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

Forest Ecology and Management 258 (2009) 794–803

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The impact of former agriculture on habitat conditions and distribution patterns of 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

A B S T R A C T

Article history: Received 5 March 2009 Received in revised form 19 May 2009 Accepted 19 May 2009

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. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Recruitment limitation Competitive exclusion Ancient forests Soil conditions Groundwater level Illumination Forest recovery

1. Introduction The impact of former agricultural land-use on the soils of forests currently existing in the landscape has become an object of growing interest to ecologists, since it is widely known that historical land-use can noticeably influence present-day soil characteristics, and through that also the diversity patterns of forest plant species (Tyler, 1989; Dzwonko and Gawron´ski, 1994; Koerner et al., 1997; Honnay et al., 1999; Compton and Boone, 2000; Kolb and Diekmann, 2004; Hipps et al., 2005; FalkengrenGrerup et al., 2006). Most studies indicate that soils in postagricultural forests show long-lasting evidence of their previous

* Tel.: +48 32 359 15 48. E-mail address: [email protected]. 0378-1127/$ – see front matter ß 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 higher values of pH and a lower content of carbon and organic matter (Koerner et al., 1997; Bossuyt et al., 1999). Besides, many authors report higher levels of phosphorus and nitrogen in recent forest soils compared to ancient woodland sites, which is explained as the consequence of former cultivation of post-agricultural forest soils (Koerner et al., 1997; Honnay et al., 1999; Dzwonko, 2001; De Keersmaeker et al., 2004). The influence of an increased level of soil nutrients on the colonization of recent woods by woodland flora depends on soil type and its fertility. The colonization capacity of woodland species in poor soils on an acidic substrate is enhanced due to the surplus nutrient content (Koerner et al., 1997), whereas in fertile, rich soils, additional levels of plant-available nutrients lead to the increased development 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

1999; Verheyen et al., 1999; De Keersmaeker et al., 2004). Furthermore, high fertility may sometimes also facilitate the dynamic growth of trees, which in turn, through dense tree canopy, reduces the light intensity below 2% in the forest interior. Thus, in such circumstances, despite the fact that soils are rich in nutrients, light becomes a limiting factor, reducing the number of shadetolerant, woodland herbs (Dupre´ et al., 2002). Nevertheless, in most recent woodland sites, the illumination level is higher than in ancient woods. Thus, a competitive exclusion by vigorous herbs is a more probable mechanism responsible for the reduced number of true forest species successfully establishing their populations in the herb layer of recent woods. In general, results of studies indicate that the direction and pace of changes in the soil chemistry of recent woodland sites are diverse and dependent on many factors, including the duration of the former agricultural use (Verheyen et al., 1999), time since afforestation (Bossuyt et al., 1999), habitat type and the type of former agricultural use (pastures, gardens, croplands) (Koerner et al., 1997, 1999). In addition, secondary succession is not only driven by the combined influence of the edaphic and light conditions, but also depends 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 wet and too dry, but show a clear affinity for sites with a moderate level of this abiotic factor (Hermy et al., 1999; Dzwonko and Loster, 2001). This feature, together with other specific life-history traits of true forest herbs, is responsible for their low colonization capacity in recent, post-agricultural woods. Despite the growing interest in the influence of former agricultural use of soils in recent woods on the forest recovery process, little is known about the relations between habitat conditions and herbaceous layer composition in post-agricultural black alder (Alnus glutinosa) woods representing the most fertile and the wettest forest habitats among the European temperate forest types. A. glutinosa belongs to the actinorhizal species, thus, due to symbiosis with Frankia alnii, responsible for N2-fixation from the atmosphere, it contributes to the enrichment of the soil with this nutrient by approximately 50 kg ha1 yr1 (Schaede, 1967). According to other sources, the efficiency of this process is estimated at 50–130 kg ha1 yr1 (Pancer-Kotejowa and Zarzycki, 1980), 10–300 kg ha1 yr1 (Sprent and Sprent, 1990), or even several hundred kg ha1 yr1 (Binkley, 1986). Enrichment of soils with nitrogen is also possible due to the ‘open’ management type of nutrition typical for black alder. This means that it does not withdraw its nutrients but retains much foliar nitrogen in the leaves until they fall in the autumn. Therefore, most mineral elements which either were absorbed from the soil during the vegetation season or from the nodules fixing N2 from the atmosphere, are returned to the soil (Zimka and Stachurski, 1976). The rate of leaf decomposition is very quick for alder (k = 0.908 yr1 which corresponds to a mass loss of 66% during the period of 16.5 months) (Pereira et al., 1998). Thus, habitats with stands composed of A. glutinosa are eutrophic. Such characteristics make black alder an excellent pioneer species, capable of colonizing soils poor in nutrients. Furthermore, in contrast to other early successional trees, A. glutinosa tolerates heavy, waterlogged soils (McVean, 1953; Obidzin´ski, 2004). Due to these features, it is a common practice in Polish forestry to use black alder in afforestation on former meadows where agriculture has been abandoned because of the permanently wet soils. In many cases recent A. glutinosa plantations are located in the proximity of ancient forests with alder as a main canopy species. The latter ones, including typical wet alder woods, alder-ash carrs, and the wettest types of oak-hornbeam communities, are still common in forested areas of many regions of Poland. Their large species richness can be

