LEGUME AND GRASS DENSITY UNDER VARIOUS TREATMENTS IN A MEDITERRANEAN GRASSLAND IN MACEDONIA, NORTHERN GREECE - T. Merou^[24] and V.P. Papanastasis^[25]

SUMMARY

Legume and grass densities were measured in Mediterranean grassland subjected to the following treatments: prescribed burning, irrigation, ploughing, cutting, P fertilization and control. The grassland was dominated by annual species such as Avena spp., Trifolium campestre, T. scabrum, Onobrychis aequindentata and Medicago minima. It is located on a calcareous substrate at about 150 m elevation, in the region of Drama, Macedonia, in Northern Greece. Treatments were applied at the beginning of the growing season and the measurements of species density were taken in the late autumn and in the spring for four years. It was found that only P fertilization positively affected the legume density, while all the other treatments affected it negatively. On the contrary, grass density was reduced by all treatments. The results suggested that human activities significantly affect species structure of Mediterranean grasslands. This effect may lead to species deterioration if climatic conditions worsen.

Keywords: legume, grass, density, Mediterranean grassland, prescribed burning, irrigation, ploughing, cutting, P fertilization.

INTRODUCTION

Grasslands are a major vegetative type in Greece. They cover 1.7 million ha and represent 13 percent of the total area of the country. Half of these grasslands are found in Northern Greece, in a variety of environments, from sea level up to the alpine zone. Structurally, they do not differ significantly from other grasslands of the world; grasses dominate both in cover and biomass (Papanastasis, 1981). In those grasslands located in the lower elevation zone (i.e. below 800 m altitude) and characterized as Mediterranean-type, the majority of species are annual plants. The dominance of annuals is attributed to their ability to complete their life cycle during the rainy period, better exploiting the winter rains.

It is well known that Mediterranean ecosystems suffer from disturbances such as fires, overgrazing and periodical ploughing. On the other hand, P fertilization is a common management practice for herbage improvement. The Mediterranean climate also affects these ecosystems with the sequence of wet and dry years.

Fire is a well known factor affecting vegetation of the Mediterranean ecosystems (Trabaud et al., 1993). Le Houerou (1981) reports that the structure of grasslands, after fire, is the effect of the reaction of every single species. Menke (1989) states that lower density in grasses was observed after fire. Among the various groups, leguminous species are the ones most common after fire (Papavasiliou and Arianoutsou, 1993).

In the Mediterranean environment, moisture is the limiting factor affecting primary productivity, which fluctuates in the short- (within the year) and long-term (from year to year) (Naveh, 1982). Irrigation, as an imitation to increased precipitation, seems to favour annual species but for a very short time, as perennials eventually dominate (Powell et al., 1990).

Ploughing is a more serious disturbance than grazing, as it totally destroys the biomass (Harper, 1977). Doer et al. (1984), report that the lighter the disturbance, the sooner the grasses recover.

Grazing is a factor which drastically alters the structure and function of a range ecosystem. It may reduce production and affect its composition (Tsiouvaras et al., 1993). By increasing grazing intensity, there is a subsequent decrease of grasses and an increase of legume abundance, while in high grazing intensities the completely undesirable species finally dominate (Naveh and Whitaker, 1979).

Fertilization is an effective means of quantitative and qualitative improvement of herbage (Menke, 1989). P is essential for the legume establishment, though it is known that in dry areas, response to P fertilization is expected only in wet years. Generally, P fertilization has a positive effect on legumes and a negative effect on grasses (Papanastasis, 1980, Osman et al., 1991).

The aim of the present study was to investigate the influence of fire, ploughing and grazing as well as P fertilization and irrigation on annual legume and grass densities in order to assist the proper management of Mediterranean-type grasslands.

MATERIALS AND METHODS

The research was conducted in Mediterranean grassland in Macedonia, Northern Greece. It is located on a calcareous substrate at about 150 m of elevation. Mean monthly temperature ranges from 3°C in January to a July high of 25°C. Average annual total precipitation is 604 mm with 70 percent falling during the growing season. The dry period is from July to September, whereas the driest month is September. The climate is usually mild but winters are very often too cold. During the course of this study, 1996 and 1997 were years with extreme weather conditions (with late autumn and spring frosts).

The growing season begins with the first effective rain and ends in May, when the dominant annuals set seed and dry. Only a few perennial species (e.g. Dichanthium ishaemum) retain green shoots into the dry summer.

