Late Quaternary history of spruce in southern Europe

Abstract

The late Quaternary history of fossil spruces in southern Europe (Picea abies (L.) Karsten and Picea omorika (Pancic) Purkyne) is based on 163 selected pollen, charcoal and macrofossil records. The timing of immigration of P. abies is estimated from data where the Picea curve passed the threshold value of 4%. P. abies occupied the southern European mountain ranges – excluding the Pyrenees – during the middle part of the last interglacial. Spruce reached its late Quaternary maximum expansion during the early Weichselian, after which it retired from central Europe and expanded in southern Europe during the middle Weichselian interstadials. A general decline in geographical distribution occurred during the last glacial maximum, and populations were most restricted during the Alpine deglaciation. The concept of ‘glacial refugia’ does not apply to residual populations because current climatic reconstructions relate periods of maximum spruce decline to maximum continental dryness during the growth season, rather than to full glacial conditions. Spruce took part in late glacial and early Holocene tree expansions in the eastern Alps and Carpathian, but failed to spread from residual populations in the Apennines and the Pirin–Rila–Rhodopes Mountains. These differences are explained by the influence of oceanic air masses on upper forest belts with relation to geographic location and maximum elevation of mountain ranges. Late glacial spruce expansion in the Alps coincides with the abrupt warming at 14 700–14 500 yr cal BP. High migration rates were reached in the upper forest belts (e.g. 1500–2300 masl) during the early Holocene, and decreased since about 6 kyr cal BP, as a result of climatic cooling during the Neoglaciation (treeline depression), ecological competition with other tree species (Abies alba), climatic and physical setting of the highest ranges in western Alps, and human impact. The long late Quaternary fossil history of presently isolated spruce stands from the Apennines accounts for their state of genetic differentiation, which could not be fully understood from the shorter time interval of postglacial events.

Introduction

Norway spruce (Picea abies Karst.) is one of the most widespread tree species of Europe, and it forms an important component of boreal and mixed conifer – broad-leaved forests. Because of its wide utilization in forestry, there is much interest in its ecological requirements and genetic structure at the population level. Climatic change and increased air pollution have drawn special attention to the spruce decline in central Europe (Schulze et al., 1989) and to rapid shifts in the position of treeline at high latitudes (Kullmann, 1986, Kullmann, 1993) and in the Alps (Holtmeier, 1993). Knowledge of the late Quaternary history of this species provides a basis for comparing historical provenances and late Quaternary migration pathways with patterns of genetic variation among living populations. Indeed the genetic variability is structured by variations in geography, ecology, and reproductive systems but also by historical events (Lagercrantz and Ryman, 1990, Taberlet et al., 1998).

In the present work, spruce is chosen in an investigation of paleoecological problems of past biome reconstruction, tree migration, refugial patterns induced by climate change, and the influence of history on genetic variation among living populations. Several studies have described the late glacial and Holocene history of spruce and the impact of climatic change and human activity on forest history in the Alps (Markgraf, 1970, Tallentire, 1973, Burga, 1988, de Beaulieu et al., 1993, Lang, 1994). Late glacial timing and patterns of migration also depend on the location and extent of tree-survival areas during the last glacial, a question hampered by the difficulty of detecting small tree populations by paleobotanical methods. Detection of spruce at particular sites benefits from the moderately high production of pollen (Markgraf, 1980, Hicks, 1994), its anemophilous pollen+seed dispersal, and the easy identification of the pollen (Beug, 1961). Huntley and Birks (1983) suggest that the area where a tree taxon first appears in the postglacial corresponds with its survival area during the full glacial, i.e. a ‘glacial refugium’ according to Huntley and Birks (1983) and Bennett et al. (1991). These authors used informally the term ‘glacial refugium’ to refer to an area of any size in which a taxon persisted, during a cold phase. In the present work the term refugium is used in a stricter sense as an area in which a taxon persists during a disturbance event that caused its extinction in contiguous areas where it previously occurred (see also Lynch, 1988). A survival area is here defined as the region to which a taxon is reduced during unfavorable periods. The identification of refugial and survival areas requires evaluating the history prior to the full glacial, using the last interglacial as a starting point to highlight processes induced by the late Quaternary phases of coldest climate on tree dynamics and migration.

This synthesis has been aided by several new paleobotanical investigations, including radiocarbon-dated sites from the Alps, the Apennines, and the Balkan peninsula, and recent progress in refining the late Pleistocene stratigraphy (e.g. van Kolfschoten and Gibbard, 2000). In addition, late glacial spruce pollen and macrofossils have recently been described from the eastern Italian Pre-Alps (Avigliano et al., 2000), a poorly studied area where spruce had not been reported, and late Pleistocene spruce records are available from maar deposits in central Italy (Allen et al., 2000, Magri, 1999, Magri and Sadori, 1999). Possible centers of survival in the eastern Po Plain, the Adriatic depression, the eastern Alpine and Slovenian ranges, and the Pannonian plain during the last glacial maximum (LGM) can also be discussed thanks to information provided by Sercelj (1996), Schmidt et al. (1998, 2000), Combourieu-Nebout et al. (1999) and Willis et al. (2000).