Comparison of community structure and soil characteristics in different aged Pinus sylvestris plantations and a natural pine forest

Forest Ecology and Management 247 (2007) 35–42 www.elsevier.com/locate/foreco

Comparison of community structure and soil characteristics in different aged Pinus sylvestris plantations and a natural pine forest J.A. Marcos a, E. Marcos a,*, A. Taboada b, R. Ta´rrega a a

Area de Ecologı´a, Facultad de Ciencias Biolo´gicas y Ambientales, Universidad de Leo´n, Campus Vegazana, 24071 Leo´n, Spain b Area de Zoologı´a, Facultad de Ciencias Biolo´gicas y Ambientales, Universidad de Leo´n, 24071 Leo´n, Spain Received 8 September 2006; received in revised form 13 February 2007; accepted 9 April 2007

Abstract The objective was to determine the change in ecosystem characteristics in terms of time passed after replanting with Pinus sylvestris, and to compare this with a natural forest. Five P. sylvestris stands of each age group: 2–3, 10–12, 40 and 80 years old, were selected and other five in the natural pine forest in Lillo (Leo´n, NW Spain) were sampled. The dimensions of the pines (height, perimeter and mean crown diameter), their density (estimated from middle distance), characteristics of the understory (cover and richness of both herbaceous and woody species) and soil (pH, organic matter, total nitrogen and available nutrient content) were studied at each stand. No differences were observed in the dimensions of the pines in the 40- and 80-year-old stands, although the crown diameter and mean trunk perimeter were larger in the natural forest. The coefficient of variation of these variables was significantly larger in the natural forest, indicating greater variability in the age of the pine trees and making more heterogeneous conditions. Spatial distribution of the pines presented an aggregated pattern at most stands in the natural pine forest, whilst it was random in the 40- and 80-year-old plantations and uniform in the most recent ones. There was no clear trend in terms of age in the understory characteristics, with the 10-year-old plantation differing the most due to its lower herb and higher woody species cover. With regards the soil, the natural forest has a lower pH and higher organic matter and phosphorus content, whilst the 2–3- and 10–12-year-old plantations were generally associated with a higher K content. The CCA carried out for global comparison of all the results showed separation between the two most recently planted groups and the mature ones, more similar to the natural forest. The complex gradient represented by the first two axes of the CCA presented a significant correlation with the characteristics of the trees and some of the understory, like herb richness and total species richness. # 2007 Elsevier B.V. All rights reserved. Keywords: Diversity; Plantation; Scots pine; Soil characteristics; Stand structure; Tree variability; Understory

1. Introduction Forest plantations have increased in area considerably in the last decades all over the world and this is predicted to continue in order to satisfy the increased global demand for wood (FAO, 1999; Moore and Allen, 1999; Hartley, 2002). Since the beginning of the 20th century and above all the 1940s there has been massive planting of pine trees and other rapid growth species, to obtain wood for industrial purposes, in forests formerly of oak and other slower growth species (Groome, 1987; Luis-Calabuig et al., 2000; Martinez, 2002). At present tree planting for production continues but there is an attempt to

* Corresponding author. Tel.: +34 987 291567; fax: +34 987 291409. E-mail address: [email protected] (E. Marcos). 0378-1127/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2007.04.022

combine it with improving the environment and protecting the soil in degraded zones (Ministerio de Medio Ambiente, 2002). The negative effects of conifer plantations on soil properties have been largely discussed (Binkely, 1995). It is generally assumed that even-aged forest plantations are negative from the viewpoint of biodiversity conservation, or at least, that diversity is less than that of natural forests (Peterken, 1996). One of the main causes is the uniformisation and loss of both horizontal (spatial heterogeneity) and vertical (stratification) structural diversity, fundamental components of forest biodiversity (Magurran, 1989; Malcolm et al., 2001; Kint, 2005). Several authors have compared the stand structure in plantations and natural forest (Summers et al., 1999; Hartley, 2002), as well as the effects of the type of management on structural diversity (Hunter, 1999; Lindenmayer and Franklin, 2002; Montes et al., 2005; Rouvinen and Kuuluvainen, 2005). These aspects have also been related to the characteristics of the plant community

