© INRA, EDP Sciences, 2009

1. INTRODUCTION

During the last few centuries, natural forest areas have decreased considerably in all of Europe (Barbaro et al., 2005). Among the principal causes of this decrease are forest clearing due to an increased demand for cultivable land and the construction of infrastructures (Hanski, 2005). Tree farming is also a major cause of the decline in the size and extent of natural forests (Rescia et al., 1994).

The substitution of broadleaf species by conifers in forests can reduce biodiversity because coniferous forests generally provide less diverse vascular understories than broadleaf forests (Barbier et al., 2008). Silviculture and disturbance regimes can also effect changes in plant diversity (Roberts, 2004). Furthermore, the recent increase in the fragmentation of natural forests is one of the major threats to species diversity (Honnay et al., 2005). For instance, in Central Europe, human intervention has reduced the proportion of native broadleaf forests from 66% to 33% of the forested area (Kenk and Guehne, 2001). As a result, old-growth deciduous forests in Western Europe consist mostly of small tracts bearing little resemblance to the original forests (Rozas, 2006). The situation is similar in the south of Europe where, in countries such as Spain, forests have been widely subjected to communal use since the Middle Age (Pardo et al., 2004).

The aim of this study was to compare understory vegetation between two types of native species of forests (mixed oak and beech stands) and Pinus radiata plantations. The objective was to evaluate the use of pine plantations for restoring native diversity in a zone where native forests are very fragmented. We compared the plot-scale plant species composition of the two types of natural forests with that of plantations of P. radiata. Landscape context around each forest was considered in order to evaluate any potential links with forest plant diversity.

2. MATERIALS

2.1. Study site

The study was carried out in the Basque region in the north of Spain, at the Ibaizabal river basin (43° 07’ N, 2° 51’ W). Since the beginning of the twentieth century, a considerable percentage of the native deciduous woodlands in this region has been substituted by plantations of fast-growing conifer. Although a few natural forest stands remain in the region, P. radiata plantations comprise approximately 66% of the forested area (Ruiz de Urrestarazu, 1992; Amezaga and Onaindia, 1997).

The Ibaizabal river basin covers 48320 ha and the elevation ranges from about 50 m to approximately 1200 m (Gesplan, 2002), and includes valleys and mountains (Aizpuru et al., 1999). The area is heavily populated and industrialized (Rallo and Orive, 1998), but some zones of great ecological interest are found there, such as the Gorbea and Urkiola Natural Parks (Gesplan, 2002).

The climate of this region is Atlantic, with a mean annual precipitation of 1200–2000 mm and a mean temperature of 14°C. The potential vegetation in virtually the entire river basin is that of mixed oak forests of Quercus robur and Fraxinus excelsior in lowland areas and the beech Fagus sylvatica in the higher zones. The distribution of the vegetation has been considerably altered and reduced by human activity. In the basin, many oak forests have been replaced by pine plantations, of P. radiata in particular (Aizpuru et al., 1999; Schmitz et al., 1998), to such an extent that native forests occupy only about 3.5% of the area of the river basin (Onaindia et al., 2004). Extensive deforestation occurred during the eighteenth and nineteenth centuries (Azkona, 1989). Later, the production of charcoal and the associated increase in logging made marked contributions to the reduction of the forested area. During the twentieth century, economic interests inspired the replanting of these forests with large pine plantations (García et al., 2004). Consequently, the native forests (mixed oak and beech forests) are now highly fragmented and surrounded by mature (more or less) pine plantations.

The mixed oak forests have a canopy dominated by Q. robur and F. excelsior. Other characteristic species are Castanea sativa, Corylus avellana, Crataegus monogyna and Frangula alnus (Loidi and Vascones, 1995). Beech forests are dominated by Fagus sylvatica; other characteristic species are Betula celtiberica, Laurus nobilis, Oxalis acetosella and Vaccinium myrtillus (Loidi and Vascones, 1995).

Sampling stand selection was based on the EUNIS habitat classification using the vegetation map on a 1:10000 scale (Davies and Moss, 2002). Twenty-one sites representative of native forests and pine plantations were selected: seven mixed oak forests, seven beech forests and seven P. radiata plantations. The plots were located on sandstone soils, had similar soil conditions, and were on slopes of less than a 30% grade. Ages of forests were approximated by the Basque Forest Administration (Gesplan, 2002). The oak and beech forests were secondary forests at different stages of natural succession from previous logging. Pine plantations have a short rotation of about 40 years. Canopy cover was estimated as the percentage of surface area that was covered by overhanging vegetation (Appendix 1, available at www.afs-journal.org).