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used as a potential reservoir of rare plants for the colonization of recent woods planted in their proximity. Thus, the studies on the recovery of the herbaceous layer in recent black alder woods, resulting in calculating migration rates of 51 forest herbs, were undertaken. These studies have shown that the process of colonization of the herb layer by woodland flora in such forests proceeds faster compared to less fertile and drier habitats. Nevertheless, herb layer recovery is limited by the abundant growth 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 dispersal and recruitment limitation, and that the pace of secondary succession is strongly influenced by habitat type. Thus, the studies focusing on abiotic conditions responsible for colonization mechanisms present in alder woodlands were undertaken. More specifically, edaphic, hydrological and light conditions present in adjoining ancient and recent alder woods were compared and related to differences in species composition observed in the course of detailed floristic research. 2. Study area description The studies on vegetation composition and habitat conditions in black alder forests differing in their management history were carried out in two neighbouring geographical regions of southwestern Poland, namely the Oles´nica Plain (518040 N; 178430 E), a part of the Opole Silesia, and the Z˙migro´d Valley (518280 N; 168540 E), belonging to Lower Silesia. In both regions ancient forests with a high level of naturalness reach a distinctive proportion. Among them the stands with A. glutinosa are well represented. As a result of cartographic studies, based on the first old map available for this part of Europe (the Schmettausche map of 1765–1780) (Orczewska, in press-a) the status of ancient forests, where the ecological research was carried out, was confirmed. These woodlands have existed continuously in the landscape for the last 230 yr. Among recent woods of post-agricultural origin, reaching ca. 15% of the total forested area, black alder woodlands, planted on former damp meadows, represent a high proportion as well. The existence of such specific situations in nature offered suitable sites for research on the rate 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 samples taken from the 33 transects, of approximately 80 m in length by 4 m in width, laid out perpendicularly across the boundary between the ancient and recent woodlands. Post-agricultural black alder plantations, representing the following age classes: up to 10, 10–20, 20–30, 30–40, and 40–50 yr (older alder stands were available in only two cases), were situated in direct proximity of the following types of ancient forests: wet types of oak-hornbeam wood (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.-Go´rn. (1975) 1987). In total 11 transects were set up in the habitat of oak-hornbeam forest, 12 in alder-ash carr, and 10 in wet alder wood. The quadrat plots of 16 m2 were laid out at intervals of 4 m. Transects consisted of four plots located in ancient forest and six in recent woodland, which gave 28 m of each transect located in the ancient forest, and 44 m into the recent wood. Such a layout of the plots permitted a calculation of the colonization rates of woodland species migrating into recent alder woods (for more details on transect design and calculations of migration rates see Orczewska, in press-b).

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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 species with, in each case, an estimate of their percentage cover (1%, 5%, 10%, and then at 10% intervals), was recorded. A similar scale was applied to estimate the total cover of the shrub layer, whereas the cover of the stand canopy was assessed using the spherical densiometer, with four readings per plot taken and their mean value used in further analyses. Floristic research was followed by studies on the chemical properties of soils, on groundwater level, and on light conditions in alder woods. To describe edaphic conditions in each plot where vegetation was studied, five soil samples from the topsoil, without the litter layer, representing the main rooting area of most herb layer plants (approximately upper 10–15 cm) were taken and bulked. After air-drying and sieving over 2 mm, they were analysed for: pH, measured potentiometrically in H2O and in 1N KCl, total organic 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 exchange capacity (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) (Lityn´ski et al., 1976; Ostrowska et al., 1991). Besides, soil profiles were dug to describe the soil types, one in each of the adjacent forests. Soil samples from each horizon of the profiles were taken and analysed for the same set of chemical parameters as the bulk samples from the transects’ plots. The humus form, following the classification given by Prusinkiewicz (1988), was also estimated. Humus form is one of the components reflecting soil nutrient levels and the availability of soil nutrients to plants, since it plays a role in nutrient storage and cycling. That is why it is one of the most important determinants of the assessment of forest soil quality. For these reasons, plants with growth strictly dependent on humus form may work as indicators of humus form quality (Klinka et al., 1990). In order to characterize the groundwater level in recent forests and their surroundings, piezometers were installed (ø = 5 cm, h = 50–200 cm, one piezometer per transect), where the oscillations of the water level (cm) were recorded at monthly intervals over the whole hydrological year. However, for the purpose of the statistical analyses, the mean values of groundwater level during three months in the spring were used. The symbol ‘+’ indicates water stagnating on the surface, whereas ‘’ refers to the level of water below the ground. To characterize the light conditions inside the woods, at ground level, 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 fu¨r Daten-, Mess- und Pru¨ftechnik – DMP, Fehraltofr, Switzerland), and expressed as a percentage of the PAR level measured in the open area at the same time. Besides, in statistical analyses, canopy (A_cover) and shrub cover (B_cover) estimates were also used as environmental variables reflecting the light availability on the forest floor.

