Mauri Timonen

The supra-long Scots pine tree-ring record for Finnish Lapland: Part 1, chronology construction and initial inferences

This paper reviews the development of the current ‘supra-long’ pine chronology for northern Finnish Lapland. In the forest-tundra ecotone region of northern Finnish Lapland over 250 samples from living Scots pines (Pinus sylvestris L.) and over 1700 samples of subfossil pines have been collected for dendrochronological studies. In addition, over 1400 subfossils have been sampled from the forested area of Finnish Lapland. The goal of the research was to build a more than 7000-year long continuous pine ring-width chronology. The construction of the chronology is now completed. The intensive phase of the data collection and chronology building lasted about 10 years, 1989 to 1999. The major part of the Finnish Lapland master curve was con structed several years ago, but it was extremely difficult to bridge the c. 300-year gap, prior to 165 bc between the ‘absolute’ younger part of the chronology and the “‘ oating’ older part. The crucial samples were identified and assembled in the chronology in early 1999, and there is now an unbroken pine chronology about 7500 years long constructed from the subfossil forest-limit pines of northern Finnish Lapland. The severe growth depression centred on 330 bc is likely to have been caused by increased wetness. A brief summary is presented of inferred tree-line changes from the location of the samples.

Old World megadroughts and pluvials during the Common Era

An atlas of megadroughts in Europe and in the Mediterranean Basin during the Common Era provides insights into climate variability. Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fennoscandia in the coming decades largely as a consequence of greenhouse gas forcing of climate. To place these and other “Old World” climate projections into historical perspective based on more complete estimates of natural hydroclimatic variability, we have developed the “Old World Drought Atlas” (OWDA), a set of year-to-year maps of tree-ring reconstructed summer wetness and dryness over Europe and the Mediterranean Basin during the Common Era. The OWDA matches historical accounts of severe drought and wetness with a spatial completeness not previously available. In addition, megadroughts reconstructed over north-central Europe in the 11th and mid-15th centuries reinforce other evidence from North America and Asia that droughts were more severe, extensive, and prolonged over Northern Hemisphere land areas before the 20th century, with an inadequate understanding of their causes. The OWDA provides new data to determine the causes of Old World drought and wetness and attribute past climate variability to forced and/or internal variability.

Sub-Milankovitch solar forcing of past climates: Mid and late Holocene perspectives

A comparison was performed of solar activity and terrestrial temperature records, both derived from tree rings (i.e., without dating uncertainties), with identification of detailed and highly quantified time- and timescale-dependent characteristics of solar forcing on climate through the current interglacial in the context of oceanic variability. The tree-ring–derived temperature record from high latitudes of Europe (Lapland) exhibits persistent annual-to-millennial–scale variations, with multidecadal to multicentennial periodicities reminiscent of the Sun9s periodicities. At millennial scales, cool temperatures coincided with large-scale glacial maxima. Moreover, millennial and bimillennial modes of climate variability were correlative with variations in sunspot numbers on similar scales, with near-century and near-zero lags, respectively. Although they were subtle in amplitude, the sub-Milankovitch–scale changes in the reception of the Sun9s energy could thus suffice to noticeably modulate interglacial climate variations. The relative significance of timescale-dependent, Sun-climate linkages has likely varied during the mid and late Holocene times, respectively. Thus, the warmer and cooler paleotemperatures during the Medieval Climate Anomaly and Little Ice Age were better explained by solar variations on a millennial rather than bimillennial scale. The observed variations may have occurred in association with internal climate amplification (likely, thermohaline circulation and El Nino–Southern Oscillation activity). The near-centennial delay in climate in responding to sunspots indicates that the Sun9s influence on climate arising from the current episode of high sunspot numbers may not yet have manifested itself fully in climate trends. If neglected in climate models, this lag could cause an underestimation of twenty-first–century warming trends.

MULTICENTENNIAL RING-WIDTH CHRONOLOGIES OF SCOTS PINE ALONG A NORTH-SOUTH GRADIENT ACROSS FINLAND

Four regional Scots pine ring-width chronologies at the northern forest-limit, and in the northern, middle and southern boreal forest belts in Finland cover the last fourteen centuries. Tree-ring statistics and response functions were examined, and tree-ring width variation was also compared to North Atlantic Oscillation (NAO) and volcanic forcing. The tree-ring statistics show evidence of an ecogeographical gradient along a north–south transect. The three northernmost regional chronologies share a positive response to mid-summer temperature, and all four chronologies show positive and significant correlation to early-summer precipitation. Moreover, a positive and significant relationship to winter NAO was detected in three out of four regional chronologies. NAO also drives the common (inter-regional) growth variability. Years of known cool summers caused by volcanic forcing exhibit exceptionally narrow tree rings in the three northernmost regional chronologies.

Growth Variability of Scots Pine (Pinus sylvestris) along a West-East Gradient across Northern Fennoscandia: A Dendroclimatic Approach

Abstract We performed a spatiotemporal analysis of a network of 21 Scots pine (Pinus sylvestris) ring-width chronologies in northern Fennoscandia by means of chronology statistics and multivariate analyses. Chronologies are located on both sides (western and eastern) of the Scandes Mountains (67°N–70°N, 15°E–29°E). Growth relationships with temperature, precipitation, and North Atlantic Oscillation (NAO) indices were calculated for the period 1880–1991. We also assessed their temporal stability. Current July temperature and, to a lesser degree, May precipitation are the main growth limiting factors in the whole area of study. However, Principal Component Analysis (PCA) and mean interseries correlation revealed differences in radial growth between both sides of the Scandes Mountains, attributed to the Oceanic-Continental climatic gradient in the area. The gradient signal is temporally variable and has strengthened during the second half of the 20th century. Northern Fennoscandia Scots pine growth is positively related to early winter NAO indices previous to the growth season and to late spring NAO. NAO/growth relationships are unstable and have dropped in the second half of the 20th century. Moreover, they are noncontinuous through the range of NAO values: for early winter, only positive NAO indices enhance tree growth in the next growing season, while negative NAO does not. For spring, only negative NAO is correlated with radial growth.