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J.A. Marcos et al. / Forest Ecology and Management 247 (2007) 35–42

and soil (Bobiec, 1998; Ferris et al., 2000; De Keersmaeker et al., 2004; Modrego and Elena, 2004). Pinus sylvestris is one of the species most often used for reforestation but also the pine species with the largest natural distribution area, extending from Arctic latitudes in Norway to warm southern areas of Spain (Costa et al., 1998; RobledoArnuncio et al., 2005). According to the Second National Forest Inventory, P. sylvestris covers more than 1 million ha in Spain (M.A.P.A., 2002) with more than 300,000 being plantation (Costa et al., 1998). In our study zone (Leo´n province, NW Spain), the surface reforested with this species is 21,794 ha (Espinosa, 2001) and there are no natural forests, except for the Lillo pine forest, which is one of the few autochthonous (not introduced) relict pine forests from the Tertiary age on the southern slopes of the Cantabrian Range (Garcı´a Anto´n et al., 1997; Robledo-Arnuncio et al., 2005). The existence of a sequence of reforestation age groups, in which the effect of time is superposed with the different methods of planting and management, allows the temporal dynamic of these plantations, with a non-autochthonous species, to be compared with a natural pine forest of the same species remaining in the zone since the Tertiary age. However, it is necessary to take into account, the methodological problems of chronosequence approach which are similar to use chronoseries to infer successional patterns (Picket, 1989; Morin, 1999). The objective of this study is to determine the changes in the characteristics of the vegetation and soil in P. sylvestris plantations in terms of the plantation age from 3 to 80 years and compare them with the relict pine forest (hereafter referred to as natural forest). Specifically, we intend to show the existence of temporary tendencies in the structure of the arboreal strata by studying its density, spatial distribution pattern and variability in the dimensions of the pines, as well as in the characteristics of the understory and the edaphic variables, and whether there is a trend towards greater similarity to the characteristics of a natural P. sylvestris forest relict in the zone. 2. Materials and methods 2.1. Study area Twenty-five P. sylvestris stands, located in Leo´n province, NW Spain (428510 –43840 N, 48550 –58160 W), between 1200 and 1627 m above sea level and on siliceous soils, were selected. The climatic characteristics are similar in all the stands, with annual precipitation of over 1200 mm, mean temperatures between 8 and 9 8C, with a mean minimum temperature of 3 8C in the coldest month and a mean maximum of 25 8C in the warmest month. The frost-free period lasts 4 months and there is a short summer dry period in July (Ministerio de Agricultura, 1980). Twenty of these were plantations divided into four age groups: 2–3, 10–12, 40 and 80 years old approximately since planting, with five replicates of each. The most recent planting was done manually in zones free from trees and dominated by shrubs, which was either not eliminated or slightly cleared. There are fewer data available on the characteristics prior to planting for those over 40 years old, but

the planting method usually was by terraces. The natural forests close to these plantations are oak (Quercus pyrenaica and Quercus petraea), beech and birch stands (Penas et al., 1995). The other five stands studied were located in the only natural pine forest existing in this zone for over 1700 years, the Lillo pine forest, which currently covers about 160 ha (Garcı´a Anto´n et al., 1997) and is surrounded by beech and birch forests, mixed with the pines in some areas. The different aged stands were scattered throughout the sampling area and the age groups were not clustered together, except for the natural pine forest. There are no detailed records on the specific forestry practices developed in the study area. Probably, in the older plantations only scarce forestry management practices have been applied (selective thinning). In the natural pine forest, structural variability may be a result of various and uneven historical uses (cattle grazing, timber and firewood exploitation). 2.2. Sampling methods and data analysis The surface area of each stand was between 6.7 and 160 ha. Since the objective was not to characterise the stands exhaustively but to compare them, a similar surface was sampled in all of them, using a systematic type method. In order to determine the density and dimensions of the pines, sampling followed a transect, situated in the centre of each stand and parallel to the level curves. The closest tree (reference tree) was selected every 10 m and measurements were taken of the closest pines in the four quadrants, determined by the transect direction and its perpendicular axe (modification of the pointcentre-quarter method, Cottam and Curtis, 1956). Distances between reference tree and their closest neighbours were measured in order to minimize bias, as density estimation by PCQ method was biased in opposite form when spatial distribution pattern was aggregated and when was uniform (Bryant et al., 2004). Five points were studied in each plot so data were available on 20 trees. Four measurements were taken for each tree: height (estimated from the base to the top of the crown), trunk perimeter (1.3 m above ground in the older stands and below the first branches in the younger ones), mean crown diameter (average of the crown diameter in the transect direction and perpendicular) and distance from the reference tree. Density was estimated as the inverse of the quadrat of the mean distance between pairs of trees. These data were also used to determine the spatial distribution pattern of the tree population, analysed using the chi-squared test (alpha = 0.05, n < 51) if variance/mean was significantly different than 1 (Krebs, 1989). For the understory vegetation study all the woody species present in the tree sampling transect were recorded. In addition, three 1 m2 quadrats were sampled, recording the cover of each species, visually estimated as a percentage of their vertical projection, always by the same researchers (so that the bias, if it existed, was similar in all the study stands). The cover values over 100% were due to strata superposition. Five soil samples were taken, one from each of the pine measuring points, collecting the first 5 cm, which were

