Spring Grass Burning : an Alleged Driver of Successful Oak Regeneration in Sub-Carpathian Marginal Woods
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PRAcE GEOGRAfIczNE, zeszyt 146 Instytut Geografii i Gospodarki Przestrzennej UJ Kraków 2016, 67 – 88 doi : 10.4467/20833113PG.16.018.5548 Spring graSS burning : an alleged driver of SucceSSful oak regeneration in Sub-carpathian marginal woodS. a caSe Study Jan Ziobro, Magdalena Koziarz, Serhiy Havrylyuk, Mykola Korol, Bernadetta Ortyl, Paweł Wolański, Andrzej Bobiec Abstract : Wooded pastures or pastured woods are disappearing from European landscapes. It is caused by the cessation of traditional farming ( in particular traditional pasturing ), lack of proper protection, forestry and agriculture intensification. Oak is one of the most common trees in such ecosystems, where it successfully regenerates, in particular due to conducive light conditions. According to the studies carried out in North America and Mediterranean zone in Europe, grass burning is one of important factors contributing to the establishment of open and semi-open habitats fostering oak regeneration. Our goal was to check the potential and progress of oak regeneration in marginal woods neighboring with grasslands in Ostoja Przemyska ( SE Poland ) and in Rozhniativ District ( W Ukraine ). In Poland the traditional silvopastoral management was ceased after the World War II ( finally in the 1970s ), while in Ukraine oak woods are still subject to occasional burning and pasturing. The inventory of oak regeneration, accompanied with the measurement of photosynthetically active radiation and the phytosociological assessment of plant communities, revealed a relatively abundant oak regeneration in the studied Ukrainian woods ( on average 4750 saplings ha-1 ), contrasting with the absence of young oaks in the Polish stands ( on average 30 saplings ha-1 ) . This coincided with the sharp difference in both light conditions and vegetation characteristics between the two studied landscape units. Occasional spring grass burning in Ukrainian woods is considered an important factor contributing to the oak regeneration success. Keywords : woodpastures, marginal woods, oak regeneration, grass burning 68 Prace GeoGraficzne, zeszyt 146 introduction Semi-opened pastures ( woodpastures, grasslands with silvopastoral woodlots ) used to dominate European landscapes over hundreds of years ( Rackham 1980 ; Vera 2000 ; Hansson 2001 ). Shaped by the traditional rural economy, involving grazing, hay-making, coppicing, etc., they were subject to the continuous migration of herds and people, which contributed to the exceptional floristic richness ( Bruun, fritzbøger 2002 ; Bonn 2005 ). Traditional management regime also contributes to vegetation structures fostering high biodiversity. When grazing and other forms of traditional extensive farming ( e.g. mowing or periodic burning ) are stopped, species richness is decreasing ( Bugalho et al. 2011 ; Hartel et al. 2013 ). Nowadays, many habitats of such high-conservation value rural landscapes have reached in Europe the extinction verge due to the change of land management systems and lack of adequate conservation schemes. For their disappearance Bergmeier et al. ( 2010 ) blame the legal systems responsible for nature protection, both at national and community levels, ignoring specific character and conservation needs of such habitats. With forestry and agriculture intensification, many wooded pastures and pastured woods have been actively transformed into full-stock timber stands or, abandoned, spontaneously developed dense undergrowth, making their phytoclimate totally unsuitable for the regeneration of light-demanding species. In the same time, the increasing competition for light, water and nutrients contributes to gradual disappearance of plant species valuable for livestock ( Vera 2000 ). Quercus robur and Q. petraea belong to the most common trees in wooded pasture lands in Central Europe. With such ecological adaptations as zoochoric dispersal, securing a high spatial independence of the location of parent trees ( e.g. Bossema 1979 ; Kollmann, Schill 1996 ; Olrik et al. 2012 ) and large starch-rich seeds enabling seedlings to grow despite heavy competition from grasses and herbs ( ziegenhagen, Kausch 1995 ; Welander, Ottosson 1998 ), oaks could be considered “wood pasture specialists ”. Therefore openness and dense graminoid-dominated sward are among the habitat characteristics favouring oak regeneration. While the former secures the high level of photosynthetically active radiation ( PAR ), a crucial environmental factor of oak seedlings successful growth ( ziegenhagen, Kausch 1995 ), the latter prevents invasion of light-seeded trees, the potential oak competitors ( Bobiec et al. 2011 ). Both characteristics are developed and sustained by