Causes and consequences of past and projected Scandinavian summer temperatures, 500-2100 AD.

Tree rings dominate millennium-long temperature reconstructions and many records originate from Scandinavia, an area for which the relative roles of external forcing and internal variation on climatic changes are, however, not yet fully understood. Here we compile 1,179 series of maximum latewood density measurements from 25 conifer sites in northern Scandinavia, establish a suite of 36 subset chronologies, and analyse their climate signal. A new reconstruction for the 1483–2006 period correlates at 0.80 with June–August temperatures back to 1860. Summer cooling during the early 17th century and peak warming in the 1930s translate into a decadal amplitude of 2.9°C, which agrees with existing Scandinavian tree-ring proxies. Climate model simulations reveal similar amounts of mid to low frequency variability, suggesting that internal ocean-atmosphere feedbacks likely influenced Scandinavian temperatures more than external forcing. Projected 21st century warming under the SRES A2 scenario would, however, exceed the reconstructed temperature envelope of the past 1,500 years.

Finnish supra-long tree-ring chronology extended to 5634 BC

Dendrochronological crossdating was used to expand a pre-existing multi-millennial tree-ring chronology for Finnish Lapland. Twelve tree-ring series from sedimentary archives of five small lakes contributed to the oldest part of the chronology, extending the record to 5634 BC (previously 5520 BC). Today, the chronology is the longest conifer tree-ring chronology in Eurasia. The geologically oldest pine megafossils were found the eastern part of the region, i.e. north-east Finnish Lapland, and this is also where the authors expect that other very old pine megafossils may be found in the future. Cool climatic conditions c.8 ka ago may have prevented pine regeneration in the tundra-forest ecotone. This may explain the practical difficulty of making the chronology longer than its present form in the study region.

Comparison of living-tree and subfossil ringwidths with summer temperatures from 18th, 19th and 20th centuries in Northern Finland

Summary This work seeks to analyse the importance of summer-temperatures an the tree-ring growth of Scots pine ( Pinus sylvestris L.) during the past three centuries. Three living-tree chronologies, subfossil pine chronology and one composite tree-ring chronology were constructed from latitudinal and altitudinal forest-limits of pine in northern Finland and compared with meteorological data comes from three localities. These data include early instrumental temperature observations from 18 th and 19 th centuries. The modern meteorological data covers the period from 1860 to present. Response functions were derived by means of Pearson correlations using five subperiods as follows: 1738–1748, 1802–1822, 1825–1835, 1861–1926 and 1927–1992. It was demonstrated that the correlations between ringwidths and mid-summer (July) temperatures did not vary significantly as a function of time. Early(June) and late-summer (August) mean temperatures were secondary in relation to mid-summer temperatures in controlling the radial growth. Early-summer temperatures governed pine radial growth most clearly during the 19 th century, whereas late-summer temperatures had strongest influence an ring-widths during the 18 th century and later part of the 20 th century. There was no clear signature of temporally reduced sensitivity of Scots pine ring-widths to mid-summer temperatures over the periods of early meteorological observations. Subfossil pine chronology, constructed using pines recovered from small Jakes along the forest-limit zone, showed a consistent pattern of response to summer-temperatures in relation to living-tree chronologies.

Forest Management and Regeneration Success in Protection Forests near the Timberline in Finnish Lapland

Sustainable forestry is practised in Finnish Lapland north of latitude 69°, i.e. farther north than anywhere else in the world. The Protection Forests Act was passed in 1922 to guarantee the stability of timberline forests in this climatically extreme, unique area. This review, based on earlier studies, covers forest and climate statistics, and legislation, regulations and recommendations of forest management, and the effects of management practices and climatic trends on the results of natural and artificial regeneration of conifer forests, as well as on shifts in the timberline. The review is mainly based on Finnish research, but references from the other Fennoscandian countries have also been used. Temperature plays the main role in forest regeneration success, and the effective temperature sum has been found to express well the location of the Scots pine (Pinus sylvestris L.) timberline. Natural regeneration of pine has been satisfactory, especially when combined with soil preparation. In many cases the pine forests regenerate in the form of advance growth without any silvicultural action. The success of artificial regeneration seems to be associated with favourable, warm periods lasting for at least 20 yrs. The effects of cutting, forest fires, reindeer herding, biotic damage and air pollution on forest regeneration and the conifer timberlines are discussed.

Growth Trends of Scots Pine (Pinus sylvestris, L.) in Unmanaged and Regularly Managed Stands in Southern and Central Finland

Annual ring data collected from strict nature reserves and long-term permanent plots of the Finnish Forest Research Institute (METLA) did not reveal any long-term trend in the radial increment of Scots pine (Pinus sylvestris) in the southern part of Finland during the last 100 years. This result was supported by the analysis of standwise increment data from permanent thinning experiments.