ancient forestswere characterized by a higher content of total N, although thesedifferences were significant only in the wet alder wood habitats(Table 3). The forest floor of recent woods receives more light thanin ancient woodland sites. However, these differences weresignificant only in the habitats of oak-hornbeam forest andalder-ash carrs (Table 3).

When habitat conditions among recent woodland sites wereanalysed, the general tendency was that soil conditions in alder-ash riparian forests were distinctively different compared to the

ent woodlands.

ent

Mean

(SD) – A-A_recent

(N = 72)

Mean

(SD) –W-A_ancient

(N = 40)

Mean

(SD) –W-A_recent

(N = 60)

5.81 � 0.57* (a) 5.68 � 0.74* 5.31 � 0.91 (b)

5.04 � 0.68y (a) 5.02 � 0.85** 4.51 � 0.99 (b)

0.18 � 0.44 (a) 0.30 � 1.27 0.94 � 1.44* (b)

0.75 14979.28 � 15336.60 (a) 9446.43 � 7760.05 9486.32 � 9770.48 (b)

2.50 � 1.40 (a) 2.42 � 1.70**** 1.10 � 0.79 (b)

27.16 � 14.06 (a) 30.05 � 14.62**** 15.32 � 10.26 (b)

0.08 � 0.07 (b) 0.22 � 0.12**** 0.12 � 0.09 (c)

0.15 � 0.11 (a) 0.17 � 0.04**** 0.11 � 0.05 (b)

29.88 � 15.18 (a) 32.86 � 16.04**** 16.64 � 10.98 (b)

38.97 � 16.82 (a) 42.37 � 14.09**** 27.11 � 9.74 (b)

40.31 � 32.13 84.23 � 79.72* 53.24 � 48.71 (b)

49.56 � 35.37y (b) 58.67 � 17.83*** 44.72 � 19.84

256.59 � 93.81 (a) 242.29 � 126.22**** 141.06 � 106.13 (b)

9.23 � 5.69 (a) 9.39 � 3.65**** 5.90 � 2.83 (b)

0.84 � 0.51 (a) 0.81 � 0.32**** 0.51 � 0.23 (b)

11.19 � 3.11 (b) 12.12 � 3.89 11.60 � 1.95 (b)

17.26 � 15.86* (b) 13.67 � 7.02 14.83 � 7.08

81.44 � 16.62 (b) 91.69 � 5.61 90.49 � 6.82 (a)

9.82 � 11.88 (b) 5.95 � 10.21 3.18 � 7.28 (a)

wet alder wood. In the case of recent woods the same letter in brackets means no

Table 4Mean and standard deviation values of environmental variables in forest-age classes of recent woods.

Variable Oak-hornbeam habitat Alder-ash carr Typical wet alder wood

<20 (N = 36) 21–40 (N = 24) 41–60 (N = 6) r2 <20 (N = 24) 21–40 (N = 33) 41–60 (N = 12) r2 <20 (N = 30) 21–40 (N = 18) 41–60 (N = 12) r2

pH_H2O 5.50 � 0.52 (b) 5.26 � 0.45 (b) 4.33 � 0.22 (a) �0.37**** 5.71 � 0.49 5.98 � 0.55 5.66 � 0.66 5.77 � 0.79 (a) 4.99 � 0.95 (b) 4.67 � 0.39 (b) �0.37****

pH_KCl 4.58 � 0.55 (b) 4.43 � 0.42 (b) 3.44 � 0.19 (a) �0.28**** 4.92 � 0.70 5.20 � 0.63 4.90 � 0.79 4.99 � 0.93 (a) 4.17 � 0.99 (b) 3.85 � 0.47 (b) �0.32****

Al+3 0.29 � 0.36 (b) 0.30 � 0.40 (b) 3.93 � 1.79 (a) 0.27 � 0.74 (a) 0.13 � 0.14 (b) 0.14 � 0.17 0.67 � 1.54 (a) 1.21 � 1.17 (b) 1.20 � 1.54 (b)

Fe2+ 5681.56 �3366.61 (b)

6908.46 �5342.47 (b)

16006.67 �2724.65 (a)

0.22**** 13793.75 �15583.67 (b)

9195.55 �3874.95 (b)

35520.42 �19508.15 (a)

8199.47 �5610.64 (b)

