0421-B3

Wildfire impacts on soils in Slovenian black pine (Pinus nigra Arn.) forests

Mihej Urbančič ¹

Abstract

Within five areas of one month to 5 years old fire places which are located in the western Slovenia ten pair-compared plots with size of 20 x 20 m were established, half in burnt forests and half in nearby preserved black pine stands. On plots soil conditions and morphological, physical and chemical soil properties were studied. On lime-stones and dolomites soil units Folic Histosol, Lithic and Rendzic Leptosol, Eutric and Chromic Cambiso, are developed. Because of wildfire effects organic horizons of burnt plots had smaller thicknesses and masses than of compared plots in preserved stands and in mineral parts of soils in spots differences in some chemical parameters (as increase of pH values and electrical conductivity, increased contents of carbonates, pant available K, P and Mg nutritions, exchangeable base cations, etc.) were discovered.

Inroduction

In Slovenia are black pine stands the most threatened by wildfires. The goal of this research was to research site conditions in different black pine forests, to analyse soil properties and classify them and to assess the influence of wildfires on these soils.

Four research areas were grounded in secondary black pine forests in the Low Kras. About 150 years ago the Low Kras (Karst) was a treeless, stony and rocky barren landscape. In 1875 forests covered only 18 % of this lowland, 24,3 % in 1957 while today over 50 % of the area is forested. An important role in this reforestation of abandoned pastures and other agricultural areas was the allochtonous species of the Austrian pine (Pinus nigra Arn.). Its more or less clear stands on limestone and dolomite cover about 11000 ha within the Low Karst region.

One research area is located in primary south-Alpine black pine forest. In Slovenia are its sites rather infrequent and cover in all 192 ha only.

Materials and methods

In Primorje, region in Slovenia with the maximal numbers of forest wildfires and by fires threatened stands, five areas with one month to five years old fire places for research of wildfire impacts on forests were established. Four research areas (Mlave, Podgovec, Vremščica, Kojnik) were grounded in secondary black pine forests (Seslerio autumnalis-Pinetum nigrae Zupančič 1997 (nom. prov.)) on limestones and dolomites of Low Karst and one (named Govci) in primary south-Alpine black pine forest (Fraxino orni-Pinetum nigrae Martin-Bosse 1967 var. geogr. Primula carniolica Dakskobler 1998) on steep dolomite slopes in the altitudinal belt between about 1050 and 1150 m a. s. l. in the northwestern part of the Trnovski gozd plateau. In chosen areas ten pair-compared plots with size of 20 x 20 m were established, half in burnt forests and half in nearby preserved black pine stands.

Table 1: Names of research locations, dates (month and year) of fire appearance and of field soil research, the type of wildfire (U = underground, G = ground, T = trunk, C = crown fire), and designations of research plots on objects in preserved (_U) and in burnet (_F) forests

Table 2: Symbols of soil layers and short descriptions of their characteristics

On plots soil conditions were examined with a gauge soil sound and representative soil profiles were dug. After the description of profiles soil samples were taken and analysed in the pedological lab. On plots of research areas Podgovec, Kojnik and Govci one by one representative soil profile was digging out and from its genetic layers samples for analyses were taken off. On each of both compared plots of research areas Mlave and Vremščica on three to five places with aid of woody square frames (size of 25 cm x 25 cm) and tubiform sound (diameter of 5 cm) so-called "quantitative" samples of organic (O) subhorizons and "quantitative" samples of mineral parts of soil from layers with the fixed depths (0 - 5 cm, 5 - 10 cm, 10 - 20 cm) were taken off so that their volumes and masses are known. Samples were dried on air, blended and sieved through the 2 mm mesh sieve before analysis. For each soil sample the pH was determined photentiometricaly in the supernatant suspension with water and 0,01 mol/l CaCl₂. The content of CaCO₃ was determined after treatment of sample with 10 % HCl by using Scheibler calcimeter. The total carbon content was determined after dry combustion of sample at 1050_ C. Total nitrogen was analyzed according Kjeldahl method. Contents of exchangeable cations (K ⁺ , Ca ²⁺ , Mg ²⁺ , Al ³⁺ , Fe ³⁺ , Mn ²⁺ and H ⁺ ), cation exchange capacities, base saturation and texture classes were determined for samples from mineral parts of soils. For determination of exchangable cations samples were first extracted with a 0,1 mol/l BaCl2. Exchangable acidity was determined potentiometricaly, while cations were determined by FLAAS. The soil particle size fractions were determined by the pipette method and the textural classes by using the USDA soil textural triangle (Manual... 1994).

Results and discussion

South-Alpine black pine forest of the research area Govci overgrows shallow and less developed organic soils (Lithic Leptosols and Folic Histosols) on dolomites. Four research areas were grounded in secondary black pine forests (Seslerio autumnalis-Pinetum nigrae) on limestones and dolomites of Low Karst. On their plots are soil conditions rather variable. On them five soil units (Folic Histosol, Lithic Leptosol, Rendzic Leptosol, Eutric Cambisol, Chromic Cambisol) were found (table 3).

