Midsummer (July) temperatures are reconstructed for the last 7500 years using the long ring-width chronology of Scots pine (Pinus sylvestris L.) from northern Finland. The chronology was built using regional curve standardization (RCS), which allows for long-term (low-frequency) variability to be extracted from this annually resolved record of 1087 samples from living trees and subfossil timber. Short-and long-term changes in reconstructed July temperatures are presented. The regression model accounts for 37% of the dependent instrumental temperature variance between ad 1879 and 1992. The warmest 30-year periods were ad 560–531, ad 560–531, 1190–1161 bc and ad 1541–1570, and the coldest 5240–5211, 5150–5121 and 3710–3681 bc. The warmest 100-year periods were ad 1501–1600, 600–501 bc and 300–201 bc, and the coldest 5200–5101, 2500–2401 and 1500–1401 bc. Broad comparisons are made with dendrochronological, lacustrine and glacial proxy evidence.

A 1200-year multiproxy record of tree growth and summer temperature at the northern pine forest limit Published by: http://www.sagepublications.com can be found at: The Holocene Additional services and information for AbstractCombining nine tree growth proxies from four sites, from the west coast of Norway to the Kola Peninsula of NW Russia, provides a well replicated (> 100 annual measurements per year) mean index of tree growth over the last 1200 years that represents the growth of much of the northern pine timberline forests of northern Fennoscandia. The simple mean of the nine series, z-scored over their common period, correlates strongly with mean June to August temperature averaged over this region (r = 0.81), allowing reconstructions of summer temperature based on regression and variance scaling. The reconstructions correlate significantly with gridded summer temperatures across the whole of Fennoscandia, extending north across Svalbard and south into Denmark. Uncertainty in the reconstructions is estimated by combining the uncertainty in mean tree growth with the uncertainty in the regression models. Over the last seven centuries the uncertainty is < 4.5% higher than in the 20th century, and reaches a maximum of 12% above recent levels during the 10th century. The results suggest that the 20th century was the warmest of the last 1200 years, but that it was not significantly different from the 11th century. The coldest century was the 17th. The impact of volcanic eruptions is clear, and a delayed recovery from pairs or multiple eruptions suggests the presence of some positive feedback mechanism. There is no clear and consistent link between northern Fennoscandian summer temperatures and solar forcing.

In the Okavango Delta (about 28,000 km 2 ) the number of identifi ed species is 1,300 for plants, 71 for fi sh, 33 for amphibians, 64 for reptiles, 444 for birds, and 122 for mammals. The local occurrence of different species of these taxonomic groups in the Okavango Delta is mainly due to a hydrological gradient from permanent streams and swamps to seasonal fl oodplains, riparian woodlands, and dry woodlands. This level of species diversity is normal for the southern African region, and all analyzed aquatic groups are composed of ubiquitous species with an additional signifi cant proportion of species originating from northern, more tropical systems. Cyclical variations in climate over thousands of years have created a huge wetland complex in the upper Zambezi and Okavango Rivers during wet phases. This wetland complex has fragmented into the Okavango Delta and other large wetlands in Zambia during dry phases. There are no endemic species in the Okavango Delta while the Southcentral African wetland complex is a centre of endemism. Species diversity of the Okavango Delta is a consequence of this unique environment, with dynamic shifts in fl ooding patterns that in turn force constant changes in patterns of plant succession and dependent animals. Temporal variations in fl ooding also cause accumulation and sudden mobilization of nutrients which are readily used by well adapted plant species. As a consequence, locally high biological productivity occurs, which in turn results in high numbers of grazing mammals.

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 constructed several years ago, but it was extremely dif cult 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 identi ed 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.

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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.

Helama, S., Timonen, M., Holopainen, J., Ogurtsov, M. G., Mielikäinen, K., Eronen, M., Lindholm, M. and Meriläinen, J. 2009. Summer temperature variations in Lapland during the Medieval Warm Period and the Little Ice Age relative to natural instability of thermohaline circulation on multi-decadal and multi-centennial scales.ABSTRACT: New tree ring-based analysis for climate variability at a regional scale is presented for high latitudes of Europe. Our absolutely dated temperature reconstruction seeks to characterise the summer temperatures since AD 750. The warmest and coolest reconstructed 250-year periods occurred at AD 931-1180 and AD 1601-1850, respectively. These periods share significant temporal overlap with the general hemispheric climate variability due to the Medieval Warm Period (MWP) and the Little Ice Age (LIA). Further, we detect a multi-decadal (ca. 50-to 60-year) rhythm, attributable to instability of the North Atlantic Deep Water, in the regional climate during the MWP but not during the LIA. Intensified formation of the North Atlantic Deep Water further appeared coincident to the initiation and continuation of MWP, the mid-LIA transient warmth occurring during the period AD 1391-1440, and to recent warming. Our results support the view that the internal climate variability (i.e. thermohaline circulation) could have played a role in the earlier start of the MWP in several proxy reconstructions compared to the externally forced model simulations.

Dendrochronologically dated changes in the limit of pine in northernmost Finland during the past 7.5 millennia SAMULI HELAMA, MARKUS LINDHOLM, MAURI TIMONEN AND MATTI ERONEN Helama, S., Lindholm, M., Timonen, M. & Eronen, M. 2004 (August): Dendrochronologically dated changes in the limit of pine in northernmost Finland during the past 7.5 millennia. Boreas, Vol. 33, pp. 250-259. Oslo. ISSN 0300-9483.Postglacial changes in the occurrence of Scots pine in northernmost Finland and adjacent areas are indicated in data sets of megafossils dated to an accuracy of 1 year using analysis of tree rings. After adjustment for land uplift, results in the altitudinal and latitudinal extent and in the structure of the pine limit are compared in the context of megafossil and pollen evidence from Fennoscandia, the Kola Peninsula and Scotland. Temporal changes in tree density at the forest limit are estimated from chronology sample size. The record from the subregion of Enontekiö shows a long-term retreat of pine, corresponding to summer cooling caused by orbital forcing. This long-term trend is superposed by shorter-term fluctuations, which is consistent with other proxy evidence of pine. An abrupt decline in the forest limit occurred during the first millennium BC in two major steps, with no indication of highaltitude pine germination between 800 BC and AD 100. The subregions of Inari and Utsjoki show significant fluctuations at the forest limit in tree density only.