6315.61 �2234.50 (b)

17459.50 �18193.38 (a)

Mg2+ 1.11 � 0.73 1.44 � 0.87 0.67 � 0.24 2.02 � 1.28 2.77 � 1.41 2.71 � 1.63 1.20 � 0.73 1.09 � 1.06 0.87 � 0.29

Ca2+ 12.15 � 6.39 16.13 � 10.65 8.31 � 4.43 21.15 � 9.95 (a) 27.66 � 14.52 34.67 � 16.24 (b) 0.23**** 18.46 � 9.98 (a) 14.83 � 11.68 8.19 � 2.84 (b)

Na+ 0.04 � 0.02 0.05 � 0.04 0.03 � 0.01 0.11 � 0.10 (a) 0.06 � 0.04 (b) 0.07 � 0.03 0.12 � 0.10 0.09 � 0.07 (a) 0.18 � 0.10 (b)

K+ 0.10 � 0.08 (b) 0.09 � 0.05 (b) 0.14 � 0.04 (a) 0.20 � 0.15 (a) 0.09 � 0.05 (a) 0.19 � 0.10 (a) 0.10 � 0.04 0.10 � 0.05 0.13 � 0.04

Base cations 13.40 � 6.80 17.70 � 11.42 9.15 � 4.58 23.48 � 10.93 (a) 30.58 � 15.83 37.64 � 17.35 (b) 0.23**** 19.88 � 10.66 (a) 16.11 � 12.73 9.36 � 3.19 (b)

Cation exchange

capacity

20.72 � 7.23 (a) 27.73 � 14.23 30.43 � 7.81 (b) 0.18*** 30.42 � 10.88 (a) 38.81 � 17.11 51.46 � 15.90 (b) 0.35**** 26.31 � 9.04 30.21 � 11.04 24.44 � 8.91

P2O5 37.15 � 26.23 (b) 27.83 � 11.39 (b) 53.43 � 8.80 (a) 29.18 � 41.84 (a) 42.51 � 23.61 (b) 56.58 � 27.15 (b) 36.28 � 49.55 (b) 44.31 � 22.94 (b) 109.10 � 33.58 (a) 0.21***

K2O 36.74 � 39.22 (b) 31.21 � 19.54 (b) 67.87 � 15.82 (a) 61.42 � 24.45 (a) 25.85 � 19.65 (b) 94.75 � 36.95 (c) 42.89 � 18.51 (b) 39.17 � 21.46 (b) 57.60 � 15.98 (a)

MgO 134.67 � 74.65 (b) 211.54 � 57.84 (a) 118.82 � 34.38 (b) 229.92 � 108.03 257.17 � 87.90 306.08 � 75.23 0.12*** 172.78 � 77.36 (a) 133.48 � 152.62 (b) 73.12 � 27.64 (b) �0.23****

Corg 4.12 � 1.50 8.50 � 6.19 5.77 � 1.30 0.15*** 8.26 � 5.08 (b) 7.00 � 3.38 (b) 15.76 � 6.43 (a) 0.22**** 5.05 � 2.55 (a) 6.72 � 2.79 (b) 6.78 � 3.12 (b)

Ntot 0.40 � 0.13 (a) 0.83 � 0.54 (b) 0.61 � 0.11 (b) 0.19*** 0.75 � 0.45 (b) 0.67 � 0.31 (b) 1.36 � 0.67 (a) 0.21**** 0.49 � 0.26 0.51 � 0.17 0.56 � 0.22

C/N 10.53 � 2.38 9.78 � 2.01 9.35 � 1.34 11.28 � 3.86 10.77 � 2.85 (a) 12.21 � 2.42 (b) 10.62 � 1.49 (a) 13.05 � 2.20 (b) 11.88 � 0.93 (b)

PAR 11.29 � 8.49 11.28 � 7.27 7.99 � 2.98 19.85 � 19.96 15.96 � 12.97 18.34 � 15.73 16.48 � 5.41 (a) 6.85 � 1.70 (b) 22.67 � 2.90 (c)

A_cover 80.06 � 13.41 (a) 85.26 � 7.32 89.56 � 1.58 (b) 76.84 � 26.49 84.25 � 5.13 81.02 � 12.20 87.54 � 8.12 (b) 92.01 � 3.24 (b) 95.60 � 1.78 (a)

B_cover 2.81 � 5.47 (a) 19.33 � 23.30 (b) 10.00 � 9.49 4.46 � 8.21 (a) 11.45 � 11.32 (b) 10.58 � 12.18 5.40 � 9.52 1.61 � 3.27 0.00

O-H – aok-hornbeam forest; A-A – alder-ash carr; W-A – typical wet alder wood. n.s. – not significant. r2 values according to the linear regression analysis.*** Significance level according to Kruskal–Wallis test, 0.0001 < p < 0.001.**** Significance level according to Kruskal–Wallis test, p < 0.0001.