4. Data analysis To estimate whether there are differences between ancient and recent forests in their richness and total cover of ancient woodland indicator species (species were selected on the basis of a list given by Dzwonko and Loster, 2001), the t-test was applied. Differences between environmental conditions (soil chemical features and illumination level on the forest floor) in ancient and recent forests were also compared with the t-test. ANOVA and the Tukey significance test were used to compare these environmental variables among post-agricultural woods representing different habitats, whereas for a comparison of variables in woods of different age classes of their stand, the Kruskal–Wallis test was applied. For the last analysis recent woods were grouped in three age classes, with 20 yr intervals, with the first, including stands up to 20 yr old, the second – 21–40, and the third – 41–60 yrs. Furthermore, the linear regression analysis was used to assess the influence of a recent woodland age on the chemical parameters of soils. However, this statistical procedure was limited only to those variables which showed a linear relationship with woodland age (without or after square root or logarithmic transformation). These statistical procedures were applied separately for each type of habitat. In order to study the vegetation gradients in recent alder woods and their relation with the habitat conditions, canonical correspondence analysis – CCA was implemented (the length of the gradient of the first axis in the detrended correspondence analysis – DCA allowed for the selection of a unimodal CCA technique). The data from the plots of the recent woods, again separately for each habitat type, were analysed. Among the CCA explanatory variables those which correlated with each other, such as Ca2+, Mg2+, Na+, K+ with cation exchange capacity, were excluded from the ordination analysis. Thus, the following quantitative environmental variables were included in the CCA: age of a recent wood, canopy (A) and shrub (B) cover, PAR level, mean water level in spring months, pH (in KCl), Al3+, Fe2+, P2O5, K2O, MgO, CEC, organic C, total N, whereas humus type represented a qualitative variable (0, 1), and included: moist mull, wet mull, moist moder-mull, and wet moder-mull. An automatic forward selection procedure was applied to select the environmental variables contributing to the species composition in recent woods. The Monte Carlo permutation test under the reduced model was used to examine the statistical significance of the explanatory variables. The CCA ordination technique was run with the application of Canoco 4.5, whereas other statistical analyses were done using the Statistica 8 package. 5. Results The number and total cover (calculated as a sum of covers of all the individual species) of true woodland herbs were higher in ancient woods than in adjoining recent ones, regardless of forest type (Table 1). However, individual differences among the habitats were observed. In alder-ash carrs, the most fertile forest types studied, ancient woodland species in recent woods reached a much higher abundance than in the other two remaining habitats. When

Table 1 Mean and standard deviation values of the number and total cover of ancient woodland indicator species present in ancient and recent woodlands.

Number of ancient woodland species (mean  SD) Total cover of ancient woodland species (mean  SD) ** ****

Oak-hornbeam habitat

Alder-ash carr habitat

Ancient

Recent

Ancient

Recent

Ancient

Recent

14.1**** (5.4) 96.4**** (48.8)

7.5 (4.4) 53.9 (47.5)

13.4**** (5.1) 153.1**** (51.0)

8.8 (4.0) 100.7 (61.7)

11.1**** (3.7) 71.3** (45.8)

8.1 (3.0) 47.4 (32.9)

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

Typical wet alder wood habitat

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

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Table 2 Relations 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

Saprihistic gleysols GLhis Luvic gleysols GLIv Mollic gleysols GLmo Stagni-haplic gleysols GLha-st Stagni-eutric gleysols GLeu-st Haplic gleysols GLha Ferri-umbric gleysols Glum-fr Umbric gleysols GLum Sapric histosols HSsa Humic gleysols GLhu

O-H recent

A-A ancient

A-A recent

W-A ancient

W-A recent

x x x

x

x

x

x

x

x

x x

x

x

x

x

x

x

x x x x x

x x

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

only ancient woodlands are compared, the lowest number of true forest species was observed in wet alder woods. On the other hand, if the woodland species number in recent woods is expressed as a percentage of their richness in ancient woodland sites, recent wet alder woods have the highest proportion of ancient woodland species present in the neighbouring ancient forests (73%), whereas their proportion in recent alder-ash carrs reaches 65.7%, and in oak-hornbeam 53.2% of that present in migration source ancient woods. Similar relations and proportions are observed in the total cover of true woodland species, with ancient wet alder woods having the lowest (71.3%), and alder-ash carrs the highest values (153.1%). An intermediate value is achieved in oak-hornbeam habitats (96.5%). The total cover of ancient woodland species present in recent oak-hornbeam forest reaches 56% of that recorded in ancient woods, whereas it is 66% in the case of wet alder wood and alder-ash carrs. Studies on soil types present in alder forests revealed that mesotrophic and eutrophic soils, on moist, wet or periodically waterlogged sites predominate. They mainly include a wide spectrum of gleysols and, to a far lesser extent, also histosols,

with humic gleysols and ferri-umbric gleysols (following the WRB nomenclature, 1998) being the most frequent, and present in all habitats (Table 2). Many properties of soils in ancient and recent woodlands differed significantly, regardless of habitat type (Table 3). Such differences 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 in ancient woods in the habitats of oak-hornbeam forests and in alder-ash carr, whereas pH level in those cases reached lower values compared to recent woods. Besides, soils in ancient forests were characterized by a higher content of total N, although these differences were significant only in the wet alder wood habitats (Table 3). The forest floor of recent woods receives more light than in ancient woodland sites. However, these differences were significant only in the habitats of oak-hornbeam forest and alder-ash carrs (Table 3). When habitat conditions among recent woodland sites were analysed, the general tendency was that soil conditions in alderash riparian forests were distinctively different compared to the

Table 3 Mean and standard deviation values of the environmental variables in ancient and recent woodlands. Variable

Mean (SD) – O-H_ancient (N = 43)

Mean (SD) – O-H_recent (N = 66)

Mean (SD) – A-A_ancient (N = 48)

Mean (SD) – A-A_recent (N = 72)

Mean (SD) –W-A_ancient (N = 40)

Mean (SD) –W-A_recent (N = 60)

pH_H2O pH_KCl Al+3 Fe2+ Mg2+ Ca2+ Na+ K+ Base cations Cation exchange capacity (CEC) P2O5 K2O MgO Corg Ntot C/N PAR A_cover B_cover

4.81  0.86 3.97  0.84 2.11  2.47**** 4669.11  3217.76 1.167  0.77 12.15  8.11 0.07  0.08y 0.13  0.07* 13.51  8.73 28.79  9.67*

5.31  0.58*** (b) 4.42  0.57** (b) 0.62  1.22 7066.35  5022.24** (b) 1.19  0.78 (b) 13.25  8.35 (b) 0.04  0.03 (a) 0.10  0.07 (b) 14.58  8.93 (b) 24.15  10.93 (b)

