Materials and Methods

2.1. The Area and the Communities Studied

This study deals with the pastures situated on several small hills located in the Plana de Vic basin (Catalonia, Spain; 41° 53' N, 2° 15' E; Figure 2), lying between the Mediterranean, Catalanidic mountains south- and eastwards, and the pre-Pyrenean ranges northwards. The main outcrops are flat, lime rich, Tertiary rocks, chiefly Eocene marls. The macroclimate is submediterranean axeromeric (Bolos and Vigo, 1984), a transition type between Mediterranean and montane (Pyrenean). The mean annual temperature in Vic lies between 12 and 13 °C and the annual rainfall normally ranges from 650 to 750 mm (Figure 2).

Figure 2. Location of the area studied (a), and climatic diagram of Vic, corresponding to a submediterranean climate with a continental tendency (b).

The vegetation is mainly mesoxerophilous, with xerophilous Mediterranean communities being restricted to drier habitats, such as the south facing slopes and rocky shelves. The hills and the peripheral slopes around Vic stand out from the intensively farmed surrounding plains, and are the main location of varied mosaics of semi-natural communities (Figure 3).

Figure 3. Hill slope bearing a complex mosaic of semi-natural vegetation: Quercus pubescens stands, diverse xerophilous pastures and eroded surfaces with sparse vegetation (badlands).

The climax vegetation is a deciduous, mesoxerophilous oak forest (Buxo sempervirentis-Quercetum pubescentis Br.-Bl. 1932), which remains on a few gently sloping sites. Deforested slopes, on any aspect and with soils ranging from well preserved to highly eroded, are prime locations for a wide variety of pasture communities (Figure 4).

1, Mesoxerophilous grasslands, mainly

PlàntàgifihApfiyllàrmêlurtr

2. Xerophilous pastures, mainly

Brachypodto-Aphyiiânth&tum brachypo&etosom mtusi

A. Oak stands (Buxo-Ousrcetum pubescentis)

B. Scatered holm oaks (Quercus ifek)

C. Sown fields

D. Gangue spots [Ouercetum cocciterae)

50 m -,

¿¿ft, O

JEWrFitf^

V 2 Xw,,, c

Urn 100

■jm 300 m

Figure 4. Cross-section of a hill in the area studied, with semi-natural plant communities surrounded by farmland.

Figure 4. Cross-section of a hill in the area studied, with semi-natural plant communities surrounded by farmland.

Depending on each precise habitat, plant communities include varying percentages of Mediterranean species (xerophilous plants, more abundant in drier environments), Medio-European species (mesophilous plants, more abundant in milder environments), and pluriregional species (ubiquituous plants, widespread).

Most of the pasture surfaces correspond to calcicole perennial communities, from xerophilous to mesophilous. Excluding the extremes of this range, we selected two pasture communities showing contrasting ecological conditions, structure and phenology (Table 1), which are rather extensive in the current landscape (Casas and Ninot, 1994). These communities are:

- Brachypodio-Aphyllanthetum O. Bolos (1956) 1967 brachypodietosum retusi O. Bolos (1959) 1960. Irregular pastures dominated by the chamaephytic, xerophilous grass Brachypodium retusum, bearing some dwarf shrubs, therophytes and small patches of bare soil (Figure 5). They develop on south facing slopes with stony, eroded soil.

- Plantagini-Aphyllanthetum O. Bolos (1948) 1956. Dense grasslands dominated by Aphyllanthes monspeliensis (Liliaceae) and various mesoxerophilous graminoids, such as Brachypodium phoenicoides (Figure 6). They cover deep, stable soils occurring on north facing or intermediate slopes.

2.2. Field Sampling

The two communities were sampled at two sites (Malla and Montrodon) representative of the relief discussed above. In the resulting 4 plots (2 communities x 2 sites) we recorded the standing vegetation in terms of phytosociological relevés (complete lists of the plant taxa found in 20 m2 areas, with evaluation of cover/abundance), and completed them with additional taxa occurring in pasture adjacent to the plots. Nomenclature and taxonomy of taxa follows Bolos et al. (2005) from sampling to here onwards.