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A. Orczewska / Forest Ecology and Management 258 (2009) 794–803 799

other two types of habitats (Table 3). This was expressed by highervalues of many parameters in alder-ash carr habitat type (e.g. pH,CEC, organic C, total N, Mg2+, Ca2+), which confirms the higherfertility of these sites compared to the other two forest types.

Most values of soil parameters change over time (Table 4). Someof the observed differences in the soil chemical characteristics inalder plantations of successive age classes are present in eachhabitat. These refer to Fe2+, available P, and total N, the content ofwhich increases with time. On the other hand, values of pH and Mg2+

decrease with time, although these differences are not alwayssignificant. Some chemical characteristics of the recent woodlandsoils representing the highest age class (41–60 yr) to some extentbecome similar to the soils in adjoining ancient woodlands.Nevertheless, this observation refers only to a few chemical features,whereas others remain distinctively different, despite the growingage of recent woodland (Tables 3 and 4). The results of the linearregression analyses show that in the habitats of oak-hornbeamforests the age of a recent wood had a negative effect on pH and basecations, whereas the values of Fe2+, CEC, C and N content grew withthe woodland age. In alder-ash carrs the positive effect of age onCa2+, CEC, available P, Mg, and on C and N contents was observed. Inwet alder wood habitats the positive effect was confirmed in the caseof available P and Mg, whereas the values of pH and base cationsdecreased with a recent wood age (Table 4).

The spectrum of mean spring groundwater levels reflectsdiverse water conditions in the forests investigated. As one wouldexpect, temporal submergence is a characteristic feature of alderswamp forests (typical wet alder woods), since in many study sitestheir water periodically stagnates on the surface in early spring(maximum mean level of water reaches here +18.3 cm). In othercases groundwater in wet alder woods is situated at the shallowest

Table 5Forward selection of environmental variables (p = 0.01) in a canonical correspon-

dence ordination of plots from the recent woodlands.

Habitat type Rank Variable Accumulated sum of

variation explained

(sum of all canonical

eigenvalues) (% of

total variation)

Oak-hornbeam 1 Age 0.30 (20.0)

2 Wet mull 0.49 (12.7)

3 Moist mull 0.66 (11.3)

4 Water level 0.82 (10.7)

5 MgO 0.94 (8.0)

6 P2O5 1.04 (6.7)

7 pH 1.12 (5.3)

8 CEC 1.19 (4.7)

9 Fe2+ 1.23 (2.7)

Alder-ash carr 1 Age 0.20 (14.5)

2 Water level 0.40 (14.5)

3 Wet moder-mull 0.55 (10.9)

4 MgO 0.69 (10.1)

5 K2O 0.80 (8.0)

6 A_cover 0.90 (7.2)

7 Moist mull 0.99 (6.52)

8 PAR 1.05 (4.3)

9 Al3+ 1.11 (4.3)

10 N tot 1.16 (3.6)

Wet alder wood 1 pH (KCl) 0.30 (19.5)

2 Moist mull 0.56 (16.9)

3 Wet moder-mull 0.72 (10.4)

4 A_cover 0.88 (10.4)

5 Age 1.00 (7.8)

6 Water level 1.09 (5.8)

7 PAR 1.17 (5.2)

8 MgO 1.25 (5.2)

9 N tot 1.30 (3.2)

10 K2O 1.35 (3.2)

11 Wet mull 1.39 (2.6)

levels compared to the other two habitats (�28.0 cm, whereas inalder-ash carrs groundwater level varies from �0.7 to �87.0 cm,and in oak-hornbeam habitat �16.0 to �70.0 cm).