J.A. Marcos et al. / Forest Ecology and Management 247 (2007) 35–42

homogenised to obtain a uniform vision of the characteristics of the stand as a whole. The samples were air-dried and passed through a 2 mm mesh sieve for later analysis. The soil pH was determined potentiometrically in a 1:2.5 ratio in H2O (M.A.P.A., 1994); organic matter by Walkley and Black (1934) method; total nitrogen by Kjeldahl (Bremmer and Mulvaney, 1982), method; available phosphorus by Bray-Kurtz (Kalra and Maynard, 1991) and available Ca, K, Mg and Na, extracted with ammonium acetate 1N and determined by atomic absorption spectrophotometry (M.A.P.A., 1994), were determined in each sample. Analyses of one-way ANOVA were carried out with the results of density and size of the pines, understory vegetation and parameters analysed in the soil, in order to determine whether there were any differences in terms of plantation age or compared to the natural pine forest. In all cases five replicates were considered. The Scheffe test was used for the post hoc comparisons when ANOVA detected significant differences ( p < 0.05). A Canonical Correspondence Analysis (CCA), relating the species present in the understory (those appearing in only one stand were removed) and the edaphic variables analysed, was carried out to compare all the results globally. The CANOCO (Ter Braak, 1991) program was used. In addition, a correlation analysis between the different studied variables of the tree stratum and of the understory and the first two CCA axes was carried out, also including the age of the stands, which was considered as being 200 years old in the case of the natural forest. The Kolmogorov–Smirnov test was used to confirm data normality. 3. Results No differences were detected in density in terms of plantation age, with mean values between 678 and 1092 pines/ha (Fig. 1). However, when compared with the natural pine forest, significantly lower values (F = 3.8, p = 0.02) were observed. In the case of tree height, the mean values were lower than 0.3 m at 3 years old, 2.6 m at 10 years old and about 17 m in the oldest plantations, being slightly lower (16 m) in the natural pine forest. The differences were significant (F = 148.5, p = 0.0001) up to the age of 40, but were not detected from then on. However, trunk perimeter was significantly greater (F = 33.9, p = 0.0001) in the natural pine forest, with a mean of 1.2 m, than in the 40- and 80-year-old plantations (with means of 0.8 and 0.9 m, respectively). It was less than 0.2 m in those 10 years old and 0.02 m in those 2–3 years old without any significant differences between the two most recent plantation groups. The same tendency was observed in crown diameter (F = 37.1, p = 0.0001), with significant differences between the natural pine forest (mean diameter 6.3 m), the 80and 40-year-old plantations (4.2 m and 4.5 m, respectively) and those 10 and 2–3 years old (1.1 m and 0.1 m, respectively). The variability in pine dimensions was compared by carrying out an analysis of variance of the coefficients of variation. The coefficient of variation was always greater in the natural forest than in the plantations, with statistically significant differences (F = 8.5, p = 0.001 for height; F = 9.1, p = 0.001, for trunk

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Fig. 1. Mean values and standard error of pine density, height, trunk perimeter and crown diameter in the five replicates of the studied zones. Different letters indicate significant differences by the Scheffe test. Numbers in X-axe indicate years since plantation, NF = natural forest.