The vegetation of the grassland is dominated by annuals such as Avena barbata, A. sterilis, Trifolium campestre, T. scabrum, Onobrychis aequindentata and Medicago minima.

For the present study, an area which was previously grazed by sheep and goats, was fenced off. The experiment was a completely randomized block design with six treatments and four blocks. The treatments were: prescribed burning, irrigation (imitation of increased precipitation), ploughing, cutting (imitation of grazing), P fertilization and control. All plots were 3x3 m in size and arranged randomly in the blocks.

Prescribed burning, P fertilization (100 P₂0₅/ha) and ploughing (about 10 cm depth) were applied in early autumn of 1994, 1995, 1996 and 1997. In the cutting treatment, plants were cut once at 3 cm above ground in late spring. In the irrigation treatment, 30 mm of water were applied in autumn of 1994 and 30 mm in the spring of 1995; in the growing season 1995-96, 30 and 55 mm were respectively applied, whereas 30 and 40 mm were applied in the growing season 1996-97. In each treatment, vegetation was sampled with quadrats 0.25 x 0.25 m in size for 1994 and 1995, and 0.5 x 0.5 m for 1996 and 1997. Within each quadrat the legume density was measured, late autumn and in spring, whereas grass density was measured only in the spring.

RESULTS AND DISCUSSION

During the first two years, almost in all treatments, legume density was higher in the spring than the autumn (Figure 1). In the first year, burning, ploughing and cutting, negatively affected legume density in the autumn, though only the first two treatments had significantly lower values than control. On the other hand, irrigation and fertilization produced positive results although the density values did not have significant differences as compared to control. In spring, burning and cutting, as well as irrigation, had higher but not significant values than the control, while ploughing and P fertilization had significant higher values as compared to the control. In the second year the results were almost the same as in the first year. In the autumn, burning, ploughing and cutting presented the same results as in the first year while irrigation gave, high density, though not significantly different. In spring, only ploughing resulted in significantly higher density, while all the other treatments resulted in higher but insignificant different values as compared to the control. Surprisingly, P fertilization gave low autumn and spring densities, but that effect should be due to the high litter accumulation, which prevented legume establishment.

During the first two years, burning, ploughing and cutting severely affected autumn density. This effect was compensated by spring density, which, in these treatments, was higher than the control, though not always significant. These results agree with Naveh and Whitaker (1979) and Papavasiliou and Arianoutsou (1993). On the other hand, irrigation favoured mainly autumn density, an effect which reflects the importance of precipitation for the legume establishment.

Figure 1. Legume density in autumn and spring in the various treatments for four years.

In the following two years, the extreme weather conditions (autumn and spring frosts), influenced legume densities severely, especially in the first one. It must be pointed out that in the last year of the experiment, P fertilization resulted in significantly higher autumn and spring densities, than the control, but much lower than in the beginning of the experiment. Similar results were found by Papanastasis (1980) and Osman et al. (1991). On the other hand, irrigation resulted in a very low legume density, probably because perennials were favoured and dominated the plot as Powell et al. (1990) had already pointed out.

As far as grass density is concerned, burning, ploughing and cutting affected them in a different way (Figure 2). Burning resulted in a little higher density than the control, while ploughing and cutting resulted in a significantly lower one. On the contrary, irrigation favoured significantly grass density.

P fertilization produced high density but not significant, probably due to the litter accumulation which negatively affected legume density, but not the grasses. In the following two years very low densities in all treatments were recorded. In the second year, cutting resulted in significantly lower value than the control, probably because the dominant annuals Avena spp., are sensitive to cutting as Rosiere (1987) had already reported. P fertilization and burning resulted in significantly higher values. However, in the last year, all treatments had lower densities than the control and only burning and P fertilization resulted in insignificant differences as compared to control. In irrigation, the perennials dominated the plot, as Powell et al.. (1990) had already stated. The reduction of grass density in the last two years might have also been affected by the extreme weather conditions.

CONCLUSIONS

Disturbances, like burning, ploughing and cutting negatively affected legume density in the autumn and spring. This effect was more severe in years with extreme weather conditions. Irrigation, also, resulted in low densities as it favoured the dominance of perennials. P fertilization, on the other hand, positively effected legume density, even if the weather conditions were not very favourable.

Figure 2. Spring grass density in the various treatments for three years.

On the contrary, grass density was negatively affected by all treatments, even by irrigation which allowed the dominance of perennials.

In conclusion, deterioration of both legume and grass densities in the Mediterranean ecosystems is possible under intense human activities, especially if the weather conditions are not very favourable.

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