The results of the CCA ordination showed that in oak-hornbeamforest habitat the first four axes explained 31% of the species datavariance, and 60% of the species–environment relations. In alder-ash carrs the values were similar (31% of species data, and 59% forspecies–environment relationships), whereas in wet alder woodhabitats 39% of the species data variance and 64.5% of the species–environment relations were explained by the four CCA axes.

CCA results also showed that the combination of variablesresponsible for the distribution pattern of species in recent woodsdiffered among the habitats (Table 5). In the case of oak-hornbeamforests and in alder-ash carrs woodland age accounted for most ofthe variation explained (20% and 14.5%, respectively). However, inthe wet alder wood habitat pH was the variable explaining thebiggest share of the total variance in recent woods (19.5%),whereas forest age was responsible for only 7.8% of the variation.Humus type was among the environmental factors explainingproportionally high variation in species pattern in all three habitats(wet and moist mull in oak-hornbeam, wet moder-mull in alder-ash carr, and moist mull in wet alder wood). Besides, the springlevel of groundwater made a relatively high contribution inexplaining the ordination model, especially in the case of oak-hornbeam and alder-ash carr forest habitats (10.7% and 14.5%,respectively). Similar tendencies were noted for available Mg inoak-hornbeam and alder-ash carr forest types, explaining 8% (0.12;1.5 in total) and 10.1% (0.14; 1.38 in total) of the total variation inspecies composition in recent woods.

Fig. 1. The CCA biplot illustrating the ordination of ancient woodland species in

relation to the statistically significant environmental variables for the plots in the

recent oak-hornbeam habitats. Bold italic letters indicate ancient woodland species,

whereas Urtica dioica (Urt.dioi), Galium aparine (Gal.apar) and Poa trivialis (Poa.triv)

are as underlined capital letters. Species abbreviations are based on the three letters

of genus and four of species: Ado.mosc – Adoxa moschatellina, Aeg.poda – Aegopodium

podagraria, Aju.rept – Ajuga reptans, Ane.nemo – Anemone nemorosa, Ane.ranu –

Anemone ranunculoides, Ant.niti – Anthriscus nitida, Ath.fili – Athyrium filix-femina,

Chr.alte – Chrysosplenium alternifolium, Cic.lute – Circaea lutetiana, Dry.cart –

Dryopteris carthusiana, Fes.giga – Festuca gigantea, Fic.vern – Ficaria verna, Geu.urba –

Geum urbanum, Imp.noli – Impatiens noli-tangere, Mer.pere – Mercurialis perennis,

Mil.effu – Milium effusum, Moe.tri – Moehringia trinervia, Oxa.acet – Oxalis acetosella,

Ran.auri – Ranunculus auricomus, Rum.sang – Rumex sanguineus, Scr.nodo –

Scrophularia nodosa, Ste.holo – Stellaria holostea, Ste.nemo – Stellaria nemorum.

A. Orczewska / Forest Ecology and Management 258 (2009) 794–803800

In oak-hornbeam habitats ancient forest species were found onthe negative side of the first canonical axis, corresponding to mostof the environmental variables significantly contributing to theexplanation of species composition in the ordination model(Fig. 1). These variables included: woodland age, wet mull humustype, groundwater level, CEC, P2O5, MgO and Fe2+, all negativelycorrelated with the first axis. Most ancient woodland speciesavoided soils with moist mull humus and high pH level, the twovariables positively correlating with the first and negatively withthe second axis. Some clear affinities of individual species withsome environmental variables were observed, for example anassociation of Circaea lutetiana and Athyrium filix-femina with ahigh level of available Mg and groundwater. On the other handMercurialis perennis, Milium effusum, Stellaria nemorum, Moehringia

trinervia and Adoxa moschatellina preferred sites with high CEC,available P, and moist mull, whereas Oxalis acetosella, A. filix-femina

and Dryopteris carthusiana were clearly associated with woodlandage (Fig. 1).

