Scots Pine (Pinus sylvestris L.) establishment success under climate change: Effect of site, stock type and planting time

1 Age and diameter distributions of nonpyrogenic, virgin Scots pine (Pinus sylvestris) populations were studied at six different sites of the Empetrum-Vaccinium type along a latitudinal gradient (65?54'-68?15'N) in northern Sweden. 1910 trees, including saplings and seedlings, were aged at their root collars in order to reconstruct regeneration patterns. 2 All populations had multimodal age structures. Obvious regeneration pulses occurred in the 1770s-80s and 1950s-70s. The three southernmost stands exhibited an additional regeneration pulse in the 1870s-80s. A regeneration cycle of about 100 years in the south and 200 years in the north were thus clearly expressed. Correlation with temperature changes over the past 300 years showed that recruitment peaks of Scots pine lagged 20-30 years behind the warm climate periods and fell to zero in the intervening cold periods. 3 A much smaller number of experimentally planted pine and spruce seeds established and survived in vegetation dominated by Empetrum hermaphroditum than in that dominated by Cladina spp. After two growing seasons, fresh weights of pine seedlings were also much lower in E. hermaphroditum vegetation than in Cladina spp. Spruce established less frequently than pine in both vegetation types. Naturally established pine seedlings were found almost exclusively in vegetation dominated by Cladina spp. 4 The wave-like regeneration pattern of pine may therefore result from successful establishment in periods with good seed production on ground dominated by Vaccinium spp. and Cladina spp. This vegetation type is favoured by warm dry conditions but its establishment possibly lagged behind climate change. The extreme amplitude between regeneration failures and successes found over the period studied may be caused by the specific complex of vegetation responses to climate variability found in this forest type. 5 The results have implications for predicting the effect of climatic warming.

Mixed-species forests have become widely studied in the recent years because of their potential to mitigate risks associated with climate change. However, their growth dynamics are often difficult to predict because species interactions vary with climatic and edaphic conditions, stand structure and forest management. We examined species interactions in mixtures of Scots pine (Pinus sylvestris) and pedunculate oak (Quercus robur) under climate change and for varying soil conditions in the Netherlands, over a period of 30 years. We parameterized, calibrated and validated the 3-PGmix model for mixing effects in Scots pine and oak mixtures and analysed these effects under climate change. 3-PGmix performed well for the variety of forest stands examined throughout the Netherlands. Furthermore, it was also able to reproduce mixing effects for each species in mixtures compared to monocultures for the growing conditions examined. Simulated climate change resulted in lower productivity of oak and higher productivity of Scots pine, compared to the current climate. This was observed for both monospecific stands and mixtures. The mixture of Scots pine and oak showed clear but limited overyielding (mixture yield greater than the mean of the monocultures), which was mainly attributed to oak. This was maintained under the most extreme climate scenario for 2050, implying that for oak, increased growth due to mixing with Scots pine was larger than the reduction in productivity under the future climate. On resource-limited soils, Scots pine competitiveness was increased, and this was maintained under a warmer and drier climate. Our results suggest that projected changes in climate will influence species interactions and result in increased Scots pine productivity, notably on poor sandy soils, which are typical of the Netherlands.

