Evolución de fracciones de gramíneas, leguminosas y otras familias en pastizales de zona de dehesas

Laburpena

Within the branches of investigation carried out in the Praticulture Section of CEBA, Salamanca, special interest has been given to the study of production and primary growth of pastures in the grazing-lands of Salamanca. 21 allotments were controlled over a period of five consecudve years (1974-78). They include the wide range of grazing-lands in this semi-arid área of the península with its most distinctive characteristics. In the last four years of this period, the contributions of the three big definable groups, par excellance, in the herbaceous stratum, were likewise quantified: Gramineae, Leguminous and other families. Analising independently each of these sections throughout primary growth and in different observation years, one can see how, in the evolution of the gramineae of all the communities, the productions obtained in the same cutting periods show great variability. This becomes progressively greater as the production cycle of the pastures advances. The highest production of gramineae is obtained in mid-June, The dominant species are Poa bulbosa (in «majadal» pastures), Anthoxanthum aristatum, Vulpia bromoides, Bromus tnollis and in greater proportion, Agrostis castellana (fundamentally in «vallicar» pastures). Throughout the growth period, the gramineae are almost uniformly dominant. In the average valúes trajectory, two máximums appear: one of which is relative, owing to the annual gramineae and the other, wich is absolute, due to biannual and perennial contributions. The máximum production of leguminous plants are obtained in June, although great irregularity is noticeable, both between the different communities in the same year and between the different years íor the same community. The species best represented is Trifolium striatum. Ornithopus compressus and Trifolium subterraneum are also important. Leguminous grasses are rarely dominant. When dominance does exist, it is usually in May. In many cases, their contribution is scarcely noticeable, even for the whole annual cycle. In the case of the section which groups the other families, variability is still greater than in the previous ones. Máximum are reached in the period berween the second fortnight in May and the first in July, although the valúes vary greatly from one community to another. The dominant species are less definable: the commonest are Convulvulus arvensis, Plantago lanceolata, Tuberaria guttata, Eringium campestre, etc. The proportion of gramineae is, save in very few exceptions, over 20 % in the cutting of máximum production and we obtain similar results in the higher activity period (May-July), when valúes as high as 100 % are reached. In the same conditions, the leguminous plants do not exceed 50 %, except for the valúes obtained for some allotments in 1977. As an equally generalizable characteristic, the other families do not exceed a 60 % contribution in dry weight. The evolution of different fractions expressed in percentages, in the last phases of the production cycle, tends towards higher valúes in gramineae, and, at the same time, the other two sections decrease (leguminous plants in greater intensity). When it comes to considering the average valúes of the communities grouped in the same types of pasture, the inter-annual differences of that general tendency are quite clear: in 1975, the highest percentages in gramineae were reached and in 1977, the lowest. These fluctuations are due only to variations in the proportion of leguminous plants, since the other families remain between narrower margins. The statistical tests, carried out in order to determine the degree of correlation or independence between the productions of dry material obtained for the three sections in the controlled communities and for the four years of study, show interannual independence and aleatory correlation between section.