perimeter; F = 3.3, p = 0.048 for mean crown diameter). The maximum value of the coefficient of variation of the distance between trees was in the natural forest. However it was not possible to detect significant differences when comparing with the oldest plantations (40 and 80 years old). The minimum values corresponded to the most recent plantations (2–3 and 10–12 years old), with significant differences in comparison with the former (F = 4.7, p = 0.008). According to these results the spatial dispersion pattern of the pine population was uniform in all the most recent plantations (2–3 and 10–12 years old), random in all the 40-year-old and 4 of the 80-year-old plantations, aggregated in the 5th. With regards the natural forest, four of the stands studied presented aggregated dispersion and the fifth random dispersion. The understory characteristics compared in Fig. 2 did not show any clear temporal tendency. Woody species cover was significantly greater (F = 12.0, p = 0.0001) in the 10-year-old plantations, with a mean greatly exceeding 100% due to strata superposition. No differences were detected amongst the rest, although mean woody cover oscillated between 40% in the 80year-old plantations and 80% in the 2–3-year-old ones, with the natural pine forest having an intermediate value (63%). Herb cover presented a similar, though opposite, dynamic. It was significantly lower (F = 7.0, p = 0.001) in the 10-year-old plantations with 20% mean cover, and there were no significant differences among the others, with mean values of between 90 and 95% in the natural pine forest and the 80-year-old plantations, respectively, and between 65 and 75% in the 40 and 2–3-year-old plantations, respectively. Woody species richness was significantly greater (F = 5.2, p = 0.005) in the 10-year-old

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J.A. Marcos et al. / Forest Ecology and Management 247 (2007) 35–42

Fig. 2. Mean values and standard error of woody and herb species cover and richness in the understory in the five replicates of the studied zones. Different letters indicate significant differences by Scheffe test. Numbers in X-axe indicate years since plantation, NF = natural forest.

plantations (with a mean of 4.5 species/m2) than in the older ones and the natural pine forest (with means between 2 and 2.4 woody species/m2), with the 2–3-year-old plantations presenting an intermediate value (3.3 species/m2). No significant differences could be detected (F = 0.9, p = 0.47) in herb species richness, which oscillated between 3.8 species/m2 in the 10year-old plantations and 6.5 species/m2 in the 2–3-year-old ones. Plantation age had variable effects in terms of the soil characteristic analysed (Table 1). The pH showed a significantly lower value in the natural pine forest than in the plantations, although no temporal tendency was found in them. The organic matter tended to increase with plantation age, except in the 2–3-year-old one, although significant differences were only detected between the natural pine forest and the

10–12-year-old plantation. Total nitrogen content was similar in all the study zones without any significant differences. With regards available nutrients, there was no global temporal trend. The calcium and magnesium content varied according to the plantation, although without any temporal trends and the differences were not significant. The highest calcium values were observed in the oldest plantations (40 and 80 years old), whilst the most recent one and the natural forest had the highest magnesium content. Sodium did not show any clear temporal trend, although the highest content appeared in the natural forest with significant differences in comparison with the 10– 12, 40- and 80-year-old plantations. The level of potassium tended to decrease over time with the lowest value in the natural forest and significant differences being detected between the most recent plantations and the rest. The phosphorus content tended to increase significantly with plantation age, except in the 2–3-year-old ones, reaching its maximum value in the natural forest. The canonical correspondence analysis was carried out with the edaphic variables and the understory species (54 species, after removing those only appearing in one stand). Axis I (explained variance of species-environmental relation: 31%) separated the more recent plantations (2–3 and 10–12 years old) from the mature ones and the natural forest, associating them with higher pH and K+ values and with greater cover by shrub species, like Chamaespartium tridentatum, Erica australis, Halimium alyssoides, Halimium umbellatun, Genista micrantha and Arctostaphylos uva-ursi (Fig. 3a and b). The natural forest stands associated with high organic matter, P and Na+, and species like Galium saxatile, Conopodium denudatum, Melampyrum pratense, Asphodelus albus and Pteridium aquilinum were grouped in the negative part of axis II (explained variance of species-environmental relation: 16%). Most of the stands corresponding to the oldest plantations (40 and 80 years old), associated with high Ca2+ content, abundant herb cover and woody species typical of forest zones, like Q. pyrenaica, Crataegus monogyna, Cytisus scoparius and Erica arborea, were situated in the positive part of axis II. One of the 2–3-year-old stands (A2) was located close to the oldest plantations because of its greater herb species cover (Brachypodium sylvaticum, Festuca paniculata, Helleborus foetidus, etc.).