2.2. Plant species sampling

Plant species sampling took place at the selected twenty-one sites between August and September 2006. At each site, two 50 m wide transects were arranged perpendicularly, representing a total area of 90 m 2that was divided into nine sub-plots of 5 m × 2 m (the optimum plot size was determined by the species/area curve method; Onaindia et al., 2004). In each sub-plot, the percent cover of each plant species (vascular and pteridophyte plants) was estimated at four different strata: 0–0.2 m, 0.2–2.5 m, 2.5–10 m and > 10 m. Species identification was according to “Flora del Pais Vasco” (Aizpuru et al., 2007; Aseguinolaza et al., 1988). At the time of sampling, Gramineae had not yet flowered, making their identification difficult. Consequently, all Gramineae in each sampling plot were identified only at the family level, as Poaceae, and were quantified as a single species. Regardless, only two species of Poaceae (Brachypodium sylvaticum and Bromus ramosus) are usually found in the forests evaluated in this study (Aseguinolaza et al., 1988); therefore, the lack of species-level Gramineae identification should have little effect on diversity values.

The mean percent cover of each species was calculated according to plot and forest type. Several biodiversity indices were calculated: richness (number of different species), Shannon’s diversity (H′ = − ∑ p_ilog₂p_i, where p_i is the relative abundance of species i); and Simpson’s diversity ().

2.3. Parameters at the landscape level

A GIS database was constructed and used to examine the spatial distribution patterns of species in a given area. Maps of land use based on the EUNIS habitat classification were generated from 1:5000 scale orthophotos provided by the cartography service of the County Council of Bizkaia (year 2002). These maps were enhanced by manual digitalization and automatic scanning of the orthophotos and were prepared and saved in GIS in vector format using the ARC/INFO program.

Four parameters describing patch (forest containing the plot) characteristics and landscape context were calculated for the 21 sites using the FRAGSTATS software (McGarigal et al., 2002).These parameters were total area (A), fractal dimension (FD), proximity index (Pi) and distance to the nearest patch of the same type (DNP). The FD provides information about the shape of the patch and was calculated as FD= 2ln P/ ln A, where A is the surface area of each patch in m² and P is the perimeter in meters (McGarigal et al., 2002). The Pi measures the degree of isolation of the patch and is calculated by the formula , where A_ir is the surface area of the patch (i) from one of the same type found within a radius (r), and h_ir is the distance to this patch. The value of Pi increases as the distance between the patches decreases. In order to determine the value of the index at different landscape levels, we calculated Pi for different radii at different vegetation-type levels (50 m, 500 m, 1000 m and 2000 m).

2.4. Data analysis

The mean percent cover of species, richness and diversity indices of the different forest types were compared by ANOVA using SPSS statistical software (data were normalized using the cosine function). The Levenne test was used to compare variances in homogeneity. In the ANOVA, the Games-Howell test was used for homogenous variance data and the DMS test was used for non-homogenous variance data. Fisher’s test was used to compare means (Sokal and Rohlf, 1981). Cluster analysis was performed to classify plot affinities using coverage data for each species according to plot. The IndVal 2.0 method was used for grouping species that best characterized the different forest types (Dufrene and Legendre, 1997). This method is based principally on the combination of the relative abundance of each species and its relative frequency in each group.

Landscape indices of the different forests were compared using the Kruskal-Wallis non-parametric test and comparisons between two means were made using the Mann-Whitney U test. Spearman correlations were used to evaluate relationships between biodiversity indices of the different forests and the landscape parameters.

3. RESULTS

3.1. Diversity indices and affinity of sites

The plots of P. radiata plantations had greater mean richness values than the mixed oak and the beech forests; however, the differences were not significant. The P. radiata plantations also had greater indices of diversity (Shannon and Simpson) than the mixed oak and beech forests. The differences in the Shannon and Simpson diversity indices between the beech forests and the pine plantations were significant (p < 0.05). The differences in the Simpson diversity index values tended to be larger than the differences in the Shannon index values; the difference in the Simpson index between the mixed oak and the beech forests was significant (p < 0.05; Tab. I).

