The forest plays a significant role in providing ecosystem services that benefit society, including biodiversity, wood production, recreation, hydrology, and climate regulation [1,2]. The relevance of anthropogenic impacts in the context of climate change is becoming an increasingly pressing issue for both society and science [3,4,5,6]. In light of climate goals and the rising demand for renewable materials, the utilization of natural resources is expected to increase, particularly to meet the growing need for timber and the expansion of forested areas. Forest vegetation represents a vital natural resource in Latvia, covering 53% of the country’s territory. This extensive forest area highlights the importance of researching forest vegetation dynamics and developing well-considered forest management plans [7,8,9].

Changes in environmental conditions are influenced not only by annual mean temperature and precipitation but also by seasonal variations and extremes, which significantly affect vegetation phenology and plant productivity. For example, fluctuations in winter and spring temperatures have been linked to vegetation development [10,11,12,13]. During drought years, the growth of boreal forests has been primarily hindered by temperature [14]. As the climate continues to change, understanding past climate and vegetation data—including trends, drivers, and consequences—serves as a crucial resource for assessing potential future forest composition [15,16]. Some studies suggest that to establish a robust next generation of trees, it is crucial that the species planted today are climatically suitable for the entirety of the twenty-first century [17]. Species such as beech, oak, and ash are generally more resilient to changing conditions, while others, including larch, silver birch, and Norway spruce, exhibit a higher vulnerability to shifting climates [18].

Given the importance of forest ecosystem services, it is essential to understand how climate change may impact forests in Latvia by the end of this century, when average temperatures are projected to be at least 2–3.5 °C warmer than present. Geological evidence indicates that the Latvian territory experienced similar climatic conditions during the Middle Holocene, characterized by air temperatures that were 2–3.5 °C higher than those today [19]. The features of historical climate and environmental variability are preserved in the natural record, which varies in data resolution and the temporal extent of the information it conveys [20].

The aim of this study is to reconstruct past vegetation changes in a sandy coastal region of Central Latvia to understand how climate change during the Holocene has influenced forest composition. Gaining long-term insights into these shifts is essential for assessing which tree species may become dominant and which are likely to decline in the future. Given that sandy soil areas are currently utilized primarily for forestry rather than agricultural purposes, it is anticipated that similar land management practices will continue to be relevant in the future. The study site was selected to determine whether the climate serves as the primary factor influencing forest dynamics, or whether other potential agents also play significant roles. This study specifically aims to consider local soil and geological factors, which have often been overlooked in research that focuses exclusively on the role of the climate in shaping forest composition in the pan-Baltic region [21,22,23,24]. We hypothesize that climate change did not significantly alter the long-term composition of dominant tree species, such as pine, birch, and alder, within the forest during the Holocene. Furthermore, we propose that soil and geological factors played a more critical role in maintaining the long-term stability of these species than the climate did.

To characterize vegetation during the Holocene, pollen analysis is used in this study, allowing the reconstruction of past vegetation relationships that allow for the identification of dominant trees in a given area. Pollen is one of the most abundant microfossils preserved in sediment archives, such as lakes, whose sedimentary assemblages are related to regional and local vegetation. Pollen analysis is the primary method for determining past vegetation responses to climate change and human impact [25,26,27]. At the same time, we are aware that pollen can travel from a wider area; therefore, our reconstructions describe more regional vegetation than that strictly local. Lake sediments accumulate relatively slowly but continuously (there may be breaks in sediment accumulation in peatlands and soils), preserving information from a specific time (younger sediments accumulate on top of older sediments), which allows for reconstructing the evolution of terrestrial vegetation and environmental changes over time. In this study, we selected Lake Lilaste as the main study site characterizing vegetation dynamics during the Holocene. Lake sediments were chosen as a primary data source due to their relatively stable depositional environment, which has been less affected by, for example, wave-induced water or wind erosion [20]. Lake Lilaste is large, and such sites tend to reflect the regional relative forest cover [28,29].

