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is the most significant economic conifer species in the Republic of Croatia, accounting for a share of 9.4% of the growing stock of Croatian forests (Meštrović 2001). Silver fir is the most damaged conifer tree species in Croatia, with 64% trees exhibiting loss of over 25% needles (Potočić et al. 2017). Dieback of silver fir in Croatia, especially in the Dinarides area, is a result of the complex and variable activity of abiotic and biotic factors. Dieback can be individual, in smaller groups, or may occur in complete stands over a forest area (Anić et al. 2002).
Forest gaps can be formed naturally by wind breakage, ice breakage, dieback due to interspecies competition or due to abiotic or biotic factors. Gaps can also be created artificially, by human activities such as silvicultural practices (Muscolo et al. 2007, Albanesi et al. 2008).
By Croatian law (Anon 2006), the cutting of dead and damaged trees is mandatory and is carried out each year from July to September. By salvage cutting dead or damaged trees, larger or smaller forest gaps are formed in the stand canopy density. Gaps in the stand, which are the result of disappearance of trees, create conditions different from those in the neighbouring canopy-covered stand (Hubbell and Foster 1986, Muscolo et al. 2007), and this affects regeneration (Brown 1993). The removal of trees result in a change of the microclimate (Aussenac 2000, Ritter et al. 2005, Vilhar et al. 2005, Muscolo et al. 2007, Albanesi et al. 2008). Microclimate conditions have an important role for the processes in the forest ecosystem. It is important for development of plants, seed germination, growth and reproduction, for fauna and for the implementation of the main ecosystem processes, such as photosynthesis and the nutrient cycle (Gray et al. 2002, Ritter 2005, Arx et al. 2012).
The microclimate of forest gaps, formed naturally or as a result of silvicultural practices, can strongly affect forest regeneration and the diversity of habitat conditions compared to those within the forest stand (Albanesi et al. 2008, Latif et al. 2010). Temporal and spatial variations of microclimate conditions in and around forest gaps have rarely been investigated in managed or natural fir forests with varying climates and species compositions (Vilhar et al. 2005, Muscolo et al. 2007, Rozenberger at al. 2007, Albanesi et al. 2008, Muscolo et al. 2010). To date, no such investigations have been carried out in managed fir forests in Croatia, while Rozenberger et al. (2007) carried out an investigation in Croatian beech-fir old growth forests. Considering the significance of fir forests for the Croatian forestry, there is a need to better understand the causes of threat to these ecosystems, and to determine measures for their rehabilitation. The high sensitivity of fir and beech-fir forests, and the occurrence of tree dieback and decline of entire stands, was the motive for conducting this study in the Gorski Kotar region of Croatia. The objective of this study was to describe microclimate conditions in and around a large and small forest gap in beech-fir forests, to compare the microclimate conditions of large and small forest gaps and determine the status of natural regeneration and determine the relationship of microclimatic elements in terms of the number and species of plants in natural regeneration.
MATERIALS AND METHODS
MATERIJAL I METODE
Study area – Područje istraživanja
The study was carried out in the mountainous Gorski Kotar region of Croatia, on the area of the Management Unit Kobiljak - Bitoraj in Forest Office Fužine, which is dominated by Dinaric beech-fir forest stands. According to the Köppen classification, Gorski Kotar is classified as having a Cfsbx climate type. This is a moderately warm rainy climate, without a dry season. The average annual air temperature for the study area is 7.2°C, with an average precipitation of approximately 2000 mm (Seletković 2001). The parent material is comprised of limestones, dolomites and sandstones of varying age, and the most common soils are calcomelanosol, brown and epimerized soil, rendzina, district brown soil and podzolic soil. The relief of the study area is highly irregular and diversified. It is interrupted by convex slopes, ditches, dales and rocks (Pernar 2001). The study investigated stands of decreased vitality and stability. Due to dieback of silver fir trees, the percentage share of common beech trees increased in the growing stock of these forests. The average dieback intensity in the research area for the period 1995–2007 was 18.3 m3 ha-1, with a range from 0.2 and 113.0 m3 ha-1 (Ugarković et al. 2011).
Forest gaps and control plots – Šumski otvori i kontrolne plohe
Three large gaps and three small gaps in the beech and fir association (Omphalodo – Fagetum Marinček et al. 1993), and three 50 x 50 m control plots in the Kobiljak-Bitoraj Management Unit (Fužine Forest Office) were investigated. A forest gap is a ´hole´ in the forest through all levels down to an average height of 2 m above ground level (Runkle 1982). These forest gaps were created by the dieback of silver fir (Abies alba Mill.) trees following salvage cutting three years previously, and had an elliptical shape. Forest gaps were three years old. The large gap was surrounded by 59% fir trees and 41% beech trees. The percentage share of trees surrounding the small gap was 54% fir, 38% beech and 1% spruce.
The control plots were located in the forest stand in the vicinity of forest gaps, in the same habitat conditions (soil, geological substrate or parent material relief). The gap area was defined by the vertical projection of the crowns of