Open Access

Peatlands are wetland ecosystems covering a relatively small area of the World (~3%), but at the same time storing excessive amounts of carbon for a very long time (equivalent to the four times global annual net primary production). As carbon sinks, peatlands work in spite of their slow growth, absorbing carbon dioxide (CO2) through the photosynthetic activity of the peatland plants and their low growth rates, and because high groundwater table removes oxygen from the soil and slows down the decomposition of the dead plant matter. Because of the relative lack of the oxygen in the peat, especially compared to the mineral soils, methanogen populations in the peatlands are abundant, and releasing methane (CH4), a potent greenhouse gas, to the atmosphere. Therefore, peatlands are generally at the same time significant carbon sinks and stores as well as the methane sources. The balance among the two peatland gass fluxes (CO2 and CH4) will dictate the impact of any given peatland on the global climate and primarily driven by hydrology, in the form of the groundwater table levels. Because of the slow decomposition rates, and from radiocarbon dating of the peat as well as the subfossil records buried in it, carbon stored in peatlands is locked for a very long time (centuries to millennia). It is, therefore, crucial to gain insights into the development of peatlands and their gas balance through time. One way to get both is by studying peatland hydrology in the form of the groundwater table levels and their historical variations. Unfortunately, intensive monitoring of peatland groundwater table, when available, is an only a recent endeavor. Therefore, we need to employ proxies to reconstruct the past by leveraging the present. In statistics, proxy variables are often used when the observations of the variable of interest, are either missing or too difficult to obtain. In this thesis, I tested whether we can use the radial growth of the Scots pines growing on peat as proxies to the peatland hydrology. To that end, I studied growth responses of the peatland Scots pines. Other proxies can and are used for the reconstructions of the groundwater table levels, but tree-growth is widely used as one of the proxies to reconstruct past environments which is at the same time annually resolved. First, I examined the growth ecology of the peatland Scots pines by looking at their intra-annual development and trying to find relationships between it and environmental factors while at the same time comparing it with the Scots pines growing at the forest sites. I first tried with wood anatomy and found that, unfortunately, peatland Scots pines do not form enough wood cells, and consequently do not have high temporal resolution, necessary to investigate the intra-annual patterns of the radial growth. Initial results from wood anatomical investigations were interesting none-the-less, indicating that peatland Scots pines might have smaller cell features than the Scots pines from forests, but might at the same time maintain Early/Latewood ratios of those same features. After I found that wood anatomical series were not resolved enough I decided to go with dendrometers, linear displacement sensors which constantly monitor the variations of stem radius, to get insights into the intra-annual growth patterns of the peatland Scots pines. Before using dendrometers for ecological investigations, I was involved in implementing routines commonly used in the analysis of the dendrometer signals and bringing them to R in the form of the dendrometeR package. At one peatland complex, I installed dendrometers on ten trees in total at both peatland and forest sites and compared the pattern of the standardized signal. I inferred from the comparisons and classifications that the signal from two sites was indistinguishable for the dendrometer series shorter than five days. Furthermore, the most important environmental factor driving the radial variation at the peatland site was hydrological, daily relative humidity, indicating further that peatland hydrology might indeed be the driver behind peatland Scots pine growth. Finally, I looked at the growth responses of peatland Scots pines from central Estonia using dendrochronological methods. Peatland hydrology, in the form of the groundwater table levels, was indeed the environmental factor with the strongest, and also stationary, correlations with the radial growth of the peatland Scots pine. That relationship indicated that peatland Scots pines are indeed possible proxies for reconstructing past levels of the peatland groundwater tables. My study further indicated that the growth response of the peatland Scots pines was non-linear, further complicating the reconstructions of the past peatland hydrology. However, the strength of the growth response was proportional to the general hydrological regime, expressed as median groundwater table level. As the hydrological regime of the peatland does not vary considerably on the annual scales, but more on decadal it might be more appropriate to find another, independent, proxy to the hydrological regime first, and than use annually resolved radial growth of the peatland Scots pine to reconstruct past levels of the peatland groundwater table.

