In forest trees, reproductive dynamics and success are intricately linked to seed or fruit production, which varies significantly across demographic stages and depends heavily on adequate environmental conditions (Younginger et al. 2017; Moran and Clark 2012). Fragmentation and climate change profoundly impact seed quality and dispersal, critical prerequisites for species survival (McConkey et al. 2012). Empirical research underscores the complex relationship between seed size and germination time. Studies have shown positive correlations between seed size and germination percentage and energy (Parker et al. 2006; Jones and Reekie 2006; Upadhaya et al. 2007; Souza and Fagundes 2014; Kaliniewicz et al. 2018), negative correlations (Simons and Johnston 2000; Gomez 2004; Tiscar and Lucas 2010; Hojjat 2011), and instances where no correlation exists (Larson 1963; Vaughton and Ramsey 1998). Serbian spruce populations with smaller seeds, such as those in Tesla and Panjak with over 600.000 seeds per kilogram, exhibit poor germination rates, posing a threat to their already endangered status. This underscores the positive correlation observed between seed size and germination efficiency in Serbian spruce. Larger seeds likely store more nutrients, facilitating rapid germination and early seedling growth (Domic et al. 2020). In conclusion, understanding these relationships is crucial for conservation efforts aimed at preserving Serbian spruce populations. Enhancing seed quality and ensuring favorable germination conditions are essential strategies in mitigating the impact of environmental stressors on this endangered species.
When comparing Serbian spruce seeds to those of other species, it becomes evident that while seed shape may be similar across species, significant differences exist in terms of absolute mass and moisture content. These variations are influenced by genetic factors of the parent tree and environmental conditions during seed development (Dech and Maun, 2006; Li and Li, 2016). Wyse and Hulme (2021) have documented a strong relationship between seed mass and dispersal potential in various wind-dispersed Pinus species. Their findings indicate that within species, there are generally no significant differences in seed mass, which may reduce selective pressure. In environments where resources are limited, the allocation of resources towards seed mass may prioritize seedling vigor and survival over dispersal efficiency.
Pintarić's studies (1956; 1957; 1969) in the mid-20th century provided foundational insights into the germination behavior of Serbian spruce seeds. His findings indicated exceptionally high germination rates under optimal conditions. Seeds aged one year exhibited germination rates as high as 97–98%, with germination energy reaching 94% after 7 days and nearly 100% after 10 days of testing. Moreover, seeds stored at room temperature in glass bottles maintained high germination rates over extended periods (Pintarić 1970). Even seeds up to 3 years old maintained around 96% germination, demonstrating robust seed viability under controlled storage conditions. However, germination rates gradually declined with increasing seed age, with 86.9% germination observed at 6.5 years. Thus, Pintarić concluded that proper seed storage can preserve the germination capacity of Serbian spruce seeds over extended periods. However, the origins of the seeds used in Pintarić's studies were not specified, which may impact the generalizability of his findings. This gap highlights the importance of understanding seed provenance in seed germination studies to ensure the relevance of the results to specific populations or genetic lineages of the species.
The studies by Tucović and Isajev (1985) indeed highlight the significant genetic variability found in Serbian spruce. They explored spontaneous and induced variability in Serbian spruce seedlings from 12 test trees in a 23-year-old planted forest. Their research emphasized significant genetic variability among the analyzed traits of maternal trees or half-sib lines. The findings underscored that genetic variability was far greater than what could be inferred from mere observations of morphological variability in parent trees. Isajev (1987) analyzed the individual variability of several trees from three planted forests of Serbian spruce in Serbia. Morphometric analysis of 1.000 cones harvested in 1981 and 1983 showed the existence of spontaneous individual variability between trees for length, width, and number of grains per cone, while the shape of the cone was significantly less variable. The range of variability for the length of cones was, on average, from 20 to 70 mm, for widths from 10 to 23 mm, while the mean number of grains varied from 20 to 113 per cone. The size of the cone in the same tree varied within 5% between different harvest years, which indicates strong genetic control of this trait (Isajev 1987). The obtained results showed that the Serbian spruce seeds from all three planted forests are characterized by very good quality (average germination rate was 90%).
In conclusion, while historical studies by Pintarić and subsequent work by Tucović and Isajev have contributed valuable insights into the germination and genetic variability of Serbian spruce, there remains a need for more comprehensive studies that consider the genetic diversity and specific ecological contexts of this endangered species. Understanding these factors is crucial for effective conservation and management strategies, particularly in ensuring the successful propagation and survival of Serbian spruce in natural and managed environments.
Cvjetković et al. (2013) tested the germination of seeds collected from 17 trees, specifically: 5 trees from the planted forest, 7 trees from the natural population Veliki Stolac, and 5 trees from the natural population Gostilja. Fresh seeds originating from the planted forest population showed the highest germination rate, averaging 84%. Seeds from the natural populations, germinated on average at 58,57% and 65,15%, respectively, with an average seed dormancy of 8,68 and 9,72 days (retrospective). Seeds stored in a refrigerator at ± 4°C for 6 months showed better germination results by approximately 20% compared to fresh seeds and 28% compared to fungicide (captan)-treated seeds. The higher vigor and faster germination in seeds stored for 6 months can be interpreted as a form of stratification (Cvjetković 2011). These studies indicate significant differences in the germination of seeds depending on whether they were collected in natural or planted forests.
Ostojić and Dinić (2009) investigated germination under natural conditions. A series of experiments involving the sowing of Serbian spruce seeds in different habitats revealed poor seed germination and subsequent seedling failure in the first year (Dinić 1988, 1989). The authors attribute these results to the decline in tree numbers at this location - from 40 trees recorded in the early 1950s, only four remain today, as confirmed by later comparative experiments examining seed germination and seedling growth (Dinić 1990). The results highlight the sensitivity of Serbian spruce seeds and seedlings to numerous abiotic and biotic environmental factors. Factors such as early spring and autumn frosts, dense forest canopy, thick layers of litter, dense herbaceous cover in clearings, and soil overheating during summer dry periods significantly impact seed germination (Ostojić and Dinić 2009).
Arguments explaining the observed failure in the natural regeneration of Serbian spruce are primarily attributed to frequent fires (Aleksić et al. 2022), limited availability of suitable habitats, and strong competition with other species (Tucic and Stojković 2001; Ostojić 2005). Recent studies also suggest possible dispersal limitations due to poor genetic connectivity, even at small spatial scales (Aleksić and Geburek 2010; Mataruga et al. 2020). However, genetic diversity monitoring between two generations of Serbian spruce did not reveal negative effects on the genetic distribution of the young generation, despite only 25% of adult trees from the same population contributing to its formation, and 66% of pollen arriving from unknown sources (Aleksić et al. 2022). These findings underscore the interdependence between the number of adult trees and seed quality, which forms the basis for the poor natural regeneration and survival of this species in critical habitats. The weak germination of seeds from endangered populations with a small number of adult trees further supports the existence of mechanisms to prevent inbreeding in this species (Mataruga and Milanović 2020).