To some extent a similar distribution pattern of ancientwoodland indicator species, concentrated on the negative sideof the first axis, was also observed in the alder-ash carr foresthabitats (Fig. 2). Species presence was related to woodland age,groundwater level wet moder-mull humus, available Mg and K,moist mull, and total N content, all negatively correlated with thefirst axis. Most woodland species avoided sites with high Al3+

content and illumination level (high PAR). Similarly to oak-hornbeam forest habitats M. perennis, S. nemorum, M. effusum, andalso Scropularia nodosa preferred sites with wet moder-mull

Fig. 2. The CCA biplot illustrating the ordination of ancient woodland species in

relation to the statistically significant environmental variables for the plots in the

recent alder-ash carrs habitats. Bold italic letters indicate ancient woodland species,

whereas Urtica dioica (Urt.dioi), Galium aparine (Gal.apar) and Poa trivialis (Poa.triv)

are as underlined capital letters. Species abbreviations are based on the three letters

of genus and four of species: Ado.mosc – Adoxa moschatellina, Aeg.poda – Aegopodium

podagraria, Aju.rept – Ajuga reptans, Ane.nemo – Anemone nemorosa, Ane.ranu –

Anemone ranunculoides, Ant.niti – Anthriscus nitida, Bra.sylv – Brachypodium

sylvaticum, Chr.alte – Chrysosplenium alternifolium, Cic.lute – Circaea lutetiana,

Dry.cart – Dryopteris carthusiana, Fes.giga – Festuca gigantea, Fic.vern – Ficaria verna,

Geu.urba – Geum urbanum, Imp.noli – Impatiens noli-tangere, Mer.pere – Mercurialis

perennis, Mil.effu – Milium effusum, Moe.tri – Moehringia trinervia, Oxa.acet – Oxalis

acetosella, Par.quad – Paris quadrifolia, Ran.auri – Ranunculus auricomus, Rum.sang –

Rumex sanguineus, Scr.nodo – Scrophularia nodosa, Sta.sylv – Stachys sylvatica,

Ste.holo – Stellaria holostea, Ste.nemo – Stellaria nemorum.

humus, whereas O. acetosella was an indicator of a growing age ofrecent wood. C. lutetiana was associated with a high level ofgroundwater, whereas D. carthusiana, Stachys sylvatica, Festuca

gigantea and Paris quadrifolia showed higher affinity to soils rich inavailable Mg. The last four species noticeably avoided sites withhigh illumination (PAR) level. Rumex sanguineus preferred siteswith high N content (Fig. 2).

Distribution of ancient woodland species in wet alder woodswas not as obvious as in the case of the other two habitats.However, some true forest species showed a clear pattern, relatedwith some environmental variables significantly contributing tothe explanation of the ordination model. Thus, most woodlandspecies avoided sites with high groundwater level, but preferredwell shaded places (M. effusum, O. acetosella, D. carthusiana and F.

gigantea), with moist mull humus (S. nemorum, Anemone nemorosa

and A. moschatellina), and rich in available K (M. trinervia). Thepresence of some species also corresponded to high woodland age(D. carthusiana), whereas others preferred sites with high pH leveland available Mg (Ranunculus auricomus) (Fig. 3).

The behaviour of U. dioica, Poa trivialis and Galium aparine, thethree vigorously growing herbs of high nutrient demand andcompetitive ability, is worth mentioning. Although it is widelyreported that these species are the indicators of soils rich innutrients, in the case of fertile recent alder woods, with a goodsupply of nutrients, their distribution pattern is not predominantlydependent on N or P availability, but primarily they noticeablyavoid sites with a high level of groundwater, combined with poorlight conditions. It should be emphasized that such a distributionpattern was observed regardless of habitat type (Figs. 1–3).

Fig. 3. The CCA biplot illustrating the ordination of ancient woodland species in

relation to the statistically significant environmental variables for the plots in the

recent wet alder wood habitats. Bold italic letters indicate ancient woodland

species, whereas Urtica dioica (Urt.dioi), Galium aparine (Gal.apar) and Poa trivialis

(Poa.triv) are as underlined capital letters. Species abbreviations are based on the

three letters of genus and four of species: Ado.mosc – Adoxa moschatellina, Aju.rept –

Ajuga reptans, Ane.nemo – Anemone nemorosa, Ath.fili – Athyrium filix-femina,

Cax.elon – Carex elongata, Chr.alte – Chrysosplenium alternifolium, Cic.lute – Circaea

lutetiana, Dry.cart – Dryopteris carthusiana, Fes.giga – Festuca gigantea, Fic.vern –

Ficaria verna, Geu.urba – Geum urbanum, Imp.noli – Impatiens noli-tangere, Mil.effu –

Milium effusum, Moe.tri – Moehringia trinervia, Oxa.acet – Oxalis acetosella, Ran.auri –

Ranunculus auricomus, Rum.sang – Rumex sanguineus, Scr.nodo – Scrophularia nodosa,

Ste.nemo – Stellaria nemorum.

