Growth rates and different bedrock condition
After 1950, notable increments in Norway spruce radial growth were observed in both limestone and granitic bedrock conditions. Generally, spruce growth increases with rising temperatures, particularly at higher elevations (Cienciala et al. 2018; Schurman et al. 2019; Pretzsch 2021; Bosela et al. 2021) and decreases especially at lower elevations due to drought stress or competition (Lévesque et al. 2013; Zang et al. 2014; Seidl et al. 2017). Also pollution reduction can lead to growth increases. (Hauck et al. 2012) for example observed a rapid growth increase in Norway spruce in the Harz Mountains in Germany since the 1990s, and they relate it to the dramatic reduction in SO2 concentrations.
However, the observed variations in Norway spruce growth rates between limestone and granitic bedrock conditions in our study over different time periods may be attributed to complex interactions between geological, ecological, and historical factors. Specifically, the recent 90-year period (1930–2020) in forest plots and the 65-year period (1955–2020) in treeline plots reveal a distinct pattern, with trees from limestone bedrock conditions exhibiting higher absolute growth rates. This may result from the beneficial effect of climate warming or the legacy effects of historical land-use practices gradually diminishing over time, with the inherent soil properties of limestone-derived soils becoming more prominent drivers of tree growth (Melo et al. 2013; Fritts 2014; Monger et al. 2015). Limestone soils typically possess better drainage, aeration, and nutrient availability, creating a more conducive environment for root development and nutrient uptake, thus promoting enhanced tree growth (Morford et al. 2011; Vestin et al. 2013; Hahm et al. 2014; Zhang et al. 2015; Rempe and Dietrich 2018; Jiang et al. 2020; Pastore et al. 2022). However, it's also important to note that specific tree species, their genetic makeup, local climate, age, competition, and other environmental factors can also influence tree growth, leading to exceptions or variations within these general patterns (Baker et al. 2003; Büntgen et al. 2007; Zielonka and Malcher 2009; Bowman et al. 2013; Filotas et al. 2014; Gourlet-Fleury et al. 2023).
On the other hand, the similarities in growth rates between the two geological settings in forest plots over the earlier 120 years (1810–1930) may be attributed to historical land-use practices. Research indicates that human activities, including mining, grazing, and wood and oil extraction, have significantly impacted the natural environment of the Tatra Mountains since medieval times (Kłapyta and Kołaczek 2009; Valsecchi et al. 2010; Rączkowska 2019; Piscová et al. 2023). Especially, grazing in the Belianske Tatras reached its peak in the 1800s but ceased by the mid-1900s (Zelina 1965; Bohuš 1994), likely had significant impacts on vegetation dynamics, potentially altering soil properties, nutrient cycling, and competition dynamics, leading to comparable growth rates between limestone and granitic settings during this time period (Frank et al. 2003; van der Wal et al. 2004; Mitchell 2005; Wei et al. 2011; Yu et al. 2019; Ramirez et al. 2018; Bernes et al. 2018; Solár and Solár 2020).
Growth rates and size classes
Detecting long-term tree growth trends by means of dendroecological analyses poses a number of challenges (Bowman et al. 2013; Peters et al. 2015; Duchesne et al. 2019; Trouillier et al. 2020). Trees which grow slow, tend to live longer (Bigler and Veblen 2009) and therefore are overrepresented in a sample (“slow-grower survivorship bias”). This then can lead to a “false-positive growth trend” of the chronology when combining older slow growing trees and younger fast growing trees, as young slow-growing trees are likely to be too small to be considered for sampling (“big tree-selection bias”) (Brienen et al. 2012). We therefore used size class categories and sampled small and large trees following (Nehrbass-Ahles et al. 2014; Jochner et al. 2017). In our study, growth rates of trees differ between different size classes, with bigger trees growing radially much more than smaller trees (Fig. 2b and c). This is not what could be expected based on the ‘slow-growth survivorship bias’ (Büntgen et al. 2005; Black et al. 2008; Bigler and Veblen 2009; Bowman et al. 2013; Alexander et al. 2018), which would lead to an overestimation of growth of younger trees, since the older fast growing trees are lost to a large degree from a contemporary sample. Narrow rings in smaller trees in comparison to larger trees in our study sites might be due to 1) smaller and presumably younger trees being more susceptible to environmental stresses like drought and temperature extremes (van Mantgem et al. 2009). 2) bigger trees have extensive root systems, are often dominant which allows them to access more resources like water, nutrients and sunlight (Pretzsch et al. 2014; Jochner et al. 2017). 3) Climatic or environmental conditions at earlier time periods were more conducive to tree growth setting the now larger trees on a different growth trajectory (Melo et al. 2013; Solár and Solár 2020). The comparison between the forest and higher elevation treeline plots showed that spruce trees from treeline are growing, as expected, slower than those from the forest, which was also found by (Körner 1998) indicating small scale variability towards the edge of the distribution of this species. In general, climatic limitations (Körner 1998) and limited access to resources in harsh treeline environments hinder tree growth (Ellison et al. 2019; Hagedorn et al. 2019).
