PHYSIO-ECOLOGICAL STUDY ON THE BEECH (« FAGUS SILVATICA » L.) OF THE CANSIGLIO PLATEAU (VENETIAN PRE-ALPS)

Abstract

In the Cansiglio National Forest (Fig. 1) a comparative physio-ecological research has been carried out on sun and shade leaves of beech-trees of the same age growing at different exposure (N-NE and S-SW) at a distance of 70–80 meters only. In this paper the data referring to the daily and seasonal trends of the leaf water status in connection with microenvironmental conditions, are reported. The collection of samples and recording of meteorological data were carried on at 20–25 days' intervals during the beech growth season. After the description of the Materials and Methods (Fig. 2) employed, the soil characteristics, the average climatic conditions (Fig. 3) together with those referring to 1965 and 1966 (Fig. 4 and Fig. 5) are shortly described. The microclimatic differences between North and South exposures are discussed more in detail (from Pig. 6 to Fig. 9, Table 1 and Table 2). They have put in evidence in the North slope: higher relative humidity, lower average temperature, lower daily global radiation, lower evaporation values and reduced microclimatic fluctuations in comparison with the opposite slope. The soil temperature data (Table 3) point out that the autumnal cooling occurs earlier for the North slope, owing to the reduced insulation period; a delayed soil heating in spring, in connection with the longer persisting snow blanket was also observed. Pedological observations carried out on soil profiles of both exposures (Table 4 and Table 5), have put in evidence a shallower soil at the North and therefore it will perhaps dry easier in spite of the North aspect of the slope. For comparative study of the water status of sun and shade leaves of both exposures, the daily and seasonal trends of the following indexes have been determined: Transpiration, Transpiration Power, Water Content (in % of dry weight), Water Saturation Deficit, Water Retention Power, Leaf Area, Stomatal Density, Degree of Succulence, Degree of Consistency, and Development of Leaf Area. After an analysis of daily data (from Fig. 10 to Fig. 18) the Authors put in evidence the most striking features of the daily trend of the main physio-ecological indexes: Transpiration and Transpiration Power: a different transpiration behaviour between sun and shade leaves was evident and the influence of external and internal conditions besides an exposure effect, is pointed out. The daily trend of LIVINGSTON'S Relative Transpiration or Transpiration Power (T/E = Transpiration/Evaporation) shows three stages: 1) considerably high T/E in the early hours of the day; 2) a decrease at noon; 3) a resumption in the afternoon. Under unfavourable climatic conditions this pattern is not evident and T/E follows a bell-shaped curve. A very low and scarcely fluctuating Transpiration Power (T/E) in newly opened (May, 1966) and young (June, 1965) leaves is pointed out. Water Content (in % of dry weight): poor correspondence between Water Content and Transpiration, higher water contents in shade leaves than in the sun ones in both exposures and more marked fluctuations in water content (Table 6) in the North tree (probably in connection with edaphic conditions), were noted. In newly opened and in young leaves a rather steady daily water content, to be connected with their water-holding power which is very high, and therefore a considerable protection against excessive water loss, was put in evidence. These findings agree with the low T/E values of newly opened and youngleaves. Water Saturation Deficit: the AA. emphasize the influence of the amount of osmotically active cell metabolites upon WSD values and with regard to this they point out the concept of «metabolites WSD». This component of WSD accounts for the frequently observed discrepancies between the daily trends of WSD and of Water Content. In Cansiglio beech, during the day the WSD followed either a bell-shaped curve with the maximum in the central hours of the day, or a curve with two maxima, one in the morning and the other in the afternoon or evening. During the night in the North tree a WSD increase in sun leaves and a decrease in the shade ones were observed; in the South tree the exactly opposite behaviour generally occurred. The different WSD values, calculated according to CATSKY'S and STOCKER'S methods in newly opened leaves, are quoted in Table 7. Greater daily fluctuations of WSD were observed in shade leaves than in sun ones in both slopes (Table 8). Finally a comparison between the daily fluctuations of Water Content and of WSD, points out that the maximum steadiness of Water Content occurred in the sun leaves South, while the maximum steadiness of WSD was often registered in the sun leaves North. Water Retention Power (Fig. 19): this index was determined only once; it was higher in sun than in shade leaves and maximum water-holding power was registered in the sun leaves North. After having analyzed the average seasonal trends of the physio-ecological indexes taken into consideration critically stressing also the difference of Transpiration values on leaf area (Tab. 9 and Fig. 20) and on dry weight basis (Tab. 10 and Fig. 21), and after having reported average data referring to some morpho-ecological indexes, the Authors point out the following interesting aspects. Higher Transpiration rates (Fig. 20) in sun leaves than in the shade ones and a reduction of Transpiration activity (especially in the sun leaves) during the period of lowest summer rain, were observed. The whole Transpiration behaviour of the North and South beeches (Fig. 22) are fundamentally identical; the North tree showed greater T rates until the middle of July, after which time the South plant taked advantage. The mean seasonal Transpiration Power (T/E, Fig. 23) puts in evidence an interdependence and a water self-regulation between sun and shade leaves. In the light of this index the Transpiration values in relation to leaf age and to plant...