A. Orczewska / Forest Ecology and Management 258 (2009) 794–803 801

6. Discussion and conclusions

Ancient and recent alder woodlands differed in their soilproperties, and in light availability level on the forest floor. Tracesof former agricultural use of soils under recent woods are stillpresent, despite the fact that such forests are very vigorous ingrowth; thus changes in their soils are far more dynamic than insites of lower fertility and moisture. Some of these differences insoil parameters could be attributed to former land-use. Alterationsin soil conditions on sites formerly used as meadows, expressed ina lower content of organic C and higher pH level, are in accordancewith those reported in earlier studies (Koerner et al., 1997, 1999;Wilson et al., 1997; Verheyen et al., 1999; Compton and Boone,2000; Dzwonko, 2001; Falkengren-Grerup et al., 2006). Aprogressive accumulation of organic matter observed afterafforestation with black alder, and reported by the above-mentioned authors, leads to acidification (Wilson et al., 1997;Richter et al., 1994). Intensive growth of trees is an acidifyingprocess owing to the exchange of base cations, taken up by plantswhich in turn release hydrogen ions into the soil (Nilsson et al.,1982; Binkley, 1986). Furthermore, intensive nitrification coupledwith nitrate leaching is among the processes determining theinfluence of N-fixing alder stands on acceleration of soil acidity(Binkley, 1986). Furthermore, a higher pH level in post-agriculturalsoils may be due to former liming. Larger amounts of available Mgand exchangeable Ca2+ in recent oak-hornbeam and alder-ash carrswoodland soils may also be evidence of this practice. Besides, thedecrease in exchangeable Ca2+ content in oak-hornbeam and wetalder wood forest types with woodland age, due to acidification,was confirmed. The relations in amounts of exchangeable andavailable K between recent and ancient woodland sites, withhigher values observed in ancient woods, were statisticallyconfirmed in each habitat. According to Binkley (1986) and Richteret al. (1994), potassium is a very mobile cation, as opposed to Ca2+

and Mg2+. Therefore, in forests a relatively rapid recycling of K+ isobserved. It is re-supplied to surface soils by a combination of itsrelease from mineral weathering and root uptake and rapidrecycling to the soils via leaching of canopies and forest floors.These observations are in agreement with the results obtained byKarkanis (1975) who estimated the rate of nutrient release fromthe alder leaf-litter to the soils, and observed that K release was thefastest. Although, as Binkley (1986) states, vigorous forests onrelatively young soils lose about 5–10 kg ha�1 yr�1 of K+, vialeaching, in general inputs exceed outputs, so this element limitsforest growth only in some very isolated situations, for example onextremely poor, sandy soils. However, its amounts, both K+

retained on cation exchange sites and available K, in recent alderwoods were increasing with the age of the woodland.

Larger quantities of P in post-agricultural woods compared tocontinuously forested sites, so often reported by many authors(Koerner et al., 1997; Honnay et al., 1999; Dzwonko, 2001; DeKeersmaeker et al., 2004; Falkengren-Grerup et al., 2006), were notconfirmed in the case of the black alder forests. Similar to thefindings of Wilson et al. (1997) soils under ancient forests werericher in available P than recent alder woods. Furthermore, agradual increase in P-available content with woodland age wasrecorded. These observations are in contrast with the arguments ofDe Keersmaeker et al. (2004) who suggested that fertilization leadsto an increase in labile inorganic forms of P, successivelyimmobilized by the formation of Ca and Al phosphates, whichtakes place after afforestation. Besides, progressive soil acidifica-tion in such pH ranges should reduce the amount of available P,whereas in recent alder woods the tendency was the opposite. Thequestion arises which mechanism is responsible for higherphosphorus availability observed in ancient woods compared toneighbouring recent stands, and for its gradual increase with age in

recent woodlands? According to Karkanis (1975) leaf-litter of aldercontributes to a distinctive increase of available K and P content insoils. Another potential issue worth discussing is whether the samephysiological feature of alders – open type of nutrition manage-ment where nitrogen is not withdrawn prior to leaf-fall (Zimka andStachurski, 1976) – which is responsible for the enrichment of soilwith nitrogen, works in the case of phosphorus as well. However, afurther probable explanation lies in what Binkley (1986) statesabout the influence of N-fixing plants on cycles of other nutrients,especially on phosphorus. According to this author P availabilityunder alders may be increased through the production ofphosphatase enzymes, which accelerate P release from litter.The results obtained for black alder forests seem to confirm hisconclusion indirectly, although more detailed biochemical studiesin this respect would be required.