Species of the tree genus Pine (Pinus L.) exist all over the world and no other group contains so many attractive forms (Curtis & Bausor, 1943) . Scots pine (Pinus sylvestris L.) is currently the most widely distributed pine and occurs throughout all of Eurasia. In the central alpine valleys, Scots pine is growing at the dry border of its distribution range, which involves overcoming periods with extreme low water availability. Although the species is known for its ability to grow on dry and nutrient poor soils, several extreme droughts during the last two decades have caused a 50% dieback of Scots pine in the dry valleys of the Central Alps in Switzerland. The ability of trees to survive drought is determined by their initial health and their resilience to drought, as well as on the characteristics of a drought event – i.e. timing, duration and intensity. The mechanisms underlying drought-induced mortality are still unclear, as well as the recovery process after soil rewetting. Furthermore, possible mitigation or aggravation of drought effects by elevated nutrient availability in the soil has not been studied before. The carbon (C) balance in trees is used as an indicator for C assimilation, growth, defense and storage processes. When trees are exposed to drought, to changes in soil nutrition or sudden defoliation, the C balance may change. In this thesis, the main objective was thus to combine effects of drought and fertilization to study the C and nitrogen (N) dynamics in Scots pine trees.
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\nIn the first chapter, I give an overview of the state-of-the-art in research on drought-affected C and N dynamics in trees. The aim of the second chapter was to assess the effects of long-term drought release on growth and non-structural carbohydrate (NSC) concentrations of adult P. sylvestris trees. A long-term (13 years) irrigation experiment was conducted in the Pfynwald, a Scots pine dominated forest located at the dry distribution margin of the species in southern Switzerland. I measured growth, NSC, N and phosphorus (P) concentrations, as well as the natural abundance of 13C isotopes on trees with different leaf area in control and irrigation plots. Irrigation resulted in higher growth rates and carbon isotope discrimination, but did not alter NSC levels. Growth and NSC decreased with lower leaf area in both control and irrigated trees, but NSC did not correlate with leaf-level gas exchange indices such as foliar δ13C, which is an indicator for water use efficiency, N or P, which are both stimulants of photosynthesis. Trees with initially low leaf area had limited ability to respond to the long-term irrigation, indicating a legacy effect of previously low crown condition. The NSC constancy across treatments suggests that carbohydrate storage may stay constant when changes in climate are slow enough to allow acclimation. Moreover, total leaf area, rather than leaf gas exchange per unit leaf area, drives variation in whole-tree carbohydrate dynamics in this system.
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\nThe main focus of the third chapter was the mitigation or aggravation of drought effects by nutrient availability in the soil. Three year-old P. sylvestris saplings were exposed to drought during two subsequent years, using four different water and two soil nutrient regimes, and drought was released thereafter. In addition, partial and full needle removal was performed in order to assess effects of changes in source:sink ratio. Biomass, leaf gas exchange and tissue NSC were measured during and after the first and second growing season. Extreme drought reduced stomatal conductance, photosynthesis, biomass and NSC, whereas intermediate drought only slightly affected biomass and NSC. Defoliation stimulated photosynthesis and fertilization increased growth and root biomass fraction, but mainly in the two intermediate drought levels. Only extreme drought pushed P. sylvestris trees to mortality. The third chapter concludes that tree mortality under severe drought periods will not be mitigated, but that the effects of low intensity drought stress could be compensated by increased nutrient availability and decreased source:sink ratio. 
\nThe aim of the fourth chapter was to assess the C and N allocation underlying the biomass changes that were found in chapter 3. I hypothesized that, during drought, increased soil nutrient availability stimulates root metabolism and carbon allocation to belowground tissues under drought stress. I therefore conducted a 15N and 13C labelling experiment in July and August 2016 respectively, on the saplings described above. 15N labelling was conducted with fertilized saplings from all water regimes, while 13C labelling was only conducted with saplings (both nutrient regimes) from two out of four water regimes (well-watered and mild drought). I assessed the abundance of 15N and 13C in the roots, stem and needles after the first growing season and during the second year. C uptake was slightly lower in drought stressed trees, and extreme drought inhibited largely the N uptake and transport. Carbon allocation to belowground tissues was decreased under drought, but not in combination with fertilization. The results indicate a potential positive feedback loop, where fertilization improved the metabolism and functioning of the roots, stimulating source activity and hence C allocation to belowground tissues. We can thus conclude that soil nutrients might play an important role in mitigating drought stress of trees.
\nOverall this thesis shows that the impairment of tree functioning and mortality can be explained with thresholds: long-term drought causes a reduction in tree vigor and leaf area, and if a threshold of approximately 60 – 70% loss of leaf area is reached, trees may follow a trajectory towards mortality, even if drought is released in the soil. In the controlled experiment, soil moisture thresholds were visualized. The impairment of C allocation belowground under mild drought, the reduction of NSC in and impairment of 15N uptake by the roots under extreme drought indicate that roots might be the first tissue to lose function and eventually die off during drought stress. Additional nutrient supply can sustain root functioning under drought, indicating that soil moisture tipping points are not fixed, but can be modified. In general, trees have a strongly coordinated supply – demand regulation for C and N, enabling homeostatic C balances as long as changes in climate are slow or mild enough for trees to acclimate. 
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In this study, we explore how Sphagnum mosses and Scots pine, Pinus sylvestris, interact on different spatial and temporal scales in a boreal bog ecosystem. We were particularly interested in relationships between the occurrence of Sphagnum‐dominated habitats and the occurrence of Scots pines of different age and size. Juvenile and adult pines occurred in different habitats. While juveniles mainly occurred in Sphagnum‐dominated habitats, predominantly with Sphagnum rubellum, adult pines were found in habitats dominated by lichens, or with a sparse vegetation cover. Examination of surface peat cores sampled close to adult pines revealed that almost all pines (97%) had established in a Sphagnum‐dominated environment and that the habitat had changed since pine establishment. Scots pine is thus capable of changing and exterminating the Sphagnum‐dominated environment preferred for germination and establishment. Pines impede Sphagnum growth and peat accumulation significantly once they have reached a stem diameter of approximately 20 mm. It takes from 30 to 90 yr for a pine to reach that size. Our results show the importance of interactions between Scots pine and Sphagnum mosses in bog ecosystems. We conclude that interactions between trees and Sphagnum mosses are important driving forces behind the vegetation change that has characterised boreal bogs during the Holocene.