Cluster analysis grouped the forest plots by forest type, with the exclusion of plot number 10, a young beech forest that was grouped with a young oak forest near the pine plantation plots (Fig. 1). There was a significant positive correlation between the age of plantations and species richness, and a negative correlation between the age of plantations and the Simpson diversity index. However, there were no correlations between diversity indices and the age of plots for natural forests. There was a significant negative correlation between species richness and the cover canopy for natural oak forests (Appendix 2, available at www.afs-journal.org).

Figure 1 Affinities between plots. Oak = mixed oak forest; beech = beech forest; pine = P. radiata plantation. Plot numbers are as explained in Appendix 1, available online at www.afs-journal.org.

3.2. Plant species composition

One hundred and fifteen different plant species were recorded: 79 different species in the mixed oak forests, 73 in the beech forests and 61 in the pine plantations. Some species were specific to each forest type (Appendix 3).

The greatest mean percent cover of species per plot was found for the pine plantations (24.8 ± 2.9 species/plot), followed by the mixed oak (24 ± 0.8 species/plot) and beech forests (21.5 ± 3.3 species/plot). However, there were no significant differences in mean percent cover between forest types. The indicator species analysis (IndVal) showed that several species occurred in practically all of the plots: Q. robur, Rubus sp., Hedera helix and C. monogyna. There were some species more associated with the mixed oak forests, including Smilax aspera, C. avellana, L. nobili and Cornus sanguinea. The species F. sylvatica, Viola riviniana, Geranium robertianum, Oxalis acetosella, Vaccinium myrtillus and Veronica officinalis were more common in the beech forests, while Pteridium aquilinum, Potentilla erecta, Lonicera periclymenum, Frangula alnus and Daboecia cantabrica were more common in the pine plantations (Fig. 2).

Figure 2 Distribution of plant species according to its affinity (%) with each forest type (results from IndVal 2.0 method).

Twenty species comprised more than 2% of the cover in the mixed oak forests, as did nine and 16 species in the beech forest and pine plantations, respectively. In general, species that had a higher percent cover occurred more frequently in the plots. In the oak forest plots, the dominant species was Q. robur with a mean percent cover of 41.7 ± 14.1%, followed by C. avellana, C. sativa and F. excelsior. The tree species L. nobilis had low percent cover but its frequency was relatively high. The percent covers of the shrubs Ulex galii, Rubus sp. and Rhamnus altaernus, and the ferns Dryopteris affinis and Polystichum setiferum were also substantial (Tab. II). There were some species characteristic of oak forests that appeared at low frequency and low cover: Anemona nemorosa, Asplenium scolopendrium, Erica arborea, Geum urbanum, Laurus nobilis, Ligustrum vulgare, Quercus pyrenaica, R. alaternus and Sanicula europaea (Appendix 3, available online at www.afs-journal.org).

In beech forests, the dominant species was F. sylvatica, with a mean percent cover of 94.9 ± 12.3%, much higher than any other tree species including Q. robur, C. sativa and B. celtiberica. There were also some natural forest-typical species that were present in high frequency but with low coverage, such as V. myrtillus, O. acetosella, G. robertianum and V. officinalis (Tab. III). Some other species had a low frequency and low cover, such as Anemona nemorosa, Athyrium filix-foemina, Helleborus viridis, Saxifraga hirsuta, Sibthtorpia europaea, Sorbus aria and Stellaria alsine (Appendix 3).

In the pine plantations, the dominant species was P. radiata (mean percent cover = 51.5 ± 6.3%), followed by P. aquilinum, Rubus sp., Robinia pseudoacacia, U. galii, D. cantabrica and C. avellana (Tab. IV). Pine plantations also contained other species characteristic of natural forests such as Arbutus unedo, A. filix-foemina, C. sanguinea, F. excelsior, Ilex aquifolium, Polystichum setiferum, Q. ilex, Q. pyrenaica and Vaccinium myrtillus (Appendix 3).

There were no significant correlations between species cover and forest age or the percent cover of canopy.