Table 1. Main aspects of the communities studied (summarised from Casas, Ninot 1994). Percentages of life forms and phytogeographic types refer both to species number and to relative cover, respectively.

Table 1. Main aspects of the communities studied (summarised from Casas, Ninot 1994). Percentages of life forms and phytogeographic types refer both to species number and to relative cover, respectively.

Plantagini-Aphyllanthetum

Brachypodio-Aphyllanthetum brachypodietosum retusi

Life forms (%)

Hemicryptophytes: 60 / 78

Hemicryptophytes: 47 / 50

Chamaephytes: 20 / 20

Chamaephytes: 31 / 47

Geophytes: 5 / 2

Geophytes: 3 / <1

Therophytes: 13 / <1

Therophytes: 19 / 3

Dominant species

Brachypodium phoenicoides: 64

Brachypodium retusum: 68

(mean cover)

Avenula pratensis: 35

Thymus vulgaris: 30

Aphyllanthes monspeliensis: 32

Koeleria vallesiana: 17

Carex caryophyllea: 11

Teucrium polium: 13

Plantago media: 11

Aphyllanthes monspeliensis: 12

Aspect, slope (°)

N (any), 4

S (SW-SE), 17

Topographic

Toeslopes, gentle slopes

Abrupt slopes

situation

Cover(%)

99

89

Phytogeographic

Mediterranean: 28 / 53

Mediterranean: 56 / 78

types (%)

Medio-European: 44 / 34

Medio-European: 21 / 16

Pluriregional: 28 / 13

Pluriregional: 23 / 6

Figure 5. Stand of the xerophilous pasture (Brachypodio-Aphyllanthetum) on a dry slope, showing typical patchy structure and dominance of Brachypodium retusum and dwarf shrubs (Santolina chamaecyparissus, Teucrium polium, etc.).
Figure 6. Aspect of the mesoxerophilous grassland ((Plantagini-Aphyllanthetum) in fulll bloom of Aphyllanthes monspeliensis.

We collected 10 soil samples from each plot using a steel cylinder, 5.5 mm in diameter, knocked 10-12 cm into the soil. Each cylindrical sample (core) was cut into two samples (0 to 5 cm and 5 to 10-12 cm deep) representing shallow and deep soil horizons, respectively, as in other studies addressing soil seed banks (Thompson, 1993). Thus, for each sampling date we obtained 80 samples, which were laid out to dry for 15 days or more in the laboratory, to prevent both germination and loss of viability.

Soil sampling took place three times during the year, with the aim of obtaining data related to the three basic seasonal states of the soil seed bank in temperate regions, following Thompson and Grime (1979): at the end of spring, when the seeds in the soil correspond to the persistent bank; at the end of summer, when the currently produced seeds have been incorporated; and in winter, after the autumn germinating species may have left the soil bank. Samples were collected in February, at the end of June and in September (October) of 2000. Moreover, we took an extra set of samples in June of 1999 (5 cores per plot, total 40 samples), to provide some estimate of the inter-annual variability of the persistent bank.

2.3. Laboratory Analyses

Given the difficulties and constraints involved when analysing the composition of soil seed banks (Gross, 1990; Recasens et al., 1991; Russi et al., 1992), we used two complementary methods. From each set of samples, one-half (40 samples: i.e., 5 samples from each plot and depth) was laid out to germinate in the greenhouse, and the other half was hand sorted to separate and identify seeds. It is known that direct observation of soil samples involves the inventorying of non-viable seeds, and may neglect a number of cryptic seeds

(small, irregular, brown). On the other hand, germination tests can result in under-estimates, because this procedure may miss a number of species with special germination requirements, or with dormancy mechanisms (Grime et al., 1981; Gross, 1990).

In this paper, what we call seed corresponds more precisely to the diaspore or dispersule: i.e., the seed of a legume, the entire fruit of a compositae, or the fruit with the concealing lemma and palea of a grass.

2.3.1. Hand Sorting and Visual Observation

The soil samples were washed and hand screened through a column of sieves decreasing in mesh width (1, 0.5 and 0.2 mm) resulting in three sub-samples per sample. The material retained in each sieve contained diverse plant materials (debris, seeds) and mineral particles (aggregates, sand); these were particularly abundant in the medium and finest sieves. The three sub-samples obtained from each sample were maintained separately to facilitate their handling; this gave 120 sub-samples per sampling date.

