4.1 Litterfall production
Litterfall production is controlled by a complex of factors related to vegetation (species composition, age, density, health condition), climatic conditions (mainly temperature and precipitation), site characteristics (soil fertility, nutrient bioavailability, water regime, contamination, elevation, slope exposition, etc.), and forest management practices (transformation of tree species composition, logging, care treatments, fertilization, etc.). The role of some of these factors as sources of litterfall variability on a global scale has been highlighted in the meta-analyses of Bray and Gorham (1964), Matthews (1997), Berg et al. (1999), Liu et al. (2004), and Shen et al. (2019). According to the latter review, the annual production of aboveground litterfall in worldwide forests ranges from around 0.15 to nearly 27 t ha− 1, with values of between 2 and 6 t ha− 1 being the most common. The annual litterfall production varied in the present study from 2,576 to 5,293 kg ha− 1, depending on the stand and year (Table 3). Thus, the values are in the range of those most typical of temperate forests. The relatively wide variability observed here should have been expected, considering at least two factors: 1) the varied age and density of the stands; and 2) the varied soil fertility (sandy and loamy soils). The role of stand characteristics was confirmed by statistical analysis. The annual litterfall production was a function of tree age (R2 = 0.497 for 2020 and 0.695 for 2021). This observation confirms the findings of Lebret et al. (2001); however, Berg et al. (1999) observed an opposite tendency. At the same time, the differences resulting from site fertility (sandy and loamy soils) were less clear, and correlations with stand density were weak (R2 = -0.377 for 2020 and − 0.445 for 2021). Generally, litterfall production in the studied stands was comparable to that reported in the literature for various birch species; however, data are scarce. Aussenac et al. (1972) estimated the annual litterfall production to be 2,200 kg ha− 1, whereas Tripathi et al. (2006) determined it to be 3,770 kg ha− 1. The silver birch litterfall consisted mainly of leaves (Fig. 2), which fell predominantly in the early autumn. This is a typical observation for temperate forests and has been reported by several authors (e.g., Parzych and Trojanowski 2009; Jonczak 2011; Staelens et al. 2011a; Jonczak et al. 2016). Binkley (1986) followed by Lebret et al. (2001), proved that the share of this fraction also depends on stand age, showing a negative correlation. This relationship was not confirmed in this study. The share of twigs in litterfall generally oscillates around 10–20% of its total mass (Bray and Gorham 1964; Jonczak 2011; Lebret et al. 2011; Jonczak et al. 2016), being strongly affected by the health condition of the stand and the weather, particularly wind (Staelens et al. 2011b). In this study, twigs constituted 5.7–13.3% of the total litterfall in 2020 and 9.8–20.8% in 2021. The lowest contribution to the litterfall was from flowers.
The studied litterfall components showed relatively low variability from year to year. Temporal variability in litterfall production is mainly driven by weather conditions (Pedersen and Bille-Hansen 1999; Jonczak and Czarnecki 2008), mast years (Kowalkowski and Jóźwiak 2003; Jonczak 2011), and insect infestations (Dziadowiec and Plichta 1985).
4.2 Litterfall chemistry and nutrient influx to the soil
Litterfall controls the biogeochemical cycling of elements in forest ecosystems (Nordén 1994; Augusto et al. 2002; Parzych and Trojanowski 2009; Staelens et al. 2011a), including their bioavailability, soil pools and forms, and profile distributions. However, its importance varies among elements and habitats, being the greatest for deficient nutrients (particularly N and P) and poor sites, such as those associated with inland dunes (Jasińska et al. 2020). The role of litterfall in the carbon cycle looks somewhat different when compared to the nutrients. First of all, the TOC content in litterfall shows low variability, constituting approximately 50% of the dry mass, regardless of plant species, organ, or season. The C constituents of litterfall cover a wide spectrum of organic structures with varied susceptibility/resistance to decomposition. Susceptible components (i.e., proteins, amino acids, monosaccharides, etc.) are a labile, easily degradable fraction of the SOM, playing an important ecological role as a source of energy for the soil microbiota (Campbell et al. 2022). Additionally, molecular characteristics strongly influence the microbial biomass, its community structure, and functions (Wu et al. 2018; Zhalnina et al. 2018). Decomposition-resistant cellulose, hemicellulose, and lignin play a key role in humus formation, an inherent process of pedogenesis (Fustec et al. 1989).
