Carbon and nitrogen accumulation in common Alder forest (Alnus glutinosa Gaertn.) in plain of Drava river

Istraživanja su provedena u 95-godišnjoj visokoproduktivnoj sastojini crne johe (Alnus glutinosa Gaertn.) u Podravini. Cilj ovoga rada je istražiti glavna obilježja akumulacije i dinamike ugljika i dušika u sastojini crne johe, kao indikatora stabilnosti ekosustava i održivosti gospodarenja ovom sastojinom. Analiza dinamike i akumulacije ugljika i dušika tijekom godine obuhvatila je istraživanje tla, posebice šumske prostirke i A- horizonta, drva i kore crne johe te lista neposredno nakon odbacivanja. Masa šumske prostirke kreće se od 4,71 Mg ha–1 nakon odbacivanja lista, preko 3,36 Mg ha–1 u proljeće, do 0,51 Mg ha–1 u jesen prije ponovnog odbacivanja lista. Od jeseni do proljeća raste sadržaj ugljika i dušika u A- horizontu, a zatim ponovno opada. Akumulacija ugljika u nadzemnoj biomasi sastojine sredinom ljeta je 214,6 Mg ha–1, a u tlu je prosječno 143,5 Mg ha–1. S druge strane akumulacija dušika veća je u tlu – u nadzemnoj biomasi je 2 Mg ha–1, a u tlu dosiže čak i preko 14 Mg ha–1 (prosječno 13,8). Sveukupna akumulacija organskog ugljika u ekosustavu je 359,5 Mg ha–1, a dušika 15,8 Mg ha–1. Istraživanje je pokazalo da se radi o izuzetno vitalnoj sastojini, očito s izuzetno dobrim ekološkim uvjetima za rast crne johe. Njena se vitalnost i dugoročna stabilnost najbolje odražavaju u postojanom indeksu širine godova u posljednjih 80 godina, što potvrđuje i vrlo velika drvna zaliha, koja za sastojinu starosti 93–98 god. iznosi 751 m3 ha–1 (iznad taksacijske granice od 7 cm).This research was conducted in a 95-year-old, highly productive stand of black alder growing in the Drava plain in Croatia. The goal was to investigate the main features of carbon-nitrogen accumulation and dynamics in the stand of black alder (Alnus glutinosa (L.) Gaertn.), as an indicator of ecosystem stability and sustainability of managing such stands. Soil physiography was investigated in a permanent sample plot sized 1 ha with growing stock of 751 m3 ha–1. In the course of one season of leaf litter decomposition, the forest floor and soil from the A horizon were sampled on three occasions; the first sampling took place after leaf drop in 2009, the second at the beginning of vegetation (April 2010), and the third before leaf drop (beginning of November 2010). The forest floor mass was determined and so was the carbon and nitrogen content in the forest floor, in the bark, in the physiologically mature leaf and in the soil, as well as the soil pH value. Dendrochronological analysis was performed on black alder wood specimens to determine wood density and carbon and nitrogen content. Data from piezometers set up within the plot for the period 1997–2000 were used to interpret the water regime. According to the research, the soil is Haplic Gleysol (Endoarenic), with a relatively shallow A horizon, of sandy-clayey to loamy-sandy texture, of weakly acidic and in the deeper part weakly alkaline reaction. The water regime is characterized by a shallow and stable groundwater level which occasionally causes shallow (up to 20–30 cm) flooding in the out-of-vegetation period, and does not drop below 130 cm in the vegetation period. The forest floor mass ranges from 4.71 Mg ha–1 after leaf drop, over 3.36 Mg ha–1 in the spring, to 0.51 Mg ha–1 in the autumn before renewed leaf drop, which indicates complete decomposition of black alder leaf litter in a yearly cycle. During this period the C:N ratio decreases from 19 to 14.8. Carbon and nitrogen content in the A horizon increases from autumn to spring, but drops again afterwards. Simultaneously, the pH value significantly drops from autumn to spring, but rises again by autumn. In mid-summer, carbon accumulation in the above-ground biomass of the stand is 214.6 Mg ha–1, while in the soil it is 143.5 Mg ha–1 on average. On the other hand, nitrogen accumulation is higher in the soil: while it is 2 Mg ha–1 in the above-ground biomass, it reaches over 14 Mg ha–1 (13.8 on average) in the soil. Overall accumulation of organic carbon in the ecosystem is 359.5 Mg ha–1, and of nitrogen it is 15.8 Mg ha–1. The research has confirmed that this is an exceptionally vital stand, evidently with excellent ecological conditions for the growth of black alder. The vitality and long-term stability of this stand is best reflected in the stable ring width index in the past 80 years, as confirmed by the very large growing stock amounting to 751 m3 ha–1 for a stand aged 93–98 (above the taxation limit of 7 cm)