One may assume that since forests show high P demand, lossesof P via leaching are minimal. Besides, N cycling and availability inN-fixing stands is increased, which in turn accelerates cycling of P,tied up in tree biomass. Furthermore, Al and Fe ions may retainphosphorus efficiently (Binkley, 1986). In conclusion, site historywould mainly affect the total P levels, whereas the recordeddifferences in available P between ancient and recent woodlandsites reflect current soil conditions, to a great extent attributable totheir current vegetation.

Total nitrogen content was always higher in ancient woodlandsoils than in recent ones. Such relations are in accordance withthose reported by Honnay et al. (1999), Verheyen et al. (1999),Dzwonko (2001), and Falkengren-Grerup et al. (2006), but oppositeto those obtained by Koerner et al. (1997, 1999), and DeKeersmaeker et al. (2004). Such variation in the differencesobserved may depend on soil fertility, the types of formeragricultural use, its duration and time since abandonment. HigherN content in ancient woods, similar to C content, may be the resultof the larger biomass of accumulated litter, also containing morealder leaves, for which the rate of decomposition is very high(Karkanis, 1975; Pereira et al., 1998). Besides, alder leaves containlarge amounts of N, which is rapidly recycled and returned to thesoils as mentioned before. Finally, the fact that recent alder woodswere planted on former meadows may be partially the reason forsuch differences, since Compton and Boone (2000), who studiedthe impact of agriculture on soil C and N, reported higher amountsof these nutrients in pastures than on former arable land.

Alder-ash forests had the greatest amount of total N in theirsoils among all three forest types studied. The lower level of total Nin ancient wet alder woods than in the alder-ash carrs may becaused by two factors. One is related with the water managementtype of alder-ash carrs, depending on lateral water fluxes, thussupplying the ecosystem with additional nutrients. In contrast, wetalder woods rely either on groundwater or water from rainfall. Aloss of N from wet alder woods is the second factor contributing totheir lower fertility compared to alder-ash carrs, since in wet alderwoods close proximity (a mosaic pattern) of aerated andperiodically saturated soil beds provide ideal conditions fordenitrification, and N removal through its volatilization. Besides,pure vigorous stands of N-fixing alder in wet alder woods causehigh rates of N mineralization, and consequently nitrate lossesthrough leaching (Binkley, 1986).

An important factor influencing nutrient cycling, especially Nand P, in alder woods is the activity of arbuscular mycorrhizal (AM)fungi, since the studies of Monzon and Azcon (2001) show that AMfungi enhance N and P uptake by alder. Phosphorus is of greatimportance in N-fixation, because its severe deficiency markedlyinhibits that process. Thus, AM mycorrhizas have a great effect ontotal N yield, working in two ways: one through P-mediatedmechanism which improves N-fixation, and the second directly byenhancing N uptake from the soil (Monzon and Azcon, 2001).

