Sufrace Runoff and Soil Loss in Burnt Stands of Aleppo Pine (Pinus halepensis Mill.) Growing on Colluvial Soils

U radu su prikazani rezultati istraživanja površinskog otjecanja oborinske vode i erozije tla na opožarenoj površini alepskog bora te progre sija vegetacije nakon požara i njen utjecaj na zaštitu tla od erozije. Sta cionarni šum ski pokus postavljen je u kolovozu 2002. godine u okolici Spli ta, u slivu Žrnov nice, na opožarenoj površini alepskog bora, koju je požar za hvatio 11. kolovoza 2001. godine. Ploha se nalazi na koluviju s pretežno detritusim kamenja, na nadmorskoj visini od 83 m i nagibu od 30° (GPS kordi nate: N 43° 31’, E 16° 22’). Istraživanja su obuhvatila razdoblje od kolovoza 2002. godine do kraja prosinca 2005. godine, tijekom koje je registrirano 265 kiš nih dana. Površinsko otjecanje oborina i gubici tla zabilježeni su u 53 kiš na dana, a izazvale su ih oborine od 8,2 mm (visok intenzitet) do 133,7 mm (vr lo vi sok intenzitet). Mjesečne vrijednosti površinskog otjecanja oborina u istraživanom razdoblju kreću od nule do maksimalno 3,03 mm/m2u rujnu 2002. godine. Godišnje vrijednosti površinskog otjecanja oborina na ovoj plohi iznosile su: 4,71 mm/m2 u 2002. godini, 2,08 mm/m2 u 2003., 4,40 mm/m2 u 2004. i 3,41 mm/m2u 2005. godini.Srednji koeficijent površinskog otjecanja oborina iznosio je 0,0042. Postotak površinskog otjecanja oborinske vode je nizak, inter cepcija, isparavanje i infiltracija vode u tlo iznosi i do 99,6 %. Mjesečne vri jednosti gubitaka tla kreću od nule u ljetnim mjesecima, do maksimalno 9,50 g/m2u rujnu 2002. godine. Godišnje vrijednosti erozijskog nanosa na ovoj plohi iznosile su: 10,83 g/m2u 2002. godini, 0,016 g/m2u 2003., 0,123 g/m2u 2004., 0,246 g/m2u 2005. godini. Koeficijent površinskog otjecanja u erodibilnim danima kretao se od 0,0020 do maksimalno 0,1165. Sukcesija prirodne vege tacije prisutna je na opožarenoj površini, na njoj se obilno razvijaja travni pokrov, dok se alepski bor jako slabo obnavlja, javlja se samo u tragovima. Uz travni pokrov razlog ovako malim gubicima tla su i geološke i pedološke karakteristike plohe, koja je postavljena na izrazito skeletnom koluviju, on je izrazito vodopropusan i djeluje kao “sito”, pa su gubici zanemarivi, gotovo ih i nema. Uz ovakva obilježja tla i vegetacije na ovoj površini nisu izražena po vršinska otjecanja i erozija tla.Severe degradation of forest ecosystems in the Mediterranean karst area is largely responsible for their lessened anti-erosion function. Almost 60% of the area is covered with coppices, scrub, maquis and bare land, while high forests of mostly Aleppo and black pine account for only 9.9 % of the forested area (Topić, 1994). In addition, fires in this area are generally of alarming proportions, making it possible to attribute soil erosion, to a certain extent, to the impact of forest fires. This paper presents the results of research into erosion and surface runoff in the Aleppo pine area burnt in a fire on 11August 2001. Post-fire vegetation progression was monitored and so was its impact on soil protection from erosion and surface runoff. Research was conducted in the Žrnovnica river in a wider area of the town of Split. This is the eu-Mediterranean phytogeographic region characterized by the Mediterranean climate with distinct oscillations of monthly temperatures and precipitation. Mean annual air temperature is 15.9 °C and mean monthly precipitation is 826 mm. Annual precipitation is not uniformly distributed across all the months and seasons. The cold part of the year is responsible for 63 % of the mean annual precipitation quantity. A stationary forest experiment, set up in August 2002, involved a burnt area of Aleppo pine growing on colluvial soil with predominant stone detritus at an altitude of 83 m and slope of 30° (GPS coordinates: N 43° 31’, E 16° 22’). Research encompassed a period spanning from August 2002 to the end of December 2005. Research was centered round a 20 m x 5 m experimental plot set up parallel with the slope. The plot was furnished with an erosion collection tank and fenced off with a tin fence. The positioning of the fence prevented the reception of lateral water and/or soil suspension, as well as uncontrolled water or soil suspension loss. Precipitation quantity, precipitation intensity and surface runoff were measured in the plot, and so was the quantity of erosion sediment as the basic indicator of the effect of forest vegetation and vegetation in general. A pedological profile opened in the plot provided soil samples that were analyzed in the laboratory. The measure of the association between surface runoff, soil loss and precipitation characteristics was estimated with Pearson’s correlation coefficient (Sokal, 1995). A significance level of 5 % was considered statistically significant for all statistical analyses. All statistical analyses were made using the STATISTICA 7.1 software (StatSoft, Inc. (2006). During the study period (2002 – 2005) there were 265 rainy days with total precipitation of 3,244.1 mm. Mean annual precipitation reached 973.2 mm. Surface runoff and soil losses were recorded in 63 rainy days. They were caused by 24 low-intensity precipitation events, 20 moderate intensity events, 9 high intensity events and 2 very high intensity precipitation events. No data on precipitation intensity for 8 days with runoff are available because there was no ombrograph in the plot at the very beginning of monitoring. Surf ace runoff was caused by precipitation between 8.2 mm (high intensity) and 133.7 mm (very high intensity). Precipitation less than 10 mm caused surface runoff in only two cases. In almost 50 % of the cases the quantity of surface runoff was equal to or less than 0.1 mm/m2. Total surface runoff in this plot for the study period (2002–2005) was 14.6 mm/m2; 4.71mm/m2in 2002, 2.08 mm/m2in 2003, 4.40 mm/m2in 2004 and 3.41 mm/m2in 2005). Monthly values of surface runoff ranged from zero to maximal 3.03 mm/m2in September 2002. The mean surface runoff coefficient was 0.0042. It is evident that the surface runoff percentage in the burnt area on skeletal colluvial soil was low and that interception, evaporation and infiltration of water in the soil reached as much as 99.6 %. The total erosion sediment quantity in the study area in the period 2002 to 2005 was 11.22 g/m2; (10.83 g/m2in 2002, 0.016 g/m2in 2003, 0.123 g/m2in 2004, and 0.246g/m2in 2005). Monthly soil loss values ranged from zero in summer months to maximal 9.50 g/m2in September 2002. The surface runoff coefficient ranged from 0.0020 to maximal 0.1165 in erodible days. The highest surface runoff and soil loss was concentrated in autumn and winter months, which coincides with the seasonal distribution of precipitation in the study area. The burnt area is covered with natural grass vegetation. In contrast, Aleppo pine regenerates poorly and occurs only in traces. It is the grass cover, occurring as early as the first year after the fire, and the features of the soil in which the plot was set up that are responsible for such low surface runoff and soil loss values. The grass cover protects the soil from raindrops hitting the ground, while its dense roots guard it against erosion. Such low soil losses can also be attributed to the geological and pedological characteristics of the plot, which was set up in distinctly skeletal colluvial soil that is exceptionally water permeable and acts as a “sieve”, thus making the losses negligible or almost non-existent. Surface runoff and soil erosion in such soil and vegetation are not distinct