Kristina Sohar

Dendroclimatic signals of pedunculate oak (Quercus robur L.) in Estonia

This study investigates the climate impact on the radial increment of pedunculate oak (Quercus robur L.) in Estonia at the species’ northern distribution limit. Tree-ring width series of 162 living oaks were compiled into three regional chronologies—western (1646–2008), northeastern (1736–2011), and southeastern Estonia (1912–2011). Although these regional growth patterns are similar to each other and even to the growth patterns in adjacent regions, spatial differences in growth responses to climate were established. Thus, oaks growing on shallow soil in western Estonia are positively influenced by summer (June–August) precipitation, and oaks on the deeper soil in northeastern Estonia are favoured by June temperature, while oaks in the southeastern part of the country depend on both July precipitation and temperature. These relationships are pronounced especially in pointer years. However, due to the impact of regional weather fluctuations on tree growth, there is a lack of correspondence between the local and the pan-European pointer years. In addition, our research presents the first tree-ring-based palaeoclimatic reconstruction for the country. Although the created model has relatively low predictive skill describing less than a quarter of the variance in actual summer precipitation in western Estonia, it has passable capacity of detecting past rainfall extremes.

Oak decline in a southern Finnish forest as affected by a drought sequence

We investigated the decline of a pedunculate oak (Quercus robur L.) forest growing on shallow soil at the northern distributional limit of the species in southern Finland, using the dendroclimatic approach. About 200-year-old trees in three vigour classes — healthy, declining and dead — were sampled in 2008. Annual tree-ring, earlywood and latewood widths were measured and chronologies were established. The tree-ring data were correlated with monthly and seasonal climate data. Radial increment of oaks was positively related to the June and July precipitations. This was expressed especially in total ring width and latewood width, whereas the earlywood was more influenced by the warmer winter and spring. Furthermore, the correlation between the current year earlywood width and the preceding year latewood width was higher than between the earlywood and latewood of the same year. The analyses showed that the dead oaks and part of the declining oaks had ceased growing during 2005–2007 after a decadelong summer drought series. This indicates a time lag in the oak dieback. The radial growth of the declining and the dead oaks had dropped already since the 1990s, while the healthy oaks had better longterm growth and higher adaptive capacity to climate variation.

Reconstruction of precipitation variability in Estonia since the eighteenth century, inferred from oak and spruce tree rings

There is plenty of evidence for intensification of the global hydrological cycle. In Europe, the northern areas are predicted to receive more precipitation in the future and observational evidence suggests a parallel trend over the past decades. As a consequence, it would be essential to place the recent trend in precipitation in the context of proxy-based estimates of reconstructed precipitation variability over the past centuries. Tree rings are frequently used as proxy data for palaeoclimate reconstructions. Here we use deciduous (Quercus robur) and coniferous (Picea abies) tree-ring width chronologies from western Estonia to deduce past early-summer (June) precipitation variability since 1771. Statistical model transforming our tree-ring data into estimates of precipitation sums explains 42% of the variance in instrumental variability. Comparisons with products of gridded reconstructions of soil moisture and summer precipitation illustrate robust correlations with soil moisture (Palmer Drought Severity Index), but lowered correlation with summer precipitation estimates prior to mid-nineteenth century, these instabilities possibly reflecting the general uncertainties inherent to early meteorological and proxy data. Reconstructed precipitation variability was negatively correlated to the teleconnection indices of the North Atlantic Oscillation and the Scandinavia pattern, on annual to decadal and longer scales. These relationships demonstrate the positive precipitation anomalies to result from increase in zonal inflow and cyclonic activity, the negative anomalies being linked with the high pressure conditions enhanced during the atmospheric blocking episodes. Recently, the instrumental data have demonstrated a remarkable increase in summer (June) precipitation in the study region. Our tree-ring based reconstruction reproduces this trend in the context of precipitation history since eighteenth century and quantifies the unprecedented abundance of June precipitation over the recent years.

Oak decline as illustrated through plant-climate interactions near the northern edge of species range

This paper investigates historical growth and climate records among the oak sites representing the northern edge of species range in northernmost Europe (Finland). This is to characterize plant–climate interactions for a multitude of sites where oak decline has recently been observed and understand this most recent decline in the context of the past decline studies elsewhere. Further, our paper demonstrates the procedures the tree-ring data can be used in isolating those factors significantly contributing to decline. Our findings point towards complex tree mortality dynamics. Compared to oaks that remain healthy, the declining and dead oaks represent the trees clearly having suffered from competition and edaphic position within their site. This was indicated by their reduced growth rates and more drastic growth disturbance, with indications of reduced resilience. Growth of these trees was also deteriorated by cold soil temperatures during the dormancy in addition to summer droughts. By contrast, the growth of healthy oaks has been notably ameliorated by springtime soil warming over the past decades. The results demonstrate the climatic determinants for observed decline in the northern oak sites, which may become increasingly vulnerable to higher background tree mortality rates and die-off in response to future warming and drought, although their habitats are not normally considered water-limited.

Global Boundary Lines of N2O and CH4 Emission in Peatlands

Predicting N2O (nitrous oxide) and CH4 (methane) emissions from peatlands is challenging because of the complex coaction of biogeochemical factors. This study uses data from a global soil and gas sampling campaign. The objective is to analyse N2O and CH4 emissions in terms of peat physical and chemical conditions. Our study areas were evenly distributed across the A, C and D climates of the Koppen classification. Gas measurements using static chambers, groundwater analysis and gas and peat sampling for further laboratory analysis have been conducted in 13 regions evenly distributed across the globe. In each study area at least two study sites were established. Each site featured at least three sampling plots, three replicate chambers and corresponding soil pits and one observation well per plot. Gas emissions were measured during 2–3 days in at least three sessions. A log-log linear function limits N2O emissions in relation to soil TIN (total inorganic nitrogen). The boundary line of N2O in terms of soil temperature is semilog linear. The closest representation of the relationship between N2O and soil moisture is a local regression curve with its optimum at 60–70 %. Semilog linear upper boundaries describe the effects of soil moisture and soil temperature to CH4 best.

Nitrogen-rich organic soils under warm well-drained conditions are global nitrous oxide emission hotspots

Nitrous oxide (N2O) is a powerful greenhouse gas and the main driver of stratospheric ozone depletion. Since soils are the largest source of N2O, predicting soil response to changes in climate or land use is central to understanding and managing N2O. Here we find that N2O flux can be predicted by models incorporating soil nitrate concentration (NO3−), water content and temperature using a global field survey of N2O emissions and potential driving factors across a wide range of organic soils. N2O emissions increase with NO3− and follow a bell-shaped distribution with water content. Combining the two functions explains 72% of N2O emission from all organic soils. Above 5 mg NO3−-N kg−1, either draining wet soils or irrigating well-drained soils increases N2O emission by orders of magnitude. As soil temperature together with NO3− explains 69% of N2O emission, tropical wetlands should be a priority for N2O management.In a global field survey across a wide range of organic soils, the authors find that N2O flux can be predicted by models incorporating soil nitrate concentration (NO3–), water content and temperature. N2O emission increases with NO3– and temperature and follows a bell-shaped distribution with water content.