COMPARATIVE STUDY OF FLORISTIC DIVERSITY IN SOWN GRASSLANDS AND PERMANENT PASTURES FROM GALICIA (NORTHWEST SPAIN) - M.I. Fraga, L.R. Calvo and J.C. Baleato

Departamento de Biología Vegetal, Facultad de Biología, Universidad de Santiago de Compostela 15706 Santiago de Compostela, Spain

SUMMARY

A comparative study between diversity of sown grasslands and permanent pastures was carried out on the basis of proportional abundance of the species, using the Shannon diversity index. A total of 110 pasture plots was surveyed, 74 sown grasslands and 36 permanent pastures. Although the number of species recorded for both leys (111 species) and pastures (119 species) was nearly the same, the diversity index was higher in permanent pastures, for a significance level of 99 percent. This difference can be explained by the fact that in the sown grasslands only a few of some commonly sown species, particularly Lolium multiflorum, stand out as the most abundant species and were largely dominant, whereas in permanent pastures a greater number of species, like Agrostis stolonifera, Holcus lanatus, Lolium multiflorum, Trifolium repens and Plantago lanceolata reached the higher abundance levels and shared the dominance.

Keywords: diversity, Shannon index, permanent pastures, sown grasslands

INTRODUCTION

The floristic diversity of Galician pasture ecosystems is closely related to management practices and environmental factors. Permanent pastures are composed principally of native species that are grazed year after year and, in many places, the sward is cut once or twice a year. Some of these species have a considerable interest as plant genetic resources because of their ability to grow on infertile acid soils.

According to Snaydon (1987) the dominance of sown cultivars in sown grasslands is kept for only a short time after sowing and very soon a high percentage of the sward is composed of invading indigenous species. These species come from soil seed banks or from other pastures, crops or natural plant communities of the surrounding areas.

At the present, knowledge about Galician pastures mainly concerns herbage yield and agricultural characters of some cultivars (Piñeiro and Perez, 1993; Piñeiro, 1994, Mosquera and Gonzalez, 1995). The floristic diversity is poorly documented and the main aim of this paper is to improve the information about how management practices can affect the floristic composition of sward.

MATERIAL AND METHODS

A total of 110 pasture plots were surveyed. Seventy-four were sown grasslands, one to five years old, with cultivars of Lolium multiflorum and Trifolium repens as the major sown species. These grasslands were fertilized and had a cutting/grazing management. The remaining 36 plots were permanent pastures, more than 15 years old, composed by autochthonous species, with very low fertilization levels or no fertilization and with grazing as main management practice. Ecological characteristics were quite similar for both sown and permanent pastures.

Floral inventories were drawn up in the period 1996-97 (autumn, spring and summer). The abundance of each species was a visual estimation of the number of plants per m² in the plot. As the abundance of the species recorded was very unequal, the diversity was analysed using Shannon's diversity index (Magurran, 1989). This index, based on the proportional abundance of species, provides a measure of diversity more complete than a simple species richness. The first step was to calculate the Shannon diversity index by the corresponding formula, for the whole of the sown grassland plots and the whole of permanent pasture plots. The second step was an estimation of the evenness of the two classes of pastures, followed by an estimation of the variance in diversity of them. Finally a t test allowed the diversities of the two pastures to be compared.

Table 1. Diversity of permanent pastures was significantly higher than diversity of sown grasslands.

t value = 4.78
Freedom degrees = 9057
*Significantly different (p>0.001)

RESULTS

The species richness was similar for both types of pastures, 119 species were recorded in permanent pastures and 111 in leys. Most of the species recorded were common for both. However, diversity, estimated by Shannon's index, was significantly higher for permanent pastures than for sown grasslands, for a significance level of 99 percent (Table 1).

The general model of sward composition was nearly the same for sown grasslands as permanent pastures. In both cases only very few species were abundant, some had medium abundance and most were represented by only a few individuals. The proportional abundance of the main species are shown in Table 2.

For grasses, Lolium multiflorum was dominant in leys, followed by Dactylis glomerata and Poa annua. In permanent pastures a greater number of species, like Agrostis stolonifera, Holcus lanatus, Lolium multiflorum, followed by Lolium perenne, Agrostis capillaris and Poa annua had reached the higher abundance levels and shared the dominance. This fact is the major component of the highest diversity of permanent pastures.

Table 2. Proportional abundance of the major species.

Among legumes, cultivars of Trifolium repens in sown grasslands and the same autochthonous species in permanent pastures have reached similar abundance levels, while cultivars of Trifolium pratense were more than two times abundant in grasslands than their wild relatives in permanent pastures. Other medium abundant species for the two classes of pastures were Plantago lanceolata, Bellis perennis, Hypochoeris radicata and Rumex obtusifolius.

In grasslands several successful invaders, such as Stellaria media, Amaranthus blitum and Raphanus raphanistrum had reached medium abundance levels, while in permanent pastures these were represented by only a few individuals, because these species generally have high requirements for soil fertility and in the permanent pastures the fertility conditions were usually poor. On the contrary, Ranunculus repens was only recorded in permanent pastures where it attained a medium abundance level. Crepis capillaris, Chamaemelum mixtum and Taraxacum officinalis were also more abundant in permanent pastures than in sown grasslands.

In summary, the main differences of diversity between leys and permanent pastures from Galicia are the abundance levels of the major species, but these results only represent partial information that will be checked in more extensive future research.

REFERENCES

Magurran, A.E. 1989. Diverdidad ecológica y su medición. Ed. Vedrá.

Mosquera, R. & González A. 1995. Estudio de la composición botánica en sistemas lecheros sometidos a distinta carga. Actas Congreso Sociedad. Española de. Malherbología: 73-76.

Piñeiro, J. & Pérez, M. 1993. Mezclas pratenses para la España húmeda. Hoja divulgadora 8/92. MAPA. Secretaría General de Estructuras Agrarias.

Piñeiro, J. 1994. Praderas sembradas en Galicia: especies que las componen. Cuadernos del área de Ciencias Agrarias 6:27-44.

Snaydon, R.W. 1987. The botanical composition of pastures. In “Ecosystems of the world” 17B:81-87. Ed. R.W. Snaydon. Elsevier Publishers.