Table 1 Mean values and standard error of analysed soil variables in the five replicates of each plantation age and the natural Forest (N.F.) pH

2–3 years 10–12 years 40 years 80 years N. F. F-Test p-Value

OM

N

P

Ca

Mg

K

Na

Mean

S.E.

Mean

S.E.

Mean

S.E.

Mean

S.E.

Mean

S.E.

Mean

S.E.

Mean

S.E.

Mean

4.67 5.05 4.75 4.79 3.80

0.16 0.08 0.11 0.11 0.04

19.3 12.4 15.2 16.2 21.8

1.13 1.26 1.28 2.21 1.46

0.48 0.39 0.43 0.39 0.40

0.03 0.09 0.05 0.04 0.07

5.50 1.53 2.18 3.82 9.27

0.93 0.18 0.39 0.59 3.05

1.56 0.76 1.98 2.07 1.12

0.33 0.08 0.63 0.46 0.12

0.63 0.35 0.39 0.46 0.52

0.15 0.03 0.10 0.07 0.08

0.65 0.48 0.36 0.44 0.28

0.05 0.04 0.04 0.04 0.03

0.084 0.052 0.060 0.054 0.096

a a a a b

19.26 0.0001

ab b ab ab ac 5.69 0.0032

0.43 0.7854

ab b b ab ac 4.48 0.0096

2.11 0.1171

1.42 0.2650

a ab bc bc c 12.99 0.0001

S.E. ac b ab ab c

0.010 0.003 0.007 0.007 0.006

9.32 0.0002

O.M. = organic mater (%), N = total nitrogen (%), P = available phosphorous (ppm), and available Ca, Mg, K and Na (mequiv./100 g). ANOVA results (F-test and pvalue) are also included. Different letters in each column indicate significant differences among different stand ages and natural forest for this variable. When there were no significant differences, letters are not included.

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Fig. 3. (a) Location of study zones and soil variables in the plane defined by the first two axes in the canonical correspondence analysis. (b) Location of species in the plane defined by the first two axes in the canonical correspondence analysis.

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Table 2 Correlation analysis between some of the studied variables

TH SP CD cvTD cvTH cvSP cvCD UC HC WN HN TN Age CCA1 CCA2

TD

TH

SP

CD

cvTD

cvTH

cvSP

cvCD

UC

HC

WN

HN

TN

Age

CCA1

0.54* 0.75* 0.77* 0.18 0.07 0.19 0.09 0.36 0.67* 0.28 0.17 0.02 0.78* 0.53* 0.26

0.92* 0.90* 0.65* 0.62* 0.13 0.15 0.64* 0.51* 0.55* 0.10 0.16 0.61* 0.84* 0.13

0.99* 0.52* 0.48* 0.05 0.13 0.56* 0.57* 0.48* 0.11 0.11 0.78* 0.81* 0.10

0.52* 0.43* 0.13 0.15 0.52* 0.55* 0.47* 0.08 0.13 0.81* 0.80* 0.14

0.20 0.38 0.73* 0.48* 0.22 0.47* 0.04 0.23 0.54* 0.60* 0.05

0.78* 0.54* 0.53* 0.36 0.25 0.27 0.11 0.10 0.34 0.46*

0.83* 0.03 0.04 0.18 0.31 0.35 0.63* 0.18 0.70*

0.21 0.09 0.30 0.11 0.24 0.61* 0.35 0.35

0.74* 0.68* 0.30 0.05 0.37 0.66* 0.32

0.28 0.68* 0.43* 0.47* 0.59* 0.23

0.27 0.64* 0.35 0.56* 0.24

0.91* 0.02 0.21 0.46*

0.16 0.07 0.47*

0.66* 0.33

0.08

N = 25. Marked correlations are significant at p < 0.05. TD = mean tree distance, TH = tree height, SP = stem perimeter, CD = crown diameter, cvTD = mean tree distance coefficient of variation, cvTH = tree height coefficient of variation, cvSP = stem perimeter coefficient of variation, cvCD = crown diameter coefficient of variation, UC = understory woody cover, HC = herbaceous cover, WN = woody spp. Number/m2, HN = herbaceous spp. Number/m2, TN = total spp. Number/m2, age = stand age, CCA1 = first axes of CCA, CCA2 = second axes of CCA.

