Introduction
Dead trees play an important role in the structure and function of forest ecosystems (Harmon et al. 1986). In many countries, intensive forest management has significantly decreased the amount of coarse woody debris (CWD). In boreal Finland, the average volume of dead trees is 60-90 [m.sup.3] x [ha.sup.-1] in unmanaged forests and 1-20 [m.sup.3] x [ha.sup.-1] in managed stands (Tomppo et al. 1999; Siitonen et al. 2000; Jalonen and Vanaa-Majamaa 2001; Rouvinen et al. 2002). The amount of CWD is, however, increasing because current forest management practices tend to leave more retention trees on harvested sites.
Few studies have focused on the nutrient dynamics of CWD because it decomposes slowly and is thus methodologically problematic (Laiho and Prescott 2004). The role of CWD in the nutrient fluxes of forests may generally be minor (Laiho and Prescott 2004). Woody debris has, e.g., a low nitrogen (N) content and a high carbon to nitrogen (C/ N) ratio compared with foliar and root litters (Berg and McClaugherty 2003). In boreal forests where N is generally the growth-limiting nutrient (Tamm 1991), CWD may play an important role in the N dynamics, however. Decaying stems may retain N for a long time and act as significant N pools (Lambert et al. 1980; Foster and Lang 1982; Arthur and Fahey 1990; Alban and Pastor 1993). Nitrogen retention in CWD may prevent N leaching after disturbances and hence be an important mechanism of nutrient retention (Zimmerman et al. 1995; Carlyle et al. 1998; Brais et al. 2006). In this context, the magnitude and timing of N release from CWD are the key issues.
Nitrogen content of CWD has often been observed to increase in the early stages of CWD decomposition and then to decrease slowly (Holub et al. 2001; Laiho and Prescott 2004). Such a pattern suggests that retention of N by CWD might indeed be an important mechanism for retaining site fertility and productivity. However, net release from early on has also been observed in several cases (Lambert et al. 1980; Krankina et al. 1999; Laiho and Prescott 1999; Holub et al. 2001; Creed et al. 2004; Brais et al. 2006). Accumulation of new N, manifesting an increase in N content (relative to the N content at the time of death), may result from translocation from the soil by fungal hyphae (Berg 1988; Boddy and Watkinson 1995), fixation of [N.sub.2] by nonsymbiotic microorganisms (Larsen et al. 1978; Granhall and Lindberg 1980), and immobilization of mineral N from rainwater (Downs et al. 1996). Generally, it seems that accumulation of new N and retention of N in CWD depend on its initial N concentration as well as the N status of the site (Alban and Pastor 1993; Laiho and Prescott 1999). The lower the initial N concentration of CWD, the longer and the more N is retained. The type of dead wood may further affect the N dynamics, since fallen logs are in greater contact with the soil, thus facilitating direct access, as well as better water and nutrient availability, for the wood-colonizing fungi in comparison with standing snags (see Yatskov et al. 2003).
The N dynamics of decomposing wood thus vary between geographical areas, site types, tree species, and type of CWD (fallen logs versus standing snags) (Laiho and Prescott 1999; Laiho and Prescott 2004). It is still poorly known how much N may be retained in decomposing stems of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.), and silver birch (Betula pendula Roth.), the most common tree species in northern European boreal forests, and when their N is eventually released. To our knowledge, only one study on the nutrient dynamics in the CWD of these species has been conducted (Krankina et al. 1999). Methodological difficulties related to the long time scale hamper comparisons between studies and generalizations of the patterns observed (Holub et al. 2001; Creed et al. 2004; Laiho and Prescott 2004). In a few studies, the initial volume or mass of the decaying stems has been measured, allowing for direct estimation of changes in N contents.
Our aim was to study the N dynamics in decomposing Scots pine, Norway spruce, and silver birch logs and snags. We hypothesized that the N content increases at the early stages of decomposition and thereafter is released slowly and that this pattern is clearer in logs than in snags. Because birch generally decomposes more rapidly (M5kinen et al. 2006), we also expected N to be released more rapidly from birch than from pine and spruce stems. We used data from the study of M5kinen et al. (2006) that were earlier used for developing decomposition models for these tree species.
Materials and methods
Study sites
The material was collected from long-term thinning experiments in southern and central Finland established and maintained by the Finnish Forest Research Institute (Makinen and Isomaki 2004x, 2004b; Makinen et al. 2006). The stands were even-aged pure or almost pure Scots pine, Norway spruce, and silver birch stands. The sites were Oxalis acetosella--Vaccinium myrtillus (OMT; fertile), Vaccinium myrtillus (MT; relatively fertile), and Vaccinium vitis-idaea (VT; relatively infertile) forest site types (Cajander 1949), and additionally, part of the birch stands were planted on former agricultural fields. The soils represented a wide range of the typical soil types found in Finland from clay to coarse till. The altitude of the sites varied from 38 to 250 m and the temperature sum from 825 to 1373 degree-days. At the time of the sampling, stand age ranged from 29 to 131 years. The experiments encompassed 36 pine, 13 spruce, and 9 birch stands (Table 1). Square or rectangular plots (1000-1600 [m.sup.2]) surrounded by a 5 m wide buffer zone were established in each stand and subjected to thinning treatments of varying intensity. Following establishment, the experiments were remeasured 2-12 times, depending on the age of the experiment. Tree species, vitality (live/dead), …
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