Matskási István (szerk.): A Magyar Természettudományi Múzeum évkönyve 79. (Budapest 1987)

Debreczy, Zs.: Fluctuating-dynamic equilibrium of photophil, xerophil rupicolous plant communities and scrub woods at the lower arid woodland limit

tion of water in the shallow, skeletal soil formed on the dolomite debris and beds and then its rapid oversaturation, the author's experiments at irrigation show that 10 mm (10 1) of "precipitation" spread on an area of 1 m

2

, 3 1 could be retained in the form of surface water in the sample square. The rapid initial decline found in the measurements of soil humidity (Fig. 12) can be attributed not only to the more rapid evaporation on the already warm soil, but also and primarily to gravitational seepage within the soil. Further water spread on the soil disappeared even more rapidly and to an even greater extent from the absorbing zone of the rupicolous vegetation. All these measurements and observations show that in the summer half-year only 1/3 of the average volume of precipitation (less in summer, slightly more in spring and autumn) is available to the rupicolous vegetation, while the mean average temperature for the summer half-year is 26 °C and can rise to 31.2° in the warmest three months ! However the loss of water by seepage applies only to the skeletal soils: the greater part of the water lost here remains in the scrub wood patches in the black rendzina soil with good water retention, that acts like a sponge. We made measurements and calculations concerning the possible limits of water retention. Jakucs found a hygroscopic coefficient of 10.51 and 6.20 for the rendzina and skeletal soils, respectively (JAKUCS 1961, p. 65; see also Fig. 19). Our calculations showed that 1 dm

3 of pure (stone-free) rendzina is capable of retaining 5 dl of water, while the figure for the same quantity of skeletal soil from here is 1.6 dl. A patch of scrub wood 4 m in width, covering an area of 18.84 m

2 with rendzina soil an average of 30 and 40 cm deep (5652 and 7500 dm

3

) is capable of absorbing 2826 and 3750 litres of water respectively, that is, after a rainfall of 30 mm (301/m

2

) (565 1/18.84 m

2

) it is capable of absorbing five to six times the quantity! It does indeed act like a sponge. The greater the proportion of areas with skeletal soil above the scrub wood on the slope, the greater the water collecting area of the scrub wood patches with rendzina soil below them : where these proportions are 50% each, the 2/3 quantity of water lost from the rupicolous associations increases the quantity of water reaching the scrub woods by more than one and a half times (by 1.7-fold) ! Fig. 20 shows the values calculated for an actual field site. This clearly shows that the quantity of water running off a rupicolous area of 20 m

2 (calculating a loss of 1/2 and 2/3) increases the quantity of water available for use in the scrub woods from the 100-150 rnm/m

2 of the rupicolous associations to 387-428 mm/m

2 for the slightly larger (25 m

2

) scrub wood patch, that is an increase of 2.8 and 3.9-fold, respectively! The scrub wood patches intermingled with rupicolous associations thus may receive a larger quantity of water than the climate-zonal (macroclimatic)level (387-424 mm instead of 300 mm) and thus in many years their crown size limited by the drier or drought periods may be capable of utilising only a part of the water available. This fact also indicates that the scrub woods are in a far more favourable situation than the rupicolous associations and that a veritable ("edaphicmicroclimatic") climate line can be drawn between them. The sustainable foliage surface The important role of dry summers is clearly illustrated by comparative calculations of the foliage surface of climate-zonal Omo-Quercetum f orests and the scrub woods which clearly show that the 1/3 levels in dry summers compared to the average rainfall (precipitation in the summer half-year of 1983 and 1986 was 118 and 120 mm respectively, as against 358 mm) are definitely reflected in the greatest foliage crown surface. In the model shown in Fig. 21 on the basis of a cenological survey in an actual area of 20 x 20 m, the greatest crown diameter of the dominant trees is expressed in globe surface values. The leaf surface calculations for the two dominant oak species and the manna ash showed a similar proportion between the globe