According to the correlation analysis carried out (Table 2), increasing stand age meant increased tree dimensions and distance among them (density decreased), and the coefficient of variation of the distance and the coefficient of variation of the trunk perimeter and crown diameter also increased. Pine size was positively correlated to greater herb cover and negatively to woody species richness and cover. The complex environmental gradient represented by the first axis of the CCA showed negative correlation with stand age and consequently with tree height, trunk perimeter and mean crown diameter, as well as distance among trees and its variability (estimated by the coefficient of variation). The correlation was positive between the first axis and woody species richness and cover, and negative for herb cover. In contrast, axis II did not show a significant correlation to age or tree dimensions. A positive correlation was observed between this second axis and herb richness and total richness, and a negative correlation with trunk perimeter and tree height variability. 4. Discussion In the results obtained, there was a superposition of both age and planting method in an intercorrelated gradient. On analysing the effect of reforestation age the most recent plantations were logically ruled out, as their characteristics correspond to those of the initial soil and vegetation since the young pines have no capacity for modifying them, given their small dimensions. No differences were observed in the characteristics of the trees (height, trunk perimeter and crown diameter) among the plantations from the age of 40. With regards density significant differences were detected between natural forest and the rest. However, no significant differences were observed in terms of age, although mean density was lower in the oldest plantations in comparison with the 10–12year-old ones. The planting method in the most recent ones (2– 3 years old) was different, as lower density was used because

the main objective was not production. Kint (2005) stated that density declines with stand age, which would support these results. However, age had a clear effect in terms of spatial distribution pattern, which started by being uniform in the most recent plantations, as a result of the planting method, and became random in the older ones, probably due to management (thinning), although it could also be a result of intraspecies competition. Rozas (2006) stated that uniform planting method is contrary to the natural regeneration, as in natural forests the distribution pattern trend is from aggregated in juveniles to random in old trees. Kint (2005) also pointed out that spontaneous structural development at the stand level in ageing P. sylvestris stands is characterised, in the absence of major disturbances, by a tendency towards a random spatial point pattern. When compared plantations from the age of 40 with the natural pine forest, no differences were observed in tree height, but were in trunk perimeter and crown diameter, which was significantly greater. This could be due to the lower density which resulted in less intraspecies competition and thus permitted greater crown development (Summers et al., 1999; Are´valo and Ferna´ndez-Palacios, 2005). Another characteristic of the natural pine forest was greater variability in tree dimensions, which coincided with the results of other authors (Summers et al., 1999; Kint, 2005; Rouvinen and Kuuluvainen, 2005), who recorded the presence of larger trees and of regeneration in natural forests, whilst intermediate size trees predominate in plantations. Natural pine regeneration was only observed in the natural forest among the stands studied. If the 2–3-year-old plantations are ruled out, a decrease in woody cover and an increase in herb cover in terms of age are observed, although no differences were detected from 40 years on. The same tendency seems to be observed in species richness, although the small sampling size did not allow the richness of each stand to be characterised, but rather the number of species per square meter to be estimated approximately. It could be assumed that the shade effect determined the decrease

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in sun-seeking woody species in the understory as canopy became closed. For example, Andre´s and Ojeda (2002) recorded greater woody species richness and cover in shrublands without a tree canopy than in understory under pine plantations. However, other authors found that the most recent P. sylvestris plantations were characterised by low diversity whilst the most mature ones spontaneously increased biodiversity, although species richness was very limited in all age groups (Lust et al., 1998). The changes occurring in the soil characteristics in the first stages of development of the pine stands depended on the planting method to a great extent. The most recent ones (2–3 years old) presented higher organic matter and nutrient values when compared with the rest, due to the fact that planting was manual and impact on vegetation was minimal. There were thus basically no modifications in the soil characteristics, which would correspond to a mountain heathland. However, in the 10– 12-year-old plantations, which had the same type of shrub vegetation, a decrease in organic matter and nutrient content was observed, probably due to machinery being used for planting in most stands. Charle and Elena (1993) and Modrego and Elena (2004) found that treatments involving soil movement decreased organic matter content and fertility. The mean values for all these characteristics recovered after 40 years, although no significant differences were observed. Several authors have stated that organic matter and total nitrogen content increase with increasing stand age (Charle and Elena, 1993; Alriksson and Olsson, 1995; Coˆte´ et al., 2000; De Keersmaeker et al., 2004; Modrego and Elena, 2004). This coincided with the results obtained in the case of organic matter, but total nitrogen showed hardly any variation and no temporal tendency. The increase in calcium and magnesium, though not significant, coincided with that found in other studies (Alriksson and Olsson, 1995; Mohr et al., 2005). When the soil of the plantations was compared with that of the natural forest, a higher organic matter, phosphorus and sodium content and decreased pH and potassium values were found. The plantations over 40 years old tend to be more like the natural forest and the 10–12-year-old ones the most different. In these older plantations, as in the Lillo natural pine forest, colonisation by deciduous species (Fraxinus, Fagus, Betula, Quercus, etc.), which appeared in the forest surrounding these stands (Marcos, 2005), was observed, and this could affect soil conditions due to changes in litter composition. There was also greater similarity in the understory species, with lower cover by heathland species, like E. australis, H. alyssoides or C. tridentatum. Many authors have pointed out the influence of understory vegetation on soil characteristics (Sanger et al., 1998; Priha and Smolander, 1999; Mohr et al., 2005). Although it was not always possible to detect differences when comparing the different variables individually, joint comparison of all the results showed more clearly the effect of age, in addition to the tree stratum characteristics, on understory and soil composition. However, although the synthesized complex gradient on the first axis of the CCA presented a significant correlation with stand age, this effect