Lake Lilaste (183.6 ha) is located in the Coastal Lowland (Figure 1) at an altitude of 0.5 m a.s.l., with a catchment area of 140 km². The average water depth is 2 m, but the max depth is 3.2 m. It is a flow-through lake with river Melnupe entering the lake on the eastern side, connected with Lake Dunezers in the south, and it has an outlet to the Baltic Sea (Riga Bay) on the northwestern part. The average mean temperature is +8 °C, the summer mean air temperature is +18.5 °C, and the winter mean air temperature is −2.5 °C. The average amount of annual precipitation is 650 mm. Rather poor soils comprise the area within 4 km, including Pv (Luvisols/Alisols), 83% of all soils; Tp (Hemic histosols), 11%; Pg (Dystric Arenosols/Alisols/Dystric cambisols, retisols, regosols), 3%; and Vg (Gleysols), 3%. Vegetation and landcover within 4 km comprise coniferous forest, 54% of all areas; sparsely vegetated areas, 11%; transitional woodland-shrub, 8%; mixed forest, 7%; broad-leaved forest, 3%, beaches, dunes, and sands, 2%; inland marshes, 3%; and peat bogs (wetland), 2% [29]. The total tree stock within 4 km is 945,037 m³. The area around Lake Lilaste is surrounded by Holocene eolian sediments—sand—influenced by the location of the lake in the area of the seaside plain, where the southeastward flow of wind drift occurs and a zone of eolian sediment accumulation is formed. The direction of the wind-driven debris flow is indicated by the dunes and foredunes. The relief consists of undulating plains and undulating dune hills, ranging in height from 10 to 20 m. Quaternary sediments within 4 km comprise lgQ3ltv (glaciolacustrine sand and silt with sand), 61.6%; vQ4 (eolian sand), 17.7%; mQ4ltv (marine sand), 8.4%; aQ4 (alluvial sand), 7.7%; and bQ4 (peat), 4.5%. Lake Lilaste is located in a coastal area and its development has been influenced by the stages of the Baltic Sea [30,31].

Our findings suggest that, with the exception of spruce, the composition of dominant tree species in central Latvia has remained relatively stable throughout the Holocene. In contemporary Latvian forests, the most prevalent tree species include Scots pine (Pinus sylvestris), comprising 33%; Norway spruce (Picea abies) at 19%; birch (Betula pubescens and pendula) at 30%; and black and gray alder (Alnus glutinosa, incana) making up 10%. Although Picea has been present in Latvia since the Late Glacial [21,42,43], it reached its highest relative abundance during the Middle and Late Holocene. Picea is one of the dominant tree species in Eurasia and it has expanded across Northern Europe in a somewhat asynchronous manner [44]. The spread of spruce in Central Latvia may have been influenced by interspecific competition with other tree species. It is likely that Picea competed with Corylus and Tilia, as indicated by the mirrored patterns of increasing and decreasing percentages between these species (Figure 3). According to Seppä et al. [44], the decline in Tilia populations during the Middle Holocene was not primarily driven by climate, but rather by competitive replacement due to overlapping ecological niches with Picea.

Vegetation changes in the study area were observed (Figure 6), but they were less significant than expected. During the Middle Holocene, broadleaved trees exhibited limited maximum distribution ranges and represented only a minor component of the overall vegetation (Figure 3). This period corresponds with warm and dry climatic conditions, as well as the peak expansion of broadleaved trees in the Baltic region [19]. In contrast, other studies examining long-term vegetation changes across different regions in Latvia report more substantial variations in pollen ratios within sediment records than our findings. For instance, the study on Lake Kūži [45] shows a rapid and sustained increase in hazel, alder, lime, and elm populations during the Middle Holocene. Tilia, specifically, has been present in Latvia since 9500 cal BP [43,45]. Although other studies suggest an earlier and more pronounced shift in broadleaved tree populations during the Middle Holocene, our findings indicate only minor variability in broadleaf representation (Figure 3, Figure 6 and Figure 7). A key difference between our study and others is the predominant Quaternary sediments and soil types in the study sites. Previous studies appear to have concentrated on sites situated in regions with till, clay, and silt sediments [19,44,45,46], where the soil and geological conditions are conductive to the growth of broadleaved trees. In contrast, our study site is located in a sandy region characterized by nutrient-poor soils, which may limit the growth and competitiveness of broadleaved trees compared to conifers. Sandy, nutrient-poor (xeric) mineral soils play a crucial role in sustaining specific forest types and associated ecological processes [47,48]. For instance, Tweiten et al. [48] investigated the influence of soil, climate, and fire in the pine and oak forests of northern Wisconsin, demonstrating that climate change impacts vegetation differently depending on soil type.