5.53  0.77 4.79  0.84 0.51  1.13* 12604.38  14750.75 2.80  1.91 27.47  17.67 0.07  0.05 0.25  0.27** 30.59  19.44 47.58  32.20y

5.81  0.57* (a) 5.04  0.68y (a) 0.18  0.44 (a) 14979.28  15336.60 (a) 2.50  1.40 (a) 27.16  14.06 (a) 0.08  0.07 (b) 0.15  0.11 (a) 29.88  15.18 (a) 38.97  16.82 (a)

5.68  0.74* 5.02  0.85** 0.30  1.27 9446.43  7760.05 2.42  1.70**** 30.05  14.62**** 0.22  0.12**** 0.17  0.04**** 32.86  16.04**** 42.37  14.09****

5.31  0.91 (b) 4.51  0.99 (b) 0.94  1.44* (b) 9486.32  9770.48 (b) 1.10  0.79 (b) 15.32  10.26 (b) 0.12  0.09 (c) 0.11  0.05 (b) 16.64  10.98 (b) 27.11  9.74 (b)

40.95  25.26 71.50  52.18**** 155.70  91.53 8.49  4.76** 0.64  0.32 13.10  3.17**** 7.77  8.21 84.96  10.54 19.47  23.06**

35.24  21.80 (a) 37.56  32.91y (a) 161.18  75.94 (b) 5.86  4.37 (b) 0.58  0.39 (b) 10.15  2.19 (a) 10.99  7.69* (a) 82.81  11.25 (b) 9.47  16.60 (b)

40.06  19.23 93.71  110.11** 251.04  135.05 11.79  7.90* 0.99  0.64 11.90  2.08 11.43  9.31 84.86  8.07 18.83  20.20**

40.31  32.13 49.56  35.37y (b) 256.59  93.81 (a) 9.23  5.69 (a) 0.84  0.51 (a) 11.19  3.11 (b) 17.26  15.86* (b) 81.44  16.62 (b) 9.82  11.88 (b)

84.23  79.72* 58.67  17.83*** 242.29  126.22**** 9.39  3.65**** 0.81  0.32**** 12.12  3.89 13.67  7.02 91.69  5.61 5.95  10.21

53.24  48.71 (b) 44.72  19.84 141.06  106.13 (b) 5.90  2.83 (b) 0.51  0.23 (b) 11.60  1.95 (b) 14.83  7.08 90.49  6.82 (a) 3.18  7.28 (a)

n.s. – not significant. O-H – aok-hornbeam forest; A-A – alder-ash carr; W-A – typical wet alder wood. In the case of recent woods the same letter in brackets means no 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.

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

Oak-hornbeam habitat

pH_H2O pH_KCl Al+3 Fe2+

5.50  0.52 (b) 4.58  0.55 (b) 0.29  0.36 (b) 5681.56  3366.61 (b) 2+ Mg 1.11  0.73 Ca2+ 12.15  6.39 Na+ 0.04  0.02 K+ 0.10  0.08 (b) Base cations 13.40  6.80 Cation exchange 20.72  7.23 (a) capacity P2O5 37.15  26.23 (b) K2O 36.74  39.22 (b) MgO 134.67  74.65 (b) Corg 4.12  1.50 Ntot 0.40  0.13 (a) C/N 10.53  2.38 PAR 11.29  8.49 A_cover 80.06  13.41 (a) B_cover 2.81  5.47 (a)

<20 (N = 24)

21–40 (N = 24)

41–60 (N = 6)

r

5.26  0.45 (b) 4.43  0.42 (b) 0.30  0.40 (b) 6908.46  5342.47 (b) 1.44  0.87 16.13  10.65 0.05  0.04 0.09  0.05 (b) 17.70  11.42 27.73  14.23

4.33  0.22 (a) 3.44  0.19 (a) 3.93  1.79 (a) 16006.67  2724.65 (a) 0.67  0.24 8.31  4.43 0.03  0.01 0.14  0.04 (a) 9.15  4.58 30.43  7.81 (b)

0.37**** 5.71  0.49 0.28**** 4.92  0.70 0.27  0.74 (a) 0.22**** 13793.75  15583.67 (b) 2.02  1.28 21.15  9.95 (a) 0.11  0.10 (a) 0.20  0.15 (a) 23.48  10.93 (a) 0.18*** 30.42  10.88 (a)

27.83  11.39 (b) 31.21  19.54 (b) 211.54  57.84 (a) 8.50  6.19 0.83  0.54 (b) 9.78  2.01 11.28  7.27 85.26  7.32 19.33  23.30 (b)

53.43  8.80 (a) 67.87  15.82 (a) 118.82  34.38 (b) 5.77  1.30 0.15*** 0.61  0.11 (b) 0.19*** 9.35  1.34 7.99  2.98 89.56  1.58 (b) 10.00  9.49

29.18  41.84 (a) 61.42  24.45 (a) 229.92  108.03 8.26  5.08 (b) 0.75  0.45 (b) 11.28  3.86 19.85  19.96 76.84  26.49 4.46  8.21 (a)

Typical wet alder wood <20 (N = 30)

21–40 (N = 18)

41–60 (N = 12)

r2

5.66  0.66 4.90  0.79 0.14  0.17 35520.42  19508.15 (a) 2.71  1.63 34.67  16.24 (b) 0.23**** 0.07  0.03 0.19  0.10 (a) 37.64  17.35 (b) 0.23**** 51.46  15.90 (b) 0.35****

5.77  0.79 (a) 4.99  0.93 (a) 0.67  1.54 (a) 8199.47  5610.64 (b) 1.20  0.73 18.46  9.98 (a) 0.12  0.10 0.10  0.04 19.88  10.66 (a) 26.31  9.04