The sorting and identification of the seeds was done under a stereoscopic microscope, at 10-40X magnification. Seeds showing evidence of damage (broken cover, anomalous appearance) were rejected. Identification was supported by illustrations or descriptions of seeds (Hanf 1982; Javorka-Csapody, 1991; Bolos and Vigo, 1984-2001; Castroviejo et al., 1996-2004; etc.) and by comparison with herbarium samples or by seeds collected ad hoc.

Sorting the sub-samples retained in the 0.2 mm sieve proved a very hard task, and yielded almost no seeds. Thus, we laid a selection of these samples in a culture chamber with standard spring conditions (14 light hours at 25 °C and 10 dark hours at 15 °C), watered with a 10M giberelic solution. Seedling emergence obtained after one month was less than 1 per sub-sample, and none coincided with the few species found in the pastures with diaspores smaller than 0.2 mm (Blackstonia perfoliata, Hypericum perforatum, Helichrysum stoechas and Sedum sediforme, and Orchidaceae). Consequently, given the time needed and the almost sterile results expected, we abandoned the exhaustive sorting of these samples, basing this part of the study on the remaining 80 samples retained in the 1 mm and 0.5 mm sieves.

2.3.2. Germination

For this treatment the 5 samples from each plot, depth and date were pooled, and then used as a single soil sample. Each of these samples was spread out in a 40 x 60 cm container, forming a layer a few mm deep, on a standard substrate made of peat, mica and perlite. The containers were laid on greenhouse tables, sprayed with water daily, and protected with a fine net to avoid contamination by new seeds.

The samples obtained were cultivated for 6 months, from November, 2000, to April, 2001. During this period the seedlings were identified as soon as possible, recorded, and removed to avoid interference with other possible germinations. The identification of seedlings was done by means of comparison with herbarium material, when available, or with the help of some specific studies (Hanf, 1982; Javorka-Csapody, 1991). Seedlings that were not easily identified were grown on in small containers until identification was possible.

2.3.3. Morpho-FunctionalAnalysis

This analysis addressed the seeds of the entire taxa pool of the pastures; i.e., taxa found in the standing vegetation of the plots studied and taxa found only in the soil seed bank, which produced a list of 95 taxa. The analysis is based on the classification of diaspores given by Grime et al. (1981), and refers to their general morphology, surface and appendages. We introduced small modifications to the given categories (Table 2), consisting of the union of certain traits not clearly differentiated, and also in the omission of a few not occurring in our local Flora. In addition, the attribute 'teeth and hairs' was moved into the group of appendages. We also measured length, width and thickness of the seeds, averaged from 5 samples, following Thompson (1993).

Table 2. Classification of diaspores into categories relating to general morphology, appendages and surface (slightly modified after Grime et al. 1981)

Morphology

Appendages

Surface

1. Spherical, or nearly so

2. Ovoid, rhomboidal, turbinate

3. Trigonous or triquetrous

4. Lenticular, reniform or subulate

5. Cylindrical or ligulate

6. Clavate

7. Winged

8. Tadpole-shaped

2. Straight awn(s) or spine(s)

3. Hygroscopic awn or spine

4. Persistent pappus or calyx

5. Large hook(s) or barbed spine(s)

6. Elaiosome

7. Wing

8. Antrorse hairs or teeth

1. Smooth

2. Rugose, muricate, tuberculate or reticulate

3. Striate

4. Hairy

5. Striate and hairy

6. Mucilaginous

In the classification of the diaspores, we took into account previous studies dealing with the morphology and function of seeds (Grime et al., 1981; Hanf, 1982; Javorka-Csapody, 1991) and also standard floras (Bolos and Vigo, 1984-2001; Castroviejo et al., 1996-2004). For taxa not included in those studies, or for data not recorded previously, we examined a number of seeds from samples available in the Herbarium of the University of Barcelona (BCN), or obtained from the field. We omitted from our analysis a few species, namely the Orchidaceae since, due to the very small size and specific germination requirements or their seeds, they would not be detected in germination tests, or by hand sorting.

Was this article helpful?

0 0
Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment