Reliability of temperature signal in various climate indicators from northern Europe.

We collected relevant observational and measured annual-resolution time series dealing with climate in northern Europe, focusing in Finland. We analysed these series for the reliability of their temperature signal at annual and seasonal resolutions. Importantly, we analysed all of the indicators within the same statistical framework, which allows for their meaningful comparison. In this framework, we employed a cross-validation procedure designed to reduce the adverse effects of estimation bias that may inflate the reliability of various temperature indicators, especially when several indicators are used in a multiple regression model. In our data sets, timing of phenological observations and ice break-up were connected with spring, tree ring characteristics (width, density, carbon isotopic composition) with summer and ice formation with autumn temperatures. Baltic Sea ice extent and the duration of ice cover in different watercourses were good indicators of winter temperatures. Using combinations of various temperature indicator series resulted in reliable temperature signals for each of the four seasons, as well as a reliable annual temperature signal. The results hence demonstrated that we can obtain reliable temperature information over different seasons, using a careful selection of indicators, combining the results with regression analysis, and by determining the reliability of the obtained indicator.

Spatial variability and temporal trends in water-use efficiency of European forests.

The increasing carbon dioxide (CO2 ) concentration in the atmosphere in combination with climatic changes throughout the last century are likely to have had a profound effect on the physiology of trees: altering the carbon and water fluxes passing through the stomatal pores. However, the magnitude and spatial patterns of such changes in natural forests remain highly uncertain. Here, stable carbon isotope ratios from a network of 35 tree-ring sites located across Europe are investigated to determine the intrinsic water-use efficiency (iWUE), the ratio of photosynthesis to stomatal conductance from 1901 to 2000. The results were compared with simulations of a dynamic vegetation model (LPX-Bern 1.0) that integrates numerous ecosystem and land-atmosphere exchange processes in a theoretical framework. The spatial pattern of tree-ring derived iWUE of the investigated coniferous and deciduous species and the model results agreed significantly with a clear south-to-north gradient, as well as a general increase in iWUE over the 20th century. The magnitude of the iWUE increase was not spatially uniform, with the strongest increase observed and modelled for temperate forests in Central Europe, a region where summer soil-water availability decreased over the last century. We were able to demonstrate that the combined effects of increasing CO2 and climate change leading to soil drying have resulted in an accelerated increase in iWUE. These findings will help to reduce uncertainties in the land surface schemes of global climate models, where vegetation-climate feedbacks are currently still poorly constrained by observational data.

Dependence of tree ring stable isotope abundances and ring width on climate in Finnish oak.

We measured ring widths and isotopic abundances of carbon, oxygen and hydrogen (delta(13)C, delta(18)O and delta(2)H) from the latewood of tree rings of pedunculate oak (Quercus robur L.) in its distributional northern limit in Southern Finland. Ring width was observed to be related to precipitation and relative humidity but not significantly to temperature. delta(13)C and delta(18)O were significantly related to all studied climatic variables, most strongly to cloud cover. Variations in delta(2)H were discovered to be complex combinations of signals from biochemical and physical processes. The results suggest that oaks in Finland can be used as a source of climate information. delta(18)O was discovered to be especially promising as it showed the strongest climate signal and highest common signal between trees. The relationship between climate and ring width indicates that water availability is the main control of ring radial growth. This is supported by the isotope data. High correlation between delta(13)C and delta(18)O time series indicates that photosynthetic carbon assimilation is limited by stomatal control. Therefore, in contrast to the expected temperature limitation, our data indicate that drought limits oak growth more than cold temperatures on the border of its northernmost distribution range.