4.99  0.95 (b) 4.17  0.99 (b) 1.21  1.17 (b) 6315.61  2234.50 (b) 1.09  1.06 14.83  11.68 0.09  0.07 (a) 0.10  0.05 16.11  12.73 30.21  11.04

4.67  0.39 (b) 3.85  0.47 (b) 1.20  1.54 (b) 17459.50  18193.38 (a) 0.87  0.29 8.19  2.84 (b) 0.18  0.10 (b) 0.13  0.04 9.36  3.19 (b) 24.44  8.91

0.37**** 0.32****

56.58  27.15 (b) 94.75  36.95 (c) 306.08  75.23 0.12*** 15.76  6.43 (a) 0.22**** 1.36  0.67 (a) 0.21**** 12.21  2.42 (b) 18.34  15.73 81.02  12.20 10.58  12.18

36.28  49.55 (b) 42.89  18.51 (b) 172.78  77.36 (a) 5.05  2.55 (a) 0.49  0.26 10.62  1.49 (a) 16.48  5.41 (a) 87.54  8.12 (b) 5.40  9.52

44.31  22.94 (b) 39.17  21.46 (b) 133.48  152.62 (b) 6.72  2.79 (b) 0.51  0.17 13.05  2.20 (b) 6.85  1.70 (b) 92.01  3.24 (b) 1.61  3.27

109.10  33.58 (a) 0.21*** 57.60  15.98 (a) 73.12  27.64 (b) 0.23**** 6.78  3.12 (b) 0.56  0.22 11.88  0.93 (b) 22.67  2.90 (c) 95.60  1.78 (a) 0.00

21–40 (N = 33)

41–60 (N = 12)

5.98  0.55 5.20  0.63 0.13  0.14 (b) 9195.55  3874.95 (b) 2.77  1.41 27.66  14.52 0.06  0.04 (b) 0.09  0.05 (a) 30.58  15.83 38.81  17.11 42.51  23.61 (b) 25.85  19.65 (b) 257.17  87.90 7.00  3.38 (b) 0.67  0.31 (b) 10.77  2.85 (a) 15.96  12.97 84.25  5.13 11.45  11.32 (b)

2

r

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.

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

<20 (N = 36)

Alder-ash carr 2

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other two types of habitats (Table 3). This was expressed by higher values of many parameters in alder-ash carr habitat type (e.g. pH, CEC, organic C, total N, Mg2+, Ca2+), which confirms the higher fertility of these sites compared to the other two forest types. Most values of soil parameters change over time (Table 4). Some of the observed differences in the soil chemical characteristics in alder plantations of successive age classes are present in each habitat. These refer to Fe2+, available P, and total N, the content of which increases with time. On the other hand, values of pH and Mg2+ decrease with time, although these differences are not always significant. Some chemical characteristics of the recent woodland soils representing the highest age class (41–60 yr) to some extent become 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 growing age of recent woodland (Tables 3 and 4). The results of the linear regression analyses show that in the habitats of oak-hornbeam forests the age of a recent wood had a negative effect on pH and base cations, whereas the values of Fe2+, CEC, C and N content grew with the woodland age. In alder-ash carrs the positive effect of age on Ca2+, CEC, available P, Mg, and on C and N contents was observed. In wet alder wood habitats the positive effect was confirmed in the case of available P and Mg, whereas the values of pH and base cations decreased with a recent wood age (Table 4). The spectrum of mean spring groundwater levels reflects diverse water conditions in the forests investigated. As one would expect, temporal submergence is a characteristic feature of alder swamp forests (typical wet alder woods), since in many study sites their water periodically stagnates on the surface in early spring (maximum mean level of water reaches here +18.3 cm). In other cases groundwater in wet alder woods is situated at the shallowest

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levels compared to the other two habitats (28.0 cm, whereas in alder-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-hornbeam forest habitat the first four axes explained 31% of the species data variance, and 60% of the species–environment relations. In alderash carrs the values were similar (31% of species data, and 59% for species–environment relationships), whereas in wet alder wood habitats 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 variables responsible for the distribution pattern of species in recent woods differed among the habitats (Table 5). In the case of oak-hornbeam forests and in alder-ash carrs woodland age accounted for most of the variation explained (20% and 14.5%, respectively). However, in the wet alder wood habitat pH was the variable explaining the biggest 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 explaining proportionally high variation in species pattern in all three habitats (wet and moist mull in oak-hornbeam, wet moder-mull in alderash carr, and moist mull in wet alder wood). Besides, the spring level of groundwater made a relatively high contribution in explaining the ordination model, especially in the case of oakhornbeam and alder-ash carr forest habitats (10.7% and 14.5%, respectively). Similar tendencies were noted for available Mg in oak-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 in species composition in recent woods.

Table 5 Forward selection of environmental variables (p = 0.01) in a canonical correspondence 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 2 3 4 5 6 7 8 9

Age Wet mull Moist mull Water level MgO P2O5 pH CEC Fe2+

0.30 0.49 0.66 0.82 0.94 1.04 1.12 1.19 1.23

(20.0) (12.7) (11.3) (10.7) (8.0) (6.7) (5.3) (4.7) (2.7)

Alder-ash carr

1 2 3 4 5 6 7 8 9 10

Age Water level Wet moder-mull MgO K2O A_cover Moist mull PAR Al3+ N tot

0.20 0.40 0.55 0.69 0.80 0.90 0.99 1.05 1.11 1.16

(14.5) (14.5) (10.9) (10.1) (8.0) (7.2) (6.52) (4.3) (4.3) (3.6)

1 2 3 4 5 6 7 8 9 10 11

pH (KCl) Moist mull Wet moder-mull A_cover Age Water level PAR MgO N tot K2O Wet mull

0.30 0.56 0.72 0.88 1.00 1.09 1.17 1.25 1.30 1.35 1.39

(19.5) (16.9) (10.4) (10.4) (7.8) (5.8) (5.2) (5.2) (3.2) (3.2) (2.6)

Wet alder wood

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.