3.3. Landscape-level characteristics

There were significant differences in patch size among the different types of forests (p < 0.05). Mixed oak forests had the smallest mean patch size (8.6 ± 4.6 ha), followed by beech forests (64.2 ± 17.5 ha). The mean patch size for P. radiata plantations was much greater (364.8± 124.1ha). There were no significant differences in FDamong the different types of forests. However, there were significant differences in the DNP among the different forest types (p < 0.05). The beech forests had the largest DNP (94 ± 69.7 m), whereas the P. radiata plantations had the smallest DNP (16± 0.1m). Interestingly, the Pi showed that mixed oak forests were most isolated (65± 31.4) as compared with beech forests (343 ± 275.1) and P. radiata plantations (8829± 3477.5; p < 0.05;Tab. V).

No correlations between plant diversity indices and landscape parameters were found for oak forests or for beech forests. However, there was a significant negative relationship between the Shannon diversity index and the FDfor pine plantations (r = − 0.761; p ≤ 0.05; Appendix 2). Analysis of relationships between species (percent cover) and landscape parameters revealed only a negative correlation between Q. robur and the patch size (r = − 0.52, p < 0.01), and the DNP (r = − 0.48, p < 0.05). There was also a negative correlation between C. sanguinea and the patch size (r = − 0.47, p < 0.05), and between S. aspera and the patch size (r = − 0.50, p < 0.05).

4. DISCUSSION

4.1. Contribution of each stand type to diversityconservation

Beech forests tend to be less diverse forests because the F. sylvatica canopy creates a heavy shade that inhibits the growth of many species. There tend to be few types of species per plot, comprising a small percent of the cover as compared with the dominant F. sylvatica (Coroi et al., 2004). Although we might expect natural forests to be more diverse than plantations (Coroi et al, 2004; Wulf, 2002), in this study, the pine plantations tended to be more diverse than the natural forests. The P. radiata plantations had greater mean richness and higher diversity values than the mixed oak and beech forests. This might have been due to the presence of generalist colonizer species in addition to some species characteristic of mixed oak forests. However, the total number of species found was higher in natural forests than in the pine plantations.

There was a significant positive correlation between the age of the plantation and species richness, which suggests that forest age can be an important parameter in explaining the species composition of forests. Coniferous plantations should become slightly more similar in plant species to the natural stands with increasing age, as has been reported for Canadian plantations (Roberts, 2002) and Belgium forests (Godefroid et al., 2005). There were no correlations between the age of plots and species richness for natural forests, possibly due to the low number of plots and the small age range of the natural forests. This could also be due to an interaction between time and patch size, because plantations had greater patch sizes and were younger than the natural forests. The effects of time on species richness could not be separated from patch area, and patch area and time clearly interacted (Jacquemyn et al., 2001).

The negative correlation between species richness and the cover canopy for oak natural forests was probably due to the limited amount of light available for forest succession, which can cause a decline in plant species richness (Howard and Lee, 2003; Gondard and Romane, 2005).

4.2. Use of pine plantations to restore native forests

Despite the fact that the Ibaizabal river basin is a degraded and fragmented area, it has a relatively high floristic richness. One hundred and fifteen different species of vascular plants were identified. The largest number of plant species was found in the mixed oak forests (79 different species). Many species were present in each of the different forest types studied, such as the hawthorn Rubus sp. Previous studies have shown that the conditions in P. radiata plantations favor the growth of Rubus sp. (Amezaga and Onaindia 1997).

We found slightly more pioneer species in the plantations than in the natural forests (Zerbe, 2002), including D. cantabrica, Calluna vulgaris and P. aquilinum (Aizpuru et al., 1999). Mixed oak forests and plantations had many species in common, such as the pioneer species B. celtiberica (Onaindia and Amezaga, 2000), C. sativa and P. aquilinum. In contrast, the beech forests had a canopy dominated by F. sylvatica, which is usually the only tree species in beech masses in the region. Most of the species identified by IndVal analysis as characteristic of pine plantations, such as P. aquilinum, P. erecta, L. periclymenum, F. alnus and D. cantabrica, are broad-ranged generalist species (Aizpuru et al., 2007). Also, the presence of the fern D. affinis subsp. Affinis, a species typical of mature forests (Bossuyt et al., 1999; Honnay et al., 1999; Onaindia et al., 2004), is an indicator of the advanced restoration level of the pine plantations. Moreover, the high percent cover of C. avellana and C. sativa and the presence of some species representative of natural forests, such as A. unedo, A. filix-foemina, C. sanguinea, C. monogyna, F. alnus, F. excelsior, I. aquifolium, P. setiferum, Q. ilex, Q. pyrenaica, Q. robur and V. myrtillus, suggest a considerable degree of maturity in the plantations.