The low abundance of broadleaved species at Lake Lilaste likely accounts for the absence of a distinct 8.2 ka cooling event signal, which has been previously identified in Latvia [19,44] and across Northern Europe [49]. Pollen studies from Northern Europe indicate that this cooling event may not be as apparent in records with lower ratios of broadleaved trees, potentially due to local geological and geographical factors [49,50,51].

Our findings indicate that tree biomass peaked during the Middle Holocene, aligning with a period of rising mean air temperatures (Figure 7). Supporting this, Matisons et al. [52] recently demonstrated that warmer climate conditions in the northeastern Baltic region enhanced the growth increment in Scots pine, bolstering its competitiveness and long-term sustainability in the area. A significant decline in tree biomass around Lake Lilaste was detected during the Late Holocene, coinciding with a period of reduced average temperatures (Figure 7). The findings demonstrate a clear correlation between dominant tree biomass and temperature, underscoring the influence of mean air temperature fluctuations on vegetation. This relationship is less apparent in the relative or percentage-based pollen data derived from Lake Lilaste sediments. Future climate projections indicate a potential increase in mean temperatures by at least 2 °C, suggesting that the region surrounding Lake Lilaste may experience a rapid rise in tree biomass, similar to conditions during the Middle Holocene. However, caution must be taken when interpreting biomass reconstructions. Modern pollen calibration to current tree biomass values in Latvia have shown relatively low correlations [29]. Therefore, further research is needed before drawing definitive conclusions about tree biomass trends around Lake Lilaste.

Land-use changes are most evident from the emergence of cultivated plant species during the Late Holocene. The anthropogenic influence around Lake Lilaste has been studied extensively, encompassing both vegetation changes and the presence of charcoal in sediment records. Evidence indicates that forest burning and deliberate human activities in the area date back to around CE 740. The increase in charcoal particles in the sediment layers aligns with a concurrent decline in pine populations [53,54]. Pollen analysis revealed cultivated plant species, such as rye and oat, appearing approximately 100 years after the earliest evidence of fire, suggesting a progressive increase in human impact over time. However, due to variations in the calibration of chronological data across different studies, this time lag may not be directly comparable. The findings indicate that the immediate surroundings of Lake Lilaste were not heavily utilized for agriculture, as evidenced by the relatively low representation of herbaceous, ruderal, and cultivated plant species in the local vegetation. Both afforestation and deforestation on sandy soils can have more pronounced effects than on forests growing on other soils, as they are relatively more sensitive to biodiversity variability [55,56]. This points to the importance of studying the sensitivity of sandy soils other than conventional farmland to environmental change, and of determining whether changes in climate and environmental conditions are relevant in the context of these areas.

It is important to acknowledge that modern vegetation patterns are unlikely to be directly attributed solely to historical climatic conditions, as they have been shaped by a range of influencing factors. These factors include the interactions between Lake Lilaste and various stages of Baltic Sea development, episodes of extreme coastal weather, and the gradual rise in agriculture accompanied by diverse land management practices [30,31,57]. Consequently, drawing direct parallels between past and present conditions may lead to inaccuracies. However, the current landscape and forest structure are the outcomes of millennia of interactions among past climate variations, natural disturbances, and human activities. Therefore, evaluating historical trends in forest composition and biomass provides crucial insights into long-term forest dynamics. Integrating this understanding with interdisciplinary studies and diverse methodological approaches can enhance predictions of future ecological changes.

While our study shows general vegetation changes, it does not fully support the assumption that the composition of dominant trees will shift with climate change. In contrast, Dyderski et al. [18] quantified the projected range changes and threat levels of 12 European forest tree species by 2061–2080, considering three climate change scenarios. Their findings suggest that pine, spruce, and birch will be significantly affected by a warming climate. However, in the area around Lake Lilaste, pine trees have dominated throughout the Holocene, with no notable deviations in the composition of the dominant taxa. This may be due to pine’s adaptation to grow on mineral-poor, sandy soils, whereas on richer soils, spruce and broadleaf species compete with pine [58]. The sandy soil along the Lake Lilaste shoreline provides ideal conditions for extensive pine populations, and their dominance is likely to persist under future warmer climatic conditions.