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In oak-hornbeam habitats ancient forest species were found on the negative side of the first canonical axis, corresponding to most of the environmental variables significantly contributing to the explanation of species composition in the ordination model (Fig. 1). These variables included: woodland age, wet mull humus type, groundwater level, CEC, P2O5, MgO and Fe2+, all negatively correlated with the first axis. Most ancient woodland species avoided soils with moist mull humus and high pH level, the two variables positively correlating with the first and negatively with the second axis. Some clear affinities of individual species with some environmental variables were observed, for example an association of Circaea lutetiana and Athyrium filix-femina with a high level of available Mg and groundwater. On the other hand Mercurialis 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 woodland age (Fig. 1). To some extent a similar distribution pattern of ancient woodland indicator species, concentrated on the negative side of the first axis, was also observed in the alder-ash carr forest habitats (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 the first axis. Most woodland species avoided sites with high Al3+ content and illumination level (high PAR). Similarly to oakhornbeam forest habitats M. perennis, S. nemorum, M. effusum, and also 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 of recent wood. C. lutetiana was associated with a high level of groundwater, whereas D. carthusiana, Stachys sylvatica, Festuca gigantea and Paris quadrifolia showed higher affinity to soils rich in available Mg. The last four species noticeably avoided sites with high illumination (PAR) level. Rumex sanguineus preferred sites with high N content (Fig. 2). Distribution of ancient woodland species in wet alder woods was not as obvious as in the case of the other two habitats. However, some true forest species showed a clear pattern, related with some environmental variables significantly contributing to the explanation of the ordination model. Thus, most woodland species avoided sites with high groundwater level, but preferred well 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). The presence of some species also corresponded to high woodland age (D. carthusiana), whereas others preferred sites with high pH level and available Mg (Ranunculus auricomus) (Fig. 3). The behaviour of U. dioica, Poa trivialis and Galium aparine, the three vigorously growing herbs of high nutrient demand and competitive ability, is worth mentioning. Although it is widely reported that these species are the indicators of soils rich in nutrients, in the case of fertile recent alder woods, with a good supply of nutrients, their distribution pattern is not predominantly dependent on N or P availability, but primarily they noticeably avoid sites with a high level of groundwater, combined with poor light conditions. It should be emphasized that such a distribution pattern 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.

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6. Discussion and conclusions Ancient and recent alder woodlands differed in their soil properties, and in light availability level on the forest floor. Traces of former agricultural use of soils under recent woods are still present, despite the fact that such forests are very vigorous in growth; thus changes in their soils are far more dynamic than in sites of lower fertility and moisture. Some of these differences in soil parameters could be attributed to former land-use. Alterations in soil conditions on sites formerly used as meadows, expressed in a lower content of organic C and higher pH level, are in accordance with 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). A progressive accumulation of organic matter observed after afforestation with black alder, and reported by the abovementioned authors, leads to acidification (Wilson et al., 1997; Richter et al., 1994). Intensive growth of trees is an acidifying process owing to the exchange of base cations, taken up by plants which in turn release hydrogen ions into the soil (Nilsson et al., 1982; Binkley, 1986). Furthermore, intensive nitrification coupled with nitrate leaching is among the processes determining the influence of N-fixing alder stands on acceleration of soil acidity (Binkley, 1986). Furthermore, a higher pH level in post-agricultural soils may be due to former liming. Larger amounts of available Mg and exchangeable Ca2+ in recent oak-hornbeam and alder-ash carrs woodland soils may also be evidence of this practice. Besides, the decrease in exchangeable Ca2+ content in oak-hornbeam and wet alder wood forest types with woodland age, due to acidification, was confirmed. The relations in amounts of exchangeable and available K between recent and ancient woodland sites, with higher values observed in ancient woods, were statistically confirmed in each habitat. According to Binkley (1986) and Richter et al. (1994), potassium is a very mobile cation, as opposed to Ca2+ and Mg2+. Therefore, in forests a relatively rapid recycling of K+ is observed. It is re-supplied to surface soils by a combination of its release from mineral weathering and root uptake and rapid recycling to the soils via leaching of canopies and forest floors. These observations are in agreement with the results obtained by Karkanis (1975) who estimated the rate of nutrient release from the alder leaf-litter to the soils, and observed that K release was the fastest. Although, as Binkley (1986) states, vigorous forests on relatively young soils lose about 5–10 kg ha1 yr1 of K+, via leaching, in general inputs exceed outputs, so this element limits forest growth only in some very isolated situations, for example on extremely poor, sandy soils. However, its amounts, both K+ retained on cation exchange sites and available K, in recent alder woods were increasing with the age of the woodland. Larger quantities of P in post-agricultural woods compared to continuously forested sites, so often reported by many authors (Koerner et al., 1997; Honnay et al., 1999; Dzwonko, 2001; De Keersmaeker et al., 2004; Falkengren-Grerup et al., 2006), were not confirmed in the case of the black alder forests. Similar to the findings of Wilson et al. (1997) soils under ancient forests were richer in available P than recent alder woods. Furthermore, a gradual increase in P-available content with woodland age was recorded. These observations are in contrast with the arguments of De Keersmaeker et al. (2004) who suggested that fertilization leads to an increase in labile inorganic forms of P, successively immobilized by the formation of Ca and Al phosphates, which takes place after afforestation. Besides, progressive soil acidification in such pH ranges should reduce the amount of available P, whereas in recent alder woods the tendency was the opposite. The question arises which mechanism is responsible for higher phosphorus availability observed in ancient woods compared to neighbouring recent stands, and for its gradual increase with age in