Pine regeneration: plasticity and acclimation in a dryer climate Increasing summer drought might limit the natural regeneration of Scots pine stands at low elevations of the Rhone valley. Common garden experiments at the forest-steppe ecotone have shown that emergence and establishment of Scots pine primarily depend on spring precipitation and, to a minor degree, on summer drought and rising temperatures. Scots pine seedlings acclimated rapidly to drought periods by favouring root to shoot growth. In the second year, the saplings were already adapted to drought so that most of them survived an extended spring and summer drought, as recorded at Sion twelve times during the last 154 years. Only an extreme summer drought – no water from June to September – killed 14.7% of the Scots pine saplings. Surprisingly, they were even able to acclimate to such extreme drought events: after the same extreme summer drought in the third year, mortality dropped below 5%. In general, the Scots pine was very plastic, i.e. seedlings and saplings changed their phenotype depending on environmental conditions. But we also found genetic adaptation: Scots pine originating from regions with pronounced summer drought, including populations from lower elevations in the Rhone valley, produced more biomass than those from moister regions in all treatment combinations. Black pine reacted similarly to the treatments like Scots pine, but it grew faster and more saplings survived the first extreme summer drought. These results show that Scots pine from low elevations of the Rhone valley is one of the most drought-tolerant provenances in Europe. Thanks to its high phenotypic plasticity and the ability of seedlings and saplings to acclimate to drought on a short time scale, natural regeneration of Scot pine at low elevations of the Rhone valley is likely to occur also under future conditions, but maybe less frequent than today.

Tree browsing by mountain hares ( Lepus timidus) in young Scots pine ( Pinus sylvestris) and birch ( Betula pendula) woodland

This study examines the spatial root development patterns of bareroot, containerized, and plug plus (plug+) saplings in hemiboreal forests of Latvia, focusing on the effects of two common soil preparation methods: mounding and disc trenching. In northern Europe, forest regeneration after clearcutting often involves planting, with soil preparation aimed at enhancing sapling survival and productivity. This study included four tree species: Pinus sylvestris, Picea abies, Betula pendula, and Alnus glutinosa. The results reveal that saplings planted in mounded sites developed more radially symmetrical root systems, while roots in trenched sites predominantly grew parallel to the furrow. This spatial root distribution was consistent across all forest types and did not show significant variation between stock types (containerized, bareroot, or plug+) or treatments (control or fertilized). Additionally, the number of main roots did not differ significantly between the soil preparation methods. These findings align with previous research and raise important questions regarding the impact of early root architecture on stand resilience at a mature stage, particularly in relation to windthrow, heavy snowfall, drought, and flooding resistance. The study underscores the need to consider root system development as a key factor in forest management practices aimed at ensuring long-term forest stability.

Tree-ring response of jack pine and scots pine to budworm defoliation in central Canada

A process-based ecosystem model was used to assess the impacts of changing climate on net photosynthesis and total stem wood growth in relation to water availability in two unmanaged Norway spruce (Picea abies) dominant stands with a mixture of Scots pine (Pinus sylvestris) and birch (Betula sp.). The mixed stands were grown over a 100-year rotation (2000-99) in southern and northern Finland with initial species shares of 50, 25 and 25% for Norway spruce, Scots pine and birch, respectively. In addition, pure Norway spruce, Scots pine and birch stands were used as a comparison to identify whether species' response is different in mixed and pure stands. Soil type and moisture conditions (moderate drought) were expected to be the same at the beginning of the simulations irrespective of site location. Regardless of tree species, both annual net canopy photosynthesis (P(nc)) and total stem wood growth (V(s)) were, on average, lower on the southern site under the changing climate compared with the current climate (difference increasing toward the end of the rotation); the opposite was the case for the northern site. Regarding the stand water budget, evapotranspiration (E(T)) was higher under the changing climate regardless of site location. Transpiration and evaporation from the canopy affected water depletion the most. Norway spruce and birch accounted for most of the water depletion in mixed stands on both sites regardless of climatic condition. The annual soil water deficit (W(d)) was higher on the southern site under the changing climate. On the northern site, the situation was the opposite. According to our results, the growth of pure Norway spruce stands in southern Finland could be even lower than the growth of Norway spruce in mixed stands under the changing climate. The opposite was found for pure Scots pine and birch stands due to lower water depletion. This indicates that in the future the management should be properly adapted to climate change in order to sustain the productivity of mixed stands dominated by Norway spruce.