traditional extensive farming, in particular free-range pasturing. However, relatively intensive grazing causes the risk of eating young oaks, which reduce reproductive success of individuals ( Bakker et al. 2004 ; Götmark et al. 2005 ). According to the North American studies, periodic fires connected with the traditional management have a positive effect on the regeneration of several oak species, including Q. alba classified as a burning-resistant species ( Dey 2002 ). sPrinG Grass burninG … 69 Because both Q. robur and Q. petraea belong to the same section of white oaks as Q. alba, a similar reaction of their regeneration to occasional grass burning can be expected. We found only one paper confirming that supposition. According to Proença et al. ( 2010 ), studying the fire resistance of trees in north-western Portugal, Q. robur is a successful post-fire re-sprouter. In Europe, studies of fire effect on oaks regeneration have been almost entirely constrained to the Mediterranean region and southern Quercus species, in particular Q. suber and Q. pyrenaica ( e.g. Calvo et al. 1999 ; catry et al. 2012 ). The goal of our study was to check the regeneration potential of oak woods occupying marginal parts of forested areas neighboring with grasslands, with respect to herbaceous layer ( presence of forest and non-forest species ), light conditions ( distribution of photosynthetically active radiation ), and the recent management history ( effect of abandonment and anthropogenic disturbances ). We compared four selected woods, two in the Ostoja Przemyska Natura 2000 site ( PLH 180012 ), SE Poland, and two in the Rozhniativ District in the Ukrainian Prykarpattya region ( fig. 1 ). We assessed the progress of oak regeneration both quantitatively and qualitatively with respect to the characteristics of entire plant communities, light conditions, and land use conspicuous traits. fig. 1. The location of the study sites ( PL1, PL2, UA1, UA2 ) 70 Prace GeoGraficzne, zeszyt 146 Study sites The study was conducted in four ca. 1 ha plots occupied by oak-dominated stands of natural origin ( natural regeneration ). The location of two woods is Ostoja Przemyska PLH 180012, SE Poland, in the close proximity of Kalwaria Pacławska ( PL1 ; 49°37’15” N ; 22°43’00” E ) and Kopysno ( PL2 ; 49°40’35” N ; 22°38’40” E ). The average annual temperature in the region is 7.4°c and annual precipitation amounts to 700 – 850 mm. The major landscape features are gentle hills ( up to 600 m a.s.l. ) with deeply incised ravines ( Janicki 1997 ). Oak woods occur on rich sites ( Eutric Cambisols ). The potential vegetation is Tilio-Carpinetum. According to local inhabitants ( unscheduled interviews ), until 1950s the area was intensively grazed by cows, as each household kept 3 – 4 cows on average. With most of fertile and accessible land cultivated for crops, more remote areas including woodlots or forest edges ( fringes ) were subject to grazing. The decline of pasturing and post-war acquisition of most of communal and private woods by the state forest holding, favouring the development of fully stocked beech-fir stands, have led to the fast loss of semi-open park-like oak woods. The two studied areas represent the best preserved, relatively large ( at least 1 ha ) oak stands of the 25 km2 landscape unit between Pacław and Rybotycze villages. PL1 ( Photo 1A; Fig. 2A ), located approximately 1 km from the village, until early 1970s was regularly visited by cattle grazing on the neighbouring grassland. After the sharp decline of pasturing the wood followed natural succession with only occasional felling of few selected oaks. With the median age of 98 years ( Bobiec, unpublished data ) oaks ( Quercus robur ) are the oldest trees in the stand, accompanied by less frequent firs ( Abies alba ), sycamores ( Acer pseudoplatanus ), wild cherries ( Cerasus avium ), and field maples ( Acer campestre ). By 2012 the plot was entirely filled with dense and high hazel ( Coryllus avellana ) understory, which in 2013 – 2014 has been experimentally removed from one third of the plot. In the intact part of the plot overlapping tree and shrub canopies cast deep shade on the ground where the scarce herb layer ( below 10 % of the cover ) consists of few shade-tolerant species. In the experimental part of the area, already one year after the intervention, the abundant ground vegetation has developed. PL2 ( Photo 1B; Fig. 2B ), bordered by the ancient local road and neighbouring with former open grasslands is located close to depopulated Kopysno village. The stand is dominated by Q. robur ( median age 148, Bobiec unpublished data ), accompanied by few firs. After selective cutting in early 1990s a very dense undergrowth of hazel, hornbeam ( Carpinus betulus ) and beech ( Fagus sylvatica ) has developed, making the development of herb layer almost impossible.