801

recent woodlands? According to Karkanis (1975) leaf-litter of alder contributes to a distinctive increase of available K and P content in soils. Another potential issue worth discussing is whether the same physiological feature of alders – open type of nutrition management where nitrogen is not withdrawn prior to leaf-fall (Zimka and Stachurski, 1976) – which is responsible for the enrichment of soil with nitrogen, works in the case of phosphorus as well. However, a further probable explanation lies in what Binkley (1986) states about the influence of N-fixing plants on cycles of other nutrients, especially on phosphorus. According to this author P availability under alders may be increased through the production of phosphatase enzymes, which accelerate P release from litter. The results obtained for black alder forests seem to confirm his conclusion indirectly, although more detailed biochemical studies in this respect would be required. One may assume that since forests show high P demand, losses of P via leaching are minimal. Besides, N cycling and availability in N-fixing stands is increased, which in turn accelerates cycling of P, tied up in tree biomass. Furthermore, Al and Fe ions may retain phosphorus efficiently (Binkley, 1986). In conclusion, site history would mainly affect the total P levels, whereas the recorded differences in available P between ancient and recent woodland sites reflect current soil conditions, to a great extent attributable to their current vegetation. Total nitrogen content was always higher in ancient woodland soils than in recent ones. Such relations are in accordance with those reported by Honnay et al. (1999), Verheyen et al. (1999), Dzwonko (2001), and Falkengren-Grerup et al. (2006), but opposite to those obtained by Koerner et al. (1997, 1999), and De Keersmaeker et al. (2004). Such variation in the differences observed may depend on soil fertility, the types of former agricultural use, its duration and time since abandonment. Higher N content in ancient woods, similar to C content, may be the result of the larger biomass of accumulated litter, also containing more alder leaves, for which the rate of decomposition is very high (Karkanis, 1975; Pereira et al., 1998). Besides, alder leaves contain large amounts of N, which is rapidly recycled and returned to the soils as mentioned before. Finally, the fact that recent alder woods were planted on former meadows may be partially the reason for such differences, since Compton and Boone (2000), who studied the impact of agriculture on soil C and N, reported higher amounts of these nutrients in pastures than on former arable land. Alder-ash forests had the greatest amount of total N in their soils among all three forest types studied. The lower level of total N in ancient wet alder woods than in the alder-ash carrs may be caused by two factors. One is related with the water management type of alder-ash carrs, depending on lateral water fluxes, thus supplying the ecosystem with additional nutrients. In contrast, wet alder woods rely either on groundwater or water from rainfall. A loss of N from wet alder woods is the second factor contributing to their lower fertility compared to alder-ash carrs, since in wet alder woods close proximity (a mosaic pattern) of aerated and periodically saturated soil beds provide ideal conditions for denitrification, and N removal through its volatilization. Besides, pure vigorous stands of N-fixing alder in wet alder woods cause high rates of N mineralization, and consequently nitrate losses through leaching (Binkley, 1986). An important factor influencing nutrient cycling, especially N and P, in alder woods is the activity of arbuscular mycorrhizal (AM) fungi, since the studies of Monzo´n and Azco´n (2001) show that AM fungi enhance N and P uptake by alder. Phosphorus is of great importance in N-fixation, because its severe deficiency markedly inhibits that process. Thus, AM mycorrhizas have a great effect on total N yield, working in two ways: one through P-mediated mechanism which improves N-fixation, and the second directly by enhancing N uptake from the soil (Monzo´n and Azco´n, 2001).

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In recent alder woods on fresh and moist soils, especially in oakhornbeam habitats, the distribution pattern of many ancient woodland 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 than its 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 but facilitating the successful establishment of typical woodland flora in recent alder woods. Besides, the litter of alder leaves decomposes very quickly; hence there is no risk of its inhibiting impact due to considerable litter thickness on the recruitment of deciduous woodland species, as opposed to some previous studies referring to trees with slowly decomposing leaves (Sydes and Grime, 1981; Dzwonko, 2001). It seems that differences in species composition between ancient and recent woods cannot be explained by a single factor, for example soil properties of these sites, but a combined effect of high nutrient and water availability, and light conditions in the forest floor. Competitive exclusion of specialist woodland herbs by vigorously growing species, such as U. dioica, taking advantage of high soil fertility seems to be a predominant form of recruitment limitation. Its inhibiting influence on the migration capacity of woodland flora into broadleaved forests on rich soils was reported in many studies, including those in alder woods (Honnay et al., 1999; De Keersmaeker et al., 2004; Hipps et al., 2005; Orczewska, in press-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 woods studied, stinging nettle distribution is much more dependent on the groundwater level, and to a lesser extent, also on light conditions, than on the high amounts of available P or total N content (Figs. 1–3). Besides, more available P was present in ancient alder woodlands, where U. dioica cover was much lower than in recent woods. It is noticeable (Figs. 1–3) that U. dioica, and some 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 wet alder woods is much lower (mean = 6.4%, max = 40%, median = 5%) than in oak-hornbeam (mean = 43%, max = 90%, median = 20%) and alder-ash carr habitats (mean = 26.5%, max = 90%, median = 10%). One cannot exclude the possibility that the low cover of stinging nettle in recent wet alder wood is a key factor responsible for the highest share of ancient woodland species occurring in the neighbouring ancient forests present in the herb layer of recent woods among all three habitats (Table 1). Recent alder woods also receive more light to the forest floor than continuously forested sites. This is not surprising, since the stands in post-agricultural plantations are subjected to different management practices, including thinning. Besides, shrub vegetation in recent woods is poorly developed compared to ancient forests. Overall, increased light availability, combined with a high level of nutrients, facilitates the development of U. dioica and other competitive species. Such a mechanism was described by Honnay et al. (1999), De Keersmaeker et al. (2004), and others. Similar conclusions were reached by Elemans (2004), who compared the plasticity of shadetolerant woodland herbs and forest-edge species. According to this author, the higher biomass production of light-demanding species at increased nutrient availability does not happen under low light conditions but only at a higher level of illumination do additional nutrients have this effect. An important conclusion, having practical application for forest management practices, can be drawn from the above observations. The studies in alder woods show that to create the best possible conditions allowing for effective forest recovery in habitats of such high fertility, it is essential to maintain good