We investigated how climate change affects the diameter growth of boreal Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) H. Karst.), and silver birch (Betula pendula Roth) at varying temporal and spatial scales. We generated data with a gap-type ecosystem model for selected locations and sites throughout Finland. In simulations, we used the current climate and recent-generation (CMIP5) global climate model projections under three representative concentration pathways (RCPs) forcing scenarios for the period 2010–2099. Based on this data, we developed diameter growth response functions to identify the growth responses of forests under mild (RCP2.6), moderate (RCP4.5), and severe (RCP8.5) climate change at varying temporal and spatial scales. Climate change may increase growth primarily in the north, with a clearly larger effect on birch and Scots pine than Norway spruce. In the south, the growth of Norway spruce may decrease largely under moderate and severe climate change, in contrast to that of birch. The growth of Scots pine may also decrease slightly under severe climate change. The degree of differences between tree species and regions may increase along with the severity of climate change. Appropriate site-specific use of tree species may sustain forest productivity under climate change. Growth response functions, like we developed, provide novel means to take account of climate change in empirical growth and yield models, which as such include no climate change for forest calculations.

The present study aims to contribute to a fine regional differentiation of Scots pine (Pinus sylvestris) response to climate change at its altitudinal margins in subarctic Finland north of 69˚ N (Utsjoki) and to find out whether a prompt establishment of new pines in response to climate change can be expected above the old pine tree limit in and above the mountain birch zone. In 10 sampling areas, distribution, site characteristics, and condition (damage degree, growth forms) of the new pines (pines that have become established since the mid 20th century) were checked in a zone 50 m to the left and right of our field routes. The field routes extended from the scattered birch forest to the treeless alpine zone and mountaintops and covered a total area of more than 4 km2. In total, 213 new pines were found. Tree height was measured and age estimated by counting the whorls. The degree of damage was estimated and then attributed to four damage classes. Pine establishment was most successful during the 1970s and up until the end of the last century. Pines younger than 10 years are rare (< 3%) in the study areas, with one exception (about 8%). Pine recruitment is comparatively intense in close proximity to old pines in the birch forest while it is sporadic within the scattered birch stands at higher elevations and in the alpine tundra. More than 80% of the new pines show disturbed growth forms due to frequent winter injury, reindeer, and moose. About 66% exhibit severe damage, and 15% have already died. On windswept terrain, microsite facilitation is essential for pine establishment. Lack of local seed sources and severe site conditions at high elevations have probably delayed pine altitudinal advance. New pine generations may become effective seed sources speeding up pine advance beyond the present seed trees. In view of the high proportions of severely damaged and dead new pines, we do not expect that climatic warming will bring about a rapid advance of the pine tree limit.

The vitality of Scots pine (Pinus sylvestris L.) is declining since the 1990s in many European regions. This was mostly attributed to the occurrence of hotter droughts, other climatic changes and secondary biotic stressors. However, it is still not well understood which specific atmospheric trends and extremes caused the observed spatio-temporal dieback patterns. In the Swiss Rhône valley, we identified negative precipitation anomalies between midsummer and early autumn as the main driver of sudden vitality decline and dieback events. Whereas climate change from 1981 to 2018 did not lead to a reduced water input within this time of the year, the potential evapotranspiration strongly increased in spring and summer. This prolonged and intensified the period of low soil moisture between midsummer and autumn, making Scots pines critically dependent on substantial precipitation events which temporarily reduce the increased water stress. Thus, local climate characteristics (namely midsummer to early autumn precipitation minima) are decisive for the spatial occurrence of vitality decline events, as the lowest minima outline the most affected regions within the Swiss Rhône valley. Mortality events will most likely spread to larger areas and accelerate the decline of Scots pines at lower elevations, whereas higher altitudes may remain suitable Scots pine habitats. The results from our regional study are relevant on larger geographic scales because the same processes seem to play a key role in other European regions increasingly affected by Scots pine dieback events.