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was due to differences between the two most recent groups (2–3 and 10–12 years old) in comparison with those over 40 and the natural forest rather than to a variation gradient. In spite of the temporal tendency to converge with the natural forest, clear differences persist, probably due to the Lillo pine forest being a relict. No direct relation was observed between plantation age and species richness. Even on the second axis of the CCA, a positive correlation appeared with herb species richness and total richness, associated with the oldest stands, but also with two of the most recent ones. The correlation between this axis and tree layer variability, associated with the natural forest, was negative. Therefore, greater structural diversity did not correspond to greater species richness. Although structural diversity of the stand has been related to ground vegetation diversity, this is not always consistent (Ferris et al., 2000; Montes et al., 2005), species diversity can even be less in open old-grown stands than closed canopy ones (Nieppola and Carleton, 1991). It must be born in mind that a lower number of species does not necessarily indicate lower stand quality. A basic aspect in terms of biodiversity is species composition, which differed in the different stands. On the other hand, stratification (vertical variability) and horizontal heterogeneity determine the existence of different niches, and thus greater fauna diversity. Taboada et al. (in press) suggest that the mature stages of the pine plantations act as secondary habitats for some forest carabid species. So, it is fundamental take into account all these aspects in order to manage this type of ecosystems for biodiversity conservation. Acknowledgements The authors wish to thank the Servicio Territorial de Medio Ambiente de la Junta de Castilla y Leo´n for the information supplied and the collaboration of the Forest Guards in the different zones. This study was partially financed by the C.I.C.Y.T. (project REN 2002-04463-C02-01) and by the Junta de Castilla y Leo´n (project 2003/25, ref. LE031/03). References Alriksson, A., Olsson, M.T., 1995. Soil changes in different age classes of Norway spruce (Picea abies (L.) Karst.) on afforested farmland. Plant and Soil 1, 103–110. Andre´s, C., Ojeda, F., 2002. Effects of afforestation with pines on woody plant diversity of Mediterranean heathlands in southern Spain. Biodiversity Conserv. 11 (9), 1511–1520. Are´valo, J.R., Ferna´ndez-Palacios, J.M., 2005. From pine plantations to natural stands. Ecological restoration of a Pinus canariensis Sweet, ex Spreng forest. Plant Ecol. 181, 217–226. Binkely, D., 1995. The influence of tree species on forest soils: processes and patterns. In: Mead, D.J., Cornforth, I.S. (Eds.), Proceedings on the trees and soil workshop. Agronomy Society of New Zealand Special Publication No. 10, Lincoln University Press, Canterbury, pp. 1–33. Bobiec, A., 1998. The mosaic diversity of field layer vegetation in the natural and exploited forests of Bialowieza. Plant Ecol. 136, 175–187. Bryant, D.M., Ducey, M.J., Innes, J.C., Lee, T.D., Eckert, R.T., Zarin, D.J., 2004. Forest community analysis and the point-centered quarter method. Plant Ecol. 175, 193–203. Costa, M., Morla, C., Sainz, H. (Eds.), 1998. Los bosques ibe´ricos. Una interpretacio´n geobota´nica, Planeta, Barcelona.

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