water conditions. A high water level inhibits the vigorous growth of expansive, nutrient-demanding species. This in turn reduces the competitive exclusion of woodland flora by such aggressive herbs. Otherwise, the drainage of such places may lead to an invasion of eutrophic species, as reported by Hermy et al. (1993). Besides, creating more shady conditions in the forest floor, partly achieved naturally due to tree growth and subsequent full canopy development, may also distinctively limit the expansion of U. dioica. This, in turn facilitates the immigration of typical woodland herbs. Acknowledgements I would like to express my gratitude to Dr. John Parkery (formerly with the Forestry Commission, Alice Holt, UK), for his support at every stage of the work, to soil scientists Mgr. Jadwiga Zalewa, Mgr. Inz˙. Stefan Zalewa, and Mgr. Inz˙. Stanisław Gaweł of the Office of Forest Managament and Forestry Geodesy in Krako´w, for digging the soil profiles and doing the chemical analyses. Much gratitude is also due to Mgr. Sabina Słomian, Mgr. Joanna Czapla, Mrs. Symeona Zyndych, Mgr. Iza Szwarc, and Mgr. Paweł Go´ras for their assistance in the field work, and to Professor Patricia Thomas, (University of Białystok), who checked and improved the language of the paper. Much gratitude is also due to the two anonymous referees for their valuable comments on the manuscript. Project supported by the Polish Ministry of Science, Grant No. 2P04F 059 29, between 2005 and 2007 References Binkley, D., 1986. Forest Nutrition Management. Wiley, New York. Bossuyt, B., Deckers, J., Hermy, M., 1999. A field methodology for assessing manmade disturbance on forest soils developed in loess. Soil Use and Management 15, 14–20. Compton, J.E., Boone, R.D., 2000. Long-term impacts of agriculture on soil carbon and nitrogen in New England forests. Ecology 81, 2314–2330. De Keersmaeker, L., Martens, L., Verheyen, K., Hermy, M., De Schrijver, A., Lust, N., 2004. Impact of soil fertility and insolation on diversity of herbaceous woodland species colonizing afforestations in Muizen forest (Belgium). Forest Ecology and Management 188, 291–304. Dupre´, C., Wessberg, C., Diekmann, M., 2002. Species richness in deciduous forests: effects of species pools and environmental variables. Journal of Vegetation Science 13, 505–516. Dzwonko, Z., 2001. Effect of proximity to ancient deciduous woodland on restoration of the field layer vegetation in a pine plantation. Ecography 24, 198–204. Dzwonko, Z., Gawron´ski, S., 1994. The role of woodland fragments, soil types and dominant species in secondary succession on the western Carpathian foothills. Vegetatio 111, 149–160. Dzwonko, Z., Loster, S., 2001. Wskaz´nikowe gatunki ros´lin starych laso´w i ich znaczenie dla ochrony przyrody i kartografii ros´linnos´ci. Typologia zbiorowisk i kartografia ros´linnos´ci w Polsce. Prace Geograficzne 178, 119–132. Elemans, M., 2004. Light, nutrients and the growth of herbaceous forest species. Acta Oecologica 26, 197–202. Falkengren-Grerup, U., ten Brink, D.J., Brunet, J., 2006. Land use effects on soil N, P, C and pH persist over 40–80 years of forest growth on agricultural soils. Forest Ecology and Management 225, 74–81. Hermy, M., Van der Bremt, P., Tack, G., 1993. Effects of site history on woodland vegetation. In: Brockmeyer, M.E.A., Vos, W., Koop, H. (Eds.), European Forest Reserves. Pudoc, Wageningen, pp. 219–233. Hermy, M., Honnay, O., Firbank, L., Grashof-Bokdam, C., Lawesson, J., 1999. An ecological comparison between ancient and other forest plant species of Europe, and the implications for forest conservation. Biological Conservation 91, 9–22. Hipps, N.A., Davies, M.J., Dodds, P., Buckley, G.P., 2005. The effects of phosphorus nutrition and soil pH on the growth of some ancient woodland indicator species and their interaction with competitor species. Plant and Soil 271, 131–141. Honnay, O., Hermy, M., Coppin, P., 1999. Impact of habitat quality on forest plant species colonization. Forest Ecology and Management 115, 157–170. Karkanis, M., 1975. Rozkład s´cio´łki pochodza˛cej z ro´z˙nych gatunko´w drzew lis´ciastych i jej wpływ na s´rodowisko glebowe. Fragmenta Floristica et Geobotanica 21, 71–97. Klinka, K., Wang, Q., Carter, R.E., 1990. Relationships among humus forms, forest floor nutrient properties, and understory vegetation. Forest Science 36, 564– 581. Koerner, W., Dambrine, E., Dupouey, J.L., Benoıˆt, M., 1999. d15N of forest soil and understorey vegetation reflect the former agricultural land use. Oecologia 121, 421–425.

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