A. Orczewska / Forest Ecology and Management 258 (2009) 794–803802

In recent alder woods on fresh and moist soils, especially in oak-hornbeam habitats, the distribution pattern of many ancientwoodland species was related to wet rather than moist humus(mull in oak-hornbeam, and moder-mull in alder-ash carrs),whereas in wet alder woods, moist mull was more important thanits wet type. Species preferences for mull humus are not surprising,but are typical for many true woodland species (Tyler, 1989).Therefore, humus cannot be among the factors inhibiting butfacilitating the successful establishment of typical woodland florain recent alder woods. Besides, the litter of alder leavesdecomposes very quickly; hence there is no risk of its inhibitingimpact due to considerable litter thickness on the recruitment ofdeciduous woodland species, as opposed to some previous studiesreferring to trees with slowly decomposing leaves (Sydes andGrime, 1981; Dzwonko, 2001). It seems that differences in speciescomposition between ancient and recent woods cannot beexplained by a single factor, for example soil properties of thesesites, but a combined effect of high nutrient and water availability,and light conditions in the forest floor. Competitive exclusion ofspecialist woodland herbs by vigorously growing species, such asU. dioica, taking advantage of high soil fertility seems to be apredominant form of recruitment limitation. Its inhibitinginfluence on the migration capacity of woodland flora intobroadleaved forests on rich soils was reported in many studies,including those in alder woods (Honnay et al., 1999; DeKeersmaeker et al., 2004; Hipps et al., 2005; Orczewska, inpress-b). Most of these authors relate the vigorous growth of U.

dioica with increased levels of P in soils under recent woods.However, the CCA results clearly show that in the alder woodsstudied, stinging nettle distribution is much more dependent onthe groundwater level, and to a lesser extent, also on lightconditions, than on the high amounts of available P or total Ncontent (Figs. 1–3). Besides, more available P was present inancient alder woodlands, where U. dioica cover was much lowerthan in recent woods. It is noticeable (Figs. 1–3) that U. dioica, andsome other competitive species such as P. trivialis and G. aparine,vigorously growing in recent alder woods, avoid the wettest sites,with the shallowest groundwater level. The cover of U. dioica in wetalder woods is much lower (mean = 6.4%, max = 40%, median = 5%)than in oak-hornbeam (mean = 43%, max = 90%, median = 20%) andalder-ash carr habitats (mean = 26.5%, max = 90%, median = 10%).One cannot exclude the possibility that the low cover of stingingnettle in recent wet alder wood is a key factor responsible for thehighest share of ancient woodland species occurring in theneighbouring ancient forests present in the herb layer of recentwoods among all three habitats (Table 1). Recent alder woods alsoreceive more light to the forest floor than continuously forestedsites. This is not surprising, since the stands in post-agriculturalplantations are subjected to different management practices,including thinning. Besides, shrub vegetation in recent woods ispoorly developed compared to ancient forests. Overall, increasedlight availability, combined with a high level of nutrients,facilitates the development of U. dioica and other competitivespecies. Such a mechanism was described by Honnay et al. (1999),De Keersmaeker et al. (2004), and others. Similar conclusions werereached by Elemans (2004), who compared the plasticity of shade-tolerant woodland herbs and forest-edge species. According to thisauthor, the higher biomass production of light-demanding speciesat increased nutrient availability does not happen under low lightconditions but only at a higher level of illumination do additionalnutrients have this effect.

An important conclusion, having practical application forforest management practices, can be drawn from the aboveobservations. The studies in alder woods show that to create thebest possible conditions allowing for effective forest recovery inhabitats of such high fertility, it is essential to maintain good

water conditions. A high water level inhibits the vigorous growthof expansive, nutrient-demanding species. This in turn reducesthe competitive exclusion of woodland flora by such aggressiveherbs. Otherwise, the drainage of such places may lead to aninvasion of eutrophic species, as reported by Hermy et al. (1993).Besides, creating more shady conditions in the forest floor, partlyachieved naturally due to tree growth and subsequent full canopydevelopment, may also distinctively limit the expansion of U.

dioica. This, in turn facilitates the immigration of typical wood-land herbs.

Acknowledgements

I would like to express my gratitude to Dr. John Parkery(formerly with the Forestry Commission, Alice Holt, UK), for hissupport at every stage of the work, to soil scientists Mgr. JadwigaZalewa, Mgr. Inz. Stefan Zalewa, and Mgr. Inz. Stanisław Gaweł ofthe Office of Forest Managament and Forestry Geodesy in Krakow,for digging the soil profiles and doing the chemical analyses. Muchgratitude is also due to Mgr. Sabina Słomian, Mgr. Joanna Czapla,Mrs. Symeona Zyndych, Mgr. Iza Szwarc, and Mgr. Paweł Goras fortheir assistance in the field work, and to Professor Patricia Thomas,(University of Białystok), who checked and improved the languageof the paper. Much gratitude is also due to the two anonymousreferees for their valuable comments on the manuscript. Projectsupported by the Polish Ministry of Science, Grant No. 2P04F 05929, between 2005 and 2007

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