Table 3: Surface shares (%) of stoniness with rockiness (K+S) and soil units (according to FAO 1989, WRB 1998) on resarch plots (denoted with abbreviations)

On all treated fire places in wildfires a great deal of soil organic horizon was burnt and that is why on all research areas burnt plots had smaller thickness of this layer in average than compared plots in undamaged stands. In the research area Govci thickness of burnt soils was, in comparison with undamaged soils, 5 years after the fire still thinner for about 5 to 10 cm. In some places soils were burnt down to the dolomite parent rock and for this reason on the scene of the fire areal shares of soils were very reduced and surface stoniness with rockiness proved to be increased from around 55 % to 80 %. On research area "Mlave" organic soil layer of one month old fire place had in average five times smaller thickness and six times smaller mass than organic horizon of compared undamaged soils. (Quantitative samples of organic horizon from fire place had in average mass of 4032 kg/ha and compared undamaged organic samples had in average mass of 24928 kg/ha). Because of partial regeneration of the horizon were these differences on other research areas with older fire places rather smaller (table 4).

In comparision with soils of preserved forests were soil layers of fire places at least in upper part of soil profile as a rule less acid, mostly had larger electrical conductivity, higher content of carbonates, total nitrogen and plant available nutritive substances, greater cation exchange capacity, sum of exchangeable base cations and exchangeable base saturation but smaller content of exchangeable acid cations. In all probability are at least some of these differences result of changes in chemical soil properties because of fire impacts. The greatest differences in chemical soil properties were found on research area "Mlave" with the youngest, only one month old fire place. Here are dominant with organic matter rich, shallow to moderate deep rendzic soils that were strongly affected by combined underground-ground- trunk-crown fire (table 5 - 7).

Table 4: Average thicknesses (in cm) of organic (O), humus-accumulative (A) and cambic ((B)) horizons of sounded soils on plots, their structure percentages, differences (_{U - F}, in cm) and relations (_U/F x 100 %) between thicknesses of compared horizons from preserved (_U) and in burnet (_F) forests

Table 5: Active (pH(H₂O) and potential (pH(CaCl₂)) acidity of soil, electrical conductivity of soil (EC), contents of organic matter (Org. s), total carbon (C_tot), carbonates (CaCO₃) and total nitrogen (N_tot), ratio between organic carbon and total nitrogen (C_org/ N_tot), amounts of total sulphur (S_tot) in soil samples of soil profiles from the preserved (U) and burnt (F) forests

Table 6: Contents of plant availible potassium (CAL-K), phosphorus (CAL-P) and magnesium (Sch-Mg) in soil samples of soil profiles from the preserved (U) and burnt (F) forests and texture of soil samples (G=clay, M=silt, I=loam, MG=silty clay, GI=clay loam, MGI= silty clay loam)

Table 7: Contents of exchangeable (Ca ²⁺ , Mg ²⁺ , K ⁺ , Al ³⁺ , Fe ³⁺ , Mn ²⁺ , H ⁺ ) cations, cation exchange capacity (KIK), sums of exchangeable base cations (S_B), sums of exchangeable acid cations (S_A) and exchangeable base saturations (V) in soil samples of soil profiles from the preserved (U) and burnt (F) forests

Conclusions

On the plots in black pine stands are soil conditions rather variable which is characteristically for limestone and dolomite parent rocks. With sounding on them five (FAO1989, WRB 1998) soil units (Folic Histosol, Lithic Leptosol, Rendzic Leptosol, Eutric Cambisol, Chromic Cambisol) were found.

On all treated fire places a great deal of in organic matter rich layers of horizon O and partly of horizon A were burnt down and charred in wildfires and that is why on all five research areas burnt plots had smaller thickesses of this layers in average than compared plots in undamaged stands. In some places soils were burnt down to the parent rock and for this reason on the scene of the fire areal shares of soils were reduced and surface stoniness with rockiness proved to be increased.

Because of wildfire effects had mineral parts of soils on burnt plots in spots increased pH values, electrical conductivities, contents of carbonates, plant available K, P and Mg nutritions and exchangeable base cations but smaller content of exchangeable acid cations.

Literature

FAO, 1990. Guidelines for soil profil description. - 3 ^rd edition (Revised). - FAO, ISRIC, Roma, 70 p.

FAO, 1989. Soil map of the world. - Revised legend. FAO, Unesco, ISRIC, Roma, Wageningen, 138 p.

Manual on methods and criteria for harmonized sampling, assessment, monitoring and analysis of the effects of air pollution on forests. 1994.- UN ECE-ICP Programme Coordinating Centers, Hamburg, Praga, 177 p.

WRB, 1998. World reference base (WRB) for soil resources. FAO, ISRIC, ISSS. Rome, (http://www.fao.org/docrep/w8594e/ w8594e00.htm).

¹ Slovenian Forestry Institute, Večna pot 2, SVN-1000 Ljubljana.
E-mail: [email protected]