Croatian Journal of Forest Engineering (Apr 2011)
Correlation of Habitat and Structural Factors with Dieback and Nutrition of Silver Fir (Abies alba Mill.) in Gorski Kotar
Abstract
Dieback of silver fir in Croatia, especially in the Dinaric part, is associated with complex changeable effects of abiotic and biotic factors. In industrial forests, the volume of dieback trees or volume of trees cut down in salvage cutting is most often used for showing tree dieback intensity, and can be used as an indicator of forest stand health status (Capecki 1981).The aim of this research was to show the correlation of silver fir dieback with habitat and structural factors, calculate the intensity of dieback according to volume, and determine tree dieback dynamics and effects of habitat factors on concentration of essential mineral nutritive elements in needles and soil.The research was carried out in the mountainous part of Croatia, in beech-fir and fir forests of Gorski Kotar. The research included the forest stands of disrupted stability and structure, at the border of the natural range of sliver fir towards a warmer area and optimum area of silver fir i.e. central part of Gorski Kotar. Data for wood mass of dieback silver fir trees were collected for the selected management units at the level of all compartments and subcompartments in the management unit. The data were being collected for twelve years (1995–2007). In the researched area, needle sampling from tree crowns was carried out on trees with three different levels of defoliation (60%). Dry needle mass was determined as well as concentration of K, Ca and Mg. Soil chemical analysis included soil reaction, and amount of phosphorus (P) and potassium (K) in the soil.The average annual silver fir dieback intensity for forest stands at the border of the natural range was 18.3 m3/ha or 9.2% of silver fir growing stock, and in forest stands in the central part of Gorski Kotar it was 6.2 m3/ha or 4.1%.The maximum annual dieback intensity in forest stands at the border of the natural range was 113.0 m3/ha, and in forest stands in the central part of Gorski Kotar it was 55.4 m3/ha. For forest stands with recorded dieback, the average annual increment was 2.7 m3/ha, and in forest stands in the central part of Gorski Kotar it was 3.3 m3/ha. In forest stands with sliver fir dieback, dieback intensity was higher than the value of the growing stock (Table 1).Intensity of the average annual silver fir dieback at the border of the natural range was from 1.6 to 18.8 m3/ha, and in forest stands in the central part of Gorski Kotar from 5.0 to 7.5 m3/ha (Fig. 1).In forest stands at the border of the natural range, the highest average annual silver fir dieback intensity was recorded in silver fir forest with hard fern (18.63 m3/ha), and the lowest in silver fir forest with small-reed (11.5 m3/ha). Dieback intensity in the central part of Gorski Kotar was lower, the highest being in silver fir forest with small-reed (14.4 m3/ha), and the lowest in the Dinaric beech-fir forest (5.8 m3/ha) (Fig. 2).In forest stands at the border of the natural range, the highest dieback intensity was recorded at west and southwest exposition, and the lowest at northwest and east exposition. In forest stands in the central part of Gorski Kotar the highest dieback intensity was at southwest, and the lowest at north exposition. In relation to altitude, the most severe dieback intensity was observed at lower, and the lowest at higher altitudes (Table 2).According to the results shown in Table 3, in forest stands at the border of the natural range, tree dieback intensity significantly and positively correlates with exposition, growing stock (0.23*) and number of silver fir trees (0.12*), and negatively with terrain slope (– 0.15*), altitude (– 0.11*), precipitation quantities in vegetation period (– 0.11*), soil water balance (– 0.11*) and increment (– 0.16*). In forest stands in the central part of Gorski Kotar silver fir dieback intensity significantly and negatively correlates with altitude (– 0.14*), precipitation quantities in vegetation period (– 0.18*) and soil water balance (– 0.12*).Calcium negatively correlates with vegetation air temperatures (– 0.23*), and positively with vegetation precipitations (0.23*), which is important for drought periods because 2007 was a »drought« year (Review No. 18). The correlation coefficients of Mg concentration show the same trend with air temperatures (– 0.48*) and precipitation quantities (0.48*).The correlation of Ca concentration and soil pH value is positive and amounts to 0.34*, while the correlation of dry needle mass and defoliation is – 0.40* (Table 4).The concentration of phosphorous and potassium in the soil correlates positively with vegetation air temperatures (0.34*, 0.29*), and negatively with vegetation precipitations (– 0.34*, – 0.29*) and altitudes (– 0.52*, – 0.50*) (Table 5).Dry needle mass (g), concentration of potassium, calcium and magnesium (mg/g) decrease with the increase of tree crown defoliation (Table 6).In the current year the mass of dry needles is 0.48 g, and in the last year it was 0.52 g. In the current year the concentration of calcium in needles is 12.34 mg/g, and in the last year it was 17.06 mg/g (Table 7).The data of Fužine and Rijeka Forest Office show that the percentage share of salvage cutting or dieback of silver fir amounts up to 100% according to prescribed yield (annual allowable cut) for the area of management units Brloško, Kobiljak-Bitoraj and Jelenje Velo (border of the natural range). On the contrary, the percentage share of salvage cuttings according to prescribed yield in the area of the management units Ravna Gora and Višnjevica (central part of Gorski Kotar) amounts up to 15%. In the area of Gorski Kotar tree dieback was recorded on all types of geological basis and forest soils, and in all silver fir forest associations. At the border of the natural range the highest dieback intensity was recorded in silver fir with hard fern, and in the central part of Gorski Kotar in silver fir with small-reed. According to our research, the highest tree dieback volumes (m3 and m3/ha) occur at the lowest altitudes and on mild slopes. Tikvić et al. (2008) reported the same results. Due to higher air temperatures and lower precipitation quantities, soil evaporation is higher at lower altitudes. During drought periods at these altitudes, soil water balance is low. Thomas et al. (2002) found positive correlation between precipitation quantities and altitude and considers that this is suitable for forest tree development at higher altitudes.In forest stands at the border of the natural range, tree dieback volume (m3/ha) is the lowest at northeast expositions, and the highest at west exposition, followed by different variants of south expositions, and compartments of different expositions. This was expected since the south and west expositions are warmer and more exposed to unfavourable currents from west Europe. The decrease of vegetation precipitation quantities significantly increases the volume of silver fir dieback. These results correspond to the results reported by other authors (Markalas 1992, Thomas et al. 2002, Tikvić et al. 2008).Even within the silver fir areal in one country, there are significant differences in factors affecting tree dieback. This can be proved by comparing the factors that affect dieback at the border of the natural range and in the central part of Gorski Kotar. Air temperature increase and precipitation quantities decrease result in the decrease of concentration of calcium and magnesium in silver fir needles, which is significant for drought periods and air temperature increase. Dry needle mass and calcium concentration are significantly higher in the last year than in the current year needles. According to the Meteorological and Hydrological Service, the year 2007 (current year needles) was a drought year. In the forest stand at the border of the natural range where dieback was recorded, the average annual dieback intensity was six times higher than the average annual increment, while in forest stands in the central part of the area, dieback intensity was two times higher than the annual increment.Tree dieback was recorded at all expositions, terrain slopes and altitudes, as well as in all forest associations. At the border of the natural range, relief, climate and structural factors have dominant effect on tree dieback, while in the central part of Gorski Kotar climatic factors have dominant effect on tree dieback. The increase of tree crown defoliation causes the decrease of dry needle mass, as well as the concentration of K, Ca and Mg. Drought resulted in lower concentration of magnesium and calcium in silver fir needles, and lower dry needle mass.