Although N generally occurs in litterfall in relatively high amounts, it is usually a deficient element in forest ecosystems––one of the major factors limiting biomass production in terrestrial biomes (Cánovas et al. 2018; Manu et al. 2022). The N content varies strongly among litterfall fractions, reflecting the effects of several factors. Generally, it occurs in its lowest amounts in strongly lignified materials, such as wood or seed coats, reaching its highest concentrations in seeds and flowers (Staelens et al. 2011a). The total and silver birch litterfall examined in this study were relatively abundant in N, reaching 12.93–17.15 g kg− 1 and 12.57–16.96 g kg− 1, respectively, in the two years of the study (Tables 4, 5). Among the birch litter fractions, the highest concentrations were typical in the flowers, followed by seeds, and then leaves (Table 6). The N contents in the silver birch litterfall were comparable to those reported by Nordén (1994) for oak, hornbeam, beech, and linden, and by Berg and Meentemeyer (2002) for white birch and silver fir. They were higher compared to beech, pine, and spruce litterfall components in a mixed stand in northern Poland (Jonczak 2011) and in Scots pine, as studied by Berg and Meentemeyer (2002), Ukonmaanaho et al. (2008), and Jasińska et al. (2020). Higher N concentrations have been reported by Berg and Meentemeyer (2002) in grey alder, by Dziadowiec et al. (2007) in black poplar, and by Jonczak et al. (2016) in black alder litterfall. The N contents in the silver birch litterfall in this study were considerably higher than reported by Kasurinen et al. (2006) for young trees of the same species growing under controlled conditions (7.53–9.31 g kg− 1). Saerens et al. (2011) reported 17.7–20.9 g kg− 1 N in silver birch leaf litterfall, depending on the year. In this study, it was 12.79–17.98 g kg− 1. It should be mentioned that the N contents in this study may have been partially affected by the post-arable character of the stands. The importance of land-use history as a factor influencing soil characteristics, including nutrient abundances, and forms has been confirmed by Pulleman et al. (2000) and Latty et al. (2004), among several other authors.
Along with N, P is considered to be limiting factor on biomass production (Elser et al. 2007). Pools and fluxes of this element in natural and semi-natural ecosystems are controlled both by the soil mineral phase (primary source) and organic matter (secondary source). Sohrt et al. (2017) reported that the entanglements between the biotic and abiotic processes that govern P cycling in temperate forests are still poorly understood, and that further and interdisciplinary studies to address this are highly recommended due to the trends observed in forests in some areas. The declining P concentrations in tree leaves in temperate forests over the last few decades (Duquesnay et al. 2000; Ilg et al. 2009) is one such trend. Fresh silver birch biomass is moderately abundant in P, with the highest contents being reported in leaves and the lowest in wood (Novák et al. 2017; Rustowska 2022). A major pool of the element is allocated in the aboveground biomass, in the branches and foliage (Ukonmaanaho et al. 2008; Gawęda et al. 2014; Novák et al. 2017). The total litterfall in the studied stands contained 1.11–1.82 g kg− 1, whereas the silver birch litterfall contained 1.05–1.68 g kg− 1 P (Tables 4, 5). Leaves were the richest source among the silver birch litterfall fractions, followed by flowers and seeds (Table 6). This is quite surprising because the highest P concentrations are usually typical of phospholipid-rich seeds (Jonczak 2011). The abundance of the studied silver birch litterfall fell into the average to high range of values reported in the literature. It was generally higher when compared to Scots pine stands in Central and Northern Europe (Ukonmaanaho et al. 2008; Jonczak 2011; Jasińska et al. 2020) and black alder stands in northern Poland (Jonczak et al. 2016), whereas it was comparable to Norway spruce and European beech stands in northern Poland (Jonczak 2011).
The soil pools of K, Ca, and Mg mainly occur in the mineral phases, and the role of SOM is lower in this area when compared to N and P. However, the contents of these elements in litterfall are generally high in relation to other elements, regardless of differences in vegetation species and litter fractions, indicating the considerable role of vegetation in the biogeochemical cycling of these elements. The K contents noted in this study in the silver birch litterfall (2.06–4.41 g kg− 1 in 2020 and 2.32–4.11 g kg− 1 in 2021) were higher when compared to conifers (Ukonmaanaho et al. 2008; Jonczak 2011), while being comparable or lower than in broadleaved stands (Norden 1994; Dziadowiec et al. 2008; Jonczak 2013b; Jonczak et al. 2016). Typically, the highest contents of the element were noted in leaves, whereas twigs and flowers contained the lowest amounts (Table 6). Similar trends were observed for Mg and Ca, their contents in the silver birch litterfall being average for broadleaved species and higher than conifers, based on the authors cited above.
The Fe content in plant biomass rarely exceeds 0.5 g kg− 1. This proportion does not reflect its much higher content in soils (usually several percent), hence the role of vegetation in its biogeochemical cycling seems to be small. On the other hand, its importance is highlighted by certain soil-forming processes (podsolization, brunification), the essence of which is the transformation of Fe compounds, including the weathering of aluminosilicates, the formation of oxides, their linkage with other soil components, and transport. These processes are considerably controlled by vegetation (Egli et al. 2008). The Fe content in the litterfall in the studied fell within the values reported in the literature (Norden et al. 1994; Ukonmaanaho et al. 2008; Jonczak 2011, 2013b; Jonczak et al. 2016), showing relatively wide variability between the stands and over time (Tables 4, 5).
Micronutrients, including Mn, Cu, and Zn, typically occur in litterfall in low concentrations. Among these elements, Mn had the highest content in the studied stands, reaching 812.1–1,916.4 mg kg− 1 in the total litterfall (Table 4) and 869.9–1,961.1 mg kg− 1 in the silver birch litterfall (Table 5), with the highest amounts occurring in leaves (Table 6). The Mn contents fell within the average values reported in the literature, being usually higher than in conifer stands (Ukonmaanaho et al. 2008), but lower than in the stands of common broadleaved species (Norden et al. 1994). The contents of Cu and Zn in the silver birch litterfall were generally much higher than noted by other authors for various tree species (Norden et al. 1994; Dziadowiec et al. 2008; Ukonmaanaho et al. 2008; Jonczak et al. 2016). Elevated concentrations of trace elements in the biomass can indicate environmental pollution, and silver birch has been used as a bioindicator in this field (e.g., Chiarantini et al. 2016; Birke et al. 2018). Considering the observed Cu levels (7.9–12.9 mg kg− 1), the environmental contamination was estimated to be low. In the case of Zn, elevated contents were expected due to the widespread opinion about silver birch being a hyperaccumulator of this element (Marguí et al. 2007; Dmuchowski et al. 2014).
The correlation coefficients between the elemental contents in the soil and total litterfall, silver birch litterfall, and silver birch leaf litterfall were calculated as a way of looking for sources of variability in the litterfall chemistry (Table 7), but no clear tendency was found based on this analysis. Although the correlations were high in some cases, they were not confirmed over the two subsequent years in the majority of cases. Also, the cluster analysis did not reveal a clear picture of any tendencies. While it did indicate that litterfall chemistry was a function of many factors, none of these had a predominant role.
Nutrient retranslocation from senescing leaves is considered to be a strategy for the effective management of deficient nutrients. The intensity of this process strongly varies across forests, reflecting nutrient demand (positive correlation) and their bioavailability (negative correlation). This study revealed that silver birch strongly retranslocates K, N, and P. The intensity of the P retranslocation was comparable to that observed by Dziadowiec et al. (2008) in poplar stands in central Poland, but lower than in Scots pine and Norway spruce stands in Finland (Ukonmaanaho et al. 2008). Calcium, Mg, Fe, Al, Mn, Cu, and Zn were usually accumulated in leaves, although the intensity of that process was not high in most cases. Only Fe was intensively accumulated.
The results of this study revealed that post-arable soils under birch stands are supplied with considerable amounts of nutrients. Typically, the annual flux of N amounts to a few dozen kilos per hectare, showing great variability among forests. In this study, it fluctuated between 33.6 and 90.7 kg ha− 1 (Table 9), mainly being a function of the stand and showing relatively low variability, year to year. However, the roles of tree age and site fertility were not made clear from this study. The recorded annual N fluxes were comparable to, or higher than, those reported by Nordén (1994) for oak (30.6–43.5 kg ha− 1), hornbeam (32.5–52.5 kg ha− 1), and beech (33.9–42.9 kg ha− 1) stands; Tripathi et al. (2006) for a Betula ermanii forest in northern Japan (47 kg ha− 1); Hansen et al. (2009) for Norway spruce (44 kg ha− 1), Sitka spruce (36 kg ha− 1), Douglas fir (35 kg ha− 1), beech (39 kg ha− 1), and oak (55 kg ha− 1) stands; and Astel et al. (2009) for Vaccinio uliginosi–Betuletum pubescentis (36.4–46.8 kg ha− 1) and Empetro nigri–Pinetum (19.2–22.1 kg ha− 1) forest ecosystems in northern Poland. The N fluxes in Stands S-40 and L-41 were even comparable to those of black alder stands located on fertile soils in a headwater ecosystem, where Jonczak et al. (2016) recorded values from 70.8 to 84.2 kg ha− 1.
Also, the annual fluxes of P, K, Ca, and Mg were usually greater than the typical values reported in the literature for common temperate tree species (Nordén 1994; Ukonmaanaho et al. 2008; Astel et al. 2009; Jonczak et al. 2016). The highest values of all elements were noted in the oldest stands, along with the highest litterfall production (Table 3). The annual fluxes of such elements as Fe, Cu, and Zn are usually below 1 kg ha− 1 (Nordén 1994; Ukonmaanaho et al. 2008; Jonczak et al. 2016; Jasińska et al. 2020). It was also confirmed that, in this study, the recorded values were usually higher than those reported by the authors cited above. Particularly higher values were noted for Zn, highlighting once again the great ability of silver birch to accumulate this element. Although this study confirmed the great importance of litterfall as a source of soil C and nutrients in silver birch stands growing on post-arable soils, its importance can be still underestimated due to the leaching of nutrients from the litterfall accumulating in litterfall traps, as evidenced by the study by Ukonmaanaho and Starr (2001).