AbstractUsing measurements from high resolution monitoring of radial tree-growth we present new data of the growth reactions of four widespread broadleaved tree-species to the combined European drought years 2018 and 2019. We can show that, in contrast to field crops, trees could make better use of the winter soil moisture storage in 2018 which buffered them from severe drought stress and growth depressions in this year. Nevertheless, legacy effects of the 2018 drought accompanied by sustained low soil moisture conditions (missing recharge in winter) and again higher than average temperatures and low precipitation in spring/summer 2019 have resulted in severe growth reductions for all studied tree-species in this year. This highlights the pivotal role of soil water recharge in winter. Although short term resistance to hot summers can be high if sufficient winter precipitations buffers forest stands from drought damage, legacy effects will strongly impact tree growth in subsequent years if the drought persists. The two years 2018 and 2019 are extreme with regard to historical instrumental data but, according to regional climate models, resemble rather normal conditions of the climate in the second half of the 21st century. Therefore the observed strongly reduced growth rates can provide an outlook on future forest growth potential in northern Central Europe and beyond.

Understanding the effects of temperature and moisture on radial growth is vital for assessing the impacts of climate change on carbon and water cycles. However, studies observing growth at sub-daily temporal scales remain scarce. We analysed sub-daily growth dynamics and its climatic drivers recorded by point dendrometers for 35 trees of three temperate broadleaved species during the years 2015–2020. We isolated irreversible growth driven by cambial activity from the dendrometer records. Next, we compared the intra-annual growth patterns among species and delimited their climatic optima. The growth of all species peaked at air temperatures between 12 and 16°C and vapour pressure deficit (VPD) below 0.1 kPa. Acer pseudoplatanus and Fagus sylvatica, both diffuse-porous, sustained growth under suboptimal VPD. Ring-porous Quercus robur experienced a steep decline of growth rates with reduced air humidity. This resulted in multiple irregular growth peaks of Q. robur during the year. By contrast, the growth patterns of the diffuse-porous species were always right-skewed unimodal with a peak in June between day of the year 150–170. Intra-annual growth patterns are shaped more by VPD than temperature. The different sensitivity of radial growth to VPD is responsible for unimodal growth patterns in both diffuse-porous species and multimodal growth pattern in Q. robur.

Abstract The role of future forests in global biogeochemical cycles will depend on how different tree species respond to climate. Interpreting the response of forest growth to climate change requires an understanding of the temporal and spatial patterns of seasonal climatic influences on the growth of common tree species. We constructed a new network of 310 tree‐ring width chronologies from three common tree species (Quercus robur, Pinus sylvestris and Fagus sylvatica) collected for different ecological, management and climate purposes in the south Baltic Sea region at the border of three bioclimatic zones (temperate continental, oceanic, southern boreal). The major climate factors (temperature, precipitation, drought) affecting tree growth at monthly and seasonal scales were identified. Our analysis documents that 20th century Scots pine and deciduous species growth is generally controlled by different climate parameters, and that summer moisture availability is increasingly important for the growth of deciduous species examined. We report changes in the influence of winter climate variables over the last decades, where a decreasing influence of late winter temperature on deciduous tree growth and an increasing influence of winter temperature on Scots pine growth was found. By comparing climate–growth responses for the 1943–1972 and 1973–2002 periods and characterizing site‐level growth response stability, a descriptive application of spatial segregation analysis distinguished sites with stable responses to dominant climate parameters (northeast of the study region), and sites that collectively showed unstable responses to winter climate (southeast of the study region). The findings presented here highlight the temporally unstable and nonuniform responses of tree growth to climate variability, and that there are geographical coherent regions where these changes are similar. Considering continued climate change in the future, our results provide important regional perspectives on recent broad‐scale climate–growth relationships for trees across the temperate to boreal forest transition around the south Baltic Sea.

Climate warming advances the onset of tree growth in spring, but above- and belowground phenology are not always synchronized. These differences in growth responses may result from differences in root and bud dormancy dynamics, but root dormancy is largely unexplored. We measured dormancy in roots and leaf buds of Fagus sylvatica and Populus nigra by quantifying the warming sum required to initiate above- and belowground growth in October, January and February. We furthermore carried out seven experiments, manipulating only the soil and not air temperature before or during tree leaf-out to evaluate the potential of warmer roots to influence budburst timing using seedlings and adult trees of F. sylvatica and seedlings of Betula pendula. Root dormancy was virtually absent in comparison with the much deeper winter bud dormancy. Roots were able to start growing immediately as soils were warmed during the winter. Interestingly, higher soil temperature advanced budburst across all experiments, with soil temperature possibly accounting for c. 44% of the effect of air temperature in advancing aboveground spring phenology per growing degree hour. Therefore, differences in root and bud dormancy dynamics, together with their interaction, likely explain the nonsynchronized above- and belowground plant growth responses to climate warming.