Some species characteristic of natural forests, such as A. nemorosa, S. europae and S. hirsuta, did not appear in plantations, possibly because the plantations were too young. This finding indicates that more succession time is needed to restore the species composition to that of the natural forests. Considering that forest restoration is an important objective for sustainable forest management in Europe (Zerbe, 2002), P. radiata plantations might serve as useful tools for restoring the original forest biodiversity in the region.

4.3. Importance of landscape structure

At landscape level, the oak forests were very small and isolated with a high degree of fragmentation. Their mean size was 8.6 ha and there was no substantially large area of mixed oak in the entire river basin. Moreover, these forests were confined to marginal zones.

The distribution of the beech forests did not appear to be altered as much as that of the mixed oak forests, possibly because beech trees grow at higher elevations that are less suitable for P. radiata plantations. Consequently, the isolation of the beech forest plots is due less to forestry than to natural causes (Rodríguez et al., 2006).

Fragmentation implies an exclusion of specialist species (Barbaro et al., 2005) and reduces the structural complexity of mature forests. However, in this study there were no significant relationships between landscape parameters and plant diversity. No direct effect of forest fragmentation on stand diversity for native forests was evident, possibly due to the small size of the natural forests and the larger size of the plantations. The negative correlations between the species Q. robur, C. sanguinea and S. aspera and patch size probably result from the fact that these species have a high percentage of cover in natural oak forests, which are the smallest forests in the area.

Some characteristic understory species (A. nemorosa, S. europae and S. hirsuta) did not appear in the plantations. This could also be due to the young age of the plantations. It may be necessary to preserve remnant old stands in order to maintain some residual native plants (Hanski, 1998; Hanski, 2005; Moola and Vasseur, 2004). These residual stands would serve as important seed sources for the dispersal of native species and promoting biodiversity in the regenerating forests. It has been demonstrated that woody species are able to establish under closed canopy in fragmented coppice forests and form a seedling bank, which may be used for natural regeneration (Gonzalez et al., 2008).

The process of fragmentation affects the forest plant richness and diversity not only by reducing patch size, but also by increasing the degree of isolation. More than a century after the onset of forest fragmentation, an extinction debt persists for species with low rates of population turnover (Vellend et al., 2006). The greatest positive effect is obtained if forests located close to remnants of biologically diverse forests are restored; this facilitates the migration of target species to the restored forests (Hanski, 2000). The regional variation in ancient forest plant species suggests that regional lists are more appropriate for assessing the conservation value of forests than global, pan-European lists (Hermy et al., 1999).

The landscape structure around the pine plantations might determine their suitability for forest restoration in the studied area. It is important to preserve the patches of natural forest between plantations to maintain a source of plant species; if plantations are too isolated from the surrounding native forests, colonization of native species will be difficult.

5. CONCLUSION

These data indicate that the pine plantations are as diverse as the mixed oak forests and much more diverse than the beech forests. Plantations contain an important community of typical natural forest species and their evolution might be considered as a natural phase in forest development. However, some characteristic understory species did not appear in plantations. Therefore, for sustainable forest management, it is necessary to maintain the plots of natural forest among the remaining plantations to promote the colonization of indigenous species.

The high degrees of fragmentation and isolation of the oak forests could be factors in their continuing degradation, and could lead to progressive colonization by generalist species and a reduction in diversity. To conserve and promote biodiversity, attempts should be made to increase the area of natural forests by regenerating the existing pine stands and connecting small patch forests to one other. The forests and plantations should be monitored to detect species characteristic of natural forests and to evaluate the diversity and landscape indices, thereby increasing our understanding of the evolution of forests.

Acknowledgments

This work was financed by the project FORSEE: gestion durable des forêts: un réseau Européen de zones pilotes pour la mise en œuvre opérationnelle. INTERREGEG III B, and Project MEC: CGL2005-08046-C03-01.

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	Figure 1 Affinities between plots. Oak = mixed oak forest; beech = beech forest; pine = P. radiata plantation. Plot numbers are as explained in Appendix 1, available online at www.afs-journal.org.
In the text

	Figure 2 Distribution of plant species according to its affinity (%) with each forest type (results from IndVal 2.0 method).
In the text