Temporal stability of the growth-climate response
When comparing climate-growth relationship over two distinct intervals, 1960–1990 and 1990–2020, we observed a notable temporal instability (non-stationarity) in these relationships over time, particularly a stronger correlation of climatic indices with tree-ring width (TRW) and maximum latewood density (MXD) chronologies during the early period (Fig. 3, 4, 5, and 6), supported by moving windows correlation analysis (Fig. S4, S5, S6, S7, S8, and S8).
The positive correlation with spring and summer month temperatures weakened, while the sensitivity to precipitation and the drought index (SPEI06) was mainly lost in the second time period in both treeline and forest sites. MXD chronologies show the sensitivity of tree growth to moisture availability more pronounced during the second period. Previous studies on the temporal variability of climatic responses of trees from other European spruce sites have also shown similar non-stationarity in climate-growth responses over time (Hasenauer et al. 1999; Wilson and Elling 2004; Büntgen et al. 2006).
We also noted that the radial growth responses to October temperatures in the year prior to ring formation became insignificant over the second period (1990–2020) at both sites, possibly due to a warming climate where forest trees are no longer limited by the amount of resources stored at the end of the growing season (Treml et al. 2012; Ponocná et al. 2016). Several studies have examined the change in the relationship between tree growth and climate, yielding variable results. For instance, (Briffa et al. 1998a; 1998b; Solberg et al. 2002; Wilson and Elling 2004; Hauck et al. 2012; Bošela et al. 2014) have reported alterations in tree ring width and density responses to temperature, suggesting potential influences from non-climatic factors, such as human-induced local emissions like SO2 or a combination of SO2 and NOx. In contrast, reduction in tree growth sensitivity to climate over the last few decades could be climatic (Brázdil et al. 2002; Wilson and Elling 2004; Friedrichs et al. 2009; Hauck et al. 2012). Similarly, the choice of detrending method (Esper and Frank 2009), the effects of tree age and variations in microsite conditions (Zhang and Wilmking 2010; Altman et al. 2017) could also be the causes of potential non-significant tree growth correlations. Additionally, the non-stationarity in these correlations may be attributed to the natural ontogenetic dynamics of individual trees as they adapt to various disturbances, competition, and changing environmental conditions (Savva et al. 2002; Smith 2008; Carrer Marco 2011; Zhang et al. 2018).
Climate growth relationship and DBH size class effects
It is commonly believed that large trees are more sensitive to year-to-year climate variations and are more suitable for dendrochronological purposes (Carrer Marco and Urbinati 2004). Consequently, tree-ring studies typically prioritize trees with larger diameters to minimize the influence of competition and to simplify cross-dating, as there are fewer missing rings (Fritts 2014). In this study, we found that trees of all diameter classes generally responded to climatic variables (temperature, precipitation and SPEI06) in similar patterns in TRW and MXD at both geological settings; however, the strength of the response varied considerably among individual size classes (Fig. 3, 4, 5, and 6). (Chhin et al. 2008) found no differences in growth–climate relationships between the diameter classes in Picea abies, but (Meyer and Bräker 2001) suggested the difference to be more related to elevational differences (subalpine versus lower montane). The overall uniformity in the response to climate variables could be attributed to similarities in genetic composition and microclimatic conditions of trees reaching the co-dominant/dominant canopy layer, and potentially the sharing of carbohydrates among trees across various diameter size classes (Fraser et al. 2005; Chhin et al. 2008). Our findings do not fully align with other studies where climate sensitivity remained constant (Esper et al. 2008; Vieira et al. 2009, 2009; Mérian and Lebourgeois 2011), or either increased or decreased with age/size (Carrer Marco and Urbinati 2004; Luis et al. 2009; Zang et al. 2012; Konter et al. 2016; Trouillier et al. 2019; Campbell et al. 2021; Schmied et al. 2022). The variability in climate sensitivity observed across studies highlights the complexity of how tree species respond to climatic factors and the reasons for these divergent conclusions which remain somewhat unclear. Differences in the relationship between growth and climate across different diameter groups can largely be attributed to factors like resource availability, competitive interactions, and physiological processes within trees (Mencuccini et al. 2005). However, most of the previously reported studies are based on comparisons among trees of different ages, while eco-physiological research suggests that these signals are more closely related to tree size rather than age (Peñuelas 2005; Luis et al. 2009). Therefore, this overlap in size and age-related effects due to environmental factors potentially contributed to the divergent findings (Luis et al. 2009).
Bedrock condition and climate growth response
The overall pattern of monthly and seasonal climate growth responses of trees growing in two different bedrock conditions (limestone and granitic) were similar (Figs. 3, 4, 5 and 6), with only slight differences in strength (Fig. S10 and S11). Even though climate sensitivity of growth might not depend on the specific site but is mainly driven by climate conditions (Oberhuber et al. 1998; Boden et al. 2014), differences among geological settings in drought tolerance could be expected because moisture conditions vary within the species’ distribution area and water availability is often the limiting factor for growth (Mäkinen et al. 2002; Andreassen et al. 2006). Similarly, in a comparison between European beech on a shallow chalk soil in north eastern Germany and those on a site with deeper and more developed soils, it was observed that beech trees on the shallow chalk soil exhibited a slightly high level of sensitivity to drought (van der Maaten-Theunissen et al. 2016).