Changing climate in the Arctic is expected to have significant effects on the pattern and distribution of terrestrial vegetation. Species characteristic of specific zones in the mountains of northern Sweden have been shown to migrate up- and down-slope with changes in climate over the Holocene. This study evaluates the potential for Scots pine (Pinus sylvestris L.) to become a treeline dominant at Fennoscandian treelines, replacing mountain birch (Betula pubescens subsp. czerepanovii (Orlova) Hämet-Ahti). Data from paired mountain birch and Scots pine tree-ring chronologies for eight locations in northern Sweden are used to develop climate – tree ring width index (RWI) relationships. Modeled climate–RWI relationships are then used to predict the relative RWI values of the two species under a suite of climate-forcing scenarios using an ensemble of three global climate models. Results indicate that mountain birch and Scots pine RWI are both correlated with summer temperatures, but Scots pine is more likely than mountain birch to be influenced by moisture conditions. Predictions of RWI under future climate conditions indicate that mountain birch is unlikely to be replaced by Scots pine within the next century.

Increased tree establishment in Lithuanian peat bogs — Insights from field and remotely sensed approaches

The historic large-scale forest conversion in the northern German lowlands resulted in a man-made dominance of Scots pine, in a landscape that would naturally be dominated by forests of European beech. Since drawbacks of pure pine forests such as their susceptibility to calamities have become clear, re-conversion to mixed and broadleaf stands has been promoted. Consequently, the share of pine is progressively declining in German forests. Nevertheless, planting pine is still a popular option from a silvicultural perspective, due to its rapid growth especially at young age, its ability to grow on nutrient-poor and dry sites, and the high demand for its wood. In the face of accelerating climate change, the ability of forests to store and sequester carbon (C) has become a focus in science, politics and forestry. The aboveground biomass represents the largest biomass fraction in the forest and can be modulated directly through management. Fine roots represent only a few percent of the tree´s biomass, but due to their fast turnover as well as through root exudation to the surrounding soil, they are the main source for soil organic carbon.
\nThe presented study therefore compared the C pools and sequestration in the above- and belowground (fine root) biomass in naturally developing, mostly European beech forests (ND) and regularly thinned Scots pine forests (YP), respectively representing the dominant natural and the dominant current forest type of the northern German lowlands. Aboveground biomass C stocks were further determined in pine forests in transition to (mixed) broadleaf stands (OP). The study was conducted in a network of 48 forests at 16 sites, distributed throughout the northern German lowlands, covering a climate continentality gradient from west to east. Aboveground biomass calculations were based on stand structural data and species-specific allometric regressions (live trees, saplings) or volume calculations and species-specific wood density (deadwood). Aboveground net primary production (ANPP) was measured in three consecutive years using permanently attached dendrometer tapes for wood increment, and litter traps for litter production. Two repetitive fine root inventories were conducted, measuring fine root bio- and necromass in the organic layer and the top 20 cm of the mineral soil. Fine root productivity was determined with the ingrowth-core approach in 0–20 cm soil depth, including the organic layer.
\nAbove- and belowground biomass C stocks were significantly higher in beech than in pine forests. A linear mixed-effects model revealed that the tree species was the most important factor in explaining aboveground biomass C stocks. Variation in stand age, with a range of roughly 100 years for both species, was surprisingly not influential. ANPP was higher in beech than in pine forests as well, which was mostly a result of higher litter production, while wood production was similar in the two forest types. Fine root productivity was also higher in beech than in pine forests, but the difference was only significant in 10–20 cm depth. The naturally dominant European beech forests thus have a considerably higher climate change mitigation potential than the Scots pine forests replacing them, although the high share of beech wood used for the production of bioenergy impairs their potential. By estimating the extent of forest conversion in the northern German lowlands, the significant loss in the C storage and sequestration potential on the landscape-scale was demonstrated. The climatic gradient of the region had only little influence in this study, but performances of beech and pine under future climatic conditions will certainly affect the functioning of the investigated forests. Evidence exists that both tree species will suffer from climate change in the study region, especially from more frequent climatic extremes. In combination with additional negative effects of pine on groundwater recharge, microclimate and soil acidity, the results of this study strongly suggest that Scots pine is not a suitable option in a silviculture focusing on the mitigation of, and the resilience against climate change.